Int J Adv Manuf Technol (2012) 60:110 DOI 10.1007/s00170-011-3582-1 ORIGINAL ARTICLE Levitation characteristics of a squeeze-film air journal bearing at its normal modes Chao Wang & Y H Joe Au Received: 13 January 2011 / Accepted: 10 August 2011 / Published online: September 2011 # Springer-Verlag London Limited 2011 Abstract A tubular squeeze-film journal bearing was designed such that it flexed its shell at its normal modes producing a triangular modal shape The shell motion was created by a single-layer piezoelectric actuator powered at 75 V AC with a 75 V DC offset and the driving frequency coincided with the modal frequency of the bearing The paper provided a theory that shows the existence of a positive pressure in a squeeze film responsible for the levitation phenomenon The various modes of vibration of the tubular bearing, made from AL2024-T3, were obtained from a finite element model implemented in ANSYS Two normal modes, the 13th and 23rd, at the respective theoretical frequencies of 16.37 and 25.64 kHz, were identified for further investigation by experiments with respect to the squeeze-film thickness and its load-carrying capacity While the bearing at both modes could cause levitation, the 13th mode has a greater load-carrying capacity because its modal shape produced a much lower end leakage Keywords Single-layer piezoelectric actuator Squeeze-film air bearing Mode shape Natural frequency Elastic hinge Introduction Bearings today need to be able to run at very high speed, providing high positional accuracy for the structure that it C Wang : Y H J Au (*) Advanced Manufacturing and Enterprise Engineering, School of Engineering and Design, Brunel University, Uxbridge, UK e-mail: Joe.Au@brunel.ac.uk supports, and requiring very little or no maintenance For this to happen, bearings must have tight tolerances and very low or zero friction during operation [1] This pushes many traditional contact-type bearings to their limits as they often fail due to friction, generating heat and causing wear By comparison, existing non-contact bearings fare better because of their very low or zero friction But some have their own problems, too For example, the fact that aerostatic bearings require an air supply means having to use a separate air compressor and connecting hoses This makes the installation bulky Aerodynamic and hydrodynamic bearings cannot support loads at zero speed Both hydrodynamic and hydrostatic bearings may cause contamination to the workpieces and the work environment because of the use of lubricating fluid A potential solution to the abovementioned problems is the new squeeze-film air bearing It works on the rapid squeeze action of an air film to produce separation between two metal surfaces This has the benefit of being compact with a very simple configuration because it does not require an external pressurized air supply, can support loads at zero speed and is free of contamination The general theory of the operation of squeeze-film type bearings has been documented by, for example, Salbu [2] who provided a basic account of the principle of squeezefilm bearing Since its publication, a variety of bearing designs based on this principle had been proposed and used The squeeze-film action was created using a piezoelectric actuator; in the 1960s, some of the ideas had even been patented in the USA [36] Scranton [11], in his 1987 patent, summarized the limitations of the design in the 1960s as: (1) the conforming surfaces are rigid and heavy; (2) the transducer which drives the surface of the bearing must be correspondingly massive; (3) the power consumption is high; (4) the noise is in the audible range; (5) the oscillatory force causes excessive vibration of the object supported by the bearing Scranton suggested bending the piezoelectric actuators in order to excite a flexural vibration mode on bearing However, he only showed a sketch of the fundamental concept without any implementation The design of the squeeze-film air bearing created by Yoshimoto [7] in 1993 involved a counterweight and two stack piezoelectric actuators The disadvantages of such a design are that the counterweight adds to the load and the stack actuators are more expensive than their single-layer counterparts, not to mention the higher power consumption Yoshimoto [7] and Storlaski [8, 9] produced designs that used what are called elastic hingesin order to create localised reduction in stiffnessresulting in a greater deformation and hence a greater variation of the squeezefilm thickness But designing elastic hinges is a complex matter and they cost more to manufacture All the designs reported in [79] excited the bearings at frequencies below that of their fundamental In 2006, Yoshimoto [10] reported the research on a newer design in which the bearing was driven by two piezoelectric actuators at the fundamental frequency, at 23.7 kHz, of the bearing When driven at this frequency, the oscillating amplitude of the bearing plate was significantly increased and, being ultrasonic, the bearing was quiet in operation Yoshimoto's work has led to the question of whether better performance can be achieved by driving a bearing at a modal frequency above the fundamental This paper attempts to explore such a possibility The purposes of work reported in this paper are: To develop a model that affirms the existence of positive pressure developed in a squeeze-film air bearing and to produce an approximate working formula for estimating the pressure; To develop a finite element model for a single-layer piezoelectric actuator that incorporates realistic boundary conditions; To develop a finite element model for the squeeze-film air bearing to study its modal shapes at various normal modes and to identify desirable modes for acceptable bearing performance; To determine by experiments the levitation performance of the squeeze-film air bearing at the desirable modes in respect of the air-film thickness and load-carrying capacity Principle of squeeze-film air bearings Consider two parallel plates of infinite lateral dimensions separated by a gap of h0; one of the plates oscillates sinusoidal normal to the other at a frequency with an Int J Adv Manuf Technol (2012) 60:110 amplitude a If the oscillating frequency is very high and the gap is very small, then, edge leakage of air is insignificant, and in addition, the process can be regarded as adiabatic Thus, pV g ẳ K; where p is the pressure, V the volume, the adiabatic constant equal to 1.4 for air and K the constant Suppose the moving plate is at the initial distance of h0 from the stationary plate, at which the air pressure in the squeeze film is ambient, denoted as po, then po Vog ẳ K: 1ị At time t, the plate moves to h ẳ ho ỵ a sinwt ịat which the pressure in the air film has changed (p+po), where p is the gauge pressure; thus, the equation of state becomes p ỵ po ịV g ẳ K: 2ị Dividing Eq by Eq and rearranging to obtain the pressure ratio as  g p V ẳ 1: 3ị p0 Vo Since the volume is proportional to the gap height, Eq can be rewritten in terms of the ratio of gap heights, as  g p h ẳ 1: 4ị p0 ho The plate moves sinusoidally such that the gap height at time t is governed by h ẳ ho ỵ a sinwtị, which on substitution into Eq gives   p h0 ỵ a sinwt ị g ẳ 1: 5ị p0 ho Introducing the non-dimensional parameters to Eq 5, namely the amplitude ratio " ẳ hao , and the time ratio t ẳ Tt , where T is the period of oscillation related to the angular frequency by T ẳ 2p w , Eq can be simplified to p ẳ ỵ " sin2pt ịịg 1: p0 6ị It is possible to show that the mean pressure ratio over a cycle of oscillation is positive, which means that the squeeze film exerts a lifting force on the plate thus causing it to float The proof is given below Using binomial expansion, the pressure ratio, Eq 6, can be represented by the infinite series pr ẳ X p g g 1ị g n ỵ 1ị n n " sin 2pt ị: ẳ p0 n! nẳ1 7ị Int J Adv Manuf Technol (2012) 60:110 The coefficients of the terms in this series are successively -, -(-1), -(-1)( -2), etc Since is positive, the sign of the coefficient alternates: negative when n is odd and positive when n is even The mean pressure ratio pr is obtained by integrating, with respect to , the series (7) term by term over the nondimensional time interval =[0, 1] In mathematical terms, Z X g g 1ị g n ỵ 1ị n n sin 2pt ịdt: " pr ẳ n! nẳ1 8ị n For odd powers of sin (2), that is, when n=2m+1, (m=0, 1, 2, ) Z sin2mỵ1 2pt ịdt ẳ 0: 9ị For even powers, that is, when n=2m, sin2m 2ptịdt ẳ 1:3:5 2m 3ị2m 1ị : 2:4:6 2m 2ị2m mẳ1 2m! 1:3:5 2m 3ị2m 1ị 2m " : 2:4:6 2m 2ị2m 11ị The series (11) contains only even power terms, and so is positive, thus confirming the existing of a levitation force in the squeeze film whose gap oscillates at high frequency in a sinusoidal manner Furthermore, since each term in Eq (11) is positive, g g1ịg2mỵ1ị 2m g gỵ1ịgỵ2m1ị , and Eq (11) can 2m the factor pr ẳ can be replaced by Cmỵ1 2m ỵ gị2m ỵ g ỵ 1ị ẳ : Cm 2m ỵ 2ị2 For =1.4, CCmỵ1 < for all positive integer values of m In m addition, since 1, the series (11a) converges The largest coefficient is C1, whose value is 0.840, as calculated earlier; other coefficients have values that are smaller than C1 This suggests another infinite series which defines the upper bound mean pressure ratio; this series is P C1 "2m and is a geometric series whose sum is pru ẳ mẳ1 pru ẳ 0:840 "2 "2 12ị Alternatively, applying numerical integration to Eq (6) over the non-dimensional time period =[0.1] with =1.4 and = 0.1 to 0.7 in 0.1 increments, the corresponding values of the mean pressure ratio pr were obtained The relationship between pr and is as shown in Fig Also shown on the graph is the upper bound mean pressure ratio pru calculated from Eq (12) It is noted that up to an amplitude ratio of 0.4, the error in the mean pressure ratio pr is less than about 1.3% Modelling of the proposed squeeze-film air bearing 3.1 Configuration of the bearing be re-written as Figure shows a squeeze-film air bearing It consists of a guideway and a squeeze-film air journal bearing with three X g g ỵ 1ị g ỵ 2m 1ị mẳ1 1:3:5 2m 3ị2m 1ị2m ỵ 1ị : 2:4:6 2m 2ị2mị2m ỵ 2ị The ratio of the two coefficients, after simplifying, is X g g 1ị g 2m ỵ 1ị g g ỵ 1ị g ỵ 2m 1ịg ỵ 2mịg ỵ 2m ỵ 1ị 2m ỵ 2ị! 10ị Substituting Eqs and 10 into (8) gives the mean pressure ratio as pr ẳ Cmỵ1 ẳ 2m! 1:3:5 2m 3ị2m 1ị 2m " : 2:4:6 2m 2ị2m 0.9 11aị The theoretical mean pressure ratio can be calculated using Eq (11a) or by performing numerical integration on Eq (6) However, it would be helpful to be able to use a simpler formula for estimating the mean pressure ratio The following derivation shows the formula In Eq (11a), the coefficient of the first term of the series ị is C1 ẳ g gỵ1 2! 2; and for = =1.4C1 =0.840 Similarly the coefficient for the mth term is given by gỵ2m1ị 1:3:52m3ị2m1ị 2:4:62m2ị2m ,and of the (m+1)th Cm ẳ g gỵ1ị2m 0.8 Mean pressure ratio Z term by 0.7 0.6 0.5 0.4 Exact 0.3 Approximate 0.2 0.1 0.1 0.3 0.5 0.7 Amplitude ratio Fig Mean pressure ratio versus amplitude ratioexact solution Eq (7) versus approximate solution Eq (12) Int J Adv Manuf Technol (2012) 60:110 Fig Squeeze air film bearing and its components: a guideway, a bearing and six piezoelectric actuators bonded to the three flat surfaces longitudinal flats on the circumference 120 apart On each flat surface were bonded two single-layer piezoelectric actuators, Fig The guideway, made from structural carbon steel, is a round rod fixed at one end and free at the other with an overhang of 130 mm; the short overhang is desired to avoid sagging due to its own weight The diameter of the round rod is 19.99 mm and the surface was ground finished The natural frequency of the round guideway set up as a cantilever was about 800 Hz The bearing, made from the material AL2024-T3, has a diameter of 20.02 mm, a length of 60 mm and a thickness of mm Three fins, each 20 mm long, are positioned 120 apart on the outer circumference of the bearing These fins are designed to provide a desirable modal shape when excited by actuators The desirable modal shape is that of a triangle (see Fig where the scale for radial displacement is grossly magnified) in the cross-sectional plane of maximum vibration This enables the air gap underneath the actuators to behave effectively as a squeeze air film The number of flats on the circumference of a tubular bearing has to be odd This way, loading in any direction can be effectively countered by the presence of a squeeze air film Three flats, and hence three fins, are preferred to five or more flats, because of the larger static and dynamic deformation that can be produced under the same driving conditions The design does not rely on complex elastic hinges to provide local flexibility, as used by other researchers [79], in order to achieve greater deflection of elements Being a simpler design, its manufacturing cost is much lower and the bearing can be adequately driven by a single-layer piezoelectric actuator (0.5 mm thick) with little power to provide the sinusoidal squeeze-film motion [10] Furthermore, the simple geometry of the design makes for subsequent simpler finite-element analysis (FEA) 3.2 Experimental setup Figure shows a schematic diagram of the experimental setup The items of equipment used were: A signal generator0 to 15 V peak-to-peak and to 100 kHz (S J Electronics) An actuator driverENP-1-1U (Echo Electronics) An actuator driver monitorENP-50U (Echo Electronics) A capacitive displacement sensor and a gauging moduleMicroSense 6810; measurement bandwidth up to 100 kHz; measurement ranges from 20 m to mm; resolution 0.25 nm rms at kHz over 50-m measurement range (Ixthus) A data acquisition cardPXI 6110 (National Instruments) The signal generator created a sinusoidal wave which was amplified by the actuator driver and shaped by the actuator monitor to provide an excitation signal, with a 75 V DC offset and a 75 V peak-to-zero AC sinusoid This excitation signal was used to drive the single-layer piezoelectric actuators The vibration response of the structure caused by the actuators was measured by the capacitive displacement sensor, whose output was sampled into a PC via the data acquisition card controlled by a LabVIEW programme 3.3 Modal analysis Modal analysis can determine the theoretical vibration characteristics, in terms of natural frequencies and mode shapes, of a structure or a machine component The natural frequencies and the mode shapes are important parameters in the design of a structure for dynamic loading conditions It is believed that certain mode shapes enhance the effectiveness of the squeeze air film in journal bearings Int J Adv Manuf Technol (2012) 60:110 Fig Schematics of the experimental setup These mode shapes have geometry that maximizes the amplitude ratio and minimizes the end leakage The tubular bearing was FEA modelled in ANSYS to establish its various modal frequencies and modal shapes The finite element Solid5 (3D Coupled-Field Solid) was chosen in order to study accurately the coupling between the piezoelectric actuator and the bearing flat From the FEA modal modelling, two candidate mode shapes were identified to have the desired geometry, namely the 13th and the 23rd modes at the respective theoretical natural frequencies of 16.37 and 25.64 kHz The mode shapes are shown in Fig where the red end of the colour spectrum denotes greater deformation It is observed that: Both modes produce flexing of the shell on the sleeve between pairs of fins which remain in the same angular orientation during the vibration; both mode shapes are triangular At mode 13, the outer edges of the round sleeve not appear to deform much while the middle section deforms noticeably (Fig 4c) At mode 23, the outer edges of the round sleeve deform noticeably while the middle section deforms not as much (Fig 4d) kind of motion cannot produce effective squeeze-film action 3.4 Static and dynamic analysis 3.4.1 Static analysiscomputer modelling and simulation The purpose of the static analysis was to determine the static deformation of the sleeve bearing when a 75 V DC voltage (0 V on the bottom and 75 V on the top surfaces of piezoelectric layer) was applied to the six single-layer piezoelectric actuators Figure shows the result of the analysis, from which a maximum radial deformation of 0.124 m is seen to occur in the middle section of the sleeve The abovementioned analysis was repeated for other driving voltages and Fig shows the relationship between the maximum static deformation and the voltage input, which is observed to be linear [9] In the FEA modelling process, the force of the piezoelectric actuators as it varies with the driving voltage was accurately represented This is unlike the approximations that most other researchers, for example [9] made by assuming that a maximum blocking force exists for all boundary conditions 3.4.2 Dynamic analysis To create these mode shapes, all six piezoelectric actuators need to be driven in synchronisation at the natural frequency of the mode shape From the FEA modelling, it was observed that the mode shapes produced at other modes besides modes 13 and 23 tend to create bending or twisting of the tubular wall This Dynamic analysis is used to determine the dynamic response of a structure under the dynamic excitation force The dynamic excitation forces are from the expansion and the compression of the piezoelectric actuators when they are loaded with an AC voltage (75 V) on top of a DC offset Int J Adv Manuf Technol (2012) 60:110 Fig End view of mode shapes: a upper leftmode 13 (16.37 kHz); b upper right mode 23 (25.64 kHz); Side view of mode shapes: c lower leftmode 13; d lower rightmode 23 (75 V) The excitation frequency should be coincident with one of the natural frequencies for either mode 13 or 23, as identified in Section 3.3, in order to achieve maximum dynamic response A dynamic experiment was performed to verify the bearing's natural frequencies and mode shapes at modes 13 and 23 The bearing was placed on a horizontal flat surface, as shown in Fig 7, and was supported at two positions near the bottom edge These points of contact were chosen to coincide with the nodal points (of no displacement) of the bearing According to the FEA simulation for mode 13, the fins on the tubular bearing have translation motion To verify this prediction, with the bearing excited at mode 13, the displacement amplitude at three pointstop, middle and bottomalong the length of the fin was measured with the capacitive displacement sensor The results confirm the prediction from FEA simulation Further measurements made on the other two fins showed the same results but stationary (zero displacement amplitude) at the both ends For this reason, subsequent dynamic experiments only attempted to measure the displacement amplitude at one single position, as shown in Fig Figure shows the dynamic deflection of a point on the fin of the bearing as measured by the capacitive displacement sensor Measurements were made ten times and it is the average that is shown on the graph; the corresponding error bar represents standard errors The narrow extent of the error bars suggests good measurement repeatability and high precision of the displacement amplitude obtained To correctly locate the natural frequency, the actuators were driven to excite the bearing over a range of frequencies from 16.28 to 16.55 kHz at three different levels of AC voltage, namely 75, 65 and 55 V The experimental natural frequency for mode 13, from Fig 8, is 16.32 kHz at which the displacement amplitude on the fin is the greatest, for example at 75 V AC, the displacement is 2.88 m Since the 75 V AC gives the greatest displacement amplitude, which in turn produces the Deflection Unit: àm 0.14 0.12 0.1 0.08 0.06 0.04 0.02 45 55 65 75 DC Unit: V Fig Static deformation of the bearing when a 75 V DC voltage was applied to the six actuators Fig Static deformation varies linearly with the applied DC voltage Int J Adv Manuf Technol (2012) 60:110 Deflection Unit: àm 2.5 1.5 75V AC 65V AC 55V AC 0.5 24.8 25 25.2 25.4 25.6 Frequency Unit:kHz Fig Deflection on the fin of the bearing at mode 23 versus the excitation frequency for the three levels of AC input Fig Setup for the dynamic response measurement greatest mean pressure ratio, Fig 1, this condition was going to be used for driving the bearing subsequently The experiment was repeated for mode 23 The measurement point, in this case, was near the end of the sleeve where the deformation is observed to be significant, Fig 4b The results are shown in Fig It is noted that the experimental natural frequency for mode 23 is 25.32 kHz From the FEA model, the theoretical displacement amplitude at the measurement point was also obtained for the different driving conditions Figures 10 and 11 show the comparison between the theoretical and experimental displacement amplitude at modes 13 and 23, respectively Load-carrying capacity experiments In these experiments, the bearing was inserted into the round guideway, set up as a cantilever with an overhanging length of 130 mm giving a natural frequency of about 800 Hz, as mentioned in Section 3.1 As shown in Fig 12, a mass was onto a wire attached to a fin of the bearing and the bearing was excited at a number of frequencies around a particular mode The capacitive displacement sensor was positioned at the top of the tubular bearing diametrically opposite to the hanging weight with the capacitive displacement sensor right over the point of maximum amplitude of oscillation as predicted by the FEA modelling Thus, for mode 13, the point was at the mid-span of the bearing's length and for mode 23, at the edge of the bearing (Fig 4c, d) The sensor was zeroed when the bearing was at rest on the round guideway As the bearing was excited by the surfacemounted piezoelectric actuators, the bearing began to float This resulted in a non-zero displacement output comprising an alternating component superimposed on a mean component This mean component is the mean film thickness; and the minimum film thickness is the mean minus the amplitude of oscillation In the experiments, the minimum film thickness was computed over a thousand cycles of oscillations For mode 13, Fig 13 shows the relationship between the minimum film thickness and load at four different excitation frequencies at and around the natural frequency of mode 13 At the natural frequency 16.32 kHz, the minimum film thickness is greater than those at other frequencies 2.5 75 V AC 3.5 65 V AC 55 V AC 1.5 0.5 16.25 Deflection Unit: àm Deflection Unit: àm 3.5 2.5 1.5 Theory Experiment 0.5 16.3 16.35 16.4 16.45 16.5 16.55 Excitation frequency Unit: kHz 16.6 55 60 65 70 75 AC Unit:V Fig Deflection on the fin of the bearing at mode 13 versus the excitation frequency for the three levels of AC input; the error bars represent standard errors Fig 10 Comparison between theoretical and experimental displacement amplitude at mode 13 (DC=75 V and variable AC) Int J Adv Manuf Technol (2012) 60:110 Unit: um Excitation frequency =16.220KHz" Experiment Theory 0.5 55 60 65 AC Unit:V 70 75 Fig 11 Comparison between theoretical and experimental displacement amplitude at mode 23 (DC=75 V and variable AC) below a load of about N However, when the load is increased beyond 2.5 N, its film thickness becomes about the same as those at other frequencies An explanation could be that with increasing load through adding mass, the natural frequency of the bearing/mass system shifts away from its original value and so the bearing is no longer being excited at its true natural frequency Figure 14 shows the relationship between the minimum film thickness and load at four different excitation frequencies at and around the natural frequency of mode 23 At the natural frequency of 25.32 kHz, the minimum film thickness is also greater in the range of loads experimented When the minimum film thickness at modes 13 and 23 are placed side by side, the difference in values is striking, showing that mode 13 is a far superior mode in terms of load-carrying capacity The comparison is made in Fig 15 where the bearing was excited at the natural frequency of modes 13 and 23 According to Fig 1, the pressure ratio increases with the amplitude ratio An increase in amplitude ratio corresponds to a decrease in minimum film thickness; an increase in pressure ratio means an increase in the load-carrying capacity of the bearing Therefore, it can be reasoned that as the minimum film thickness decreases, the bearing stiffness increases Fig 12 Direction of loading by hanging masses Minimum air film thickness 1.5 16.430KHz Excitation frequency =16.320KHz 16.520KHz 1 1.5 2.5 Load Unit: N 3.5 Fig 13 Minimum film thickness of bearing versus load at four excitation frequencies around mode 13 When the load of the bearing was increased from to N, the frequency of the modal peak at mode 13 was observed to increase from 16.32 to 16.98 kHz Figure 16 compares the two sets of results obtained, one at the fixed frequency of 16.32 kHz and the other at the resonant frequency which varied between 16.32 and 16.98 kHz as the loading changed It is noted that the latter always produces a greater minimum film thickness Discussion The model of an air film between two flat plates using the ideal gas law assuming adiabatic process proves theoretically the existence of a mean positive pressure and that this pressure increases as the amplitude ratio, Fig It does not, however, attempt to model the pressure leakage on the edges the squeeze-film air bearing Often, it is argued that when the plates oscillate at a very high frequency, the leakage effect can be ignored and the adiabatic process holds true On the issue of the end leakage, driving the bearing at its natural frequency particularly at higher modes is beneficial because the natural frequency tends to be high The design described in the paper was operated at modes 13 and 23 at the natural frequencies of 16.37 Minimum air film thickness Unit: àm Deflection Unit: àm 2.5 1.4 1.2 Excitation frequency 25.697KHz 0.8 Excitation frequency 25.477KHz 0.6 0.4 Excitation frequency 25.383KHz 0.2 Excitation frequency 25.322KHz 1.2 1.4 1.6 1.8 Load Unit: N 2.2 Fig 14 Minimum film thickness of bearing versus load at four excitation frequencies around mode 23 Minimum film thickness Unit: àm Int J Adv Manuf Technol (2012) 60:110 Mode shape 13th Mode shape 23th 1 1.5 2.5 Load unit: N 3.5 Fig 15 Comparison between modes 13 and 23 in the load-carrying capacity Minimum air film thickness Unit: àm and 25.64 kHz, respectively This compares favourably with the designs by Stolarski [9] and by Yoshimoto [7], both driving their designs at a frequency lower than or at the fundamental frequency On the issue of the mean pressure, according to Fig 1, higher mean pressure is achieved by higher amplitude ratio, which means that the structure must be such designed that the bearing surface can have large displacement The proposed design allows this to happen with relative ease Between modes 13 and 23, mode 13 has superior performance This is because of its lower end leakage due to the deflection geometry: both ends of the sleeve hardly deform while the shell in the middle section of the sleeve under the actuators are made to flex thus creating a squeeze film The ring of stagnant air film at both ends of the squeeze film minimises the leakage effect In the FEA modelling of the bearing sleeve, the piezoelectric actuators are accurately represented as a unit that expands and contracts with the driving voltage In addition, the interaction with sleeve as the actuator moves is also accounted for by including the material properties of the two parts Consequently, the analysis is more accurate Frequency without adjustment Frequency with adjustment 1 1.5 2.5 Load Unit: N 3.5 Fig 16 Relationship between the minimum film thickness and the load applied The static analysis shows the linear relationship between the input DC voltage and the deformation on the bearing The same result was also obtained by Stolarski [9] Given the same driving condition, the dynamic response is much bigger than the static response In particular, when driven at the mode 13 natural frequency, the maximum displacement at the fin is roughly m However, when excitation frequency drifts from the natural frequency, the amplitude falls, Fig The same conclusion can be drawn for mode 23 The similar result was also obtained by Yoshimoto [10], who observed vibration amplitudes of about 1.5 m at the excitation frequency of 23.7 kHz and at 70 V AC Conclusion The advantage of a squeeze-film air bearing system is its compactness because it does not require an externally pressurized air supply system The advantage of the tubular squeeze-film air bearing, as reported in this paper, is its simple design, not involving any elastic hinges, which can be difficult to manufacture The findings from this research are summarized as follows: The theory developed from using the ideal gas law shows the existence of a positive pressure in the tubular squeeze-film air bearing that causes levitation The positive pressure at any amplitude ratio can be estimated using the approximate formula, Eq (12), with an error of less than about 1.3% up to the amplitude ratio of 0.4 Two normal modes, at the 13th and 23rd, of the bearing were identified to have the desired geometry of the modal shape, namely that of a triangle The corresponding theoretical natural frequencies were found to be 16.37 and 25.64 kHz, a result confirmed also by experiments From the FEA analysis, the maximum radial deformation of the bearing when driven at 75 V DC was observed to be 0.124 m When the bearing was driven at 75 V AC with 75 V DC offset, the displacement response was 2.88 m (Fig 8) and 1.98 m (Fig 9) for modes 13 and 23, respectively The measurements were highly repeatable as is evident from the small extent of the error bars in Fig The load-carrying experiments show that when driven at the natural frequency in either mode 13 or 23, the squeeze-air film was the thickest However, comparing between the two modes, mode 13 has superior levitation performance than mode 23 (Fig 15) because 404 Int J Adv Manuf Technol (2012) 60:397407 Table Fuzzy comparison matrix with respect to WIP Table Fuzzy comparison matrix with respect to service level KB CW HB KB CW 1, 1, 3/2, 2, 5/2 2/5, 1/2, 2/3 1, 1, 2/5, 1/2, 2/3 1/2, 1, 3/2 HB 3/2, 2, 5/2 2/3, 1, 1, KB kanban, CW CONWIP, HB hybrid KB CW HB KB CW 1, 1, 7/2, 4, 9/2 2/9, 1/4, 2/7 1, 1, 2/11, 1/5, 2/9 2/5, 1/2, 2/3 HB 9/2, 5, 11/2 3/2, 2, 5/2 1, 1, KB kanban, CW CONWIP, HB hybrid On the basis of the above values, the fuzzy weights are shown by graph in Fig V Sp1 V Sp2 V Sp3 V Sp4 V Sp5 ! Sp2 ị ẳ ! Sp1 ị ẳ 0:51 ! Sp1 ị ẳ ! Sp1 ị ẳ ! Sp1 ị ẳ V Sp1 V Sp2 V Sp3 V Sp4 V Sp5 ! Sp3 ị ẳ ! Sp3 ị ẳ ! Sp2 ị ẳ :029 ! Sp2 ị ẳ ! Sp2 ị ẳ V Sp1 V Sp2 V Sp3 V Sp4 V Sp5 The fuzzy number values are compared with the help of graph shown in Fig and Eqs and as: ! Sp4 ị ẳ ! Sp4 ị ẳ ! Sp4 ị ẳ ! Sp3 ị ẳ 0:58 ! Sp3 ị ẳ 0:15 From the Eq 4, the weight vector is calculated as: d0 P1 ị ẳ min1; 1; 1; 1;ị ẳ d P2 ị ẳ min0:51; 1; 1; 1;ị ẳ 0:51 d P3 ị ẳ min0; 0:29; 1; 1;ị ẳ d P4 ị ẳ min0; 0; 0:58; 1;ị ẳ d0 P5 ị ẳ min0; 0; 0:15; 1;ị ẳ V Sp1 V Sp2 V Sp3 V Sp4 V Sp5 ! Sp5 ị ẳ ! Sp5 ị ẳ ! Sp5 ị ẳ ! Sp5 ị ẳ ! Sp4 ị ẳ Priority weights for Table as: Wp2 ẳ 0; 0:20; 1ịT and normalized weights are Wp2 ẳ 0; 0:167; 0:83ịT Priority weights for Table as: As per Eqs and 6, the priority weight of the various parameters with respect to goal: Wp3 ẳ 0; 0:20; 1ịT and normalized weights are Wp3 ẳ 0; 0:167; 0:83ịT W ẳ 1; 0:51; 0; 0; 0ịT and normalized weights are; W Priority weights for Table as: ẳ 0:662; 0:337; 0; 0; 0ịT Wp4 ẳ 0:187; 1; 1ịT and normalized weights are Wp4 Similar procedure is used to calculate the alternatives priority weights with respect to each parameters for the matrix Tables 3, 4, 5, 6, and Priority weights for Table as: Wp1 ẳ 0:187; 1; 1ịT and normalized weights are Wp1 ẳ 0:086; 0:457; 0:457ịT Priority weights for Table as: Wp5 ẳ 0; 0:20; 1ịT and normalized weights are Wp5 ẳ 0:086; 0:457; 0:457ịT ẳ 0; 0:167; 0:83ịT Table Fuzzy comparison matrix with respect to Throughput KB CW HB KB CW HB 1, 1, 5/2, 3, 7/2 7/2, 4, 9/2 2/7, 1/3, 2/5 1, 1, 3/2, 2, 5/2 2/9, 1/4, 2/7 2/5, 1/2, 2/3 1, 1, KB kanban, CW CONWIP, HB hybrid Table Fuzzy comparison matrix with respect to M/c utilization KB CW HB KB CW HB 1, 1, 3/2, 2, 5/2 3/2, 2, 5/2 2/5, 1/2, 2/3 1, 1, 1/2, 1, 3/2 2/5, 1/2, 2/3 2/3, 1, 1, KB kanban, CW CONWIP, HB hybrid Int J Adv Manuf Technol (2012) 60:397407 405 Table Fuzzy comparison matrix with respect to lost demand KB CW HB KB CW 1, 1, 7/2, 4, 9/2 2/9, 1/4, 2/7 1, 1, 2/11, 1/5, 2/9 2/5, 1/2, 2/3 HB 9/2, 5, 11/2 3/2, 2, 5/2 1, 1, Final rankings of the alternatives are obtained by adding the weights of each alternative calculated as above and multiplied with the weighs of corresponding parameter that is shown by Table Alternative E-3, i.e., HW has the highest priority weight and selected as the best production policy for the manufacturing system described before As shown by Table 8, the priority order of the alternatives is HW>CW>KB 5.2 Method As calculated with the help of Eq in the previous section, synthesis weights of the parameters with respect to goal for Table are as follows: SP1 ẳ 0:279; 0:379; 0:496ị; SP3 ẳ 0:107; 0:153; 0:211ị SP5 ẳ 0:052; 0:068; 0:090ị SP2 ẳ 0:207; 0:284; 0:378ị SP4 ẳ 0:082; 0:115; 0:159ị The center value of the SP5 =0.068, which is minimum, therefore SP5 is used as reference TFN (as explained in the proposed method 2) From the Eq 7, the distances between SP5 and SP1 =0.628, SP5 and SP2 =0.437, SP5 and SP3 =0.173, SP5 and SP4 =.0963, SP5 and SP5 =0 The weight matrix is calculated using Eq The weights for the decision Table are W=[0.696, 0.505, 0.241, 0.164, 0.068]T After normalizing, weight matrix is W=[0.416, 0.302, 0.144, 0.098, 0.041]T, while W(method 1)=(0.662,0.337, 0, 0, 0)T shows that the parameters 3, 4, and have zero weights which is not logical Similar procedure is adopted to calculate the Table Ranking table by method Alternatives E-1 (KB) Alternatives E-2 (CW) Alternatives E-3 (HW) P1 0.662 P2 0.337 P3 0.000 P4 0.000 P5 0.000 Rankings 0.086 0.000 0.000 0.086 0.000 0.057 0.457 0.167 0.167 0.457 0.167 0.358 0.457 0.83 0.83 0.457 0.83 0.582 alternatives weights matrices with respect to each parameter as follows: Synthesis weights of the alternatives with respect to WIP criteria for Table 3, SE1 ẳ 0:14; 0:2; 0:292ị SE3 ẳ 0:246; 0:4; 0:69ị SE2 ẳ 0:234; 0:4; 0:627ị The distances between SE1 and SE2 =0.416, SE1 and SE3 = 0.439, SE1 and SE1 =0 and The weight matrix is Wp1= [0.20, 0.616, 0.639]T and after normalizing, weight matrix is Wp1 =[0.137, 0.423, 0.439]T The normalized matrices with respect to service level, throughput, utilization of machines, and lost demand are Wp2 = [0.056, 0.374, 0.570]T, Wp3 =[0.086, 0.321, 0.594]T, Wp4 =[0.137, 0.423, 0.439]T, Wp5 =[0.056, 0.374, 0.570]T, respectively For obtaining final rankings, similar procedure for method is adapted which is shown by Table From Table 9, the final result is similar to that obtained by the method [6], i.e., HW has the highest priority weight and the priority order of the alternatives is HW> CW>KB But the results have been improved and look more practical and real 5.3 Method As per the methodology and Eqs and 10 explained before, the fuzzy weights for Table are as follows: Sp1 ẳ 0:28; 0:38; 0:51ị Sp2 ẳ 0:21; 0:28; 0:39ị Sp3 ẳ 0:11; 0:15; 0:22ị Sp4 ẳ 0:08; 0:12; 0:16ị Sp5 ẳ 0:05; 0:07; 0:09ị Table Ranking table by method Fig Plot of fuzzy weights of the parameters w.r.t goal Alternatives E-1 (KB) Alternatives E-2 (CW) Alternatives E-3 (HW) P1 0.416 P2 0.302 P3 0.144 P4 0.098 P5 0.041 Rankings 0.137 0.056 0.086 0.137 0.056 0.102 0.423 0.374 0.321 0.423 0.374 0.392 0.439 0.570 0.594 0.439 0.570 0.507 406 Int J Adv Manuf Technol (2012) 60:397407 Table 10 The fuzzy decision matrix P1 (WIP) P2 (SL) P3 (TH) Table 12 The fuzzy weighted normalized decision matrix P4 (MU) P5 (LD) W 0.28, 0.38, 0.21, 0.28, 0.11, 0.15, 0.51 0.39 0.22 0.08, 0.12, 0.05, 0.07, 0.16 0.09 KB 0.14, 0.2, 0.29 0.14, 0.2, 0.3 CW 0.23, 0.4, 0.63 HB 0.25, 0.4, 0.69 0.08, 0.1, 0.11 0.1, 0.12, 0.14 0.29, 0.37, 0.26, 0.34, 0.23, 0.4, 0.46 0.45 0.63 0.41, 0.53, 0.4, 0.53, 0.25, 0.4, 0.68 0.70 0.69 0.08, 0.1, 0.11 0.29, 0.37, 0.46 0.41, 0.53, 0.68 WIP SL TH MU LD KB 0.05, 0.10, 0.02, 0.04, 0.01, 0.03, 0.02, 0.03, 0, 0, 0.02 0.21 0.07 0.04 0.07 CW 0.09, 0.22, 0.09, 0.15, 0.04, 0.08, 0.03, 0.07, 0.02, 0.04, 0.46 0.27 0.14 0.16 0.06 HB 0.1, 0.22, 0.51 0.12, 0.23, 0.06, 0.11, 0.39 0.21 0.03, 0.07, 0.03, 0.05, 0.15 0.1 WIP work-in-progress, SL service level, TP throughput, MU machine utilization, LD lost demand WIP work-in-progress, SL service level, TP throughput, MU machine utilization, LD lost demand Similar procedure is adopted to obtain the alternatives fuzzy weight matrices with respect to each parameter The alternatives fuzzy weight matrix for the WIP criteria: SE1 ẳ 0:14; 0:2; 0:29ịSE2 ẳ 0:23; 0:4; 0:63ịSE3 ẳ 0:25; 0:4; 0:69ị The alternatives fuzzy weight matrix for the SL: SE1 ẳ 0:08 0:1 0:11ịSE2 ẳ 0:29 0:37 0:46ịSE3 ẳ 0:41 0:53 0:68ị The alternatives fuzzy weight matrix for the TH: SE1 ẳ 0:1 0:12 0:14ịSE2 ẳ 0:26 0:34 0:45ịSE3 ẳ 0:4 0:53 0:70ị The alternatives fuzzy weight matrix for the MU: SE1 ẳ 0:14 0:2 0:3ịSE2 ẳ 0:23 0:4 0:63ịSE3 ẳ 0:25 0:4 0:69ị The alternatives fuzzy weight matrix for the LD: SE1 ẳ 0:08 0:1 0:11ịSE2 ẳ 0:29 0:37 0:46ịSE3 ẳ 0:41 0:53 0:68ị The summarized Table 10, i.e., fuzzy decision matrix is shown where the alternatives are in row (m=3) and parameters (n=5) are in column Weight of each parameter is given in the first row Table 11 The fuzzy normalized decision matrix WIP SL TH MU LD KB 0.20, 0.29, 0.12, 0.14, 0.14, 0.17, 0.2, 0.28, 0.12, 0.14, 0.42 0.17 0.2 0.42 0.16 CW 0.34, 0.58, 0.43, 0.54, 0.37, 0.49, 0.36, 0.58, 0.43, 0.54, 0.9 0.69 0.65 0.68 HB 0.36, 0.58, 0.62, 0.79, 0.58, 0.76, 0.34, 0.58, 0.62, 0.79, 1 0.9 WIP work-in-progress, SL service level, TP throughput, MU machine utilization, LD lost demand The process of normalization is carried out using Eqs 11 and 12 The normalized fuzzy decision matrix has been shown by Table 11 Step In this step, the weighted fuzzy normalized decision is constructed using Eq 13 and is shown by Table 12 Step To determine the rankings, first distance of each alternative from fuzzy positive ideal solution and fuzzy negative ideal solution is calculated using Eqs 1418 The distances from fuzzy positive ideal solution obtained is dn=0.279, 0.730, and 0.904 and from fuzzy negative ideal solution is dp =4.751, 4.378, and 4.234 The ranking of all alternatives is then obtained by CC The rankings of the three alternatives using Eq 19 given by Chen [8] are obtained as: 0.055, 0.14, and 0.176 However, the rankings of the alternatives are also computed using Eq 20 given by Fu [10] that is 0.0034, 0.027, and 0.044 for KB, CW, and HW, respectively The results obtained from both the methods show that HW has the highest priority weight and the priority order of the alternatives is HW>CW>KB Conclusion In this paper, our first purpose is to present the three methodologies that link fuzzy logic with conventional AHP and use it to solve production control policy selection problem and to show that final result would be same if all the three used the same weights However at the microscopic level, the second new and proposed method which is also a fuzzy AHP method calculates the weight matrix in a different way because some parameters have zero weights in weight matrix in the first method and this new and proposed method seems more realistic than the first At the end, in the third method, i.e., TOPSIS has also been applied to make the decisions near to the positive ideal solution or Int J Adv Manuf Technol (2012) 60:397407 more real The two different closeness coefficients have also been used given by the two different authors in the third method and the results have been compared Our second and important conclusion is that the results obtained from all the methods show that HW has the highest priority in the ranking for the proposed experimental design and the priority order of the alternatives is HW>CW>KB The result or rankings would be different if the weight of any parameter changes and this can be done through sensitivity analysis One of the further research directions may include an implementation of the Theory of Exchange [18] in the present environment for any potential improvement in system performance/productivity In future research, it would also be interesting to see if the proposed model of experimental design continues to apply, where the work stations may be parallel or a combination of serial and parallel or split and merge production line with assembly line as in case of some of the companies Another interesting work would consist of a comparison with other policies; such as the base stock, extended kanban that is a combination of kanban and base stock policies To get the optimal solution, choosing an appropriate distribution of kanbans may give the expected results Finding the optimal card distribution is also a part of future research References Belton V (1986) A comparison of the analytic 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problem: a review and prospects Int J Adv Manuf Technol doi:10.1007/s00170-010-3094-4 Int J Adv Manuf Technol (2012) 60:409419 DOI 10.1007/s00170-011-3612-z ORIGINAL ARTICLE Realising next-generation web service-driven industrial systems Stamatis Karnouskos Received: February 2011 / Accepted: 23 August 2011 / Published online: September 2011 â Springer-Verlag London Limited 2011 Abstract Business continuity and agility form the core modus operandi of modern global enterprises Complex business processes performed in highly distributed production systems need to be efficiently integrated with the shop floor, which needs to be able to fully respond to dynamic adaptations and sophisticated interactions with the enterprise systems in a timely manner As the new generation of industrial devices coming to the shop floors features greatly improved storage, computing and networking capabilities, the factory of the future transforms itself to a system of systems, where of large numbers of heterogeneous networked embedded devices dynamically exchange information, complement each others functionality and provide new innovative capabilities that satisfy the emergent dynamic business requirements This new breed of networked embedded devices goes beyond simple passive roles, e.g being able to store and report information about themselves and their physical surroundings once queried, but execute complex computations and global logic locally, as well as dynamically adapt to fulfil goaldriven conditions They communicate in an open interoperable way, form cooperative peer-to-peer networks and strongly interact with enterprise systems This leads to highly modular, manageable and dynamic factories that will be able to adapt and optimize their behaviour to achieve the business goals pursued in a cross-layer collaborative way Keywords Service-oriented architecture ã Enterprise systems ã Web services on devices ã Dynamic discovery ã Cooperating objects S Karnouskos (B) SAP Research, Vincenz-Priessnitz-str 1, Karlsruhe, Germany e-mail: stamatis.karnouskos@sap.com URL: www.sap.com 1.1 Trends Introduction The last decade has witnessed a deep paradigm shift on the shop floor towards embedding Information and Communication Technologies (ICT) in all operational aspects of the factory in order to enhance its capabilities and qualitative aspects As sophisticated networking and computation capabilities are embedded in devices used in manufacturing and process automation, it is possible to adopt modern software engineering approaches and benefit from them Our key message is that collaboration via an information centric infrastructure is of key importance for the future industrial systems This cooperation has to be not only horizontal (e.g among devices) but also vertical among devices, systems and enterprise services Links should be created among them in an informationflat infrastructure that glues several layers directly without necessarily having to go to through the deployed hierarchies This information-driven interaction may enable rapid new application development and coupling of the real world as sensed by industrial devices to the enterprise systems Throughout this paper, we will show how such infrastructures can be realised, raise potential challenges and future directions and depict our experiences while implementing them Modern factory devices can provide their functionality as a collection of services that can be used by third 410 entities for their purposes This information-driven interaction empowers the vision of the so-called Internet of Things as described in [8], according to which billions of devices inter-connected will provide new innovative applications and systems as indicated by [11], that may significantly increase the efficiency of current systems Future shop floors are expected to be serviceoriented as envisioned in [4], where devices will offer their functionality as a service and collaborate as depicted in [1] They are expected to be highly heterogeneous integrating not only shop floor industrial-only devices as this is done today but also a large number of modern IT devices that will be used by the employees and will need to be easily integratable for better visibility and on-demand information acquisition Enabled by software services, the Internet of Things provides for virtually infinite integration of sensors, actuators, microsystems, mechatronic systems, robots, mobile devices, etc The core idea behind the amalgamating the physical and virtual (business) world is to seamlessly gather any useful information about objects of the physical world and use the information in various applications during the objects entire life cycle Collecting information and making it available, for example, about the objects and goods origin, location, movements, physical properties, usage history and context, can help enterprises improve both existing intra- and inter-company business processes and also create new ones Existing business processes may become more accurate since information taken directly from the point of action can be used to enhance decision-making procedures The continuous evolution of embedded and ubiquitous computing technologies, in terms of decreasing costs and increasing capabilities, may even lead to the distribution of existing business processes not only to the network itself but also to network edges, i.e the advanced industrial networked embedded devices, and can overcome many limitations of existing centralized approaches The emerging approach is to create system intelligence by a large population of small and smart networked embedded devices at a high level of granularity, as opposed to the traditional approach of focusing intelligence on a few large and monolithic applications This increased granularity of intelligence distributed among loosely coupled intelligent physical objects facilitates the adaptability and reconfigurability of the system, allowing it to meet business demands not foreseen at the time of design and providing real business benefits as we depict in [2] It is expected that in the future, business applications will heavily interact with the real world via the optimal timely exploitation of the information offered by devices [13] Monitoring and control Int J Adv Manuf Technol (2012) 60:409419 (M&C) heavily depends on the integration of embedded systems and is expected to grow from e 188 Bn in 2007, by e 300 Bn, reaching e 500 Bn in 2020 [5] This will have a significant impact in several domains and especially in manufacturing and process industry 1.2 Web services on devices The use of the Service-oriented architecture (SOA) paradigm implemented through Web Service (WS) technologies is not new especially for high-level systems However, due to the powerful networked embedded devices, we can now have the same capabilities at the device layer Since any device can host WSs as depicted in [12, 13, 18, 19] and make its functionality available via it, we can have integration of devices based purely on the composition of services, i.e information driven without having to focus on the devices as such and their specifics but rather only on the provided functionality This has profound implications, especially when it comes to the interaction with enterprise systems as depicted in [24] Enterprise application and service designers can greatly benefit from real-world integration, however it is not feasible nor wished to engage towards tackling the heterogeneity and idiosyncrasies of devices but rather stay at abstract functional level, i.e that of services This will enable third party service providers to provide much better and high-performance implementation of their services (since they know the specifics of the device much better) and therefore enable a decoupled parallel evolution of devices and enterprise systems coupled only by standardized services; a winwin situation WSs on devices can act as a unifying technology, empowering all levels of the enterprise, from sensors and actuators to enterprise business processes The benefits of service orientation as shown in [10] are conveyed all the way to the device level, facilitating the discovery and composition of applications by reconfiguration rather than re-programming Dynamic self-configuration of smart networked embedded devices using loosely coupled services provides significant advantages for highly dynamic and ad hoc distributed applications The goal will require the definition of new integration concepts taking into account the emerging requirements of business applications and the explosion of available information from the device level Of particular interest is the availability of near realtime event information, which will be used to specify new enterprise integration approaches for applications such as business activity monitoring, overall equipment effectiveness optimization, maintenance optimization, etc Int J Adv Manuf Technol (2012) 60:409419 411 1.3 Cross-layer SOA-driven collaboration It is already demonstrated in detail in [4, 13] that the future shop floor infrastructures can significantly benefit from service-oriented approaches, both in vertical (cross-layer) and horizontal communication A vision of this is depicted in Fig where a service-enabled information-driven collaboration is possible We have pursued the realization of such a cross-layer infrastructure (as depicted in Fig 2) within the SOCRADES project (www.socrades.eu) In such an infrastructure, new, rich services can be created by orchestrating and combining services from different system levels, i.e services provided by enterprise systems, by middleware in the network such as the one detailed by [22] and by advanced devices themselves The composed services with complex behaviour can be created at any layer (even at device layer) In parallel, dynamic discovery and peer-to-peer (P2P) communication allows for easy identification of the device and its functionality The trend is to clearly move away from proprietary connections between monolithic hardware and software systems towards more autonomous systems that interact in a more standardized, cooperative and open way Entities (devices, applications, etc.) cannot only communicate in a cross-layer way but heavily collaborate in mash-ups as envisioned by [4] Aggregated device-level services interact with higherlevel business processes situated at the level of business applicationsin particular Enterprise Resource Planning (ERP) systemsin order to demonstrate seamless integration of device-level functionality into higherorder business application scenarios as demonstrated by [13] Fig SOCRADES vision: SOA-based integration of the future factory shop floor We have to point out that this approach flattens the currently hierarchical structures within factories, enabling for instance an Enterprise service to act as a virtual device on the shop floor and vice versa a device to be directly integrated in an enterprise service This is a very powerful and innovative concept that gives more flexibility to integrate on-demand in a lightweight manner, without having to go through the whole predefined hierarchies and painful integration vendor lock-ins The convergence of applications and products towards the SOA paradigm improves shop floor integration and transparency, thereby increasing reactivity and performance of the workflows and business processes commonly found in manufacturing and logistics Events become available to any entity of the system as they happen, and business-level applications can exploit such timely information for purposes such as diagnostics, performance indications, or traceability While these vertical collaborations are beneficial for business application software, new challenges arise: direct communication with devices can be error prone or unreliable, which must be considered when critical decisions, such as branches in a workflow, depend on it Amalgamating the enterprise and shop floor landscapes Fig Towards cross-layer collaboration Significant effort has been invested into the integration of physical computing devices with standard enterprise software, such as ERP systems Planning a production order or creating a bill of materials in the ERP application is neither effective nor optimized, unless the shop floor is transparent to those applications As an example, the manufacturing industry foresees enterprise applications to consider real-time events on 412 Int J Adv Manuf Technol (2012) 60:409419 the shop floor to plan production, enhance customer relationship management, and have a healthy updated supply chain This shop floor intelligence obtained in real time allows business to adapt to the market demand and forecast shop floor breakdowns in a timely fashion Additionally as SOA approaches start to prevail, the introduction cycle of new applications could be significantly shorter This could enable exchange of real-time information across enterprises and trusted business partners, which will have an effect on the respective business decisions In order to realise the collaboration of the enterprise and shop floor systems, three main activities have to be performed: Identification of the cooperating entities (systems): the identification of the collaborative automation units that are able to expose and/or consume services, for each production scenario in a defined production domain, e.g electronics assembly, manufacturing, continuous process, etc A collaborative unit can be a simple intelligent sensor or a part/component of a modular machine, a whole machine and also a complete production system Building the system of systems: networking/bringing the entities together within a SOA or collaborative infrastructure, i.e putting the units architecturally together, and Making the system working for reaching the production goal: collaborative behaviour of the systems for reaching common objectives, i.e control objectives, production specifications, markets objectives, etc It is clear that cooperation would be beneficial for next generation production systems if it is done among devices as envisioned by [17] in the shop floor and among the devices and the enterprise systems as advocated by [1] In order to achieve that, several challenges need to be tackled and requirements to be met as already identified in ([14]) Seamless cooperation and collaboration is necessary to realise adaptive production systems Key issue in all of these approaches is to be capable of integrating any device, without taking direct consideration of its device-dependent characteristics; hence we aim at hiding its heterogeneity and focus only on its functionality As such, this abstract integration of devices based on WSs can be realised as we have demonstrated; details can be found in [12, 13]) This may indeed result to significant benefits as we also show in an initial evaluation by [2] Such benefits include cost reduction, improved performance and new opportunities but also potential drawbacks 2.1 Tackling device heterogeneity with SOA Current infrastructures are highly heterogeneous and a tremendous amount of effort has been invested in dealing with it In practice, this means to try to stick to specific vendors and products and also invest in glue solutions that enable the cross-bridging of information Several standards are used, but when investments are made with a 1015-year horizon, it is hard to predict the prevailing standards to use and invest accordingly Today we still not have long-lasting future-compatible solutions This is critical as future factory shop floor heterogeneity will increase drastically especially with respect to the networked embedded devices However, using WSs this heterogeneity can be hidden empowering information-driven integration approaches (and not device driven as done today) 2.1.1 Device cartography Device integration based on SOA has already been prototyped within the scope of the SOCRADES project However, today WSs on devices, have some nonnegligible requirements on memory, computational power and storage It might be that these not pose a barrier in the future, however they pose a barrier in the short term and for devices with older technologies that can still be found on the majority of shop floors As such migration solutions need to be found since the lifecycle of such infrastructures in some domains can be decades, and business continuity as well as return of investment must be guaranteed As pointed out not all devices are expected to have in the future the resources to run WSs natively Even if that was feasible it might not really be reasonable (or offer any business advantages) in some cases as taskspecific devices may assume a very specialized role that can be fully accomplished with no advanced capabilities (e.g a proximity sensor, and RFID tag, etc.) Generally, we can see different device categories in the future factory: Passive non-electronic devices: These are devices physically not capable of hosting WS technology as they not feature any electronic capability These passive devices can be monitored indirectly, e.g via attached RFID or barcode tags The tags themselves (e.g a wireless sensor) or wrappers around them could make it possible for such devices to participate in the future factory, e.g as depicted in [23] Resource constrained devices: These devices have the power to communicate and process informa- Int J Adv Manuf Technol (2012) 60:409419 tion However, their resources are so limited that it would not be feasible or reasonable business wise to deploy WSs on them In this situation, however, it seems very rewarding to connect them to gateway or service mediator that encapsulates the devices functionality and offers WSs to the outside world (as depicted in Fig 4) Typical such devices are today the RFID tags where the RFID reader is used as a Gateway depicting full-WS capabilities Other lightweight alternatives might offer better integration for these devices, e.g via REST ([16]) WS-capable devices: These devices have enough computing resources to retrieve, store, compute and transmit information and can stand-alone participate in the future factory infrastructure 2.1.2 WS-driven device integration Our focus for the future factory is on the serviceenabled devices Business applications will need to access device data and state preferably always through (Web) services Although other connection operations exist, only the (Web) service abstraction delivers a message-oriented asynchronous communication method truly independent from the underlying operating system and programming language The basic need is to find at which level of the architecture those complex services are provided in a form that is ready for consumption by the enterprise system The issue should not be discussed only bottom-up, i.e derived from the level of functionality that automation devices can offer, but primarily top-down, i.e defined by the data exchanged between the enterprise applications and the shop floor The enterprise system needs direct access to timely and context specific information; as such many unnecessary details should be hidden and an abstract service should capture only the desired functionality The last is possible usually as a composition of other more generic services A key issue is the integration of legacy devices, as any transition to the envisioned future factory will have a migration phase Replacement of legacy infrastructure will come gradually and therefore transition approaches need to be defined In parallel, we must guarantee coexistence of legacy and future infrastructure as well as minimization of downtimes and media breaks As such the only viable option is an evolutionary approach, where non-WS-enabled devices are softwareupdated to include a WS stack (if technically feasible), or be replaced one after the other, because they have reached the end of their lifetime, or new functionality on the physical level is required In this way, WS- 413 enabled devices can gradually replace the conventional systems of today In Fig 3, an example of a WS-enabled device is depicted Its functionality has been wrapped with WSs (i.e using the device profile for WSs (DPWS) stack) and put on the network Existing IT systems such as a computer hosting COTS Windows Vista/CE/7, can dynamically discover the device (due to WS discovery in DPWS profile), and see its data such as serial number, MAC address, IP address, model number, Unique ID, etc Furthermore, there is a standard way to access the functionality on the device and, e.g control it, or obtain its health status Since now the device can provide this information in a standardized way via WSs, other devices or services, e.g in a maintenance platform, can subscribe to the events it creates The last depicts a clear paradigm shift towards an event-driven infrastructure, where information can be dynamically discovered and pushed to the interested parties only This may have profound implications on a number of scenarios, e.g remote maintenance as depicted by [21] In [6, 12], we proposed an extensible integration architecture based on WSs and capable also of supporting legacy and Web-service-enabled devices and products The approach was realised with the help of infrastructure services implemented in a middleware which is described in detail in [22], which would enable the device-to-business integration of a variety of devices and systems existing in factories today Several prototypes following all these approaches have been demonstrated and evaluated some of which are depicted in [13] 2.2 Migration to service-driven production systems Migrating towards a fully service-based infrastructure may be the key to unleash the potential for Fig Example of dynamic discovery and presentation of a WSenabled device in MS windows 414 Int J Adv Manuf Technol (2012) 60:409419 services, so as to factor the information common to all instances and thus to save also resources, e.g memory 2.2.2 Service mediator Fig Non-service-enabled device integration: gateway vs service mediator concepts sophisticated monitoring and control in the future heterogeneous factory shop floor A gateway or a service mediator may be a viable approach for the migration phase of existing systems to the future ones The concept behind them is depicted in Fig While gateway implementations may be straightforward, the more sophisticated one, i.e the service mediator, may serve us better in the longer run 2.2.1 Gateway A Gateway is a device that controls a set of lowerlevel non-service-enabled devices, each of which is exposed by the Gateway as a service-enabled device This approach allows to gradually replace limited-resource devices or legacy devices by natively WS-enabled devices without impact on the applications using these devices This is possible since the same WS interface is offered this time by the WS-enabled device and not by the Gateway This approach is used when each of the controlled devices needs to be known and addressed individually by higher-level services or applications The Gateway approach requires some specific support, e.g from a DPWS implementation Indeed, while a standard DPWS-enabled device is only required to store and manage its own discovery, description and hosted services metadata, a Gateway needs to support a multitude of devices It is therefore necessary to introduce a registry for devices and hosted services that helps structure and manage the required information When several instances of the same device type are present, the registry distinguishes between class- and instance-level information, both for devices and hosted Originally meant to aggregate various data sources (e.g databases, log files, etc.), the Service Mediator components evolved and are now used to not only aggregate various services but possibly also compute/process the data they acquire before exposing it as a unified service Service Mediators aggregate, manage and eventually represent services based on some domain specific semantics (e.g using ontologies) In our case, the Service Mediator could be used to aggregate various non-WS-enabled devices In this way, higher-level application could communicate to Service Mediators offering WS, instead of communicating to devices with proprietary interfaces The benefits are clear, as we dont have the hassle of (proprietary) driver integration Furthermore now processing of data can be done at Service Mediator level and more complex behaviour can be created, that was not possible before from the stand-alone devices 2.2.3 Reality check: gateway vs service mediator Service Mediators can be used instead of simple Gateways whenever we want to introduce some low-level semantics and multiplex functionality Consider, for example a wireless sensor network monitoring temperature along a conveyor belt (shop floor) Such a network can be composed of tenths of temperature sensors, yet, the interesting service on the top floor is not the services offered by each and every sensor but rather the average temperature on the conveyor belt A first prototype demonstrating this concept has been realised as depicted in [20] As shown on Fig using Service Mediators introduces another level of abstraction and aggregation between the clients and devices Thus, seen from the outside, there might not be significant difference between a Service Mediator and a composite service that relies on a set of service-enabled devices A Service Mediator is a device that controls a set of lower-level non-service-enabled devices that realise a process which is exposed as a service interface Thus, the individual lower-level devices are invisible outside of the Service Mediator On the contrary the Gateway depicts as a service functionality, that can be directly related to a specific device; hence, one can directly relate and identify an explicit device as the source of a service offered by the Gateway (which might not be possible in the Service Mediator) Int J Adv Manuf Technol (2012) 60:409419 Both the Gateway and the Service Mediator can host several serviceslimited only by their internal resources Both approaches enable the functionality of the shop floor to be more accessible and tap into an event-based infrastructure where devices (indirectly via their proxy) and functionality can be dynamically discovered as depicted by [7] and used, e.g due to the WS-Eventing support of DPWS To have the two classes of systems (the SOAenabled and the conventional ones) communicate with each other the new devices could come with a dual interface, providing both WS services as well as some other (e.g vendor-specific/proprietary) protocol This option is very useful for the migration phase As soon as a significant part of the production environment has these dual-stack interfaces, the whole system could be re-configured and service interaction could be used as the single interaction method between devices This switch to SOA-based control requires the remaining non-WS devices to be integrated in the new system The preferred way of integration is to have proxy WS devices and services for the real devices As a rule of thumb, the proxies should be as close as possible (in terms of network distance) to the real devices and should be instantiated at the lowest possible layer of the system Generally, the lower the level at which a non-WSenabled device is wrapped into a WS-compliant WS, the more flexibly it can participate in the device SOA compositions The lowest feasible level would be the PLC (assuming of course that no network adapter is on the device) that could have in addition to its cyclic, real-time, control part, a second part that hosts the WS stack (or just can speak http protocol in case of REST) This very low-level integration is however very costly and would require to re-design or introduce significant changes in PLC devices Therefore, it is practically preferable to add WS support using singleboard computers or complete Gateways implemented on industrial PCs Demonstration and lessons learned 415 further lead towards strong integration of physical and software-based systems which may further stretch our capabilities as well as provide new innovations, mainly driven by cross-layer collaboration and on-demand information acquisition We consider this a critical factor to move towards collaborative industrial systems as also depicted in [15] 3.1 Implemented demonstration scenario In order to demonstrate the flexible integration and collaboration, we consider having a multi-site serviceoriented enterprise in which the assembly of electromechanical components is performed in two geographically distributed assembly systems (both of them are similar to the system depicted in Fig 5) and production orders could be allocated to different sites This allocation/re-allocation of orders is done as an evaluation result of the best production facility available at the moment when a production request is made, or whendue to external factorsthe production should be shifted to a different location (e.g for performance reasons, maintenance, etc.) The scenario depicted in Fig constitutes one of the trials designed, implemented and evaluated within the scope of the SOCRADES project by SAP, Schneider Electric and Tampere University of Technology The scenario assumes that similar production facilities are available in remote locations (e.g Schneider Electric in Germany or Tampere in Finland) The prototypes developed and hosted in Tampere (TUT) and Seligenstadt (Schneider Electric) represent two different companies that are linked with business relations These companies are inter-connected via the Enterprise SIA Architecture (Enterprise Integration) Network Enterprise Services SIA Server SAP (DE) Enterprise to shop-floor Communication Internet Local Network As we have discussed so far, WSs pose a promising approach towards tackling heterogeneity, and unleashing the full power of production system not only at individual level but in collaboration with other devices and enterprise systems We believe that the latest advances may enable us to import concepts and results from the IT domain such as service composition and apply it directly to the factory of the future, e.g servicebased composition of devices and systems This would Event Subscription LDU LDU Event Subscription TUT (FI) SchneiderElectric (DE) Fig Cross-location integration of service-enabled device discovery and integration via SIA 416 Applications that are hosted in Walldorf, Germany (SAP) All communication goes over the public Internet infrastructure while the in-cloud-based enterprise services coordinate and mediate among the physical locations Both facilities provide electromechanical assembly capabilities as envisioned by the SOCRADES architecture; this means that the components of the production systems in these locations are abstracted/wrapped and perceived externally as one or more WSs At local level, each one of the facilities acts independently and can coordinate its service-enabled production system by using on-site tools At global level, both facilities connect to a service-enabled ERP module provided by SAP, which is used for coordinating the production in the remote locations A network application (named LDU) is downloaded over the Internet and once instantiated it immediately provides discovery of devices and services (via the DPWS) on the local network and connection to the backend system via the SOCRADES Integration Architecture (SIA) middleware as we describe in detail in [22] Different versions of the LDU can add-up functionalities, e.g proxy also specific enterprise services at the local shop floor, where they can be discovered and used by the devices and other services LDUs provide a means for connecting and managing devices from different premises, without needing virtual private network connections to SAP premises The LDUs can discover local services and can interact directly with production execution systems exposed as WS This is a typical example of hosted functionality on a network server at the provider side, where business services are being implemented/updated and the remote sites (in our trial Tampere and Seligenstadt) can interact over the network, with only minimal installations at their side (in order to interact with the business services) In this specific case, the software that interconnects each site is downloaded on the fly over the network via an one-click operation from a web browser (e.g Firefox, Internet Explorer, Chrome, etc.) It is critical that the device that instantiates the LDU has network access so that the discovery can succeed in identifying the necessary devices and services The different sites involved are collaborating between them via interactions that previously were not possible or would require significant implementation efforts All communication is event driven with the enterprise services acting as the main interaction point This implies that status of services, dynamic information etc are delivered via the pub/sub model to the interested parties and decision-making processes can be started as soon as situations arise As can be seen, this Int J Adv Manuf Technol (2012) 60:409419 is an event-based approach where all sites are notified about the necessary status of the production in the other side, and where the enterprise systems have full visibility on the production and can re-arrange orders in order to meet business goals Hence our approach can act as enabling component to realise complex collaborative manufacturing, e.g as envision by [3] 3.2 Implementation technologies In order to demonstrate the Cross-layer service-driven collaboration and management of next-generation production systems, we have implemented and demonstrated in the scope of SOCRADES project a prototype involving three different geographical locations that collaborate towards realising a virtual distributed shop floor As depicted in Fig 5, in order to achieve our goals we have implemented a middleware hosting infrastructure (Web) services implemented selective enterprise services and exposed them as DPWS devices on the shop floor exposed the functionality of shop floor devices (e.g sensors, robots etc.) as WSs (implemented directly on-device or with the usage of gateways and service mediators) The developed middleware facilitates infrastructure services such as communication between shop floors as well as integration of the production systems with enterprise services All SIA components are independent and communicate through WSs both with each other and to business systems Therefore a networked device can connect to the SIA and directly participate in business processes while SIA hides the details of the underlying hardware As shown also on the right side of Fig 5, SIA is split in two parts: a local part which runs on the onsite premises and features a Local Network Discovery and Management Unit (LDU) and is running at the local network that contains the devices to be integrated, and a central system (anywhere on the network or the Internet) that hosts enterprise-level applications Many LDUs (e.g one for each shop floor location) can connect to a central system In the local subsystem at Device Layer, there are several embedded devices that are running various services SIA is able to interact with devices using several communication protocols SIA allows applications to subscribe to any events sent by the devices, offering a publish/subscribe component that supports WS Notifications It also offers Int J Adv Manuf Technol (2012) 60:409419 buffered invocations of hosted services on devices that are only intermittently connected When the device becomes available again a notification is sent to the client or the system caches an invocation message and delivers it when the device is ready to receive it On the device side, the implementation has been done by either implementing WSs on the device or using service mediators and gateways in order to capture the functionality of several devices, including (partially only) the on-demand translation, e.g from OPCUA to DPWS All of the devices were discovered in a timely manner, despite of their different DPWS stack implementations that were used, i.e the WS4D (www.ws4d.org) and SOA4D (www.soa4d.org) This resulted in three different implementations, i.e WS4D (Java) and SOA4D (Java and C) coexisting in the trial 3.3 Lessons learned The biggest issue that became apparent in the trial was that of interoperability Although discovery was fully working, the same did not hold true for the event subscription We solved this in some cases by utilizing a dual-stack approach, i.e implementing both SOA4D and WS4D stacks in a device in order to enable event subscription without problems from any client Furthermore, due to the stack-specific implementation of SOA4D, services offered by devices should be a priori known which was limiting All in all, we were able to demonstrate seamless device discovery, integration on the enterprise system, interaction with the enterprise services and cross-layer collaboration and information flow SIA has been proven in the trial an adequate way to provide cross-layer and timely SOA integration Integration of heterogeneous devices, especially their interaction with enterprise services in a timely manner, is a challenging research topic Its impact in the future Internet infrastructure and its services is expected to be significant and critical to the success of a new generation of applications that will merge the real and the business world However, we can report positive experiences in managing the infrastructure and especially with respect to asset management where devices are dynamically discovered and access to their metadata is immediate (even cross-location) reduces the need for static configurations and human error In traditional IT architectures, business process activities, applications and data are locked independently and often incompatibly Users have to navigate separate networks, applications and databases to conduct the chain of activities that completes a business process This absorbs an excessive amount of IT budget 417 and staff time to maintain Additionally, business demands enhancing reliability and real-time performance in wireless technology Thus, production systems require reconfigurability and flexibility in order to improve the efficiency of manufacturing products The SOCRADES approach targets these business needs by utilizing the SOA paradigm at the device level, which enables the adoption of a unifying technology for all levels of the enterprise This can enable a wide range of potential business opportunities but is also associated with challenges as we identify in [2] SIA allows enterprise applications to connect to devices for monitoring and management (soft control) using open standards It features several advanced services in order to free business application developers from the complexity of interacting with a highly heterogeneous and unreliable infrastructure It also helps discovering and keeping track of devices and can manage the embedded software if that is supported by the devices Additionally, legacy devices can be included by providing proxy services for them SIA is not build with a specific domain in mind, but in the scope of the SOCRADES project we have successfully demonstrated its capabilities in industrial automation domain An overview of various other efforts to demonstrate the cross-layer architectural integration can be found in [13], where we show how this can be applied in various industrial scenarios We have shown that the followed approach targets emerging business needs as depicted in [25] by utilizing the SOA paradigm at the device level, which enables the adoption of a unifying technology for all levels of the enterprise This can enable a wide range of potential business opportunities but is also associated with challenges which have already been identified in [2] and [9] Organizations that benefit most from adopting the full-blown SOA-based approaches such as the one we have depicted here, have large and complex application portfolios with a vast quantity of point-to-point interfaces like manufacturing companies, since the more complex applications and their integration architectures become, the more risky it is to change them With a high level of complexity, the impact of change cannot be assessed sufficiently, hence test cycles become longer and more defects occur during the production process Thus, the system change cannot keep pace with business changes and the organization can not maintain its competitive edge: the business cannot respond quickly enough to changes in market demand This can be prevented when adopting SOA-based architectures as we analyse in greater detail in [2] 418 Int J Adv Manuf Technol (2012) 60:409419 Conclusions References It is clear that we are heading towards an infrastructure where heterogeneity will be dominant and not all devices will have the capability of tapping directly to the future SOA dominated shop floor by implementing WSs natively and provide their functionality as a service to the others In fact, the last might not only be infeasible due to technological constraints, but it also might not make sense from the business point of view Therefore any approach proposed for the future industrial automation domain, has to make sure that all types of devices can be directly and indirectly integrated in a global communication infrastructure Ubiquitous, SOA-based device integration leading to interaction of devices with enterprise services in a timely manner is an important vision that creates substantial impact both from the perspective of research and industrial application The paradigm of services on every layer of the network will influence the structure and operation of future factory Dynamic service discovery and composition will enable a new generation of applications that will more closely couple physical environments and processes with the corresponding models in business software, which are their virtual counterparts Collaboration will emerge as a key behaviour as already identified by [1, 17] and a new breed of industrial applications will be possible We have presented the imperatives and motivation for more dynamic and flexible production lines in the factory of the future To ease legacy infrastructure transition to the SOA shop floor, we have shown how this can be realised via a Gateway or a Service Mediator approach The overall architecture and its components have been designed while taking into account a wide set of requirements, as well as the existing service bridging concepts and technologies With increasing integrated collaboration, more information exchange and 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