Chapter 15 350 Sr = Sn ± (.l0 × Sn) Temperature drift tolerances must be also calculated. Over a range of –25 to +70°C, a sensing distance drift of +10% can be expected. Over –25 to +85°C, the tolerance increases to +15%. Su = Sr ± (.10 × Sr) –25 to +70°C Su = Sr ± (.15 × Sr) –25 to +85°C Usable sensing distance (Su) of any sensor can now be estimated. Su is the distance at which the sensor will always operate. If the target-to-sensor range is greater, the sen- sor may or may not operate reliably. (See Figure 15.1.41.) Figure 15.1.41: Nominal sensing distance (Sn) versus usable sensing distance (Su). Once the usable sensing distance is determined, you need to figure in the actual ap- plication conditions. There are three factors to take into account: ■ Target material, ■ Target size, and ■ Target presentation mode. The nominal sensing distance given in inductive proximity sensor specifications is determined with a target made of mild steel (in accordance with EN 60947-5-2). Whenever a target is a different metal, a correction needs to be made to the usable sensing distance (Su). The formula is: New Su = Old Su × M M = material correction factor Position and Motion Sensors 351 The standard target is a square of steel, 1mm (.04 in.) thick, with sides equal to sen- sor diameter. To determine the sensing distances for materials other than standard, a corresponding correction factor is used. Some common materials and their correction factors are listed in Table 15.1.1. Table 15.1.1: Correction factors for non-standard target materials. Correction Factor: 400 series stainless steel 1.15 Cast iron 1.10 Mild steel (Din 1623) 1.00 Aluminum foil (0, 0.5mm) 0.90 300 series stainless steel 0.70 Brass MS63F38 0.40 Aluminum ALMG3F23 0.35 Copper CCUF3O 0.30 Mild steel targets of “standard” sizes are used to establish published sensing dis- tances. The standard size for each size and style of sensor usually is given in the manufacturer’s order guides. If your desired target is the same size or larger than the standard target, no correction factor is necessary. However, a smaller target affects sensing distance. The surface area of the application target versus the surface area of the standard target provides the correction factor. (See Table 15.1.2.) The formula is: New Su = Old Su × T T = target correction factor Table 15.1.2: Correction factors for non-standard target sizes Surface Percent of Standard: Area Sensing Distance Target Shielded Unshielded 25% 56% 50% 50% 83% 73% 75% 92% 90% 100% 100% 100% When working with capacitive sensors, the dielectric constant of the target must be determined. All materials have a dielectric constant. This constant is what increases the capacitance level of the sensor to a set trigger point. The larger the dielectric con- stant, the easier a material will be to detect. Materials with high dielectric constants can be detected at greater distances than those with low constants. This allows mate- Chapter 15 352 rials with high dielectric constant to be sensed through the walls of containers made of a material with a lower constant. An example is the detection of salt (6) through a glass wall (3.7). Each application should be tested. The list of dielectric constants in Table 15.1.3 is provided to help determine the feasibility of the application. Table 15.1.3: Dielectric constants for different targets. Material Dielectric Constant Acetone 19.5 Acrylic Resin 2.7–4.5 Air 1.000264 Ammonia 15–25 Aniline 6.9 Aqueous Solutions 50–80 Benzene 2.3 Carbon Dioxide 1.000~85 Carban Tetrachloride 2.2 Cement Powder 4 Cereal 3–5 Chlorine Liquid 2.0 Ebonite 2.7–2.9 Epoxy Resin 2.5-6 Ethanol 24 Ethylene Glycol 38.7 FiredAsh 1.5–1.7 Flour 2.5–3.0 Freon R22 & 502 (liquid) 6.11 Gasoline 2.2 Glass 3.7–10 Glycerin 47 Marble 8.5 Melamine Resin 4.7–10.2 Mica 5.7–6.7 Nitrobenzene 96 Nylon 4-5 Paper 1.6–2.6 Paraffin 1.9–2.5 Perspex 3.5 Petroleum 2.0–2.2 Phenol Resin 4–12 Polyacetal 3.6–3.7 Polyester Resin 2.8–8.1 Polypropylene 2.0–2.2 Polyvinyl Chloride Resin 2.8–3.1 Porcelain 5-7 Powdered Milk 3.5–4 Press board 2-5 Rubber 2.5–35 Salt 6 Sand 3-5 Shellac 2.5–4.7 Position and Motion Sensors 353 Shell Lime 1.2 Silicon Varnish 2.8–3.3 Soybean Oil 2.9–3.5 Styrene Resin 2.3-3.4 Sugar 3.0 Sulpher 3.4 Tetraflouroethylene Resin 2.0 Toluene 2.3 Turpentine 2.2 Urea Resin 5-8 Vaseline 2.2–2.9 Water 80 Wood Dry 2-6 Wood Wet 10–30 As shown in Figure 15.1.42, there are two target presentation modes. Published sensing distances usually are determined by using the head-on mode of actuation. The target can also approach the sensor in the slide-by mode. However, the slide-by method reduces actual sensor-to-target distance by 20%. Figure 15.1.42: Targets may be presented in head-on or slide-by mode. Inductive proximity switches are available with a choice of switching functions. Nor- mally open circuitry causes output current to flow when a target is detected; normally closed circuitry produces zero output current when a target is detected. Changeover circuitry has two sensing outputs; one conducts when a target is detected while the other will not. Applicable Standards for Proximity Sensors CENELEC (The European Committee for Electrotechnical Standardization), www. cenelec.org. Chapter 15 354 IEC (International Electrotechnical Commission), www.iec.ch/, especially IEC 60947-1 and IFC 60947-5-1, which explain the general rules relating to low-voltage switch and control gear for industrial use; IEC 529 rates the level of protection pro- vided by enclosures, using an IP (International Protection) rating system. Description of protective classes (EN 60529) common to proximity sensors: ■ IP 65: Protection against ingress of dust and liquid ■ IP 67: Protection against limited immersion in water and dust ingress under predetermined pressure and time conditions (1 meter of water for 30 minutes minimum) ■ IP 68: Protection against the effects of continuous immersion in water NEMA (National Electrical Manufacturer’s Association), www.nema.org. NEMA rates the protection level of enclosures as does IEC 529, but includes tests for envi- ronmental conditions, such as rust, oil, etc. that are not included in IEC 529. UL (Underwriters Laboratories), www.ul.com. Interfacing and Design Information for Proximity Sensors When applying capacitive sensors, it’s important to note that while shielded capacitive sensors may be flush-mounted, unshielded sensors require isolation—a material-free zone around the sensing face. Materials immediately opposite both shielded and un- shielded sensors must be removed to avoid false actuation. See Figure 15.1.43. Figure 15.1.43: Unshielded proximity sensors require isolation. Position and Motion Sensors 355 Device-to-device isolation is used when two or more sensors are mounted near each other to prevent cross talk and interference between the devices. Mounting distance between shielded capacitive proximity sensors (center to center) should be at least the diameter of the sensing face. Distance between unshielded sensors will vary and be three to four times the nominal sensing distance. When shielded or unshielded sensors are facing each other, distance between sensing faces should be at least eight times the sensing distance. To ensure that both shielded and unshielded proximity switches function properly, and to eliminate the possibility of false signals from nearby metal objects, plan for minimum distances as shown in Figure 15.1.44. Figure 15.1.44: Minimum distances for proximity sensors. For unshielded proximity switches mounted opposite to each other or side by side, the minimum allowable distances in Figure 15.1.45 apply: Figure 15.1.45: Minimum mounting distances for unshielded sensors The switching hysteresis (Figure 15.1.46) represents the difference between the switch ON and switch OFF points for axial or radial approach to a target and the sub- sequent retreat. Usually it will be 3 to 15% of the real sensing distance (Sr). Chapter 15 356 To measure the maximum switching frequency, two tests (performed in accordance with EN 60947-5-2) enable the maximum switching frequency f = l/(tl + t2) to be determined exactly from the duration of the “switch ON” period (tl) and the “switch OFF” period (t2). (See Figure 15.1.47.) Figure 15.1.46: Switching hysteresis. Figure 15.1.47: Measuring maximum switching frequency. Most DC versions employ normally open, normally closed or changeover circuitry and are available with either NPN or PNP open collector outputs. ■ Operating voltage (VB) A 5% residual ripple must not cause the operating voltage to fall below the minimum stated value. Correspondingly, a 10% ripple must not cause the op- erating voltage to exceed the maximum value quoted. Position and Motion Sensors 357 ■ Voltage drop (Vd) Maximum voltage drop at the proximity switch if the output drops to zero. ■ Residual voltage (Vr) Voltage drop at the load if the sensing output is not conducting. ■ Maximum load current (la) Under nominal conditions, the output of the proximity switch cannot be driven by a current greater than this value. ■ Residual current (lr) If the output is not conducting, Ir is the maximum current flowing through the load. ■ Current consumption without load (lo) Current consumption of the switch under nominal conditions without load. ■ Standby delay (tv) Period between the application of the operating voltage and the sensor reaching the “ready” state. It is determined by the transient behavior of the oscillator. ■ Series and parallel circuitry If required, inductive proximity switches can be connected in series or in parallel. For series connection, the voltage drops Vd of two or more 3-wire switches (DC) or 2-wire switches (AC or DC) can be significant. Care should be taken that the output voltage is large enough to drive the load. With the NPN-version, the 3-wire switches must be con- nected to a common positive terminal. With the PNP-version, connect the switches to a common negative terminal. Series connection results in an AND function. Parallel connection of 2-wire switches (AC) and 3-wire switches (DC) with open col- lector outputs is possible. The sum of the residual currents must be negligible enough to prevent the load (the holding current of a relay or magnetic switch) from being acti- vated. For 3-wire switches with a collector resistor, it is recommended to decouple the sensing outputs with diodes. An OR function is obtained by connecting the switches in parallel. Logic cards can be added to inductive proximity sensors. They receive the proxim- ity sensor signal, amplify it and modify the output to respond in a particular way (as determined by time delay, pulse, or other logic). Besides operating output devices, the logic card output signal can be used as input to another card for customer logic. This is done most often with a modular control base. Chapter 15 358 One-shot (pulsed) logic gives a single fixed pulse in response to a change at the sen- sor. This is often used as a leading edge detector for moving parts, where the first indication of presence requires a single operation to take place, but where the contin- ued presence will not cause recycling to occur. Maintained (latching) logic might be used to detect parts for manual reject. The out- put is continuous until the operator resets. After resets, the output will not trigger if the original target is still in front of the sensor. ON delay does not trigger immediately with a change at the sensor, but will trig- ger only if the input signal exceeds a preset time delay. For example, it can provide jam-up detection on a conveyor for parts feeding at specific intervals. A slow down or stoppage downstream will cause a slower rate of passage, recognized as overloading or jam-up, and will cause an output to give warning or shut down the equipment until the cause is eliminated. A similar type provides an output which stays ON even when the cause is corrected, until manually reset by the operator. ON/OFF delay is used especially for jam-up detection on vibration feeders and con- veyors. The ON delay detects a jam-up, and the OFF delay allows the needed time for the jam to clear the sensing area. Zero-speed detection provides shutdown for universal jam-up detection where the product may end up in front of the sensor for too long an interval, depending on whether the jam is upstream or downstream. If the interval exceeds a preset time, the output turns OFF or shuts down the equipment. Photoelectric Sensors Photoelectric sensors respond to the presence of all types of objects, be it large or small, transparent or opaque, shiny or dull, static or in motion. They can sense targets from distances of a few millimeters up to 100 meters. Photoelectric sensors use an emitter unit to produce a beam of light that is detected by a receiver. When the beam is broken, a “presence is detected. The emitter light source is a modulated, vibration-resistant LED. This beam, which may be infrared, visible red or green, is switched at high currents for short time intervals so as to generate a high-energy pulse to provide long scanning distances or penetration in severe environments. Pulsing also means low power consumption. The receiver contains a phototransistor that produces a signal when light falls upon it. A phototransistor is used because it has the best spectral match to the LED, a fast response, and is temperature stable. By tuning the receiver circuitry to respond to a narrow band around the LED pulsing frequency, very high ambient light and noise [...]... of your choice is typically available Electrical Output by Type (%) Digital 9 RS-232 3 4–20 mA 17 0 to 10 VDC 7 0 to 5 VDC 14 10 VDC 6 ±5 VDC 11 Voltage Divider 33 0 5 10 15 20 25 30 35 Percent Figure 15.2 .10: Typical electrical output by frequency of use Low Power, Simple Signal Conditioning CPTs, particularly analog potentiometer types, generally have low power and simple signal conditioning requirements... conductor As discussed earlier in this chapter and elsewhere in this handbook, he Hall effect may be used to measure magnetic fields (and hence in contact-free current measurement), but its commonest application is in Figure 15.3.9: Hall effect sensors motion sensors where a fixed Hall sensor and a small magnet attached to a moving part can replace a cam and contacts with a great improvement in reliability... often used to refer to these transducers are: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ cable actuated position sensor cable extension transducer cable position transducer cable sensor cable-actuated sensor CET CPT stringpot string potentiometer draw wire encoder draw wire transducer wire rope transducer wire sensor wire-actuated transducer yo yo pot yo yo potentiometer These names all refer to devices that measure... extracts from and retracts to a spring-loaded drum This drum is attached to a rotary sensor (see Figure 15.2.1) By understanding the strengths and weaknesses of CPT technology, designers, engineers, and technicians can specify and design the Precision Sensor best displacement measurement solution for their application Technology Review CPTs were first developed in the mid1960s in concert with the growth... industrial machinery and equipment applicatoins Table 15.2.1 shows a comparison of weight to range for various displacement measurement sensors Table 15.2.1: CPTs have a weight-to-range advantage over other sensor types (mass shown in oz (g)) Range (in (mm)) 2 (51) 10 (254) 40 (101 6) CPT 1 (28) 2 (57) 8 (227) LVDT 13 (369) 41 (1162) 67 (1900) Linear encoder 20 (567) 25 (709) 44 (1247) Magnetostrictive 15... moisture, and other parameters 374 Position and Motion Sensors Variety of Electrical Outputs What does your controller or data acquisition device require? 4–20 mA? 0 to 5 VDC? 0 to 10 VDC? ±5 VDC? 10 VDC? Strain gage compatible? Quadrature? RS-232? LVDT or RVDT type signals? Synchro or resolver type signals? Because CPTs can incorporate a broad range of rotary sensors and related signal conditioning, the electrical... the minimum operating light level of the sensor (normal operation) Unstable light: The Green LED changes to Red (or turns OFF) to show that the photoelectric is receiving an amount of light less than 50% extra but still greater than the minimum operating point The sensor is still operating but marginally 366 Position and Motion Sensors Certain photoelectric sensors also are equipped with an additional... Selecting Position Transducers http://spaceagecontrol.com/selpt.htm Sensor and Transducer Total Cost of Ownership (S054A) http://spaceagecontrol.com/s054a.htm 378 Position and Motion Sensors 15.3 Linear and Rotary Position and Motion Sensors Analog Devices Technical Staff Walt Kester, Editor Modern linear and digital integrated circuit technology is used throughout the field of position and motion sensing... SCHAEVITZ E100 − POSITION + − POSITION + Figure 15.3.2: Linear variable differential transformer (LVDT) A wide variety of measurement ranges are available in different LVDTs, typically from 100 µm to ±25 cm Typical excitation voltages range from 1 V to 24 V rms, with frequencies from 50 Hz to 20 kHz Key specifications for the Schaevitz E100 LVDT are given in Figure 15.3.3 380 Position and Motion Sensors... or improve its effectiveness New electronic parts have improved the overall characteristics of sensors, and more functionality is being added at the sensor level Diagnostic functions and easy-to-use calibration features are improving control systems and reducing installation time Communication modes are increasingly important in determining the right sensing technology for an application, and the ability . Distance Target Shielded Unshielded 25% 56% 50% 50% 83% 73% 75% 92% 90% 100 % 100 % 100 % When working with capacitive sensors, the dielectric constant of the target must be determined. All materials. Design Information for Proximity Sensors When applying capacitive sensors, it’s important to note that while shielded capacitive sensors may be flush-mounted, unshielded sensors require isolation—a. drift of +10% can be expected. Over –25 to +85°C, the tolerance increases to +15%. Su = Sr ± ( .10 × Sr) –25 to +70°C Su = Sr ± (.15 × Sr) –25 to +85°C Usable sensing distance (Su) of any sensor