CHAPTER Sensors and Actuators Chapter Outline Automotive Control System Applications of Sensors and Actuators Variables to be Measured 236 Airflow Rate Sensor 236 Pressure Measurements 242 Strain gauge MAP sensor 242 Engine Crankshaft Angular Position Sensor 245 Magnetic Reluctance Position Sensor 247 Engine angular speed sensor 256 Timing sensor for ignition and fuel delivery 258 Hall-Effect Position Sensor 259 The Hall-effect 260 Shielded-field sensor 262 Optical Crankshaft Position Sensor 263 Throttle Angle Sensor 265 Temperature Sensors 268 Typical Coolant Sensor 268 Sensors for Feedback Control 270 Exhaust Gas Oxygen Sensor 270 Desirable EGO characteristics 272 Switching characteristics 272 Oxygen Sensor Improvements 274 Knock Sensors 276 Automotive Engine Control Actuators Fuel Injection 284 Fuel injector signal 284 Exhaust Gas Recirculation Actuator Variable Valve Timing 286 288 V VP Mechanism Model 290 Electric Motor Actuators Brushless DC Motors 279 292 301 Stepper Motors 304 Ignition System 304 Ignition Coil Operations 305 Understanding Automotive Electronics http://dx.doi.org/10.1016/B978-0-08-097097-4.00006-0 Copyright Ó 2013 Elsevier Inc All rights reserved 233 234 234 Chapter The previous chapter introduced two critically important components found in any electronic control system: sensors and actuators This chapter explains the operation of the sensors and actuators used throughout a modern car Special emphasis is placed on sensors and actuators used for powertrain (i.e., engine and transmission) applications since these systems often employ the largest number of such devices However, this chapter will also discuss sensors found in other subsystems on modern cars In any control system, sensors provide measurements of important plant variables in a format suitable for the digital microcontroller Similarly, actuators are electrically operated devices that regulate inputs to the plant that directly controls its output For example, as we shall see, fuel injectors are electrically driven actuators that regulate the flow of fuel into an engine for engine control applications Recall from Chapter that fundamentally an electronic control system uses measurements of the plant variable being regulated in the closed-loop mode of operation The measured variable is compared with a desired value (set point) for the variable to produce an error signal In the closed-loop mode, the electronic controller generates output electrical signals that regulate inputs to the plant in such a way as to reduce the error to zero In the open-loop mode, it uses measurements of the key input variable to calculate the desired control variable Automotive instrumentation (as described in Chapter 1) also requires measurement of some variable For either control or instrumentation applications, such measurements are made using one or more sensors However, since control applications of sensors demand more accurate sensor performance models, the following discussion of sensors will focus on control applications The reader should be aware, however, that many of the sensors discussed below can also be used in instrumentation systems As will be shown throughout the remainder of this book, automotive electronics has many examples of electronic control in virtually every subsystem Modern automotive electronic control systems use microcontrollers based on microprocessors (as explained in Chapter 4) to implement almost all control functions Each of these subsystems requires one or more sensors and actuators in order to operate Automotive Control System Applications of Sensors and Actuators In any control system application, sensors and actuators are in many cases the critical components for determining system performance This is especially true for automotive control system applications The availability of appropriate sensors and actuators dictates the design of the control system and the type of function it can perform The sensors and actuators that are available to a control system designer are not always what the designer wants, because the ideal device may not be commercially available at acceptable Sensors and Actuators 235 costs For this reason, special signal processors or interface circuits often are designed to adapt an available sensor or actuator, or the control system is designed in a specific way to fit available sensors or actuators However, because of the large potential production run for automotive control systems, it is often worthwhile to develop a sensor for a particular application, even though it may take a long and expensive research project to so Although there are many subsystems on automobiles that operate with sensors and actuators, we begin our discussion with a survey of the devices for powertrain control To motivate the discussion of engine control sensors and actuators, it is helpful to review the variables measured (sensors) and the controlled variables (actuators) Figure 6.1 is a simplified block diagram of a representative electronic engine control system illustrating most of the relevant sensors used for engine control As explained in Chapter 5, the position of the throttle plate, sensed by the throttle position sensor (TPS), directly regulates the airflow into the engine, thereby controlling output power A set of fuel injectors (one for each cylinder) delivers the correct amount of fuel to a corresponding cylinder during the intake stroke under control of the electronic engine controller to maintain the fuel/air mixture at stoichiometry within a narrow tolerance band A fuel injector is, as will presently be shown, one of the important actuators used in automotive electronic application The ignition control system fires each spark plug at the appropriate time under control of the electronic engine controller The exhaust gas recirculation (EGR) is controlled by yet another output from the engine controller All critical engine control functions are based on measurements made by various sensors connected to the engine in an appropriate way Computations made within the engine controller based on these inputs yield output signals to the actuators We consider inputs (sensors) to the control system first, and then we will discuss the outputs (actuators) ENGINE CONTROL FUEL INJECTORS INLET AIR TPS MAF IGNITION SYSTEM ENGINE CRANKSHAFT/ CAMSHAFT POSITION SENSOR EGO Figure 6.1: Representative electronic engine control system COOLANT TEMPERATURE SENSOR EGR 236 Chapter Variables to be Measured The set of variables sensed for any given powertrain is specific to the associated engine control configuration Space limitations for this book preclude a complete survey of all powertrain control systems and relevant sensor and actuator selections for all car models Nevertheless, it is possible to review a superset of possible sensors, which is done in this chapter, and to present representative examples of practical digital control configurations, which is done in the next chapter The superset of variables sensed in engine control includes the following: 10 11 12 13 14 15 mass airflow (MAF) rate exhaust gas oxygen concentration throttle plate angular position crankshaft angular position/RPM camshaft angular position coolant temperature intake air temperature ambient air pressure ambient air temperature manifold absolute pressure (MAP) differential exhaust gas pressure (relative to ambient) vehicle speed transmission gear selector position actual transmission gear, and various pressures In addition to measurements of the above variables, engine control is also based on the status of the vehicle as monitored by a set of switches These switches include the following: air conditioner clutch engaged brake on/off wide open throttle closed throttle, and transmission gear selection Airflow Rate Sensor In Chapter 5, we showed that the correct operation of an electronically controlled engine operating with government-regulated exhaust emissions requires a measurement of the mass flow rate of air ðM_ a Þ into the engine (Recall from Chapter that the dot in this notation implies time rate of change.) The majority of cars produced since the early 1990s use Sensors and Actuators 237 a relatively simple and inexpensive mass airflow rate (MAF) sensor This is normally mounted as part of the intake air assembly, where it measures airflow into the intake manifold It is a ruggedly packaged, single-unit sensor that includes solid-state electronic signal processing In operation, the MAF sensor generates a continuous signal that varies as a function of true mass airflow M_ a Before explaining the operation of the MAF, it is, perhaps, helpful to review the characteristics of the inlet airflow into an engine It has been shown that a 4-stroke reciprocating engine functions as an air pump with air pumped sequentially into each cylinder every two crankshaft revolutions The dynamics of this pumping process are such that the airflow consists of a fluctuating component (at half the crankshaft rotation frequency) superposed on a quasi-steady component This latter component is a constant only for constant engine operation (i.e., steady power at constant RPM such as might be achieved at a constant vehicle speed on a level road) However, automotive engines rarely operate at absolutely constant power and RPM The quasi-steady component of airflow changes with load and speed It is this quasi-steady component of M_ a ðtÞ that is measured by the MAF for engine control purposes One way of characterizing this quasi-steady state component is as a short-term time average over a time interval s (which we denote M_ as ðtÞ) where M_ as ðtÞ ¼ s Zt M_ a ðt0 Þdt0 (1) tÀs The integration interval (s) must be long enough to suppress the time-varying component at the lowest cylinder pumping frequency (e.g., idle RPM) yet short enough to preserve the transient characteristics of airflow associated with relatively rapid throttle position changes Alternatively, the quasi-steady component of mass airflow can be represented by a low-passfiltered version of the instantaneous flow rate Recall from Chapter that a low-pass filter (LPF) can be characterized (in continuous time) by an operational transfer function (HLPF(s)) of the form HLPF ðsÞ ¼ bo þ b1 s þ /bm sm ao þ a1 s þ /an sn (2) where the coefficients determine the response characteristics of the filter The filter bandwidth effectively selects the equivalent time interval over which mass airflow measurements are averaged Of course, in practice with a digital powertrain control system, mass airflow measurements are sampled at discrete times and the filtering is implemented as a discrete time transformation of the sampled data (see Chapter 2) A typical MAF sensor is a variation of a classic airflow sensor that was known as a hot wire anemometer and was used, for example, to measure wind velocity for weather forecasting as 238 Chapter well as for various scientific studies In the typical MAF, the sensing element is a conductor or semiconductor thin-film structure mounted on a substrate On the air inlet side is mounted a honeycomb flow straightener that “smoothes” the airflow (causing nominally laminar airflow over the film element) The concept of such an airflow sensor is based upon the variation in resistance of the twoterminal sensing element with temperature A current is passed through the sensing element supplying power to it, thereby raising its temperature and changing its resistance When this heated sensing element is placed in a moving air stream (or other flowing gas), heat is removed from the sensing element as a function of the mass flow rate of the air passing the element as well as the temperature difference between the moving air and the sensing element For a constant supply current (i.e., heating rate), the temperature at the element changes in proportion to the heat removed by the moving air stream, thereby producing a change in its resistance A convenient model for the sensing element resistance (RSE) at temperature (T) is given by RSE ðTÞ ¼ Ro þ KT DT (3) where Ro is the resistance at some reference temperature Tref (e.g.,  C), DT ¼ T À Tref, and KT is the resistance/temperature coefficient For a conducting sensing element, KT > 0, and for a semiconducting sensing element, KT < The mass flow rate of the moving air stream is measured via a measurement of the change in resistance There are many potential methods for measuring mass airflow via the influence of mass airflow on the sensing element resistance One such scheme involves connecting the element into a so-called bridge circuit as depicted in Figure 6.2 R3 R1 + V1 i1 i2 R2 V2 RSE(T) – differential amplifier bridge circuit Figure 6.2: Mass airflow sensor vo Sensors and Actuators 239 In the bridge circuit, three resistors (R1, R2, and R3) are connected as depicted in Figure 6.2 along with a resistive sensing element denoted RSE(T) This sensing element consists of a thin film of conducting (e.g., Ni) or semiconducting material that is deposited on an insulating substrate The voltages V1 and V2 (depicted in Figure 6.2) are connected to the inputs of a relatively high-gain differential amplifier The output voltage of this amplifier vo is connected to the bridge (as shown in Figure 6.2) and provides the electrical excitation for the bridge This voltage is given by vo ¼ GðV1 À V2 Þ (4) where G is the amplifier voltage gain In this bridge circuit, only that sensing element is placed in the moving air stream whose mass flow rate is to be measured The other three resistances are mounted such that they are at the same ambient temperature (Ta) as regards the moving air The combination bridge circuit and differential amplifier form a closed-loop in which the temperature difference DT between the sensing element and the ambient air temperature remains fixed independent of Ta (which for an automobile can vary by more than 100  C) We discuss the circuit operation first and then explain the compensation for variation in Ta For the purposes of this explanation of the MAF operation, it is assumed that the input impedance at both differential amplifier inputs is sufficiently large that no current flows into either the þ or e input With this assumption, the differential input voltage DV is given by DV ¼ V1 À V2 R2 RSE ¼ v0 À R1 þ R2 RSE þ R3 (5) ! (6) However, it has been shown that vo ¼ GDV, so the following equation can be shown to be valid: ! R2 RSE (7) À ¼ G R1 þ R2 RSE þ R3 In the present MAF sensor configuration, it is assumed (as is often found in practice) that G >> For sufficiently large G, from Eqn (6.7), we can see that RSE is given approximately by RSE ðTÞ ¼ R2 R3 R1 (8) In this case, it can be shown using Eqn (3) that the temperature difference between the sensing element and the ambient air is given approximately by 240 Chapter kT DT ¼ R2 R3 À ½R0 þ kT ðTa À Tref ފ R1 (9) where Tref is an arbitrary reference temerature This temperature difference can be made independent of ambient temperature Ta by the proper choice of R3, which is called the temperature compensating resistance In one such method, R3 is made with the same material but possibly with a different structure as the sensing element such that its resistance is given by R3 ðTa Þ ¼ R3o þ kT3 ðTa À Tref Þ (10) where R3o is the resistance of R3 at Ta ¼ Tref and kT3 is the temperature coefficient of R3 The sensing element temperature difference DT is given by     Á À R2 R3o R2 À R0 þ kT3 À kT Ta À Tref kT DT ¼ R1 R1 (11) If the sensor is designed such that R2 kT3 ¼ kT R1 then DT is independent of Ta and is given by R2 R3o DT ¼ À Ro k T R1 ! (12) This temperature difference is determined by the choice of circuit parameters and is independent of amplifier gain for sufficiently large gain (G) The preceding analysis has assumed a steady mass airflow (i.e., M_ a ¼ constant) The mass airflow into an automotive engine is rarely constant, so it is useful to consider the MAF sensor dynamic response to time-varying M_ a The combination bridge circuit and differential amplifier has essentially instantaneous dynamic response to changes in M_ a The dynamic response of the MAF of Figure 6.2 is determined by the dynamic temperature variations of the sensing element Whenever the mass airflow rate changes, the temperature of the sensing element changes The voltage vo changes, thereby changing the power PSE dissipated in the sensing element in such a way as to restore DT to its equilibrium value An approximate model for the dynamic response of DT to changes in M_ a is given by DT_ þ where PSE ¼ i22 RSE : DT ¼ a1 PSE À a2 M_ a sSE  ¼ vo RSE þ R3 2 RSE (13) Sensors and Actuators 241 In equation 13, i2 ¼ current shown in Figure 6.2 sSE ¼ sensing element time constant and where a1 and a2 are constants for the sensing element configuration The Laplace methods of analysis in Chapter are not applicable for solving this nonlinear differential equation for the exact time variation of TSE However, a well-designed sensing element has a sufficiently short time constant sSE such that the variation in DT is negligible In this case, the change in power dissipation from the zero airflow condition is given by a1 ½PSE ðM_ a Þ À PSE ð0ފ ¼ a2 M_ a (14) It can be shown from Eqn (14) that MAF sensor output voltage varies as given below: vo ðM_ a Þ ¼ ½v2o ð0Þ þ KMAF M_ a Š1=2 (15) where KMAF is the constant for the MAF configuration As an example of this variation, Figure 6.3 is a plot of the sensor voltage vs airflow for a production MAF sensor This example sensor uses a Ni film for the sensing element The conversion of MAF to voltage is nonlinear, as indicated by the calibration curve depicted in Figure 6.3 for the example MAF sensor Fortunately, a modern digital engine controller can convert the analog bridge output voltage directly to mass airflow by simple computation As will be shown in Chapter 7, in which digital engine control is discussed, it is necessary to Figure 6.3: Output voltage for example MAF vs mass flow rate g/s 242 Chapter convert analog sensor voltage from the MAF to a digital format The analog output of the differential amplifier can be sampled and converted to digital format using an A/D converter (see Chapter 4) The engine control system can calculate M_ a from vo using the known functional relationship vo ðM_ a Þ Pressure Measurements There are numerous potential applications for measurement of pressure (both pneumatic and hydraulic) at various points in the modern automobile, including ambient air pressure, intake manifold absolute pressure, tire pressure, oil pressure, coolant system pressure, transmission actuation pressure, and several others In essentially all such measurements, the basis for the measurement is the change in an electrical parameter or variable (e.g., resistance and voltage) in a structure that is exposed to the pressure Space limitations prevent us from explaining all of the many pressure sensors used in a vehicle Rather, we illustrate pressure-type measurements with the specific example of intake manifold pressure (MAP) Although it is obsolete in contemporary vehicles, the speededensity method (discussed in Chapter 5) of calculating mass airflow in early emission regulation vehicles used such an MAP sensor Strain gauge MAP sensor One relatively inexpensive MAP sensor configuration is the silicon-diaphragm diffused strain gauge sensor shown in Figure 6.4 This sensor uses a silicon chip that is approximately millimeters square Along the outer edges, the chip is approximately 250 mm (1 mm ¼ 10À6 m) thick, but the center area is only 25 mm thick and forms a diaphragm The edge of the chip is sealed to a Pyrex plate under vacuum, thereby forming a vacuum chamber between the plate and the center area of the silicon chip A set of sensing resistors is formed around the edge of this chamber, as indicated in Figure 6.4 The resistors are formed by diffusing a doping impurity into the silicon External connections to these resistors are made through wires connected to the metal bonding pads This entire assembly is placed in a sealed housing that is connected to the intake manifold by a small-diameter tube Manifold pressure applied to the diaphragm causes it to deflect Diaphragm deflection in response to an applied pressure results in a small elongation of the diaphragm along its surface The elongation of any linear isotropic material of length L corresponds to the length becoming L þ dL in response to applied pressure For linear deformation, dL [...]... þ pvhx Š ( 46) Sensors and Actuators 261 where vex ¼ meEx ¼ electron drift velocity, vhx ¼ mhEx ¼ hole drift velocity and where n and p are the electron and hole concentrations and me and mn are the electron and hole mobilities, respectively However, when the magnetic flux density (B) is nonzero, there is a force acting on the electrons and holes known as the Lorentz force F Le (electrons) and F Lh (holes),... the Hall-effect sensor element, and the sensor output drops to near zero Note in Figure 6. 16b that the waveform is just the opposite of the one in Figure 6. 15 Sensors and Actuators 263 Figure 6. 16: Shielded-field Hall-effect sensor Optical Crankshaft Position Sensor In a sufficiently clean environment, a shaft position can also be sensed using optical techniques Figure 6. 17 illustrates such a system... proportional to the pressure), as shown in Figure 6. 5b Figure 6. 5: Example MAP sensorr circuit Sensors and Actuators 245 We illustrate the operation of this sensor with the following model The voltage at point A is denoted VA and at point B as VB The resistances R1 and R3 are given by Rn ð˛Þ ¼ Ro þ R˛ ˛ n ¼ 1; 3 (18)  dR  >0 R˛ ¼  d˛ ˛¼o (19) where For resistances R2 and R4, the model for resistance is given... fundamental equations are used in the modeling of the sensor of Figure 6. 7 and other similar magnetic sensors The path for the magnetic flux of the sensor of Figure 6. 7 is illustrated in Figure 6. 9 In Figure 6. 9, gc is the width of the gap in the pole piece and tT is the thickness of the steel disk For a configuration such as is shown in Figure 6. 9, the lines of constant magnetic flux follow paths as indicated... conductor is defined as V I EL ¼ sEA R¼ (54) Sensors and Actuators 267 Figure 6. 19: Throttle angle sensor: a potentiometer R¼ Lr A (55) where r ¼ 1/s ¼ material resistivity (ohm m) Consider now a resistive material formed in a segment of a circle of radius r as depicted in Figure 6. 19 Let the radial dimension and the thickness of the material be uniform and small compared to the circumferential distance... open circuit voltage that is proportional to the x-directed current density Jx and to the magnetic flux density Bz The operation of the angular position sensor configuration depicted in Figure 6. 13 is based upon the variation of magnetic flux density normal to the Hall element and its relationship to 262 Chapter 6 Figure 6. 15: Hall sensor output voltage waveform the terminal voltage Vo derived above... intersection of the vertical plane of symmetry of the magnet with the flat surface of the disk In Figure 6. 13, qn is the angle between the reference line and the center of the nth tab as shown Figure 6. 13: Representative Hall-effect sensor configuration  260 Chapter 6 Vs y I thickness = d Ly Ēs Jy Vo Jx x Lx Figure 6. 14: Schematic illustration of Hall-effect sensor The Hall-effect The Hall element is a thin,... within air gaps, and H g is the magnetic field intensity within air gaps Figure 6. 9: Magnetic circuit of the sensor of Figure 6. 7 252 Chapter 6 From Eqn (30) above, the following equation can be written for the contour shown in Figure 6. 9: Z H$d‘yHg ga þ Hm Lm (32) C where ga is the total air gap length along contour C, Lm is the total length along contour C within the material, and C is the closed... denoted R‘ When the Ls Rs V Vo R Figure 6. 11: Equivalent circuit for variable reluctance sensor 2 56 Chapter 6 sensor of Figure 6. 7 is connected to signal processing circuitry, the exact zero-crossing point of its terminal voltage can potentially vary as a function of RPM The variation in zerocrossing point is associated with the phase shift of the circuit of Figure 6. 11 At any sinusoidal frequency u the... any of the other position sensor techniques could be used as well Refer to Figure 6. 7 and notice that the four tabs will pass through the sensing coil once for each crankshaft revolution Sensors and Actuators 257 For each crankshaft revolution, there are four voltage pulses of a waveform depicted qualitatively in Figure 6. 10b For a running engine, the sensor output consists of a continuous stream of ... output drops to near zero Note in Figure 6. 16b that the waveform is just the opposite of the one in Figure 6. 15 Sensors and Actuators 263 Figure 6. 16: Shielded-field Hall-effect sensor Optical... within air gaps Figure 6. 9: Magnetic circuit of the sensor of Figure 6. 7 252 Chapter From Eqn (30) above, the following equation can be written for the contour shown in Figure 6. 9: Z H$d‘yHg ga þ... circuitry is denoted R‘ When the Ls Rs V Vo R Figure 6. 11: Equivalent circuit for variable reluctance sensor 2 56 Chapter sensor of Figure 6. 7 is connected to signal processing circuitry, the