UnderstandingAutomotiveElectronics8th-Chapter5

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CHAPTER

SENSORS AND ACTUATORS

CHAPTER OUTLINE

Automotive Control System Applications of Sensors and Actuators ........................................................ 184

Variables to be Measured ..................................................................................................... 185

Airflow Rate Sensor ............................................................................................................. 186

Pressure Measurements ....................................................................................................... 191

Engine Crankshaft Angular Position Sensor ............................................................................ 194

Magnetic Reluctance Position Sensor .................................................................................... 195

Hall-Effect Position Sensor .................................................................................................. 205

Optical Crankshaft Position Sensor ....................................................................................... 208

Throttle Angle Sensor ......................................................................................................................... 211

Temperature Sensors ......................................................................................................................... 213

Typical Coolant Sensor ...................................................................................................................... 214

Sensors for Feedback Control ............................................................................................................. 215

Exhaust Gas Oxygen Sensor .................................................................................................. 215

Oxygen Sensor Improvements ............................................................................................... 220

Knock Sensors ................................................................................................................................... 221

Angular Rate Sensor .......................................................................................................................... 223

LIDAR ................................................................................................................................................ 227

Digital Video Camera ......................................................................................................................... 229

Flex-Fuel Sensor ................................................................................................................................ 235

Oscillator Methods of Measuring Capacitance ........................................................................ 239

Acceleration Sensor ............................................................................................................. 244

Automotive Engine Control Actuators ................................................................................................... 247

Fuel Injection ...................................................................................................................... 251

Exhaust Gas Recirculation Actuator ...................................................................................... 253

Variable Valve Timing ........................................................................................................................ 254

VVP Mechanism Model ........................................................................................................ 257

Electric Motor Actuators ..................................................................................................................... 258

Two-Phase Induction Motor .................................................................................................. 263

Brushless DC Motors ............................................................................................................ 266

Stepper Motors .................................................................................................................................. 268

Ignition System .................................................................................................................................. 268

Ignition Coil Operations ....................................................................................................... 269

Understanding Automotive Electronics. http://dx.doi.org/10.1016/B978-0-12-810434-7.00005-3

183

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 power train

(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 syst em, sensors provide measurements of important plant variables in a format suit-

able for the digital control system (often called an ECU). Similarly, actuators are electrically operated

devices that regulate inputs to the plant that directly controls its output. For examp le, as we shall see,

fuel injectors are electrically driven actuators that regulate the flow of fuel into an engine for engine

control applications.

In Appendix A, it is explained 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 generat es 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

Appendix A) also require s measurement of some variable. For either control or instrumentation appli-

cations, 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 also can be used in instrumentation systems.

As will be shown throughout the remainder of this book, automotive electronics has man y 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 vehicular 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 idea l device may not be commercially available at acceptable costs. For

this reason, special signal proce ssors 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 expen-

sive research project to do so.

Although there are many subsystems on automobiles that operate with sensors and act uators, we

begin our discussion with a survey of the devices for power train control. To motivate the discussion

of engine control sensors and actuators, it is helpful to review the variables measured (sensors) and the

184 CHAPTER 5 SENSORS AND ACTUATORS

controlled variables (actuators). Fig. 5.1 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 6, 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 the 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 dis-

cuss the outputs (actuators).

VARIABLES TO BE MEASURED

The set of variables sensed for any given power train is specific to the associated engine control con-

figuration. Space limitations for this book preclude a complete survey of all power train control systems

and relevant sensor and actuator selections for all car models. Nevertheless, it is possible to review a set

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 set of variables sensed in engine control includes the following:

1. Mass airflow (MAF) rate

2. Exhaust gas oxygen concentration

3. Throttle plate angular position

4. Crankshaft angular position/RPM

5. Camshaft angular position

6. Coolant temperature

Inlet

air

TPS

Engine

control

Fuel

injectors

Ignition

system

Crankshaft/

camshaft

position

sensor

Coolant

temperature

sensor

MAF Engine EGO EGR

FIG. 5.1 Representative electronic engine control system.

185AUTOMOTIVE CONTROL SENSORS AND ACTUATORS

7. Intake air temperature

8. Ambient air pressure

9. Ambient air temperature

10. Manifold absolute pressure (MAP)

11. Differential exhaust gas pressure (relative to ambient)

12. Vehicle speed

13. Transmission gear selector position

14. Actual transmission gear in use

15. 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:

1. Air conditioner clutch engaged

2. Brake on/off

3. Wide open throttle

4. Closed throttle

5. Transmission gear selection

AIRFLOW RATE SENSOR

In Chapter 4, 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



into

the engine. Throughout this book, the over dot in this notation implies time rate of change. The majority

of cars produced since the early 1990s use 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

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 in Chapter 4 that a four-stroke reciprocating engine

functions as an air pump with air pumped sequentially into each cylinder every two crankshaft revo-

lutions. The dynamics of this pumping process are such that the airflow consists of a fluctuating com-

ponent (at half the crankshaft rot ation frequency times the number of cylinders) superposed on a

quasisteady 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 quasisteady

component of airflow changes with load and speed. It is this quasisteady component of

tðÞthat is

measured by the MAF for engine control purposes. One way of characterizing this quasisteady-state

component is as a short-term time average over a time interval τ (which we denote

aτ

tðÞ) where

aτ

tðÞ¼

tτ

ðÞdt

(5.1)

186 CHAPTER 5 SENSORS AND ACTUATORS

The integration interval (τ) 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 characterist ics

of airflow associated with relatively rapid throttle position changes.

Alternatively, the quasisteady component of mass airflow can be represented by a low-pass-filtered

version of the instantaneous flow rate. Appendix A explains that a low-pass filter (LPF) can be char-

acterized (in continuous time) by an operational transfer function (H

LPF

(s)) of the form

LPF

sðÞ¼

+ b

s + ⋯ b

+ a

s + ⋯ a

(5.2)

where the coefficients determine the response characteristics of the filter. The filter bandwidth effec-

tively selects the equivalent time interval over which mass airflow measurements are averaged. Of

course, in practice with a digital power train control system, mass airflow measuremen ts are sampled

at discrete times, and the filtering is implemented as a discrete time transformation of the sampled data

(see Appendix B).

A typical MAF senso r is a variation of a classic airflow sensor that was known as a hot-wire an-

emometer and was used, for example, to measure wind velocity for weather forecasting and 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, a honeycomb flow straightener is mounted 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 two-terminal

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 airstream (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 and the temperature difference betwee n

the moving air and the sensing element. For a constant supply current (i.e., heating rate), the temper-

ature at the element changes in proportion to the heat removed by the moving airstream, thereby pro-

ducing a change in its resistance. A convenient model for the sensing element resistance (R

)at

temperature (T) is given by Eq. (5.3)

TðÞ¼R

+ k

T T

ref

ðÞ (5.3)

where R

is the resistance at some reference temperature T

ref

(e.g., 0°C) and k

is the resistance/

temperature coefficient. The current i

flowing through R

causes its temperature to rise above

ambient temper ature such that T ¼T

+ ΔT. The relationship between ΔT and

is explained later

in this section of the chapter for a conducting sensing element, k

>0, and for a semiconducting

sensing element, k

<0.

The mass flow rate of the moving airstream is measured via a measurement of the change in resis-

tance. 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 Fig. 5.2.

In the bridge circuit, three resistors (R

, R

, and R

) are connected as depicted in Fig. 5.2 along with

a resistive sensing element denoted R

(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 V

and

(depicted in Fig. 5.2) are connected to the inputs of a relatively high-gain differential amplifier.

187AUTOMOTIVE CONTROL SENSORS AND ACTUATORS

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