Understanding Automotive Electronics 5E Episode 6 docx

MICROCOMPUTER INSTRUMENTATION AND CONTROL 4 UNDERSTANDING AUTOMOTIVE ELECTRONICS 139 Closed-Loop Control System Recall the basic closed-loop control system block diagram of Chapter 2. The error amplifier compares the command input with the plant output and sends the error signal to the control logic. The control logic uses the error signal to generate a plant control signal that causes the plant to react with a new output so that the error signal will be reduced toward zero. The control logic is designed so that the plant’s output follows or tracks the command input. A microcomputer can replace the error amplifier and the control logic. The computer can compare command input and plant output and perform the computation required to generate a control signal. Limit-Cycle Controller The limit-cycle controller, discussed in Chapter 2, can be readily implemented with a microcomputer. Recall that the limit-cycle controller controls the plant output so that it falls somewhere between an upper and lower limit, preferably so that its average value is equal to the command input. The controller must read in the command input and the plant output and decide what control signal to send to the plant based on those signals alone. Using a microcomputer, the upper and lower limit can be determined from the command input by using a lookup table similar to that discussed later in this chapter. The plant output is compared against these two limits. If the plant output is above the higher limit or below the lower limit, the microcomputer outputs the appropriate on/off signal to the plant to bring the output back between the two limits. With proper software, a microcomputer can replace the error ampli- fier and control logic used in the closed-loop control system. Recall that in Chapter 2 the concept of a feedback control system was introduced. There it was shown that a control system compares the value of some controlled variable with a desired value (or set point) for that variable. In such a control system, the difference between the desired and actual value is first obtained, then an electrical signal is generated. The resulting error signal is processed electronically, thereby generating a control signal that operates an actuator. The actuator changes the controlled system in such a way as to reduce the error. In Chapter 2, it was presumed that analog electronics were used for the control. A feedback control system can also be implemented using digital electronics. Figure 4.24 is a block diagram of a control system employing a computer. In this diagram, there is a physical system, or plant, that is to be controlled. The specific variable being controlled is denoted X. For example, in an automobile, the plant might be the engine and the controlled variable might be brake torque. The desired value for X is the set point S. An error signal e is obtained: e = S – X 2735 | CH 4 Page 139 Tuesday, March 10, 1998 11:06 AM 4 MICROCOMPUTER INSTRUMENTATION AND CONTROL 140 UNDERSTANDING AUTOMOTIVE ELECTRONICS The error signal is sampled, yielding samples e n (where n represents sample number; i.e., n = 1,2, . . .). In a typical digital control system, the computer generates an output y n for each input sample: where P is the proportional gain I is the integral gain D is the differential gain T is the time between successive samples The previous equation represents an example of an algorithm for the particular control strategy. The example algorithm is a form of a PID (proportional integral differential) control strategy. Figure 4.24 Typical Digital Control System FPO y n y o Pe n Ie n e n 1– e n 2– ++()∆TD e n e n 1– –() ∆T ++ += ∆ 2735 | CH 4 Page 140 Tuesday, March 10, 1998 11:06 AM MICROCOMPUTER INSTRUMENTATION AND CONTROL 4 UNDERSTANDING AUTOMOTIVE ELECTRONICS 141 After computing y n for each input sample, a digital version of y n is transmitted through the I/O to the DAC. There it is converted to analog format, providing a control signal to the actuator (A), which is presumed here to be analog. The actuator controls the plant in such a way as to cause the error to be reduced toward zero. Many examples of the application of computer- based electronic control systems in automobiles are presented in a later chapters of this book. Multivariable Systems With the appropriate control program, micro- computers have the abil- ity to sample and control multiple inputs and out- puts independently. This type of control is much more difficult to design when using analog cir- cuitry. A very important feature of microcomputer control logic is the ability to control multiple systems independently and to control systems with multiple inputs and outputs. The automotive applications for microcomputer control involve both of these types of multivariable systems. For instance, the automobile engine has several inputs (such as air/fuel ratio, throttle angle, and spark timing). All of the outputs must be controlled simultaneously because some inputs affect more than one output. These types of controllers can be very complicated and are difficult to implement in analog fashion. The increased complexity (and cost) of a multivariable microcomputer system is not much higher than for a single-variable microcomputer system, presuming the microcomputer has the capacity to do the task. It only affects the task of programming the appropriate control scheme into the microcomputer. This type of control is discussed in a later chapter. Table Lookup One of the important functions of a microcomputer in automotive applications is table lookup. These applications include: 1. Linearization of sensor data 2. Multiplication 3. Calibration conversion These applications are explained in detail in later chapters where appropriate. In this chapter, we explain the basic principle using a specific example. The concept of table lookup is illustrated in Figure 4.25, in which a pair of variables, V o and X, are related by the graph depicted therein. Also shown in Figure 4.25 is a table listing certain specific values for the relationship. The functional relationship between V o and X might, for example, be the output voltage of a nonlinear sensor V o for measuring a quantity X. If the value for V o is known, then the corresponding value for X can theoretically be found using the graph or the tabulated values. In the latter case, the nearest two tabulated values for V o are located, and the corresponding values for X are read from the table. Denoting V 1 and V 2 as the nearest values and X 1 , X 2 as the corresponding 2735 | CH 4 Page 141 Tuesday, March 10, 1998 11:06 AM 4 MICROCOMPUTER INSTRUMENTATION AND CONTROL 142 UNDERSTANDING AUTOMOTIVE ELECTRONICS tabulated values, the value for X corresponding to V o is found by linear interpolation: A microcomputer can perform the same operation using tabulated values for the relationship between V o and X (i.e., V o (X ) in correct mathematical notation). This method is illustrated using a specific example of the measurement of a variable X using a sensor output voltage, and variable X is assumed to be that which is illustrated in Figure 4.24. A microcomputer is to obtain the value for X using a table lookup operation. The portion of the microcomputer that is involved in the table lookup process is illustrated in Figure 4.26. The relationship V o (X ) is stored in ROM for representative points along the curve. This data is stored using V o values as addresses, and corresponding values of X as data. For example, consider a point (V 1 , X 1 ). The data X 1 is stored at memory location V 1 in binary format. The operation of the table lookup is as follows. The sensor S has output voltage V o . The computer reads the values of V o (using an ADC to convert to digital format) through the I/O device. Then the MPU under program control (program ROM) calculates the addresses for the two nearest values toV o , which are V 1 and V 2 (V 1 < V o < V 2 ). The computer, under program control, reads values X 1 and X 2 and then calculates X using the preceding formula. Figure 4.25 Illustration of Table Lookup and Interpolation FPO XX 1 X 2 X 1 –()V o V 2 –()/ V 1 V 2 –()+= 2735 | CH 4 Page 142 Tuesday, March 10, 1998 11:06 AM MICROCOMPUTER INSTRUMENTATION AND CONTROL 4 UNDERSTANDING AUTOMOTIVE ELECTRONICS 143 Repeated reference will be made to the table lookup function in later chapters. In particular, Chapter 7 will discuss how a typical digital engine control system frequently obtains data using table lookup. After a chapter on basics of automotive engine control and a chapter on sensors and actuators, this book will deal more specifically with particular microcomputer automotive instrumentation and control systems to show how these systems are used in the automobile to control the engine and drivetrain and many auxiliary functions. Figure 4.26 Architecture Involved in Table Lookup FPO 2735 | CH 4 Page 143 Tuesday, March 10, 1998 11:06 AM 4 MICROCOMPUTER INSTRUMENTATION AND CONTROL 144 UNDERSTANDING AUTOMOTIVE ELECTRONICS Quiz for Chapter 4 1. The parts of a computer include a. CPU b. memory c. input/output d. all of the above 2. What does a microcomputer use to interface with other systems? a. parallel interface b. analog-to-digital converter c. digital-to-analog converter d. all of the above 3. Which control line do peripherals use to get the computer’s atten- tion? a. power line b. read/write line c. interrupt line d. clock line 4. What is a data bus? a. a set of wires that carries bits to or from the processor and memory or peripherals b. a large yellow vehicle for carrying data c. a bus carrying addresses d. a set of wires for control signals 5. What are computers used for in instrumentation systems? a. signal processing b. sensor, actuator, and display linearization c. display formatting d. filtering e. all of the above 6. According to the Nyquist sam- pling theorem, a signal must be sampled at a. the highest frequency in the signal b. at least twice the highest fre- quency in the signal c. less than half the lowest fre- quency in the signal d. more than half the lowest fre- quency in the signal 7. What advantages does digital sig- nal processing have over analog signal processing? a. digital is more precise b. digital doesn’t drift with time and temperature c. the same digital hardware can be used in many filters d. all of the above 8. What advantage does analog signal processing have over digital signal processing? a. analog is always less expensive b. the same analog hardware can be used for many filters c. analog is sometimes less expensive d. high-frequency signals can only be filtered with analog filters 9. What type of memory is used to permanently store programs? a. RAM b. ROM c. MAP d. RPM 2735 | CH 4 Page 144 Tuesday, March 10, 1998 11:06 AM MICROCOMPUTER INSTRUMENTATION AND CONTROL 4 UNDERSTANDING AUTOMOTIVE ELECTRONICS 145 10. What type of memory is used to temporarily store data and vari- ables? a. RAM b. ROM c. MAP d. RPM 11. What distinguishes a computer from a fancy calculator? a. add, subtract, multiply, and divide b. stored program c. the calculators can read paper tape d. digital circuits 12. What part of the computer per- forms the arithmetic and logic functions? a. peripherals b. memory c. CPU d. address bus 13. Which computer register is the main work register? a. program counter b. stack pointer c. condition code register d. accumulator 14. A short initialization program is called what kind of program? a. subroutine b. boot program c. main program d. branch 15. Which register keeps track of pro- gram steps? a. program counter b. stack pointer c. condition code register d. accumulator 16. A programmer uses what type of statements in an assembly lan- guage program? a. op codes b. mnemonics c. machine code 17. Most microcomputers use how many bits to address memory? a. 16 b. 32 c. 4 d. 6 18. Most automotive microcomputers use how many bits in arithmetic? a. 1 b. 6 c. 4 d. 8 19. Which of the following is a short program that ends with an RTS instruction? a. main program b. interrupt c. boot d. subroutine 2735 | CH 4 Page 145 Tuesday, March 10, 1998 11:06 AM 2735 | CH 4 Page 146 Tuesday, March 10, 1998 11:06 AM THE BASICS OF ELECTRONIC ENGINE CONTROL 5 UNDERSTANDING AUTOMOTIVE ELECTRONICS 147 The Basics of Electronic Engine Control Engine control in the vast majority of engines means regulating fuel and air intake as well as spark timing to achieve desired performance in the form of torque or power output. Until the late 1960s, control of the engine output torque and RPM was accomplished through some combination of mechanical, pneumatic, or hydraulic systems. Then, in the 1970s, electronic control systems were introduced. This chapter is intended to explain, in general terms, the theory of electronic control of the automotive engine. Chapter 7 explains practical control methods and systems. The examples used to explain the major developments and principles of electronic control have been culled from the techniques used by various manufacturers and are not necessarily representative of any single automobile manufacturer at the highest level of detail. MOTIVATION FOR ELECTRONIC ENGINE CONTROL The motivation for electronic engine control came in part from two government requirements. The first came about as a result of legislation to regulate automobile exhaust emissions under the authority of the Environmental Protection Agency (EPA). The second was a thrust to improve the national average fuel economy by government regulation. Exhaust Emissions The combustion of gaso- line in an engine results in exhaust gases includ- ing CO 2 , H 2 O, CO, oxides of nitrogen, and various hydrocarbons. The engine exhaust consists of the products of combustion of the air and gasoline mixture. Gasoline is a mixture of chemical compounds that are called hydrocarbons . This name is derived from the chemical formation of the various gasoline compounds, each of which is a chemical union of hydrogen (H) and carbon (C) in various proportions. Gasoline also contains natural impurities as well as chemicals added by the refiner. All of these can produce undesirable exhaust elements. During the combustion process, the carbon and hydrogen combine with oxygen from the air, releasing heat energy and forming various chemical compounds. If the combustion were perfect, the exhaust gases would consist only of carbon dioxide (CO 2 ) and water (H 2 O), neither of which are considered harmful in the atmosphere. In fact, both are present in a human’s breath. 2735 | CH 5 Page 147 Tuesday, March 10, 1998 11:10 AM 5 THE BASICS OF ELECTRONIC ENGINE CONTROL 148 UNDERSTANDING AUTOMOTIVE ELECTRONICS Unfortunately, the combustion of the SI engine is not perfect. In addition to the CO 2 and H 2 O, the exhaust contains amounts of carbon monoxide (CO), oxides of nitrogen (chemical unions of nitrogen and oxygen that are denoted NO x ), unburned hydrocarbons (HC), oxides of sulfur, and other compounds. Some of the exhaust constituents are considered harmful and have come under the control of the federal government. The exhaust emissions controlled by government standards are CO, HC, and NO x . Automotive exhaust emission control requirements started in the United States in 1966 when the California state regulations became effective. Since then, the federal government has imposed emission control limits for all states, and the standards became progressively more difficult to meet through the decade 1970–1980. Auto manufacturers found that the traditional engine controls could not control the engine sufficiently to meet these emission limits and maintain adequate engine performance at the same time, so they turned to electronic controls. Fuel Economy Everyone has some idea of what fuel economy means. It is related to the number of miles that can be driven for each gallon of gasoline consumed. It is referred to as miles per gallon (MPG) or simply mileage . Just like it improves emission control, another important feature of electronic engine control is its ability to improve fuel economy. Electronic engine con- trol is used to reduce exhaust emissions and improve fuel economy, both of which have lim- its set by the govern- ment. It is well recognized by layman and expert alike that the mileage of a vehicle is not unique. It depends on size, shape, weight, and how the car is driven. The best mileage is achieved under steady cruise conditions. City driving, with many starts and stops, yields worse mileage than steady highway driving. The government fuel economy standards are not based on just one car, but are stated in terms of the average rated miles per gallon fuel mileage for the production of all models by a manufacturer for any year. This latter requirement is known in the automotive industry by the acronym CAFE (corporate average fuel economy). It is a somewhat complex requirement and is based on measurements of the fuel used during a prescribed simulated standard driving cycle. FEDERAL GOVERNMENT TEST PROCEDURES For an understanding of both emission and CAFE requirements, it is helpful to review the standard cycle and how the emission and fuel economy measurements are made. The U.S. federal government has published test procedures that include several steps. The first step is to place the automobile on a chassis dynamometer, like the one shown in Figure 5.1. 2735 | CH 5 Page 148 Tuesday, March 10, 1998 11:10 AM [...]... Ratios FPO UNDERSTANDING AUTOMOTIVE ELECTRONICS 163 2735 | CH 5 Page 164 Tuesday, March 10, 1998 11:10 AM 5 THE BASICS OF ELECTRONIC ENGINE CONTROL Figure 5.11 Typical Variation of Engine Performance with EGR FPO CONTROL STRATEGY The use of catalytic converters to reduce emissions leaving the tailpipe allows engines to be calibrated for better performance and still meet emission regulations 164 It is... HC and CO as functions of temperature Above about 300˚C, the efficiency approaches 98% to 99% for CO and more than 95% for HC Figure 5.12 Oxidizing Catalytic Converter FPO UNDERSTANDING AUTOMOTIVE ELECTRONICS 165 2735 | CH 5 Page 166 Tuesday, March 10, 1998 11:10 AM 5 THE BASICS OF ELECTRONIC ENGINE CONTROL Figure 5.13 Oxidizing Catalyst Conversion Efficiency versus Temperature FPO The Three-Way Catalyst... stoichiometry The operation of the three-way catalytic converter is adversely affected by lead Thus, in automobiles using any catalyst, it is necessary to use lead-free fuel 166 UNDERSTANDING AUTOMOTIVE ELECTRONICS 2735 | CH 5 Page 167 Tuesday, March 10, 1998 11:10 AM THE BASICS OF ELECTRONIC ENGINE CONTROL 5 Figure 5.14 Conversion Efficiency of a TWC FPO The TWC operates at peak efficiency when the air/fuel... (IGN) 5 Exhaust gas oxygen sensor (EGO) 6 Engine coolant sensor (ECS) 7 Engine position sensor (EPS) The EPS has the capability of measuring crankshaft angular speed (RPM) as well as crankshaft angular position when it is used in conjunction with a stable and precise electronic clock (in the controller) UNDERSTANDING AUTOMOTIVE ELECTRONICS 167 2735 | CH 5 Page 168 Tuesday, March 10, 1998 11:10 AM 5 THE... torque are also strongly influenced by timing Figure 5.9 shows that maximum torque occurs at a particular advanced timing referred to as minimum advance for best timing (MBT) UNDERSTANDING AUTOMOTIVE ELECTRONICS 161 2735 | CH 5 Page 162 Tuesday, March 10, 1998 11:10 AM 5 THE BASICS OF ELECTRONIC ENGINE CONTROL Figure 5.8 Typical Variation of Performance with a Variation in Air/Fuel Ratio FPO Operation... variable to be controlled, which assists the engine control system in meeting exhaust gas emission regulations Figure 5.9 Typical Variation of Performance with Spark Timing FPO 162 UNDERSTANDING AUTOMOTIVE ELECTRONICS 2735 | CH 5 Page 163 Tuesday, March 10, 1998 11:10 AM THE BASICS OF ELECTRONIC ENGINE CONTROL 5 Effect of Exhaust Gas Recirculation on Performance Exhaust gas recirculation greatly reduces nitrous... proportional to power The definitions for the brake-specific emission rates are BSHC = brake-specific HC concentration r HC = Pb BSCO = brake-specific CO concentration r CO = Pb 160 UNDERSTANDING AUTOMOTIVE ELECTRONICS 2735 | CH 5 Page 161 Tuesday, March 10, 1998 11:10 AM THE BASICS OF ELECTRONIC ENGINE CONTROL 5 BSNOx = brake-specific NOx concentration r NOx = Pb where rHC = HC rate of flow rCO = CO rate... quantities that are outputs from the controller These outputs are 1 Fuel metering control 2 Ignition control 3 Ignition timing 4 Exhaust gas recirculation control Figure 5 .6 Major Controller Inputs from Engine UNDERSTANDING AUTOMOTIVE ELECTRONICS 157 2735 | CH 5 Page 158 Tuesday, March 10, 1998 11:10 AM 5 THE BASICS OF ELECTRONIC ENGINE CONTROL Figure 5.7 Major Controller Outputs to Engine DEFINITION... friction This means that only about 20% is available to drive the vehicle and accessories These percentages vary somewhat with operating conditions but are valid on the average UNDERSTANDING AUTOMOTIVE ELECTRONICS 159 2735 | CH 5 Page 160 Tuesday, March 10, 1998 11:10 AM 5 THE BASICS OF ELECTRONIC ENGINE CONTROL Calibration The definition of engine calibration is the setting of the air/fuel ratio and ignition... generator absorbs all mechanical power that is delivered at the drive wheels, and the horsepower is calculated from the electrical output (7 46 watts of electrical output equals 1 horsepower.) The controls of the dynamometer can be set to simulate the UNDERSTANDING AUTOMOTIVE ELECTRONICS 149 2735 | CH 5 Page 150 Tuesday, March 10, 1998 11:10 AM 5 Emission samples are collected and measured during a simulated . 145 Tuesday, March 10, 1998 11: 06 AM 2735 | CH 4 Page 1 46 Tuesday, March 10, 1998 11: 06 AM THE BASICS OF ELECTRONIC ENGINE CONTROL 5 UNDERSTANDING AUTOMOTIVE ELECTRONICS 147 The Basics. functions. Figure 4. 26 Architecture Involved in Table Lookup FPO 2735 | CH 4 Page 143 Tuesday, March 10, 1998 11: 06 AM 4 MICROCOMPUTER INSTRUMENTATION AND CONTROL 144 UNDERSTANDING AUTOMOTIVE ELECTRONICS Quiz. corresponding 2735 | CH 4 Page 141 Tuesday, March 10, 1998 11: 06 AM 4 MICROCOMPUTER INSTRUMENTATION AND CONTROL 142 UNDERSTANDING AUTOMOTIVE ELECTRONICS tabulated values, the value for X corresponding