Page 274 - Understanding Automotive Electronics

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VEHICLE MOTION CONTROL 8

Vehicle Motion Control

Electronic controls can The previous chapter discussed the application of digital electronics to
automate some driver engine control. This chapter discusses the application of electronics to vehicle
functions that were pre- motion control systems such as cruise control, tire slip control, ride control,
viously performed man- antilock braking, and electronic power steering control.
ually.
TYPICAL CRUISE CONTROL SYSTEM
A cruise control is a Automotive cruise control is an excellent example of the type of electronic
closed-loop system that feedback control system that was discussed in general terms in Chapter 2.
uses feedback of vehicle Recall that the components of a control system include the plant, or system
speed to adjust throttle being controlled, and a sensor for measuring the plant variable being regulated.
position. It also includes an electronic control system that receives inputs in the form of
the desired value of the regulated variable and the measured value of that
variable from the sensor. The control system generates an error signal
constituting the difference between the desired and actual values of this
variable. It then generates an output from this error signal that drives an
electromechanical actuator. The actuator controls the input to the plant in such
a way that the regulated plant variable is moved toward the desired value.
In the case of a cruise control, the variable being regulated is the vehicle
speed. The driver manually sets the car speed at the desired value via the
accelerator pedal. Upon reaching the desired speed the driver activates a
momentary contact switch that sets that speed as the command input to the
control system. From that point on, the cruise control system maintains the
desired speed automatically by operating the throttle via a throttle actuator. The
plant being controlled consists of the power train (i.e., engine and drivetrain),
which drives the vehicle through the drive axles and wheels. The load on this
plant includes friction and aerodynamic drag as well as a portion of the vehicle
weight when the car is going up and down hills.
The configuration for a typical automotive cruise control is shown in
Figure 8.1. The momentary contact (push-button) switch that sets the
command speed is denoted S in Figure 8.1. Also shown in this figure is a
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disable switch that completely disengages the cruise control system from the
power supply such that throttle control reverts back to the accelerator pedal.
This switch is denoted S in Figure 8.1 and is a safety feature. In an actual
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cruise control system the disable function can be activated in a variety of ways,
including the master power switch for the cruise control system, and a brake
pedal–activated switch that disables the cruise control any time that the brake
pedal is moved from its rest position. The throttle actuator opens and closes the
throttle in response to the error between the desired and actual speed.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 261