Page 397 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 3. 1 Vehicle motion control

driver electronics to produce a large duty-cycle signal to below zero volts. The voltage V I is steady or unchanging
operate the solenoid so that most of the time the actu- only when the error is exactly zero; this is why the in-
ator cylinder chamber is nearly at manifold vacuum level. tegral gain block in the diagram in Fig. 13.1-10a can
Consequently, the piston will move against the restoring reduce the system steady-state error to zero. Even a small
spring and cause the throttle opening to increase. As error (e.g., due to a disturbance) causes V I to change to
a result, the engine will produce more power and will correct for the error.
accelerate the vehicle until its speed matches the com- The outputs of the proportional and integral amplifiers
mand speed. are added using a summing amplifier, op amp 4. The
It should be emphasized that, regardless of the actu- summing amplifier adds voltages V P and V I and inverts
ator type used, a microprocessor-based cruise control the resulting sum. The inversion is necessary because both
system will: the proportional and integral amplifiers invert their input
signals while providing amplification. Inverting the sum
1. Read the command speed.
restores the correct sense, or polarity, to the control signal.
2. Measure actual vehicle speed. The summing amplifier op amp produces an analog
3. Compute an error (error ¼ command – actual). voltage, V s , that must be converted to a duty-cycle signal
before it can drive the throttle actuator. A voltage-
4. Compute a control signal using P, PI, or PID control to-duty-cycle converter is used whose output directly
law.
drives the throttle actuator solenoid.
5. Send the control signal to the driver electronics. Two switches, S 1 and S 2 , are shown in Fig. 13.1-10a.
6. Cause driver electronics to send a signal to the Switch S 1 is operated by the driver to set the desired
throttle actuator such that the error will be speed. It signals the sample-and-hold electronics
reduced. (Fig. 13.1-10b) to sample the present vehicle speed and
hold that value. Voltage V I , representing the vehicle
An example of electronics for a cruise control system that speed at which the driver wishes to set the cruise con-
is basically analog is shown in Fig. 13.1-10. Notice that the troller, is sampled and it charges capacitor C. A very high
system uses four operational amplifiers (op amps) and input impedance amplifier detects the voltage on the
that each op amp is used for a specific purpose. Op amp 1 capacitor without causing the charge on the capacitor to
is used as an error amplifier. The output of op amp 1 is ‘‘leak’’ off. The output from this amplifier is a voltage, V s ,
proportional to the difference between the command proportional to the command speed that is sent to the
speed and the actual speed. The error signal is then used as error amplifier.
an input to op amps 2 and 3. Op amp 2 is a proportional Switch S 2 (Fig. 13.1-10a) is used to disable the speed
amplifier with a gain of K P ¼ R 2 =R 1 . Notice that R 1 is controller by interrupting the control signal to the
variable so that the proportional amplifier gain can be throttle actuator. Switch S 2 disables the system when-
adjusted. Op amp 3 is an integrator with a gain of K I ¼ ever the ignition is turned off, the controller is turned off,
1=R 3 C. Resistor R 3 is variable to permit adjustment of the or the brake pedal is pressed. The controller is switched
gain. The op amp causes a current to flow into capacitor C on when the driver presses the speed set switch S 1 .
that is equal to the current flowing into R 3 . The voltage For safety reasons, the brake turnoff is often
across R 3 is the error amplifier output voltage, V e . The performed in two ways. As just mentioned, pressing the
current in R 3 is found from Ohm’s law to be brake pedal turns off or disables the electronic control. In
certain cruise control configurations that use a vacuum-
V e operated throttle actuator, the brake pedal also
I ¼
R 3 mechanically opens a separate valve that is located in
a hose connected to the throttle actuator cylinder. When
which is identical to the current flowing into the capac- the valve is opened by depression of the brake pedal, it
itor. If the error signal V e is constant, the current I will be allows outside air to flow into the throttle actuator cyl-
constant and the voltage across the capacitor will steadily inder so that the throttle plate instantly snaps closed.
change at a rate proportional to the current flow. That is, The valve is shut off whenever the brake pedal is in its
the capacitor voltage is proportional to the integral of the inactive position. This ensures a fast and complete
error signal: shutdown of the speed control system whenever the
driver presses the brake pedal.
ð
1
V I ¼ V e dt
R 3 C 13.1.3.3 Advanced cruise control
The output of the integral amplifier, V I , increases or The cruise control system previously described is ade-
decreases with time depending on whether V e is above or quate for maintaining constant speed, provided that any

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