Page 284 - Understanding Automotive Electronics

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VEHICLE MOTION CONTROL 8

A switching, duty-cycle The force exerted by the piston is varied by changing the average pressure
type of signal is applied in the cylinder chamber. This is done by rapidly switching the pressure control
to the solenoid coil. By valve between the outside air port, which provides atmospheric pressure, and
varying the duty cycle, the manifold pressure port, the pressure of which is lower than atmospheric
the amount of vacuum, pressure. In one implementation of a throttle actuator, the actuator control
and hence the corre- signal V is a variable-duty-cycle type of signal like that discussed for the fuel
c
sponding throttle angle, injector actuator. A high V signal energizes the electromagnet; a low V signal
c
c
is varied. deenergizes the electromagnet. Switching back and forth between the two
pressure sources causes the average pressure in the chamber to be somewhere
between the low manifold pressure and outside atmospheric pressure. This
average pressure and, consequently, the piston force are proportional to the
duty cycle of the valve control signal V . The duty cycle is in turn proportional
c
to the control signal d (explained above) that is computed from the sampled
error signal e .
n
This type of duty-cycle-controlled throttle actuator is ideally suited for
use in digital control systems. If used in an analog control system, the analog
control signal must first be converted to a duty-cycle control signal. The same
frequency response considerations apply to the throttle actuator as to the speed
sensor. In fact, with both in the closed-loop control system, each contributes to
the total system phase shift and gain.
CRUISE CONTROL ELECTRONICS
In an analog cruise con- Cruise control can be implemented electronically in various ways,
trol system, an error including with a microcontroller with special-purpose digital electronics or
amplifier compares with analog electronics. It can also be implemented (in proportional control
actual speed and desired strategy alone) with an electromechanical speed governor.
(command) speed. The The physical configuration for a digital, microprocessor-based cruise
error signal output is fed control is depicted in Figure 8.8. A system such as is depicted in Figure 8.8
to a proportional ampli- is often called a microcontroller since it is implemented with a
fier and an integral microprocessor operating under program control. The actual program that
amplifier. The resultant causes the various calculations to be performed is stored in read-only
outputs are combined by memory (ROM). Typically the ROM also stores parameters that are critical
a summing amplifier. to the correct calculations. Normally a relatively small-capacity RAM
memory is provided to store the command speed and to store any temporary
calculation results. Input from the speed sensor and output to the throttle
actuator are handled by the I/O interface (normally an integrated circuit
that is a companion to the microprocessor). The output from the controller
(i.e., the control signal) is sent via the I/O (on one of its output ports) to so-
called driver electronics. The latter electronics receives this control signal
and generates a signal of the correct format and power level to operate the
actuator (as explained below).
A microprocessor-based cruise control system performs all of the required
control law computations digitally under program control. For example, a PI
control strategy is implemented as explained above, with a proportional term

UNDERSTANDING AUTOMOTIVE ELECTRONICS 271