Page 287 - Understanding Automotive Electronics

2735 | CH 8 Page 274 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

This signal is, in effect, a signed number (i.e., a positive or negative
numerical value). A sign bit indicates the direction of the throttle movement
(advance or retard). The numerical value determines the amount of advance or
retard.
The magnitude of the actuator signal (in binary format) is loaded into a
parallel load serial down-count binary counter. The direction of movement is in
the form of the sign bit (SB of Figure 8.9). The stepper motor is activated by a
pair of quadrature phase signals (i.e., signals that are a quarter of a cycle out of
phase) coming from a pair of oscillators. To advance the throttle, phase A signal
is applied to coil 1 and phase B to coil 2. To retard the throttle these phases are
each switched to the opposite coil. The amount of movement in either
direction is determined by the number of cycles of A and B, one step for each
cycle.
The number of cycles of these two phases is controlled by a logical signal
(Z in Figure 8.9). This logical signal is switched high, enabling a pair of AND
gates (from the set A1, A2, A3, A4). The length of time that it is switched high
determines the number of cycles and corresponds to the number of steps of the
motor.
The logical variable Z corresponds to the contents of the binary counter
being zero. As long as Z is not zero, a pair of AND gates (A1 and A3, or A2 and
A4) is enabled, permitting phase A and phase B signals to be sent to the stepper
motor. The pair of gates enabled is determined by the sign bit. When the sign
bit is high, A1 and A3 are enabled and the stepper motor advances the throttle
as long as Z is not zero. Similarly, when the sign bit is low, A2 and A4 are
enabled and the stepper motor retards the throttle.
To control the number of steps, the controller loads a binary value into
the binary counter. With the contents not zero the appropriate pair of AND
gates is enabled. When loaded with data, the binary counter counts down at the
frequency of a clock (C in Figure 8.9). When the countdown reaches zero, the
K
gates are disabled and the stepper motor stops moving.
The time required to count down to zero is determined by the numerical
value loaded into the binary counter. By loading signed binary numbers into
the binary counter, the cruise controller regulates the amount and direction of
movement of the stepper motor and thereby the corresponding movement of
the throttle.

Vacuum-Operated Actuator
The driver electronics for a cruise control based on a vacuum-operated
system generates a variable-duty-cycle signal as described in Chapter 6. In this
type of system, the duty cycle at any time is proportional to the control signal.
For example, if at any given instant a large positive error exists between the
command and actual signal then a relatively large control signal will be
generated. This control signal will cause the driver electronics to produce a large
duty cycle signal to operate the solenoid so that most of the time the actuator

274 UNDERSTANDING AUTOMOTIVE ELECTRONICS