Page 282 - Understanding Automotive Electronics

2735 | CH 8 Page 269 Tuesday, March 10, 1998 1:19 PM

VEHICLE MOTION CONTROL 8

whether a slot is in the line of sight from source to detector. The light detector
produces an output voltage whenever a pulse of light from the light source
passes through a slot to the detector. The number of pulses generated per
second is proportional to the number of slots in the disk and the vehicle speed:
f = NSK

where,
f is the frequency in pulses per second
N is the number of slots in the sensor disk
S is the vehicle speed
K is the proportionality constant that accounts for differential gear ratio
and wheel size
It should be noted that either a magnetic or optical speed sensor generates a
pulse train such as described here.
The output pulses are passed through a sample gate to a digital counter
(Figure 8.6). The gate is an electronic switch that either passes the pulses to the
counter or does not pass them depending on whether the switch is closed or
open. The time interval during which the gate is closed is precisely controlled
by the computer. The digital counter counts the number of pulses from the
light detector during time t that the gate is open. The number of pulses P that is
counted by the digital counter is given by:
P = tNSK

That is, the number P is proportional to vehicle speed S. The electrical signal in
the binary counter is in a digital format that is suitable for reading by the cruise
control computer.
Throttle Actuator
The throttle actuator is an electromechanical device that, in response to
an electrical input from the controller, moves the throttle through some
appropriate mechanical linkage. Two relatively common throttle actuators

Figure 8.6
Digital Speed
Measurement
System
FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 269