Page 281 - Understanding Automotive Electronics

2735 | CH 8 Page 268 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

Figure 8.5a
Digital Speed
Sensor

FPO

Figure 8.5b
Digital Speed
Signal

FPO

In a typical vehicle speed measurement system, the vehicle speed
information is mechanically coupled to the speed sensor by a flexible cable
coming from the driveshaft, which rotates at an angular speed proportional to
vehicle speed. A speed sensor driven by this cable generates a pulsed electrical
signal (Figure 8.5b) that is processed by the computer to obtain a digital
measurement of speed.
A speed sensor can be implemented magnetically or optically. The
magnetic speed sensor was discussed in Chapter 6, so we hypothesize an optical
sensor for the purposes of this discussion. For the hypothetical optical sensor, a
flexible cable drives a slotted disk that rotates between a light source and a light
detector. The placement of the source, disk, and detector is such that the slotted
disk interrupts or passes the light from source to detector, depending on

268 UNDERSTANDING AUTOMOTIVE ELECTRONICS