Page 63 - Understanding Automotive Electronics

2735 | CH 2 Page 50 Tuesday, March 10, 1998 10:55 AM

2 THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION

components. Again it should be noted that the trend in automotive electronic
systems is toward digital rather than analog realization. However, because both
realizations are used, both types of components are discussed below.

Sensor
Sensors convert one A sensor is a device that converts energy from the form of the measurement
form of energy, such as variable to an electrical signal. An ideal analog sensor generates an output voltage
thermal energy, into that is proportional to the quantity q being measured:
electrical energy.
v = K q
s
s
where K is the sensor calibration constant.
s
By way of illustration, consider a typical automotive sensor—the throttle-
position sensor. The quantity being measured is the angle (theta) of the throttle
plate relative to closed throttle. Assuming for the sake of illustration that the
throttle angle varies from 0 to 90 degrees and the voltage varies from 0 to 5
volts, the sensor calibration constant K is
s
5
K = -------- = .056 volt/degree
s
90°
Alternatively, a sensor can have a digital output, making it directly
compatible with digital signal processing. For such sensors, the output is an
electrical equivalent of a numerical value, using a binary number system as
described earlier in this chapter. Figure 2.15 illustrates the output for such a
sensor. There are N output leads, each of which can have one of two possible
N
voltages, representing a 0 or 1. In such an arrangement, 2 possible numerical
values can be represented. For automotive applications, N ranges from 8 to 16,
16
8
corresponding to a range of from 64 (2 ) to 256 (2 ) numerical values.
Of course, a sensor is susceptible to error just as is any system or system
component. Potential error sources include loading, finite dynamic response,
calibration shift, and nonlinear behavior. Often it is possible to compensate for
these and other types of errors in the electronic signal processing unit of the
instrument. If a sensor has limited bandwidth, it will introduce errors when

Figure 2.15
Digital Sensor
Configuration

50 UNDERSTANDING AUTOMOTIVE ELECTRONICS