Page 69 - Understanding Automotive Electronics

2735 | CH 2 Page 56 Tuesday, March 10, 1998 10:55 AM

2 THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION

Figure 2.20
Operational Amplifier
Circuit Used to
Compensate for the
Poor Frequency
Response of a
Sensor
FPO

CONTROL SYSTEMS
Control systems are systems that are used to regulate the operation of
other systems. For this discussion, the system being controlled is known as
the system plant. The controlling system is called an electronic controller.
In preparation for discussing the many electronic control systems in
automobiles it is worthwhile to explain in detail what a control system is. A
control system is described by its fundamental elements, which are the objectives
of control, system components, and results or outputs. The objectives of a control
system are the quantitative measures of the tasks to be performed by the system.
These describe the desired values of a variable or of multiple variables and are
normally specified by the user. The results are called outputs (or controlled
variables). Typically, the objective of a control system is to regulate the values of
the outputs in a prescribed manner by the (operator-determined) inputs through
the elements (or components) of the control system.
Control systems, which A control system should
are used to control the 1. Perform its function accurately.
operation of other sys- 2. Respond quickly.
tems, are measured in 3. Be stable.
terms of accuracy, speed 4. Respond only to valid inputs (noise immunity).
of response, stability,
and immunity from A control system’s accuracy determines how close the system’s output will come
external noise. to the desired output, with a constant-value input command. Quick response
determines how closely the output of the system will track or follow a changing
input command. A system’s stability describes how a system behaves when a
change, particularly a sudden change, is made by the input signal. Some unstable
systems will oscillate wildly if uncontrolled. Others that are normally controlled
may go out of control. Either case is undesirable, and a good controller design
will minimize the chance of unstable operation. A system should maintain its

56 UNDERSTANDING AUTOMOTIVE ELECTRONICS