Page 73 - Understanding Automotive Electronics

2735 | CH 2 Page 60 Tuesday, March 10, 1998 10:55 AM

2 THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION

plant to be controlled, actuator(s), and control electronics. In addition,
however, this system includes one or more sensors and some signal-
conditioning electronics. The signal conditioning used in a closed-loop control
system plays a role similar to that played by signal processing in measurement
instrumentation. That is, it transforms the sensor output as required to achieve
the desired measurement of the plant output. Compensation for certain sensor
defects (e.g., limited bandwidth) is possible, and in some cases necessary, to
allow for the comparison of the plant output with the desired value. Electronic
control systems are classified by the way in which the error signal is processed to
generate the control signal. The major control systems include proportional,
proportional integral, and proportional integral differential controllers.

PROPORTIONAL CONTROLLER

The component at the left of the block diagram of Figure 2.23, is the
element in which the output is actually compared to the input. An error signal
is obtained by electrically subtracting the feedback signal from the command
input. The error signal is the input to the control electronics system, which, in
turn, generates an output called a control signal. The control signal is applied to
the actuator, and the actuator moves in such a direction as to reduce the error
between the actual and desired output to zero.
In Figure 2.23, the sensor provides a measurement x of the plant output.
o
The error signal e is obtained by subtracting x from the desired value x:
o
e = x - x o
In a proportional control system, the error signal is amplified by an amplifier to
yield an output v , which is the control signal:
c
v = Ge
c
where G is the amplifier gain.
The actuator causes the plant output y to increase in proportion to v . The
c
operation of this control system is as follows. Assume arbitrarily that the plant
output (x ) is larger than its desired value. In this case the error signal e is
o
negative. The amplified error signal is applied to the actuator, causing the plant
output to decrease. Thus, x will decrease until x = x, at which point e is zero
o
o
and the output remains fixed at the desired value. A controller that generates a
control signal proportional to the error signal is called a proportional controller.
Disturbance Response

Any purely proportional control system has poor response to a
disturbance. Typically, a disturbance is caused by factors that are outside of the
plant or the control system. For example, in Chapter 8 in a discussion of cruise
control, we introduce an example of a disturbance in which cruise control is
activated on a level road. When the car encounters a hill, the change in load on
the engine is a disturbance.

60 UNDERSTANDING AUTOMOTIVE ELECTRONICS