Page 47 - Instrumentation and Measurement

FIGURE 3.20 Control of the speed of rotation of a shaft.

The control systems so far considered are essentially concerned with a single loop for which there is
a single set point, a single controlled variable and a single actuator. However, in many control
situations there are more than one variable being controlled. Modern automobiles have engine
management microcontrollers which are used to exercise control over the large number of systems
in such automobiles. The engine control unit aims to ensure that the engine operates at optimal
conditions at all times and thus exercises control over such items as fuel injection, spark timing, idle
speed and anti-knock. It does this by taking inputs from a number of sensors, interpreting the values
and then giving output to adjust the operation of the engine. There are also systems for such items as
braking and traction control, suspension control, cruise control, air conditioning, air-bag systems and
security systems.

While with control systems having more than one variable, more than one sensor and more than one
actuator it would be possible to wire each situation independent of the other with its own controller,
there are advantages in using a single controller to control the inputs for more than one sensor. This
does allow more than one variable to be taken into account in arriving at the optimum outputs for
sensors but also it is possible to simplify the wiring by using a bus connector so that each sensor and
actuator is connected locally into a common connection termed a bus (Figure 3.21). Sensors and
actuators are then in communication with the bus controller via a common connection, each such
sensor and actuator being uniquely identified to the controller so that information can be supplied
from supervisory control to specific sensors and specific actuators and receive information back from
them.

FIGURE 3.21 A bus system.

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