Tutorial 4 - Embedded Control
This tutorial will introduce two examples of embedded control systems.
The diagram below illustrates the simplest possible control scheme.
Here a single input, x, is applied to the plant, which
causes the output, y, to occur. The plant represents the process
we are trying to control.
Feedforward Control Diagram
An example would be a lawn sprinkler, if the gardener desires a large
radius of the lawn to be watered then the water valve is opened by a
large amount. Conversely, if a small radius of the lawn is to be watered,
the valve is only opened a small amount. Another highly simplified example
would be an automobile. In this case the engine of the car represents
the plant, the accelerator pedal is the input, and the speed we are
travelling is the output.
However, in the automobile situation, we are usually constantly
monitoring our current speed and compensating (either accelerating or
decelerating) according to whether we are traveling too fast or too
slow. The feedback we get through our senses helps us track a desired
speed. This leads us to our next topic, feedback control.
In the simplest case, feedback control is achieved by using a sensor
to monitor the value of the output. The sensor value can then be subtracted
from the input, or setpoint, to create the error signal. This
error signal is then fed into the controller as the new input
signal. This signal is manipulated by the controller, which then outputs
a control signal to the plant.
Feedback Control Diagram
Feedback controllers can be divided into two basic types,
analog and digital. Analog controllers are continous-time controllers
which augment the transfer function of the plant, making the output
stable about some operating point. This is usually accomplished by adding
roots or poles to the transfer function equation. Digital controllers
are discrete devices which can be used in place of their analog counterparts.
Digital controllers usually combine the controller, analog-to-digital
(A/D) converter and digital-to-analog (D/A) converter in one package,
making a compact unit. Digital controllers also have the advantage of
being very flexible devices that are not susceptible to drift. A purely
analog controller, on the other hand, is constructed from resistors,
capacitors and operational amplifiers (here
is a good example for a temperature controller) and parameter (gain)
modification is achieved through changing the position of potientiometers.
Using the Eyebot and the expansion box, you will write
programs to control two simple processes. The first is a simple temperature
regulator and the second is the position of a servo motor.