Controller Actions

Introduction

This experiment shows three different ways that a controller can react to a deviation from the setpoint.


On-Off Control

This is the simplest type of control. The algorithm which describes the action of the controller is

The specified amount in question is known as the deadzone and in the experiment is defined as a percentage of the total height of the tank.

The graphs below show that if the deadzone is small then the level is close to its setpoint but the valve is being opened and closed continuously which is unsatisfactory (why?).

On the other hand if the deadzone is large then there is not as much opening and closing of the valve but the level is not very close to the setpoint, which is also unsatisfactory.

The optimal solution depends on the application of the loop and is a compromise between the two solutions shown below.


Small Deadzone


Large Deadzone


Proportional Only Control

In proportional only control the algorithm which describes the action of the controller is

The variable which determines the proportionality is known as the proportional band. The relationship between the adjustment and error is

Thus

Note that the value at no error always stays constant. If there are changes to the conditions, either a setpoint change or other disturbance, then there must be a corresponding change in the adjustment. However, for any adjustment there must be an error. This leads to one of the disadvantages of using proportional control. At the new steady state there has to be an error or offset.

Therefore the two graphs below show that

Again the optimal solution depends on the application of the loop and is a compromise between the two solutions shown.


Small Proportional Band


Large Proportional Band


Proportional-Integral Action

The third and final controller action used in this experiment is proportional-integral control. This is an extension of the proportional controller shown above. It works by summing the current controller error and the integral of all previous errors. i.e.:

It could simply be thought of as a way of continuously adjusting the value at no error so as to eliminate the offset.

The new variable introduced to define the amount of integral action is known as the integral reset time of the controller.

Thus the advantage of using integral action with proportional action is that it can eliminate the offset. The disadvantages of proportional-integral control are that it gives rise to a higher maximum deviation, a longer reponse time and a longer period of oscillation than with proportional action alone. This type of control action is therefore used where the above can be tolerated and offset is indesirable.

The two graphs below show the effect of changing the integral reset time. Note that the proportional band is constant at a value between the two extremes used for the graphs in the previous section.


Small Reset Time


Large Reset Time


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