Setting up a Control System

The function of a control system has been described, and what variables are involved. Now it is possible to apply this knowledge to an actual control system.

The first thing to consider is the function of the control system in question. In this case the objective is to remove water from the tank which is of a specified temperature. Thus we have to

• Measure the temperature of the water
• Compare this temperature with the setpoint temperature
• Decide which action is to be taken to compensate for this difference in temperature
• And finally carry out this action and hence take note of the consequence

Note that the controller action will be discussed later.

The second thing to discuss is the hardware associated with this system.

• Sensor - this is a temperature transducer. Its electrical output changes depending on the temperature.
• Controller - this is a feedback on/off controller. How it works is described in more detail below.
• Control Element - this is an electrical heater. The controller adjusts the power supply to the heater.

The next thing is the software associated with this system. This is another name for the variables which have to be considered as listed below.

• Flowrates of inlet/outlet streams
• Temperature of inlet stream
• Setpoint temperature of outlet stream
• Power of heater

The first two could change unpredictably and so are called Disturbances. The third is the Setpoint Variable. Finally it is possible to determine the power of heater and so it is the Manipulated Variable.

For the sake of simplicity it is assumed that the temperature of the inlet stream is constant and once the flowrate of the inlet stream has been set it is also constant. Finally the tank is full and so the inlet flowrate equals the outlet flowrate and hence the volume is constant.

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