How the Control Loop is Completed

In this experiment the controller is given instructions by a computer. On a chemical plant this could be a large computer located in a control room, handling many control loops at the same time. Alternatively it could be, as in this case, a simple cheap computer dedicated to controlling one small part of a plant or piece of equipment.

Computers are electronic devices required to be attached to measuring instruments which produce electrical signals. Similarly the electrical output from the computer must be converted to a suitable form which can affect the process.

The electrical signals from any measuring device are analog signals i.e. they vary continuously with the measurement. They have to be changed into digital signals which are either on or off, represented in the computer by the presence or absence of a voltage. The device which does this is called an analog-to-digital converter or ADC. This receives a continuous electrical signal and produces a number inside the process control computer.

Similarly, the digital signal from the computer usually has to be returned as a continously varying analog signal. This is the inverse of the above and is performed by a digital-to-analog converter or DAC.


In the system studied here there are a number of stages required to convert the level in the tank to a change in flowrate.

The above procedure can be shown more clearly in a diagram.


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