Lesson 7 - Process Control Mechanisms SAQ
This lesson allows you to test yourself on the material covered in the Process Control Mechanisms lesson and determine the level to which you have met the learning outcomes.
Suggested Response - Q2
** What are the components of a control loop? Draw a diagram and explain each component. Translate this into a level control loop for a tank.
Response:
Begin by sketching a diagram to illustrate all key components of a control loop – hopefully similar to the one below!
Now let us briefly explain each of the key terms:
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The process variable (PV) is the variable in the process you wish to control. In certain cases, you cannot measure this directly and you may have another variable which is the measured variable.
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The disturbance variable or load disturbances are the variables in the process that act on the process variable.
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The setpoint value (SP) is the desired value for the process variable.
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The control mechanism is the process or decision used to effect the change in the variable value.
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The manipulated variable is the variable you can use to control the process variable. It can also be the controller output (CO).
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The error (ε) is the difference between the process variable and the setpoint. For the purposes of this lesson, we will define it as:
ε = SP – PV
Let us now turn this into a level control loop for a tank:
