Principles of Control Systems

A control system is a system of integrated elements whose function is to maintain a process variable at a desired value or within a desired range of values. The control system monitors a process variable or variables, and then causes some action to occur to maintain the desired system parameter. A good example in a practical application is a central heating unit that monitors the temperature of the house using a thermostat. When the temperature of the house drops to a preset value; the furnace turns on providing a heat source. The temperature of the house increases until a switch in the thermostat causes the furnace to turn off.

A control system input is the stimulus applied to a control system from an external source to produce a specified response from the control system. In the central heating unit example discussed above, the control system input is the temperature of the house as monitored by the thermostat.

A control system output is the actual response obtained from the control system. In the central heating unit example above, the temperature dropping to a preset value on the thermostat causes the furnace to turn on providing heat to raise the temperature of the house.

Control systems are classified by the control action which is the quantity responsible for activating the control system to produce the output. The two general classifications are open-loop and closed-loop control systems.

An open-loop control system is one which the control action is independent of the output. An industrial application example of an open-loop control system is a chemical addition pump with a variable speed control. The feed rate of chemicals that maintain proper chemistry of a system is determined by the operator, who is not part of the system changes, the pump cannot respond by adjusting its feed rate (speed) without operator action.

A closed loop control system is one in which control action is dependent on the output. A feedback is information in a closed loop control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. The information is continually fed back to the control circuit in response to control action.

A typical example of a closed loop control system; a level control in a storage tank is shown below:

This control system maintains water level in a storage tank. The system accomplishes this goal by continuously sensing the level in the tank and adjusting a supply valve to add more or less water to the tank. The desired level is preset by an operator who is not part of the control system. The actual storage tank level, sensed by the level transmitter is the feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level.

An automatic control system is a preset closed loop control system that requires no operator action. This assumes the process remains in the normal range for the control system. An automatic control has two process variables associated with it: a controlled variable and a manipulated variable.

A controlled variable is the process variable that is maintained at a specified value or within a specified range e.g. the tank level in the above example.

A manipulated variable is the process variable that is acted on by the control system to maintain the controlled variable at the specified value or within the specified range. In the example discussed earlier, the flow rate of the water supplied to the tank is the manipulated variable.

Functions of Automatic Control

An automatic control has four basic functions namely:

• Measurement
• Comparison
• Computation
• Correction

In our water tank level control system covered above, the level transmitter measures the level within the tank. The level transmitter sends a signal representing the tank level to the level control device, where it is compared to the desired tank level, the level control device then computes how far to open the supply valve to correct any difference between the actual and desired tank levels.

Functional elements of automatic control

The three functional elements needed to perform the function of an automatic control system are:

• Measurement element
• Error detection element
• Final control element

The measuring element performs the measuring function by sensing and evaluating the controlled variable. The error detection element first compares the value of the controlled variable to the desired value, and then signals an error if a deviation exists between the actual and desired values. The final control element responds to the error signal by correcting the manipulated variable of the process.

An automatic controller is an error-sensitive self-regulating device. It takes a signal from the process and feeds it back into the process. Therefore, closed loop control is referred to as feedback control.