Process Control System Elements

By Youssef Edward
June 15, 2019

Process control refers to adjusting a variable element to a certain setpoint where that element is under external influences that could deviate far from that setpoint. If those influences are strong enough deviating the element under control from the setpoint, the control system must be able to return the variable element to the setpoint in short time and with minimum transient deviation.

Two important parameters play major role in control systems: the time for the variable element to settle to the setpoint and the transient behavior. The less the time it takes to get back to the setpoint under strong external influences, the better the control system. Also the less the deviation of the variable element from the setpoint, the better the control system performance. Usually, in any control system, after abnormal environmental influences, the variable element under control oscillates back and forth in damping nature until reaching again the set point.

As an example for a simple control system, consider a tank that outlet water to instrument. The tank has inlet water that fills the tank. The control system job is to get the tank level to a predefined level. If the outlet water flow is high, the inlet flow must be increased also to make sure the tank level will not be decreased. Also if the outlet flow is low, the inlet flow must be decreased also to make sure the tank level will not saturate.

In the above example, the variable element under control is the tank level. The element that makes the actual control is the solenoid in the inlet that controls the flow rate. The mechanical element that makes the actual control is called the actuator in the context of control systems. In The current example, the actuator could also be a motor that rotate a head for a water valve. If the head is rotated in one direction, the area inside the valve would be minimized which minimizes also the inlet flow rate. In the reverse direction, the area would be maximized which maximizes the flow rate.

The device that measures the element under control is called the sensor. In the current example, a level sensor must be used that measures that tank level inside the tank. Generally, the process of measuring the variable is called instrumentation.  The instrumentation device must be able to convert the nature of the variable to an electric nature. For instance, a sensor measuring pressure must be able to convert the pressure energy into electric energy. Also measuring temperature involves converting temperature energy into electric energy, and so on. Those devices are called transducers or instrumentation devices in the context of control systems.

Generally, the transducers output very low signal in the range of MilliVolt. Those are difficult to be processed directly by the controller. So a signal conditioning device must be used. Those devices must be able to amplify the signal to a suitable level that could be processed by the controller. For instance most of controllers require a range from 0-5 V signal level to read correctly the variable. If the variable is processed directly without amplification, the processing would be useless and strange results would occur.

The most important element is the process control is the controller itself. The controller is the intelligent element that takes the decisions based on the variable value that is measured using the sensor and the setpoint adjusted by the user. If some deviation happens for the variable with respect to the setpoint, the controller first must decide in which direction the variable deviated. In the above example, if the tank level is lowered, the valve must be rotated in a direction so that the area of flow is maximized to increase the volume of water inside the tank. The amount of rotating the head of the valve is dependent on the controller nature.

Two types of controller exist: the analog controller and the programmable controller. Analog controllers are composed of analog circuits and may contain logic gates in the design. Those elements are so designed to convert the error signal into appropriate amount of output signal the will fed the actuator. The more elegant way for the design of controller is the microprocessor based controllers. Those controllers use common programming languages such as C to implement the controller behavior.

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