There are two basic actions of PID with respect to the control direction of MV. These are direct action and reverse action. Direct action will lead the MV to increase when the PV is larger than the SV.(For example, cooling application)· Reverse action will lead the MV to decrease when the PV is larger than the SV.(For example, heating application) A combination of all three of the actions described above is more commonly referred to as PID action. The waveforms of PID action are illustrated in Fig below. PID is the most often used corrective action for process control. There are however, many other types of control actions based upon PID action. Understanding the fundamentals of PID action gives a good foundation for understanding other types of controllers. The waveforms used have been idealized for ease of the explanation and are only an example of what may be encountered in practice. Loading is a function of demand and is not affected by the control functions or actions; the control function is to ensure that the variables are within their specified limits. To give an approximate indication of the use of PID controllers for different types of loops, the following are general rules that should be followed:
Pressure control requires proportional and integral; derivative is normally not required. Level control uses proportional and sometimes integral, derivative is not normally required. Flow control requires proportional and integral; derivative is not normally required. Temperature control uses proportional, integral, and derivative usually with integral set for a long time period. However, the above are general rules and each application has its own requirements.
Normally the PID function is used to control process variables such as temperature, pressure, liquid level, or flow rate. The PID controller receives the process variable (PV) and controls the manipulation variable (MV) in order to adjust the PV to match the set value (SV). The figure below shows a typical configuration for a PID control system.
In the above figure:
Process: A reaction which is controlled by physical values such as temperature, pressure, flow rate, etc.
Sensor: A detector that detects the controlled physical values. It can be a thermocouple, RTD, pressure gage, flow meter, etc.
Signal Converter: A device that transmits a weak sensor signal to the PID controller by converting it into the signal suited for the environment such as 4 – 20 mA, 1 – 5 Vdc, pulse train, etc.
Actuator: A device that regulates fluid or electric power to the process according to the signal generated by the PID controller. It can be a control valve, thyristor, variable speed drive, etc.
PV: Process variable … normally 4 – 20 mA or 1 – 5 Vdc analog signals
MV: Manipulation variable … normally 4 – 20 mA, 1 – 5 Vdc or a time-proportional pulse.
PID Equation: Controls the MV based on the magnitude of e and De and the PID tuning parameters. These parameters may be more or less aggressive based on system time responses and operator preferences.
Figure : Waveforms for proportional plus integral action and waveforms for proportional plus derivative and integral action.