PID control loop is a closed loop control system used widely in industrial automation. PID stands for Proportional-Integral-Derivative control. PID control is the combination of these three controllers.
Implementation of the control loops can be achieved using pneumatic, analog, or digital electronics.The first process controllers were pneumatic. They are replaced with electronics controllers, which are low cost and needed less maintenance.
Simple ON/OFF control:
An ON/OFF controller takes two controller actions ON and OFF actions. When the process exceeds the setpoint, controller turns OFF the process and stay ON at any stage below setpoint.
For example, In this case, the furnace temperature sensor moves a flapper that controls the air flow from a nozzle. When the temperature in the furnace reaches its set point the sensor moves the flapper toward the nozzle to stop the air flow and allow pressure to build up in the bellows. The bellows operate an air control relay that shuts OFF the air flowing to the control valve turning OFF the fuel to the furnace.
When the temperature in the furnace drops below a set level the flapper is opened by the sensor, reducing the air pressure in the bellows, which in turn opens the air control valve allowing the air pressure to drop and the control valve to open, turning ON the fuel to the furnace.
PID controller:
PID controller is a continuous control action which is a combined action Proportonal, Integral and Derivative control actions. There are pneumatic and electronic controllers, pneumatic controllers are replaced by electronic controllers.
Pneumatic controller:
PID control action is acheived using diaphragm-bellow-flapper nozzle system.
The pressure from the sensing device Pin is compared to a set or reference pressure Pref to generate a differential force (error signal) on the flapper to move the flapper in relation to the nozzle giving an output pressure proportional to the difference between Pin and Pref. If the derivative restriction is removed the output pressure is fed back to the flapper via the proportional bellows to oppose the error signal and to give proportional action. System gain is adjusted by moving the position of the bellows along the flapper’s arm,
Integral action is achieved by the addition of the integral bellows and restriction. An increase in Pin moves the flapper towards the nozzle causing an increase in output pressure. The increase in output pressure is fed to the integral bellows via the restriction until the pressure in the integral bellows is sufficient to hold the flapper in the position set by the increase in Pin, creating integral action.
Electronic controller:
PID action can be performed using either analog or digital electronic circuits.The electronic circuit diagram is built on the basis of the block diagram shown below:
The output from the process is compared with the setpoint through a comparator. The difference is fed to the integrator, differentiator, and proportional amplifier through two inverting unity gain buffer.
All the output from three proportional amplifier, differentiators and integrators are added at summing amplifier, which is a correction function. The correction signal is given to the actuators which makes a change in control action in the process.
Electronic Circuit:
The circuit implementation of the PID controller is :slight:
This is a complex circuit because all the amplifier blocks are shown doing a single function to give a direct comparison to the block diagram and are only used as an example. In practice, there are a large number of circuit component combinations that can be used to produce PID action.