PID CONTROLLERS                     

PID CONTROLLERS

PID controllers are found in a wide range of applications for industrial process control. Approximately 95% of the closed-loop operations of the INDUSTRIAL AUTOMATION SECTOR USE PID CONTROLLERS. PID STANDS FOR PROPORTIONAL-INTEGRAL-DERIVATIVE. THESE THREE CONTROLLERS ARE COMBINED IN SUCH A WAY THAT IT PRODUCES A CONTROL SIGNAL.

Working of PID controller

As a feedback controller, it delivers the control output at desired levels. Before microprocessors were invented, PID control was

implemented by analog electronic components. But today all PID controllers are processed by the microprocessors. PROGRAMMABLE LOGIC CONTROLLERS ALSO HAVE THE INBUILT PID CONTROLLER INSTRUCTIONS. DUE TO THE FLEXIBILITY AND RELIABILITY OF THE PID CONTROLLERS, THESE ARE TRADITIONALLY USED IN PROCESS CONTROL APPLICATIONS.

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Working of PID Controller

With the use of a low-cost simple ON-OFF controller, only two control states are possible, like fully ON or fully OFF. It is used for limited control applications where these two control states are enough for the control objective. However oscillating nature of this control limits its usage and hence it is being replaced by PID controllers.

PID controller maintains the output such that there is zero error between the process variable and setpoint/ desired output by closed-loop operations. PID uses three basic control behaviors that are explained below.

 

P- Controller: 

P-controller

Proportional or P- controller gives an output that is proportional to current error e (t). It compares desired or set points with actual value or feedback process value. The resulting error is multiplied by a proportional constant to get the output. If the error value is zero, then thiS the controller output is zero.

 

P-Controller Response

This controller requires biasing or manual reset when used alone. This is because it never reaches the steady-state condition. It provides a stable operation but always maintains the steady-state error. The speed of the response is increased when the proportional constant Kc increases.

 

I-Controller

 

PI controller

 

Due to the limitation of the p-controller where there always exists an offset between the process variable and set point, I-controller is needed, which provides necessary action to eliminate the steady state error.  It integrates the error over a period of time until the error value reaches zero. It holds the value to the final control device at which error becomes zero. Integral control decreases its output when a negative

error takes place. It limits the speed of response and affects the stability of the system. Speed of the response is increased by decreasing integral gain Ki.

PI Controller Response

as the gain of the I-controller decreases, the steady-state error also goes on decreasing. For most cases, the PI controller is used particularly where the high-speed response is not required. While using the PI controller, I-controller output is limited to somewhat range to overcome the INTEGRAL WIND UP CONDITIONS WHERE INTEGRAL OUTPUT GOES ON INCREASING EVEN AT ZERO ERROR STATE, DUE TO NONLINEARITIES, IN THE PLANT.

 

D-Controller

 

PID controller

I-controller doesn’t have the capability to predict the future behavior of error. So it reacts normally once the setpoint is changed. D-controller overcomes this problem by anticipating future behavior of the error. Its output depends on the rate of change of error with respect to time, multiplied by the derivative constant. It gives the kick start for the output thereby increasing system response.

 

PID Controller Response

response of D controller is more, compared to PI controller and also settling time of output is decreased. It improves the stability of the system by compensating for phase lag caused by I-controller. Increasing the derivative gain increases the speed of response.