Question

(TCO 4) Describe PID control.

Answer 1

Controllers are used to obtain the desired performance specification . The important uses of the controllers are

1. Controllers improve steady state accuracy by decreasing the steady state errors.
2. As the steady state accuracy improves, the stability also improves.
3. They also help in reducing the offsets produced in the system.
4. Maximum overshoot of the system can be controlled using these controllers.
5. They also help in reducing the noise signals produced in the system.
6. Slow response of the over damped system can be made faster with the help of these controllers.

The proportional controller (p-controller) stabilizes the gain but produces a steady state error.The integral controller reduces or eliminates the steady state error. The derivative controller reduces the rate of change of error. So to overcome these drawbacks PID controller is used where having advantages of three controllers.

Block diagram of PID CONTROLLER

A PID (Proportional Integral Derivative) controller is a common instrument used in industrial control applications. A PID controller can be used for regulation of speed, temperature, flow, pressure and other process variables. A PID controller, which continuously calculates an error value e(t) as the difference between a desired setpoint sp=r(t) and a measured process variable PV=y(t) an applies a correction based on proportional, integral, and derivative terms. The controller attempts to minimize the error over time by adjustment of a control variable u(t).

?Term P is proportional to the current value of the SP???PV error e(t). For example, if the error is large and positive, the control output will be proportionately large and positive, taking into account the gain factor "K". Using proportional control alone in a process with compensation such as temperature control, will result in an error between the setpoint and the actual process value, because it requires an error to generate the proportional response. If there is no error, there is no corrective response.

?Term I accounts for past values of the SP???PV error and integrates them over time to produce the I term. For example, if there is a residual SP???PV error after the application of proportional control, the integral term seeks to eliminate the residual error by adding a control effect due to the historic cumulative value of the error. When the error is eliminated, the integral term will cease to grow. This will result in the proportional effect diminishing as the error decreases, but this is compensated for by the growing integral effect.

?Term D is a best estimate of the future trend of the SP???PV error, based on its current rate of change. It is sometimes called "anticipatory control", as it is effectively seeking to reduce the effect of the SP???PV error by exerting a control influence generated by the rate of error change. The more rapid the change, the greater the controlling or dampening effect.

Tuning – The balance of these effects is achieved by "loop tuning" to produce the optimal control function.

PID is used to decrease the stedy state error and increase the stability, because pole at orgin and two zeroes are added.one zero compensate the pole and other zero will increase the stability.

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