PID Control

The letters P, I and D are three labels that define a controller's action in response to changing process conditions, the effect of which action is to minimise the disturbance by altering the setting of a control device, such as a valve, a vane or a motor's speed.

Defining the 'error' as the 'setpoint' minus the 'measured variable', the term 'P' refers to an action that is proportional to the error, the term 'I' refers to an action that is related to the integral of the error over a time called the integral time constant, and the term 'D' refers to an action that is related to the rate of change of error with respect to a time called the derivative time constant.

The P, I and D terms are configurable by the user in order to alter the action of the controller so as to optimise the settings. The optimum will be determined by a combination of the characteristics of the process being controlled and the user's preferences. For example, one set of settings will give an action labelled, "fastest approach to setpoint" though in the case of, say, a cracker still in an oil refinery the over-temperature may produce undesirable effects in the still's heater tubes. Another combination of settings may produce "no overshoot", being a slower approach to the setpoint without exceeding it, desirable in such a case. Zeigler & Nicholls produced a widely-used rule-of-thumb for evaluating these settings and tuning the controller, the details of which are available in any control engineering textbook, though many stand-alone controllers have an on-board algorithm for doing it known as "auto-tune", which may be considered as cheating by some.

The process of selecting the correct P, I and D terms is that of evaluating the stability of the control system, and the domain of the evaluation is that of the Controls Engineer.

Hi,

I recently purchased this book "Modern Control Systems" by

Richard C. C. Dorf, Robert H Bishop. It's a little pricey but, if you have the desire to learn more about PID, Controls etc... it's worth it.

In the mean time, follow this link below to wikipedia. You will find this information to be very useful. http://en.wikipedia.org/wiki/PID_controller

mr pwslac , define PID vey nicely and he is correct , however i will increas more as under.

m =100/ % PB(e +1/R ∫ e dt +D (de/dt)

uper equation is for PID controller

e = error

R = integral time

D = derivative time

P = proportional band

Question = how much time it will take for the output to change 5 percent if the measurement stay constant

Given

proportional band 50%

integral time = 3 minuts

derivative time = 3 minuts

set point = 40 %

measuremet = 35 %

cotrol action : increase , decrease

pls solve the above

thanks