Fundamentals of Process Control
What are the advantages and disadvantages of each?
Control schemes are algorithms that are implemented for the purpose of acting on an error signal to produce a control signal. The control objectives may be:
 This is acceptable only when the time constant of the process is large compared to the time lag of the control elements. This type of scheme is commonly used in thermostat-furnace systems. The neutral zone of approximately 2F is hardly noticed by the occupants. On-off control for a hot-water valve when taking a bath would be fine, but it would make for a very uncomfortable shower. Why? Proportional Control (P). In a proportional control scheme the change in the control signal is proportional to the error from the set point. The proportionality constant between the error and the signal is called the proportional gain, K: where C(t) is the control signal and E(t) is the error signal. Proportional control is used in applications requiring rapid action since the control signal responds to the error instantaneously (in theory). The chief disadvantage of proportional control is that it results in steady-state errors in systems that include resistance. It can only be used when the proportional gain can be set large enough to compensate for the largest expected disturbance. Transient behavior may also be very oscillatory. Proportional-Integral (PI) Control. The addition of an integral component to a proportional control scheme results in the PI scheme: 
where KP is the proportional gain and KI is the integral gain. The presence of the integral term provides an automatic reset of the controller in the presence of a disturbance. The use of two gains allows for more sophisticated specification of the transient response. Of course, one drawback of this scheme is that the integral term usually generates a control signal even if the error is zero.
More details can be found in: |
