Data Acquisition

The values of K p , K i and K d affect the characteristics of the step response. This is shownin Figure 11.4.Figure 11.4Effect of damping on the step response of a closed loop systemThe natural tendency of closed loop systems to oscillate around the required output valuecan be seen from the step response. In addition to this, there are many practical controlsystems in which it is almost impossible to entirely eliminate the error.Such systems allow for a zero crossing deadband. This adjustable deadband allows theuser to select an error range above and below the SetPoint where the output will notchange.This deadband is useful in ensuring that the output does not oscillate even though thereis a small error in the system.A feature many controllers incorporate, is output limiting (using an anti-reset windup),whereby the software acts to limit the output from the PID equation from exceeding acertain value.In terms of the PID control algorithm, the integral term is excluded from furthercalculations until the output returns to a value within the correct operating range.Where a control system allows for manual user control of the output, a return toautomatic control could cause a ‘bump’ in the controller output, and subsequently in thesystem output. Bumpless transfer allows the system to transfer from the manual mode tothe automatic mode (where the PID equation determines the output), without the outputbumping up or down. This is achieved in software by calculating a required integral termin the PID equation for automatic mode, so that no immediate 'bump' is caused to theoutput of the controller. The system then slowly adjusts back to the reference outputunder automatic control.