Integral Behaviour - Ecolab MULTRONIC Notice Technique

8.6.2

Integral behaviour

In a P controller, the no-delay relationship between the state deviation and the control variable
results in an undesirable steady-state deviation. If, instead of the measured variable, it is the
regulating speed which is influenced directly by the state deviation, then this rigid relationship
between the two variables is eliminated. The result is an integrating controller.
When the state deviation is zero, i.e. when the setpoint value and the actual value are the
same, the regulating speed is also zero. Both positive and negative state deviations can be
influenced by positive and negative regulating speeds, and the control variable cycles through
the entire control range. The control range of the I-controller is that range within which the
state deviation controls the regulating speed in a linear fashion.
The characteristic quantity of the I-controller is the reset time, T n . The reset time is the time it
takes - owing to the integrating action - for the step response to arrive at the value which a P
controller would reach without any delay. At T n = 0 the controller has no I-component.
Given a constant command variable, and provided that there is no interference from
extraneous factors, a closed loop with an I-controller will have no steady state deviation in the
steady state. On the other hand, the closed loop will be slow to respond. With a finite state
deviation, an I-controller is able to change its control variable only gradually.
For this reason, with I-controllers, the directly
controlled variable generally changes more slowly
than with proportional-action controllers. If one then
attempts to make the controller faster by reducing the
integration constant, the closed loop will develop a
slight tendency towards instability.
Control variable function for PI controller: Characteristic value Tn
NOTE
417101163_Multronic.doc
With I-controllers, the finite regulating speed means that the response, though relatively
slow, does result in the complete elimination of the state deviation.
- 67 - Rev: 5.07-08