Design and Modeling of Anti Wind Up PID Controllers - Complex System Modelling and Control Through Intelligent Soft Computations - page 4

2 Previous Work

As explained in the previous section, the windup phenomena is caused by the
integral action of a PI or PID controllers when the input of the system is saturated,
then the performance of the system is deteriorated by the increasing of the integral
action of the controller, yielding many unwanted effects such as a higher overshoot,
a long settling time and even instability. This phenomena is found in SISO and
MIMO systems in the continuous and discrete time representations when the input
is saturated due to the physical limits of different kind of actuators such as
mechanical, hydraulic and electrical systems.
In the case of SISO continuous systems there are some classical architectures
implemented to avoid this unwanted effect, some of them, are based on the back
calculation of the integral action and other are based on the feedback of the satu-
rated input to the PID controller. The tracking anti windup controller is one of the
well known control strategies implemented to avoid the deterioration of the system
when this phenomena is found; it consists of a feedback loop generated by the
saturated and non saturated inputs and then this signal is used to reduce the inte-
grator input (Bohn and Atherton 1995 ). In Fig. 1 the tracking anti windup controller
is shown where as can be noticed the difference of the non saturated and saturated
inputs are fedback to the integrator.
Another method is conditional integration, which consist in turning on and off the
integrator according on higher values of the control and error inputs (Bohn and
Atherton 1995 ). Another anti windup control architecture is the limited integrator, this
technique consist in feed the integrator output through a dead zone with high gain,
reducing the effects of windup when the input saturates (Bohn and Atherton 1995 ).
The anti windup control architectures for discrete time SISO system are similar
to their continuous counterpart, for example in Chen et al. ( 2003 ) an anti windup
cascade control technique is implemented to suppress the unwanted effects yielded
by windup in digital control systems, proving that is an ef cient control architecture

when the input is saturated. In Lambeck and Sawodny ( 2004 ) an anti windup
control architecture is derived when the input of the system is constrained, the
development of this strategy is based on the frequency response characteristics of

T d s

K

1 1

Saturation

T

s -

i

-

Fig. 1 Tracking anti windup controller