A Frequency Domain Approach to Anti-windup Compensator
Internal Report UPTEC 93024R, Dept. of Technology,
The aim of anti-windup compensation is to modify the dynamics of a
control loop when control signals saturate, so that a good transient
behaviour is attained after desaturation, while avoiding nonlinear
oscillations and repeated saturations.
Model-based anti-windup compensation is here considered for
single-output (SISO) systems and controllers in polynomial form.
The effect of input
saturation in linear time invariant control systems
is addressed. We utilize a
controller in polynomial form, with a three degrees of
freedom structure. This makes it possible to
add a saturation compensator (an anti-windup filter) separately, after
tuning of the controller properties for unsaturated signals.
Our aim is that the effect of a saturation event should decay
as quickly as possible after desaturation, while the tendency for
due to nonlinear oscillations, should be suppressed.
This is achieved by modifying the relevant loop gain
by a safety margin.
The modified loop gain is then adjusted, so that it touches
the describing function of the saturation nonlinearity.
The construction of the safety margin takes the magnitude
signals into account. The method requires only simple calculations.
It can be automated, using a one-dimensional numerical search.
Furthermore, the effect of D/A-quantization
in digital feedback systems
is also controlled by the same antiwindup scheme. The presence
of unstructured errors in a linear plant model can be
taken into account.
We also provide a discussion on how the frequency domain viewpoint
can improve the understanding of previously suggested
anti-windup methods, such as the observer-based approach,
and the conditioning technique.
- Related publications:
PhD Thesis by Jonas Öhr, 2003.
Conference paper (ECC'97)
on the corresponding design for multivariable control systems.
with more details on the multivariable case.
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