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Co
pyright ©
IF
AC 11th
Tri
ennial World
Co
ngress,
Ta
lli
nn.
ESlOn
ia, USS R. 1990
ANTI-RESET
WINDUP
FOR
PID
CONTROLLERS
L.
Rundqwist
Department
of
Automatic Cont
rol,
Lund
Institute
of
T
echno
l
ogy,
p.a. B
ox
11
8,
5-22100 Lund, Sweden
Abstract.
This
paper
describes
and
compares
a
number
of
ways
of
avoiding
integrator
windup
for
proportional-integral-derivative
(PID)
controllers.
It
covers
both
a
number
of
ad
hoc
schemes
and
a
general
procedure
to
avoid
windup,
which
admits
a
unification
of
the
ideas. Design rules for
anti-windup
in
general
purpose
PID
controllers
are
derived
and
tested
in
simulations
and
experiments.
Keywords:
PID
control, process control,
saturation,
implementation
1.
Introduction
Most
control
systems
are
designed for
operation
in
the
linear
range.
For
large
set
point
changes
and
disturbances
the
control
signal will however
be
saturated,
and
then
the
system
operates
in
open
loop
since
the
feedback
path
is broken.
If
the
controller
is
unstable
the
breakup
of
the
loop
may
give severe consequences. A
PID
controller
is a
typical
example
of
a controller
that
may
cause
instability
or
poor
transient
output
during
saturation.
This
paper
presents
design rules for
anti-
wind
up
compensators
in
general
purpose
PID
controllers.
The
rules
are
based
on
the
con-
troller's
performance
for a special
disturbance.
They
are
important
for controllers
that
may
be
utilized
on
a
variety
of
processes.
2.
Anti-windup
methods
for
PID
controllers
A few
methods
for avoiding
integrator
windup
will
be
briefly described.
They
are
described
in
more
detail
in
Astrom
and
Rundqwist
(1989).
Conditional
integration
methods
In
these
methods
the
integrator
is
updated
only
during
certain
conditions.
The
essential differ-
ences
between
the
methods
are
the
exact
con-
ditions
for
suspending
and
resuming
integration,
and
how
the
integral
part
i~
treated
when
it
is
453
Figure
1.
PIn
controller
with
anti-wind
up
based
on
tracking.
suspended.
Examples
of
conditional
integration
in
PID
controllers
are
found
in
e.g.
Fertik
and
Ross (1967),
Gallun
et
al
(1985)
and
Shinskey
(1988).
Tracking
or
Backcalculation
The
idea
of
backcalculation
was
proposed
by
Fer-
tik
and
Ross (1967) for a velocity
limited
incre-
mental
PI
algorithm.
The
idea
is
that
the
stored
value is
recomputed
so
that
the
controller
out-
put
is
exactly
at
the
saturation
limit.
In
Phe-
lan
(1977)
the
"Intelligent
Integrator"
is
ad-
justed
in
the
described
way.
During
saturation
the
controller
thus
tracks given
inputs
and
out-
puts.
It
was found
advantageous
not
to
reset
the
integrator
in
one
sampling
period
but
dynami-
cally
with
a
time
constant
Tt.
Figure
1 shows a
block
diagram
of
a
PID
controller
with
tracking.
Controllers
with
tracking
are
discussed e.g.
in
Astrom
(1987)
and
in
Glattfelder
and
Schaufel-
berger
(1983).
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