Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 14)

Optimizing Your Process System with the WATLOW Series 988
2.4
Basic Control Strategies and Terms
down to a state of equilibrium before making
another change.
• Remember that the time you need to spend
tuning the electronic controller system is
relative to the precision of control you need.
Proportional Band
The proportional band adjustment is the means
of selecting the response speed (gain) or sensi-
tivity of a proportioned controller to achieve
stability in the system. The proportional band
— whether measured in degrees, percent of
range or other units — must be wider than the
natural oscillations of the system and yet not
wide enough to dampen the system response.
The time proportioning output must be set to
switch faster than the natural oscillation of the
system, sometimes called, “system cycle time.”
The tuning procedure is very simple, if you can
use a recorder to monitor the actual process
variable. If a recorder is not available, observe
the process response and record readings over
a defined time period.
If the system oscillates when the proportional
band is at its narrowest setting (not zero), the
adjustment of the proportional band should be
increased in small increments until the oscilla-
tion stops. Because the proportional band has
been tuned (adjusted) to have the controller
seek stability of the system, very often an offset
(droop) from the set point occurs. At this point
in tuning the system, the process variable
should be in a state of equilibrium, but not
right on the desired set point.
Integral (Reset)
The reset adjustment is tuned at this point to
correct for the droop caused by the propor-
tional output. While it does automatically
correct for offset errors, automatic reset has to
be tuned to each system. Each system has its
own characteristic response time (system cycle
time), thus, the auto reset time constant
(repeats per minute) must be tuned to match
the overall system response time.
Initially auto reset should be set at the lowest
number of repeats per minute (least corrective
action). Increase the repeats per minute in
small increments, allowing the system to settle
down (stabilize) before making additional
changes, until the system just starts to oscil-
late. Then back the setting off enough to re-
establish system equilibrium.
Derivative (Rate)
Rate is the last control parameter adjustment
to be made. Rate’s function is to reduce or
eliminate overshoot (excursions above or below
set point). It has a time base (measured in
minutes) that must be tuned to work with the
overall system response time (system cycle
time). The initial setting for rate should be at
the smallest integer of minutes possible (least
corrective action). Increase the integer in small
increment, then after each adjustment increase
the set point moderately. Observe the approach
of the actual process to set point. If it over-
shoots, continue to increase the rate integer in
small increments. Then increase the set point
moderately until optimum approach to set
point is achieved. If at some point the system
becomes very sluggish or cannot reach the new
set point at all, too much rate (corrective
action) has been adjusted into the control
system. To reduce this “damping” effect,
decrease the number of minutes of rate.
Recommended Tuning Reference
There are many reference books on the art of
tuning electronic controllers to the systems
they control. If you are not an instrument
technician qualified to tune thermal systems,
we suggest that you become familiar with the
following reference before attempting to tune
your system:
Tuning of Industrial Control Systems
by Armando B. Corripio
Published by the Instrument Society of America
(ISA)
Member $48.00, list $60.00 (approx.).
Phone: (919) 549-8411