Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 38)

Enhanced Software Features
4.5
Optimizing Your Process System with the WATLOW Series 988
Dual PID sets
Overview:
Standard software units have a single set of
PID parameters. Units with enhanced software
can use two independent sets of heat/cool PID
parameters, PID A
and PID B . To
enable dual PID, enter the Global menu and set
the algorithm prompt
to dual PID .
This second set of PID parameters enables the
controller to switch between two sets of PIDs,
to compensate for changes in the system
characteristics. This need can arise from a
variety of circumstances, such as significant
set point changes (controlling at 250, then
controlling at 750), operating a furnace with
half a load versus a full load of steel, changing
the speed of a conveyor through a curing oven
or using different materials in an extruder.
The 988 family can be configured to switch
between PID A and PID B based on a process
value, a set point value or the event input
status. Use the dual PID prompt in the
Global menu, to select what will cause the
switch: process
; set point ; or none
. If process is selected at the
prompt, the PID’s will switch based on the
crossover process value. If set point
is
selected at , the PIDs will switch at the
crossover set point value, PID A used below the
crossover point and PID B above. PID crossover
can also be selected via event input by selecting
at the or prompt. PID A is used
when the event input switch is open, PID B
when closed. (Note : One event input is stan-
dard on all units, a second event input is an
option)
Requirements:
The 988 family controller needs the enhanced
software option to use dual PID sets.
TL TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
pressure
switch
temperature
sensor
heater
chiller
limit
sensor
limit
output 2
output 1
input 2
input 1
Test Chamber
Sample Application:
A test engineer needs to control the temperature in a test chamber that can
be operated at normal atmosphere or under vacuum conditions. If he tunes
the controller for normal atmospheric conditions, when he reaches the portion
of his test that requires a vacuum, he must stop the test and enter new PID
parameters to maintain stable temperatures. The system characteristics are
so very different, that one set of PID’s will not give satisfactory results under
both normal and vacuum conditions.
The 988 solves this problem with the dual PID option. Auto-tuning PID A un-
der normal atmospheric conditions, then auto-tuning PID B under vacuum
conditions, establishes PID values for two sets of system characteristics. A
pressure switch connected to the event input tells the controller when to
switch between PID A and PID B, eliminating the need to change PID values
manually.