Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 36)

Enhanced Software Features
4.3
Optimizing Your Process System with the WATLOW Series 988
In a system controlling a heater this would be
the maximum and minimum desired sheath
temperatures of the heater. Typically the range-
low term is set below the ambient temperature.
Otherwise the system could never fully cool
down.
Setup
The PID parameters for the two PID sets, PID A
and PID B, are determined with the auto-tune
function. First the PID B settings (inner loop)
are determined by setting the auto-tune
prompt
to PID B , which allows for
tight control of the energy source at the set
point determined by the primary loop. During
the tuning process the internal percent (%
int
)
value is determined by the auto-tune set point
parameter
. The default is 90%. This
generates a set point for the heater equal to the
range high times the
value. The PID A
TL TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
input 1
output 1
limit heater
oil out oil in
input 2
outer-loop
thermocouple
Lube Oil Tank
inner-loop
thermocouple
settings (outer loop) are then determined by
setting the auto-tune prompt
to PID A
. During the tuning process the set point
is determined by multiplying the value by
the set point (SP) entered via the front panel.
Once the system is stable the heater will
operate at a value greater than the final pro-
cess value (input 1). If this value is significantly
different than the set point at which the sec-
ondary loop was tuned, adjust the
param-
eter so that the tuning set point is near the
value at which the heater is controlling.
Requirements
Cascade control requires enhanced software.
Two analog inputs are required to monitor the
inner and outer loops. At least one control
output is required to control the process.
Sample Application
A Series 988 is used to heat lube oil to 125
°F with a screw-plug-style
heater. To protect the oil from breaking down and maximize its life, it is
desirable to limit the maximum heater sheath temperature to 250
° F.
The 988 is ordered with two thermocouple inputs. Input 2, the inner loop
in the cascade configuration, measures the heater sheath. Input 1, the
outer loop, measures the lube oil temperature before it leaves the tank.
The external set point is 125
°. By setting range high 2 to 250 ° the
set point for the heater sheath will be limited, thus extending the lube oil
life.
limit sensor