100
c
HAPTER
4:
Operation
Model 372 AC Resistance Bridge and Temperature Controller
The following menu navigation example illustrates the available heater ranges when
the heater output display units are configured for current. If configured for power, the
available ranges would be listed in power.
Menu navigation:
Heater Range
Q
(Off, On, 31.6 µA, 100 µA, 316 µA, 1 mA, 3.16 mA, 10 mA, 31.6 mA,
100 mA)
Default: Off
Interface command:
RANGE
4.6.1.5.10 Polarity (Sample Heater only)
When the sample heater output is used in a traditional manner (to drive a resistive
load) the heater output needs to be unipolar since reversing the current will still heat
the load. There may be some rare circumstances when the heater output is not con-
nected to a resistive load and bipolar control is desired. The heater output can be set
to control the current to plus and minus the full-scale range by setting the heater
polarity to bipolar mode. For the majority of applications and any application where
the heater output is connected to a resistive load, the unipolar mode should be
chosen.
Menu navigation:
Output Setup
Q
Sample Heater
Q
Polarity
Q
(Unipolar, Bipolar)
Default: Unipolar
Interface command:
OUTMODE
4.6.1.5.11 Control Filter
During normal operation, the PID control algorithm uses the unfiltered input read-
ings from the control channel as its feedback. This method offers the best control sta-
bility and some noise immunity because of the natural averaging that takes place in
the integral (I) part of the control equation. The control algorithm can be configured
to use filtered readings instead of unfiltered readings. This configuration can improve
noise rejection but it is sometimes at the expense of more difficult tuning or even con-
trol instability. The filter creates time lag that destabilizes the control loop. When
control filtering is enabled, the firmware uses the same filter parameters set for the
control input channel. Any control application should be tuned first with the control
filter off. Filter action should be added gradually, starting with a very short settling
time, and adjustments to P and I made be required along the way (most often P will
decrease and I will increase). The filter time constant should always remain shorter
than the I setting in seconds.
Menu navigation:
Output Setup
Q
Sample Heater or Warm up Heater
Q
Control Filter
Q
(Off,On)
Default: Off
Interface command:
OUTMODE
4.6.1.5.12 ALL OFF
The
ALL OFF
key is provided as a means of shutting down all control outputs with one
key. It is equivalent to setting the heater range parameter of all outputs to Off. This
function is always active even if the keypad is locked or when it is in remote mode.