Rockwell Automation 1771-TCM TEMPERATURE CONTROL - Manual (Page 41)

5–6 Configuring the Loops
Publication 1771Ć6.4.5 - January 1997
Once the temperature alarm bits are on, they are kept on until the
temperature drops below the high alarm by the alarm dead-band
value or rise above the low alarm by this value. You specify the
temperature alarm dead band. This dead band provides a hysteresis
effect. The dead-band value applies to the high and low temperature
and deviation alarm values.
•
Low Alarm With Dead Band — When the temperature falls
below the user-defined low alarm value, the low alarm bit is
turned on. When the temperature rises above the level of the low
alarm value but still below the level of the dead-band value, the
low alarm bit remains on. Only when the temperature rises above
the dead-band level will the alarm bit be turned off.
•
High Alarm With Dead Band — When the temperature rises
above the user-defined high alarm value, the high alarm bit is
turned on. When the temperature falls below the level of the high
alarm value but still above the level of the dead-band value, the
high alarm bit remains on. Only when the temperature falls
below the dead-band level will the alarm bit be turned off.
High Alarm Level
Low Alarm Level
Time Time alarm on
The temperature passes thru
the dead band before the alarm
is turned on or off to provide
stability to alarm indicators
without dead band with dead band
alarm off
dead band
dead band
Temperature
The ladder logic can send the TPO signal to a digital output module
to generate the control variable output signal to a digital temperature
control actuator.
X
Y
On
Off
Duty Cycle = Ċ
X
Y
TPO Bit
X = On Time
Y = TPO Period
You set the period and minimum on time for each loop’s heat and
cool TPO bits. You must set the TPO period to be less than the
system lag time (i.e. deadtime plus the time constant; this time is
available from the autotune results). If the TPO period is greater
than the lag time, the auto-tuning will calculate large gain values,
which will probably cause the temperature to overshoot the set point.
Alarm Dead Band
TPO Bit