Specifications

Digital I/O Functions 6-7
The high and low alarm limits on the D1000 sensor modules may be set to
provide on-off control with hysteresis. The two limits specify the two
control setpoints. The difference between the high limit and the low limit is
the hysteresis value. The high limit must be greater than the low limit for
proper operation. The alarm output used to control the process must be
set to the Latching mode. If the control output is turned on, it will remain on
until the input data exceeds the second alarm value. At this point the
control output is turned off.
A typical example of a controller with hysteresis is illustrated in Figure 6.5.
A J-Thermocouple input module such as a D1311 maybe used to regulate
the temperature of an oven. The thermocouple is used to sense the oven
temperature. The LO alarm output controls a solid state relay (SSR) which
in turn controls the oven heater. The Enable Alarms (EA) command must
be used to activate the alarm outputs. In this case the desired regulated
temperature is 100°C. The Lo alarm is set to 95°C in the latching mode
with the LO command. The HI alarm command is used to set the upper
limit to 105°C in the momentary mode. The total hysteresis is the differ-
ence between the two alarm values, or 10°C. In the steady state condition,
the oven temperature will oscillate between 95°C and 100°C (ideally).
Assume the oven temperature is below 95°C. This value is less than the
value loaded into the low limit, therefore the LO alarm output is turned on.
Since the low alarm is set for latching mode, the control output stays on
even as the oven temperature goes above the 95°C low limit. The control
output will stay on until the temperature reaches the value loaded into the
high limit, in this case 105°C. At this point the latched LO alarm is turned
off, turning off the heater. The control output will remain off as the oven
cools down through heat losses. When the oven cools to 95°C, the LO
alarm is again turned on, and the control process repeats indefinitely. The
control signals are shown in Figure 6.6.
In this case the high alarm was set to momentary mode. The high alarm
could have been set to the latching mode without affecting the LO alarm
output. However, the output at the HI alarm terminal would change. If the
high alarm is set to Latching, the alarm output is simply the complement of
the LO alarm. Either alarm output may be used for control depending on
which one will result in negative feedback. For example, in a refrigeration
system, the HI output may be used to control the refrigeration compressor
and the low alarm value is used only to set the desired hysteresis value.