Micromod MOD: Modcell 2050R Users Guide - Manual (Page 203)

IB-23C650
ON/OFF CONTROL
ON/OFF CONTROL
DESCRIPTION
The on/off output calculation diagram is shown in Figure 29. The local set-
point adjustment is made using the front panel keys. The calculated output
signal, either ON or OFF is displayed as a percentage. ON is indicated by
100% and OFF is indicated by 0%.
When the controller is operating in automatic, the control algorithm receives
the active set-point and process input signals. A calculated output signal is
generated based on set-point/process deviation and the differential gap
setting. The calculated output is a 2-state on/off signal as illustrated in
Figure 30. The calculated output goes to the output driver to control the
action of the output relay. The output relay on-time and off-time is determined
by the requirement to eliminate deviation between the process and set-point.
Relay chatter or hunting is eliminated by proper adjustment of the differential
gap value.
When the on/off controller is switched to manual, the calculated output value is
initialized at 0% and the relay output becomes a time proportioning signal.
This time proportioning approach provides a useful method for handling
manual operation in an on/off controller. Operation in manual is essentially the
same as manual operation on a time proportioning controller.
When the calculated output initializes at 0% following a transfer to manual, the
output relay commences continuous oscillation. The period of oscillation is
determined by the value of the cycle time tuning parameter; this parameter is
only active in manual. The duty cycle of the output relay can now be
controlled by changing the calculated output value manually via the front panel
keys. The calculated output responds continuously between 0 and 100% of
span allowing an appropriate duty cycle to be selected. Sample duty cycles
are shown in Figure 26.
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