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PRESYS
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Instruments
ON/OFF Control
ON/OFF - associates the ON/OFF control mode with the related control block,
1 or 2 (or both).
ACTION - this option determines the direction of the control action (direct or
reverse).
HYST (hysteresis) - this is the span above and below the setpoint value which
determines the lower and upper switching limits for the relay contacts (by relay we
mean the relay itself, the solid-state relay and the open collector voltage.
REV (reverse) - whenever REV is selected for control action (ACTION), the
output signal is switched to OFF when the controlled variable exceeds the setpoint
value plus the hysteresis (upper limit) and switched to ON when the variable falls
below the setpoint minus the hysteresis (lower limit).
DIR (direct) - whenever DIR is selected for a control action (ACTION), the
output signal is switched to ON when the controlled variable exceeds the setpoint
value plus the hysteresis (upper limit) and switched to OFF when the variable falls
below the setpoint minus the hysteresis (lower limit). The ON/OFF control mode with
reverse action is illustrated on figure 24 below.
Relay output
DCY - 2057 Pressure Controller
Contolled
Variable
SP+HIST
SP
SP-HYST
ON
OFF
Fig. 24 - ON-OFF control with reverse action
Time
Time
Maintenance
Page 38
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