Caution: With DC Trigger installed in your controller do not wire an external AC or DC voltage
signal to the DC Trigger terminals.
6.7 Analog Output Option
With the analog output option, the controller supplies 4-20mA to the process. Up to two 4-20
outputs can be installed (see the Wiring section for rear-terminal designations). The controller
regulates the analog outputs to bring the process (PV) to setpoint (SV).
To use the analog output, ensure that ‘Cycle Time’ (‘Ct’ in Menu 3, PID) is set to ‘0’
6.8 ON-OFF Control
ON-OFF is the simplest form of control, where the controller output is OFF at setpoint and ON
away from setpoint. To use ON-OFF control, set the ‘Pb’ or ‘CPB’ parameter (Menu 3, PID) to
‘0’. Use the Hysteresis setting ‘Hyst’ to create a ‘dead band’ around the setpoint to minimize
relay chatter. These are also found in Menu 3, PID.
FIGURE 6.1: ON-OFF COOLING AND HEATING CONTROL EXAMPLES
6.9 PID Control
ON-OFF control cannot be used for processes where setpoint overshoot or process oscillations
are not acceptable and may cause damage to the materials in the process. For more precision,
PID control is necessary. PID has three elements:
Proportional control (‘Pb’ parameter in Menu 3, PID)
Integral time (‘ti’ parameter in Menu 3, PID)
Derivative time (‘td’ parameter in Menu 3. PID)
PID control minimizes setpoint overshoot, oscillations, and disturbances in a process by
throttling the controller output automatically. The controller must first be tuned so that it
understands the speed and dynamics of the process. See the Auto Tune section.
To Next Page
Loading...
Need help?
General
