REMOTE SETPOINT OPTION
A remote 0/4 to 20 mA analog input signal can be used to change the setpoint value. This input can be scaled over a process range independent of that of the system range. In operation, the front panel “MN” indicator illuminates for local setpoint operation and is off for remote setpoint operation. In either mode, the indicator flashes during Manual Mode.
Filtering, ramp rate (velocity limiting), and local/remote transfer response options can be configured to suit the application. Local/Remote transfer operations are performed either externally (user input) or at the keypad.
If large changes in the remote setpoint signal result in undesirable controller performance, the remote setpoint can be velocity limited (ramped) by the setpoint ramp parameter (SPrP).
Similarly, if fluctuation in the remote setpoint signal also results in poor performance, the programmable filter can be used to provide a steady setpoint to the controller. A unique feature of the filter is the filter band. Fluctuations that occur that are less than the filter band value keep the filter engaged, while fluctuations outside the filter band disable the filter. This action allows the controller to respond rapidly to large changes of the remote setpoint signal.
CASCADE CONTROL
External cascade control involves the use of two controllers, one of which has a Remote Setpoint Input.
The outer loop controller (Primary controller) directs the setpoint of the inner loop controller (secondary controller) through the linear DC output.
The secondary controller has a Remote Setpoint option in order to receive the directed setpoint. See Figure 35, Cascade Control, for more details.
Note: The Remote Setpoint of the secondary controller must be scaled prior to tuning the controllers. Normally, the Remote Setpoint is scaled to equal the actual process range of the secondary. Scaled in this way, the primary controller can direct the setpoint of the secondary controller over its operating range. Normally, the secondary controller is a faster control loop.
It is able to respond quickly to disturbances before they affect the primary loop. Therefore, the control quality of the primary loop under cascade control is superior to that of a single loop controller.
Figure 35, Cascade Control
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