This loop causes 2 loop controllers to work on the same output. Each loop controller produces an action and a (minimum or maximum) comparator selects the action to be applied. The auto selector loop consists of a main loop made from a process loop and a secondary loop made from a single loop. The two loops share one single output branch.
For example, the auto selector loop is used to implement a restricted process control. The main loop is used to monitor the main value and the secondary loop is used to prevent an auxiliary value from exceeding a limit (or restriction), specified by this loop’s setpoint.
Inhibiting One of the Loops
One of the following commands can be used to inhibit a loop to have either a process loop, or a single loop: direct output 1 or direct output 2.
The 2 loop controllers have the same sampling period.
Auto Selector Loop Configuration
An auto selector loop can be configured in 2 ways:
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Case 1: one single Auto/Manual at output branch level after selector. The OUT_MAN manual command value is thus directly applied on the loop output.
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Case 2: an Auto/Manual on the output of each loop controller. Each loop controller’s operating mode is then independent. Each loop controller’s output value can be manually set upstream of the selector.
Case 1: One Single Auto/Manual at Output Branch Level
The following schema shows an auto selector loop with only one Auto/Manual at output branch level.
In this case, the 2 loop controllers are always in automatic mode and track the applied OUT_MAN command. When the loop is in automatic mode, the output of these loop controllers is accepted. If this is not the case, the output is not accepted.
As the loop controllers are trackers of the actual output, there is no risk of bump on switching if the loop controller’s integral action is used.
The initial loop operating mode and initial setpoint type (Remote/Local) for each loop controller can be configured.
Case 2: An Auto/Manual on the Output of Each Loop Controller
The following schema shows an auto selector loop with one Auto/Manual on each loop controller’s output.
In this case, in manual mode, you do not act directly on the actuator command, but at the level of each loop controller’s output (OUT_MAN1 and OUT_MAN2).
As long as at least one loop controller is in automatic mode, output selection is made during the loop sampling period. If both loop controllers are in manual mode, selection is made on each task cycle.
Both loop controllers permanently track the OUT_MAN actual output. In automatic mode, if they are using an integral action, they take the previous OUT_MAN output value into account. This signifies that on Manual/Auto switching, the loop controller will not use its last manual value, but the last OUT_MAN actual output value.
The initial operating mode and initial setpoint type (Remote/Local) for each loop controller can be configured.
Starting autotuning forces the auto selector to the direct position of the autotuned loop. At the end of autotuning, the auto selector must be reset to the required position, if it is different from the one imposed.
