App Note:
Automatic Door Control
Easy Logic Using WinScript
There are many applications where an Alcorn McBride Show Controller can perform non-safety-related duties normally delegated to a PLC. One application that immediately comes to mind is automatic theatre doors. These doors open when the main show is loading/unloading (as in the Large Theatre example). The Show Controller should automatically open/close the doors as dictated in the script except when overridden by an operator control switch.


This simple example utilizes a three-position maintained switch to provide a Show Controller with two inputs: Open and Close. The center position is Auto, and can be deduced by the absence of both Open and Close inputs. When the switch is in the Open position, the Show Controller should keep the doors open, regardless of what the show is doing at the moment. The same is true with the Close position; the doors should remain closed regardless of the status of the show. In the Auto position, the Show Controller can dynamically open and close the doors at the appropriate time. A flag will provide automated control of the doors from within any of the other sequences: the sequence will turn the flag on to open the doors and off to close the doors. If a set of doors is in Auto mode, our looping sequence should open/close them based on the state of the flag. If the doors are in Open or Close mode, the other sequences may still turn on and off the flag, but the doors will not be affected.

Show Control V16+

We will perform this function by using one Loop Enabled sequence that continuously checks the inputs and opens/closes the doors if necessary.

The figure below shows the events of our sequence, OpenCloseDoors. This sequence loops every 10 frames and updates the state of the doors based upon the switch inputs (DoorOpenInput and DoorCloseInput) and the script open/close flag (DoorFlag). The sequence first checks if either input is on. If the operator has moved the switch to the Open position, the doors should be opened. Similarly, if the operator has moved the switch to the Close position, the doors are automatically closed without regard to DoorFlag. Finally, if the doors are in the Auto position, the status of DoorFlag is checked. If DoorFlag is "on" the doors are opened, and vice versa.


Using flags as "permission" outputs allows a separate sequence to monitor the status of operator controls, check the flag, and perform the desired action ("grant permission"), if possible. As you can see, we have saved considerable Ladder Logic programming (and perhaps an entire PLC) by utilizing one sequence, one flag, and two inputs to control our automatic doors.

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