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PROGRAMMABLE LOGIC CONTROLLERS The Programmable Logic Controller (PLC) is a computer which can do timing functions. The timer instruction can be used to time events or create delays in residential applications. An example of a timer function is in the control of the garage light; on when the garage door is open and off 5 minutes after the door is closed. |
Eugene R.G.
Kowch, P.Eng. is a Control Systems Engineer with P.I.D. Consultants Inc.. Eighteen years
of providing professional integrated design and construction support for industrial,
commercial and residential projects. An active member of the Association of Professional
Engineers and Geoscientists of British Columbia. |
The Programmable Logic Controller (PLC) is a computer which can do timing functions. The timer instruction can be used to time events or create delays in residential applications. An example of a timer function is in the control of the garage light; on when the garage door is open and off 5 minutes after the door is closed.
One input (10001) to the PLC is a momentary input keypad switch located by the man door in the garage. This switch will allow for the lights to be manually operated. ( See December 1997 issue of HTINews )
The input (10002) to the PLC will be a dry contact, normally closed, switch located on the garage door. Security systems use these types of switches when monitoring the status of the garage doors. When the garage door is closed the switch is opened, when the door begins to open the switch closes.
The output (00001) from the PLC is to the garage lights. Depending on the lighting load, the output may have to go through an interposing relay.
The timer application program for a typical garage lighting control is shown below;
The first rung of logic is the typical lighting control discussed in a previous issue of HTINews.
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The third rung is the timer instruction which is set at 300 seconds (5 minutes). The timer function will not start timing until it is enabled and will not pass power until the time has elapsed.
The fourth rung is when the garage door starts to open, a positive transitional contact will pass power to the Door On state and only if the garage light is off already.
The last rung is part of the typical lighting control, with the addition of the Door On and Off states.
