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Old 06-01-2007, 11:37 AM
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markttu markttu is offline
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This is actually a pretty simple problem to solve using a PLC.

The way I see it you have a few options for determining the position of your gate:

1) Linear potentiometer (POT). Typically this isn't too difficult to physically implement and its very easy to implement in the PLC code. Offers ability to know position even after a power failure. Does require an anlog input on the PLC.

2) Simple rotary encoder. Typically this is only moderately difficult to physically implement and moderately difficult to implement in PLC code. It doesn't let you know the position after a power failure. Requires a high speed counter or quad-encoder card in the PLC.

3) Absolute position encoder. Pretty much the same thing as a simple rotary encoder, but there is more code for it with the benefit of knowing position after power failure.

4) Several photo eyes along the track. This should be easy to physically implement and fairly easy to implement in the PLC as well. Of course you won't ever know exactly where you are, but there are a few tricks for remembering where you are after a power failure. Needs a discrete (digital) input on the PLC for each photo eye.

For the PLC you can use something very simple like a DL05 or DL06 from AutomationDirect.
Depending on what kind of user interface you need you might like one of these as well.

How do you move the gate now? Will you want the PLC to move the gate to pre-determined positions for you at the touch of a button? Lots of questions rattling around in my head right now
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