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Bolt 05-31-2007 11:40 PM

Linear Position Potentiometer Wiring
I have a gate that moves in a straight line along 200 foot of track.

I need to have some means of electronicaly knowing where the gate is, and running different controls off of it. I don't need anything accurate, just 10 foot increments or so.

For example:

If the gate is at the back (<70 feet), Relays 1-6 need to be energized.

If the gate is between 70 and 60 feet, 1-5 need to be hot.

If it's between 60 and 50 feet, 1-4 need to be on.

50 to 40 feet equals 1-3.

40 to 30 feet away, relays 1-2 are on.

30 feet or closer, just relay 1 is on. (I guess this wouldn't require a control then, as it would always be hot).

These are just some generalized descriptions, but I hope someone out there understands what I mean and what I'm trying to do.

I've found this site, and many others.

But the part I am unsure about is getting the potentiometer's signal and processing it to run the relays. What do I need for such a task?

If it would help, I could probably get some drawings for you folks tommorow.

Or am I barking up the wrong tree here, is the cable pot the way to go, or maybe laser, or something else?

BBchevy396 05-31-2007 11:44 PM

How about photo-electric eyes?..........:confused:

Bolt 05-31-2007 11:55 PM

Well this application needs to be up and out of the way, where cows can't reach it, and relatively weather resistant. The problem I run into with eyes is they only know if the gate is right there, and not if it's beyond them and still needing to be "read".

BBchevy396 06-01-2007 12:15 AM

Well, Ya didn't mention!.............
that changes everything!

( just funning with ya, I really can't help....sorry)

GWIZ 06-01-2007 01:04 AM

I'm thinking.

How many relays do you need, 200' @ 10' = 20 relays.
Do you want the relays to turn OFF (in order) one by one if you close the gate every 10' ?

I assume 1-6 is relays 1 thru 6 and not #1 and #6 relay.

MXtras 06-01-2007 08:34 AM

To use something like this, you need to have something that knows how to process the signal - like a PLC (programmable Logic Controller) that can take the input from such a device and apply it to a simple program, or logic, that would produce a triggered output. Depending on exactly what you are needing to do, there is likely a more simple way. What about using limit switches and triggers? I do not know the mechanical arrangement here, but I think I understand the environment.

A cable pot is a decent idea, I think, and as I was reading your first post and saw the length, a cable transducer was the first thing to pop into my head. You might even be able to use a regular rotary encoder and get away from the cable if you elect to stay with the electronic control system.

What is the driving mechanism? Is it a chain or gear rack or ??

You might be able to put flags on the driving mechanism and then use latching circuits on relays and get away from any type of electronics other than a set of relays and a single limit switch. I can easily see this working, but then again I don't know the mechanical arrangement.

*******edit***** - after just a little thought on this arrangement - using a electromechanical arrangement with a single trigger could make it very tricky to recover from a power interruption if the gate was in motion during the failure.

What are the relays controlling and how is this gate driven?


markttu 06-01-2007 11:37 AM

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 :D

dubby 06-01-2007 04:48 PM

I got nothin...

I unbolted the garage door opener when it quit working. Problem solved. I know when it's up or down, and most times inbetween... :p

Bolt 06-01-2007 05:11 PM

1 Attachment(s)
Alright, lots of action here, there's always somebody who knows something about everything around these parts. I am also starting to think that a PLC is going to make life easier in this application.

The gate currently moves by an air motor, which grabs a roller conveyor chain (fixed ends) and moves along it. The controls are air over air, no electricity what so ever. The gate doesn't need to be controlled by anything new, the way it is is fine. So it is entirely possible and quite likely that the gate moves when the power is interupted, but our power outages rarely last more than 2 seconds, as the standby diesel generator kicks in instantly. Also, it's not supper critical that it work 100% of the time, as long as it would reset when the gate goes all the way forward again.

I am trying to relay some coils for solenoid valves. I think a PLC would work well here, that way no relays are needed, just hook them up to the outputs.

The gate has a travel of 120 feet, not 200 like mentioned earlier.

Once the gate, moving from 120 feet away towards 0, gets past 60, the back valve needs to shut off. Once the gate clears 50, another one shuts off. Once it's past 40, another valve turns off, so the back 3 are off, the front 3 are still on. 30 feet or closer the front 2 are on, the back 4 are off. 20 feet or closer, just 1 is on, the back 5 are off. The front one never turns off.

Overall, these valves need to be controlled as a group, depending on the gate's position, by a timer, something like 1 minute on, 9 off. The rest of the 9 minutes another external contactor need to be triggered. These minutes need to be fairly tweakable, the distance stuff isn't going to need to be changed too often.

Now, what do I need in a PLC and a pot to do this? What else do I need.

The valves are 24V, not sure if it's AC or DC right now, the external contactor can be 120VAC, and then the pot needs a power source.

Now that everybody is thoroughly confused, lets keep going with the discussion!

tigman250 06-01-2007 05:47 PM

crowd gate for your milking parlor? i am still confused as to what you are trying to do? all of them i have seen are manually operated, when the operator sees the critters need motivation they move the gate up. it dosen't suprise me that you are seeking this degree of automatic operation, last parlor i was in i noticed it was quite high tech, not much for the people to do any more. the cows wore "necklaces" LOL with a pendant that identified them and made sure they got fed and milked out. the operators were basically there to put the milker on.

something simpler could also be done in conjunction with a PLC, rather than eyes or lasers just use an old fashoned limit switch run off of the chain somehow, you also might want to look into air logic components, you may be able to do away with the electronic stuff all together by using limit valves and such.

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