Cheapest solution for Linear Positioning feedback to PLC ? 2

[pocketooth]

I need my PLC program to know where the object is located in order to branch out to next operation. This object that can move within 2 inches between point A and B only in straight line fashion so that ease the detection. What is the best way of "knowing" where the device is sitting between A and B in 0.1 inch resolution? Servo positioning is too expansive for my project. Can anyone tip me any better solutions ?
Thanks!

 

[ScottyUK]

Simplest method would be a linear pot driving a voltage input to the PLC. After that costs start mounting up when you want precision, ruggedness, non-contact, long life, ease of mounting, etc, etc.


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If we learn from our mistakes I'm getting a great education!

 

[robnew]

String potentiometers such as those from Celesco can be very cost effective and I have found them to be durable and reliable. They are also easy to mount in many applications.

 

[itsmoked]

Those are entirely cool!!! Never even heard of such a thing before. On inspection they are intuitively obvious. I will certainly be considering them on future positioning apps. Thanks robnew.

Let me also welcome you to Eng-Tips.

Cheers.




Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You can easily get 0.1 inch resolution on an infrared proximity sensor with an analog output. I have used some with accuracy down to 0.5mm, there may even be better ones than that out there.


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[ScottyUK]

'Smoked,

They're ok in fairly clean environments and for non-continuously-modulating service. We evaluated them for governor servo position feedback, largely because we were ordered by a cost-obsessed manager, but had some major concerns in that application. We went for a magnetostrictive sensor housed within the hydraulic cylinder. The big downsides for our application were: pot track wear, even on a conductive film pot; bearing wear for the pot shaft; marginal accuracy for what we were trying to achieve; and inevitable fine abrasive dust ingress past the seals once the dust killed the seals. We did use them for a couple of jobs where we need to quickly get reasonably accurate valve position indication for fault diagnostics, and they were easy to install, easy to set up, and were a fraction of the installed cost of a magnetostrictive type. If you are aware of both the limitations and benefits they definitely have their applications.


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If we learn from our mistakes I'm getting a great education!

 

[robnew]

Thanks for the welcome itsmoked. I agree with scottyuk's comments on the string pot's limitations. They sure are easy to apply however...especially when fitting to existing equipment.

 

[itsmoked]

Roger-that robnew.

And thanks for the additional eye opening Scotty I will keep that in mind.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Kiljoy]

We purchased a company that used the Celesco units. Mind you, it was for 120" ranges. These things went out right and left due to dirt gumming up the POT or the "string" just plain breaking. They would get way out of calibration.
We switched to Ametek/Gemco magnorestrictive on some machines and Banner LT3 lasers on others. One of our service guys actually had to duct tape two toothbrushes on the string to clean it off as it wound back up.
I think there are much cheaper short range laser sensors out there. Keyance comes to mind.

 

[itsmoked]

Hi Kiljoy. Trying to live up to your name??

Well yeah, there are engineers who thoughtlessly misapply stuff everywhere. I wouldn't hold that against the product. I do take your point though - not a good solution in some contaminating situations. Roger that!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Kiljoy]

Sorry to unload there, but I spent several years calling up machines to calibrate the Celesco stringline analog signals when they broke or gummed up. HUGE source of frustration for us. However, these machines were at recycling facilities and were covered with all sorts of "dumpster swill". I'm sure they work very well in cleaner environments.
My vote is for the magnorestrictive sensor. The lasers have a warm up time and are a bit noisy.

 

[itsmoked]

That sounds like a pretty nasty environment indeed. They'd have probably worked adequately if they'd been enclosed totally, but of course, that would probably be a bigger problem to solve. LOL

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rmayer]

Speaking of cool new technology getting cheaper...

We started using guided wave radar to indicate mechanical positions and fluid levels. Check out IFM efector.

Hope it helps. I'm usually the guy asking the questions.

 

[itsmoked]

Got a link?

Keith Cress
kcress -

http://www.flaminsystems.com