Sturdy rotary position sensor

Ken Roach · Apr 3, 2009

Can anyone recommend a reliable rotary position sensor for a slow-moving load that rotates about 70 degrees in a minute.

The current system has a 8:1 pulley and a multiturn potentiometer and 4-20 mA interface, but it's worn out after a little more than a year, cycling about 30 times daily. I need an absolute output, preferably 4-20 mA analog.

I have looked at the Celesco RT8000 series, the Gemco 1786A and the AMCI DC425 Duracoder.

I hope somebody on the Forum can say "I use model X from brand Y and it has worked for years with no trouble".

plc noob · Apr 3, 2009

Maybe this thread i started a few days ago may help ken.http://www.plctalk.net/qanda/showthread.php?t=46259

leitmotif · Apr 3, 2009

Ken

Chimed in to keep track of this. I want the same answer.
Try this outfit
http://www.beiied.com/main-product.html
I have no experience with them. Two others (Okie PC and ??) recommended them.

What failed on the existing - the pot??

Crazy idea - use a ring gear from a engine flywheel count the teeth with a prox. Ideally you would use a gear with 360 teeth.
BUT what is your positioning tolerance ie moves 70 +"x" -"xx"
Also space and mounting may be problems. Smaller gear - drive at X multiple of what you drive the "table" at to get better accuracy??

Dan Bentler

Ken Roach · Apr 3, 2009

The BEI H25 with analog output might be a good fit for this application.

The existing pot just seems to have worn out; when it gets to about 65 degrees the input value (verified by a meter) just starts jumping around between 50 to 70 degrees. I figure that's a worn spot in the pot slider.

If it were my design, I would have put an absolute encoder with DeviceNet or Grey code and been certain of the mechanism's position. There were a lot of things done with this system I would not have chosen... anyone ever seen three SLCs used to make a ControlNet standby system ? With cable TV slider switches to change the CNet trunk routing ? Thought so.

panic mode · Apr 3, 2009

hi Ken,

you mention absolute position. does it need to read over multiple turns or limited angle is ok?

I've been using Baumer MDRM sensors. They work like potentiometer and although they can be turned endlessly, sensing range is limited to +/-80deg on MDRM18U9524 (or +/-60deg for current output model MDRM18I9524). either version needs magnet 123344.

as for worn out potentiometers (with some exceptions), they are also usually meant for very small current (expected load impedance greater than few or rather tens of megaohms) so even small current like 20mA could be a problem.

Ned_Flanders · Apr 3, 2009

leitmotif said:
Ken

What failed on the existing - the pot??

Crazy idea - use a ring gear from a engine flywheel count the teeth with a prox. Ideally you would use a gear with 360 teeth.
BUT what is your positioning tolerance ie moves 70 +"x" -"xx"
Also space and mounting may be problems. Smaller gear - drive at X multiple of what you drive the "table" at to get better accuracy??

Dan Bentler

I did a very similar mod to this in a packaging machine, to replace a row of cam switches.
I used two slot sensors and counted the rising and falling edges of slots cut into a 100mm disc (So doubling the count). The second sensor is to reset the count every revolution.I fed it into a Siemens s200 plc, but you've already got a plc.
Perhaps you could alter the gearing to give a 1 degree = 1 count accuracy?

Reliability? Never failed in 4 years!!

irondesk40 · Apr 3, 2009

Here is a sensor that I would recommend, just order 8 of them to put on some dancer assemblies for some web motors. you can teach it where 4ma is and then where you want 20ma to be. Anywhere from 0-360 degrees. Cost was about 300 dollars each.

irondesk40 · Apr 3, 2009

forgot the link
Here it is

http://www.am.pepperl-fuchs.com/products/product.jsp?product_id=24055

Doug-P · Apr 3, 2009

I've only been exposed to the incremental encoder offerings from BEI but the stuff just works. Had one in a sorter that went for over fifteen years with no trouble - and it was even abused by a contractor who did a mod. and loaded down the open collector output with about one third the recommended resistance. That gave us headaches with occasional flaky sorting for awhile but the encoder just kept on chugging.

Steve Bailey · Apr 3, 2009

I used a rotary transducer from Trans Tek many years ago.

http://www.process-controls.com/intertechnology/Trans_Tek/

The application was on a paper finishing machine,and I believe we decided to mount the unit in a protective housing.

danw · Apr 4, 2009

Ken Roach said:
... anyone ever seen three SLCs used to make a ControlNet standby system ? With cable TV slider switches to change the CNet trunk routing ? Thought so.

Can you get a photo with your cell phone and share it with us? Another example for the 'good practices NOT' session . . .

CharlesM · Apr 9, 2009

I just found these today. It may be a little late but could help someone.

http://www.novotechnik.com/products/rotary/rotary_touchless.html

Sturdy rotary position sensor

Ken Roach

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plc noob

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leitmotif

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Ken Roach

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panic mode

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Ned_Flanders

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irondesk40

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irondesk40

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Doug-P

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Steve Bailey

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danw

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CharlesM

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