Opinions for optical encoder.

[a70duster]

Hello all,

I'm looking for an optical encoder for a Reliance motor (800 hp). I have switched from a tachogenerator (100 V per 1000 RPM) to a 1024 Northstar magnetic encoder. With the motor heavily loaded, noise is getting into the encoder and blurring the pulses. This appears to the drive that the motor is slowing down, so it dumps in more current to "speed" up the lift. The drive eventually trips out on a drive fault.

The encoder uses 12 volts and a A+-, B+- for signal. The A signals are twisted together as well as the B signals. The signal shielding was grounded in the drive cabinet. For giggles, the shield was grounded ONLY at the motor with no luck. Finally another cable was run away from the motor with no luck either.

My thoughts are to replace the magnetic encoder with an optical one.

Can anybody recommend a bearingless optical encoder (1024 PPR) that would simply bolt to the comm end of the motor?

Thanks!

Mike

 

[dbaird]

Check:

http://www.beiied.com/

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[llangford]

Hi, Mike,

You might check out the ones made by Keyence and also Banner. Both companies make excellent products.

LL

 

[EERN]

TR Electronics makes a good optical encoder

 

[sreid]

I think that dbaird's suggestion of BEI Encoders is excellent. BEI makes especially rugged encoders. We use the model HS35. It allows shafts up 1 inch diameter and the tether arm anti-rotation option makes mounting easy. BEI also supplies plastic shaft sleeves for shafts smaller than 1 inch.

http://www.beiied.com/PDFs2/HS35_Incremental_Encoder.pdf

 

[zappedagain]

Here are some thoughts on the magnetic encoder:

Have you checked for noise on your power supply? Additional filtering at the tachometer might help. Shunt capacitance to minimize the size of the current loop, and series inductance if the noise is getting picked up on the power leads.

The blurring makes me think you are picking up some lower frequency noise. If you trigger your scope off a motor drive signal does the blurring go away on the scope? (it didn't really go away, you just found a consistent spot to trigger on) I expect it will (inductive coupling from high currents under heavy load). If not you may have something else going on. Identifying the source may help you find creative solutions to reduce it's effect. Noise has three parts - the source, the path, and the receptor). Clobber one of the three and you can usually knock out the noise.

Have you tried mounting the tachometer on a flexible coupling that can insulate it from the motor shaft? That might eliminate a path for the noise. If you already have an insulator between the motor and the tachometer, try adding a short ground strap between the two if you can do it safely.

Did you try grounding the cable shield at both ends? Sometimes that helps too.

 

[nbucska]

a70:
what is the typeand maker of the encoder presently used?
What is the lenght of the cable?


<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS
Plesae read FAQ240-1032

 

[a70duster]

The type of encoder is a NorthStar shaft mount. The part of the encoder the couples to the armature is made of a non-conducting polymer. There is a metallic arm that bolts to the motor frame that keeps the encoder from rotating.

Power supply is clean even when the drive is sensing a fault.

I personally did not scope the encoder signal. Someone else told me about the blurring.

 

[a70duster]

Missed one - the cable length is about 70 feet.

 

[sreid]

One quick fix to try is to put a clamp-on ferrite noise suppressor on the encoder cable at the controller end. If the problem is radiated RF pick up, this might solve the problem (RS-422 differential line receivers are only good for 7 Volts of common mode noise above 5V and below 0 V).

Many encoders (and I have seen them) are really bad when it comes to capcitive coupling between either the encoder case or the encoder shaft. If the inverter PWM switching noise corrupts the internal encoder electronics (the photo diode max currents are about 10 microamps), there is nothing to be done but to switch to another brand encoder.