Choosing an encoder and a PLC for a high-precision rotary table

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Thread Starter

Yegor

We're considering a rotary table capable of two operation modes: 1) rotating at speeds up to 3 rpm and 2) positioning with precision of 60 arcseconds. That is 21600 positions per turn and 1080 positions per second at max speed. Also, during rotation, the current position must be recorded upon signal from an ultrasonic scanner (which is a blackbox at the moment) to be fetched by a computer later.

This kind of control is new to our company. I think it could be accomplished with a PLC that has high speed inputs and an optical incremental rotary encoder attached to the VFD motor shaft. Am I right or is there a better approach? Particular products? Any suggestions at all?

Thank you!
 
I have done turntable applications with Renishaw ring encoders. Look at the Signum and Resolute products. You will want to use a servo motor/amp (I suggest Copley Controls) with a motion controller. Delta Tau offers a very nice motion controller which is like a swiss army knife of the motion control world. The renishaw product is nice in that you can configure it for different amounts of encoder pulses coming out of their read head electronics. I think we have ours set up for 8x10^6 cts/rev at the moment.

You position capture requirement will need to be handled depending on your tolerance of error. That is to say that if you can tolerate one servo cycle of error (at the max velocity you will be using at this point in your move) you can use any old input and just write a quick polling loop in the motion controller (or maybe even the drive itself if your motion requirements are simple) and record the position when the input transitions. *If* however the inaccuracy is such that you need better than this you will have to have a drive and/or motion controller with a hardware position compare feature. Sometimes this requires a quadrature encoder signal because the serial encoders are cyclical, and some analog interpolation devices are inherently slow.

So without knowing more about your application, I can't say which exact config I recommend but this should at least put you in touch with the right people to help you with your application. If you haven't worked with motion before you might want to hire a consultant or work something out with your distributor.

KEJR
 
W

William Sturm

The proper selection of a motor and gearing would be the first item, the proper electromechanical actuator will make the controls job much easier. Conversely, the wrong choices of actuator will make control difficult or impossible.

Kollmorgen makes a very nice looking Direct Drive Rotary motor that may eliminate the need for any gearing and external encoders. Also, a high resolution encoder or feedback device will be helpful, at least 5 times the required resolution, preferably 10 times or more.
 
S
A few thoughts to get you started:

You probably want your final position-loop encoder on the table rather than the motor shaft. I'm sure there will be enough backlash to defeat your positioning goals if you don't.

Even if everything else is perfect, the precision and repeatability of your home position sensor could fail this app. Pay a lot of attention to the sensor selection and application.

Assuming a 3000rpm motor, you must stop the motor shaft within 16 degrees of arc. That should be doable with a VFD, especially if you have an encoder on the motor shaft for velocity feedback, but you may find you want a servomotor. But 1000:1 is a heck of a gear reduction, probably 3 or 4 stages, so you're really going to have to be careful with backlash.
 
B

Bob Peterson

Something occurred to me.

21600 counts per turn is only 60 counts per degree.

60 arc seconds is 1 arc minute or 1/60th of a degree.

you probably need a much higher resolution encoder. it is tough to position within 1 count.

--
Bob
http://ilbob.blogspot.com/
 
In general unless there are requirements dictating gearing I would go direct drive (I was thinking this but didn't mention it in my post). There are many manufacturers doing direct drive these days. Kollmorgen (as already mentioned), Yaskawa, Aerotech, etc.

The renishaw ring is nice, however, because it can be bolted directly to the turntable. You probably won't need it if you buy a nice direct drive system, however because there should only be the encoder and encoder coupling error which you can ask the manufacturer about to see if it will be a problem.
As to Steve's comment about home sensor, if you use direct drive you can home off of the encoders index pulse and it will be very accurate with any halfway decent motion controller or smart drive.

KEJR
 
S
I had forgotten about those large, cylindrical (typically hollow) direct drive motors. I used a Yaskawa one a long time ago and it seemed like a pretty good product. My app wasn't nearly as demanding as this one. Is 30 rpm a reasonable speed for direct drive systems? So he's not buying 10 times the hp he needs just to get the torque?
 
Control Technology Corporation (CTC) has a controller with built-in motion control to handle the rotary axis. It has secondary encoder inputs with high-speed registration inputs that can 'hardware-latch' and record positions. The controller also has a lot of storage space to queue the captured positions locally. With the controller's built-in Ethernet port you can offload your data using single or block-data reads using CTC's communications tools, but there's also an OPC server available to gather and present this data in standard form if you have an off-the-shelf software package in mind to analyze your data.
 
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