Hello all,

I am wondering if this is a silly question but I still cannot find the proper solution.

I have a turntable with 16 unique positions per revolution of the motor. Basically, my system requires the motor (turntable is directly mounted on the motor) to move from position 1 to 16 and back to 1 (counterclockwise direction only) again with a fix angle of 22.5 degrees. I prefer to use absolute positioning because I do not want to worry about accumulated errors.

The problem is that I perform my absolute move by multiplying my fixed step (e.g. 1000 counts) by the position number, hence I have position 0 up till 16000. I can perform one revolution but when I at position 16000 and I ask the motor to go back to position 0, it takes the long way back instead of just taking the shortest path to get back to position 0.

I am using a NextMove ES controller by the way.

please help,
Eugene

 

Usually the motion controller has a parameter option where you can tell it to make the shortest move when using a rotary motor. There also might be an option to make the moves only go clockwise or counter-clockwise. I'd try and consult the vendor of your controller if all else fails.

You have to be a little careful when talking about accurate turntables. Normal encoders don't have terribly great accuracy, and the bearings and such on the motor probably will have some runout. Of course, all of this depends greatly on your table diameter, the greater it is the worse accuracy you will get. We've had good luck with Heidenhain and Renishaw encoders on very high quality bearings using a frameless motor design.

Of course, one good way out if you can't get the accuracy is to go to teaching the 16 points and relying on repeatability of the system which is better than the accuracy.

~Ken

 

Hi, Have you ever considered a geneva mechanism?

 

The controller must have a configuration setting that will defne the axis as rotary with a roll over count.

 

First of all, if your encoder is evenly divisible by 16 (most binary values are 1024, 2048...) then you could use incremental moves and make your life much easier. Most controllers will increment from your current commanded position, not the actual position. Should work as long as you need, unless the control has some overflow issues. You are smart (or lucky) to have 16 stations, not 14 or 18 or some other non binary number.

If you really need absolute positioning, see if the control supports a rotary axis mode (some do: PMAC, for instance, has this). This mode will typically roll over back to zero, and also should take the shortest path to your target position.

Hope this helps,

Bill

 

If you can redefine absolute position on the fly, redefine position 16000 as 0 then tell it to go to 1000.
--
Steve Myres, PE
Automation Solutions
(480) 813-1145

 

Hi All,

Thanks for the replies.

1) Ken, I am basically doing what you recommend, teaching the 16 points but because of the rollover thingy, I have problems at the transition from position 16 to 1.

2) Marc, what is a geneva mechanism?

3) Bill, we have tried an incremental move but there is a drift after a few cycles. I am still looking for the rotary axis mode thingy. No success yet. Will continue search through the manual. There is a rollover of the encoder value though but the controller's counter does not roll over. I am guessing this is why the motor moves backwards instead.

4) Steve, we have tried redefining position 16 as position 1 but the trick is when to do it. We are moving the motor every 80ms, with a target indexing speed of 33ms. If the motor is still oscillating at that time, this will lead to a slight offset in the end.

Any new ideas?

thanks and regards,
eugene

 

Well, if you have no way of telling your amp/controller that it is a rotary axis and have it automatically loop over after one rotation you are stuck in the water and have to use a different amp/controller or use a standalone motion controller such as Delta Tau or Galil (unless you go incremental as described below). I'd have a serious talk with the amplifier people and try an get them to help you. If they can't then perhaps your controller can't do it.

We have done incremental moves quite easily in the past. Most of our machines run this. We have used Delta Tau PMAC and we have enough precision to not have significant rounding errors. You have to make sure that your amp/controller does an incremental from the *commanded* position and not the actual position or you will pick up tons of error. If your commanded position can't be a floating point number, then keep track of the commanded position in a seperate floating point variable and then assign it to your commanded position after calculation. This is important if your index value is not in whole encoder counts (i.e. one index is 2478.346548899999999 counts... or something like that) This way you won't truncate each index. I think we found that in our application it would take weeks or months (perhaps years, I can't recall) to accummulate error using a floating point value, and we power down our machines or do a homing sequence every day or so.

Keep in mind that if you use incremental moves you are relying on system *Accuracy* now and not repeatability, which may not work for you.

~Ken

 

> 2) Marc, what is a geneva mechanism? <

See http://www.britannica.com/eb/article?tocId=9000830

> 4) Steve, we have tried redefining position 16 as position 1 but the trick is when to do it. We are moving the motor every 80ms, with a target indexing speed of 33ms. If the motor is still oscillating at that time, this will lead to a slight offset in the end. <

Good point. I can see how that would be a problem.

 

Dear sir,
Hardware---National instrument's Motion controller+ allen bradely AC servo drive

Drives are in velocity feedback and positioning is controlled by controller.

I have to control a turntable,XC axis, the application is put a job/workpiece on table and grind its edges, the grinder is stationery at one end, the height of grinder can be adjusted manually.

How can I achive it.

Thanks,
Surender