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Author Topic: Placing the Encoder - On ballscrew vs On motor  (Read 3264 times)

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Placing the Encoder - On ballscrew vs On motor
« on: February 03, 2008, 06:11:06 PM »
Hi all,

I am in the process of retro'ing a manual machine using a package from www.lowcostcncretrofits.com, powered by Mach3. I am at the stage of interfacing the servos on the ballscrew. A lot of people go with the timing belt reduction way. I tend to like the gearhead way. Truly accurate reduction gearheads are also extremely expensive (in comparison to timing belt reduction systems). Cheaper gearheads can have an angular backlash of up to 40arc-minutes.

This is what I am thinking.

If I do get a cheaper. higher-backlash reduction gearhead and relocate the encoders away from the motor and directly onto the ballscrews themselves, wouldn't that make the high gearhead backlash a non-issue ? Regardless of the backlash on the motor-gearhead interface, if the screw don;t turn then the encoder does not register.

What do you guys think ?

Regards.

Offline Hood

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Re: Placing the Encoder - On ballscrew vs On motor
« Reply #1 on: February 03, 2008, 06:17:04 PM »
Your motor will constantly hunt back and forth, I would say not to do it but others opinions may differ. Timing belts work great and loads of commercial Industrial CNC Lathes and mills use timing belts.
Hood
Re: Placing the Encoder - On ballscrew vs On motor
« Reply #2 on: February 03, 2008, 06:21:58 PM »
I thought that the back-forth hunting of the motor would most probably happen if the glass-scale output is used to provide positional feedback. My thought is to relocate the rotary encoder, normally located on the servo motor, on the ballscrew itself.

Offline Hood

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Re: Placing the Encoder - On ballscrew vs On motor
« Reply #3 on: February 03, 2008, 06:27:07 PM »
If you have very acurate ground ballscrews then it may work but what is your reason for wanting gearboxes when timing belts are  acurate and have proved themselves on many Industrial machines that are used day in day out.
Hood

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Re: Placing the Encoder - On ballscrew vs On motor
« Reply #4 on: February 03, 2008, 06:45:52 PM »
Also just thought you might not be aware how Mach works, it sends out a series of pulses (steps) that it calculates that you need to go the distance you have asked it to go, if you then go back the way your backlash will use up some of these pulses with no movement. You will need to enable and set up  the backlash comp. If your gearbox is consistent with its amount of backlash this may not be too much of a problem. I just dont understand why anyone would deliberately add backlash into a setup when there is a cheap and eay way to do things backlash free.
Hood
Re: Placing the Encoder - On ballscrew vs On motor
« Reply #5 on: February 05, 2008, 07:11:08 PM »
^^  What he said. I've seen timing belts routinely used on high-end military antenna positioners for their effectiveness alone... trust me, the price doesn't come into it. The better ones have steel strands to prevent stretching.