Servo motor following error video?

To anyone interested in making machines with servos: This fellow I know does ornamental wood turning. One way of milling
helical grooves on a wooden cylinder works like this: A lathe has a sprocket attached to the back end of the spindle. A chain is draped over this sproket. One end of the chain is weighted, the other end is attached to a cable. Said cable is run over various pulleys and is attached to the lathe carriage. A spinning cutter is mounted to the carriage. As the carriage moves back and forth the spindle turns and the cutter, engaging the cylinder, milling a helical groove. The pitch of the helix is changed by changing the sproket. So this fellow was explaining this to me and mentioned that he wanted to do this electronically. Use an encoder on the carriage to send pulses to a servo or stepper driver which then controls the motor that spins the spindle. He spoke with an engineer who told him that following error would be so great that he couldn't move back and forth like he could with the sproket arrangement. I didn't believe it so I built a demo model with the signal generating encoder connected through a precision differential to the motor shaft. Spinning the encoder one direction causes the motor to spin in the opposite direction. If there is no following error the the center section of the differential would remain stationary. Any movement left or right would show following error. Well, the center section does swing left or right. About two or three encoder counts. At high count settings the amount of following error, in degrees of rotation, is so small that it doesn't matter, and is probably less than the windup in the mechanical version. Using encoders with switchable counts shows more movement of the diff with the lower count settings. So, after that long explanation, would anybody like me to try to post a video online demonstrating this? Eric
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On Sat, 28 Jan 2012 22:18:53 +0000, etpm wrote:

Congratulations. You've reinvented the CNC lathe.
--
Tim Wescott
Control system and signal processing consulting
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On Sat, 28 Jan 2012 16:42:12 -0600, Tim Wescott

Well, sort of. You are of course talking about electronic gearing. I didn't invent anything. Just figured out a really graphic way to show someone what the following error would be and how to change it. Maybe I should have posted the above referencing electronic gearing instead of following error. Because when I was first told about this guy's desire to eliminate the mechanical connection I thought about electronic gearing. I told him this and he brought up following error and how he believed it would be a problem. Eric
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<snip>

Not really but if you like, you can simply remove the rh way cover on your fadal, and then take the end cover off of the drive motor--inside, you will see 3 small screws that are used to secure the resolver onto the housing...
Slightly loosen all three of the screws which will allow you to rotate the resolver hand--note how the table basically moves in lock step whenever the resolver is rotated.
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On Sat, 28 Jan 2012 16:19:43 -0800, "PrecisionmachinisT"

That shows the machine responding to a perceived (by the machine) positioning error. It does not show the following error. Of interest to me and the guy doing the wood is how much lag one axis has following motion from another axis. Eric
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Following error is displayed on the fadal crt--to the right of the axis dimensional read out
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On Sat, 28 Jan 2012 22:58:08 -0800, "PrecisionmachinisT"

Well that's cool to know.
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On Sat, 28 Jan 2012 22:18:53 +0000 (UTC), snipped-for-privacy@whidbey.com wrote:

Absolutely. Please do.
-- The most decisive actions of our life - I mean those that are most likely to decide the whole course of our future - are, more often than not, unconsidered. -- Andre Gide
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