Dynomotion

Group: DynoMotion Message: 3788 From: himykabibble Date: 2/7/2012
Subject: Servo Tuning Questions....
Today I attempted to do some servo tuning, by running the encoder signals from the Geckos into the KFlop, and using the Step Response dialog in KMotion. Results are rather mixed, which makes me wonder if I'm doing it wrong.

First, I have four Geckos, three are G320s, the fourth is a G320X. The 320X is on the quill. All have been working for about 3 years with no problems whatsoever. But, with the KFlop, I'm seeing random faults, especially on the knee. Since I originally did the tuning entirely by "ear", it seems safe to assume there is room for improvement.

So, I tried several things, but settled on this rough procedure:

1) Tune by ear to get it moving reliably.
2) Use the "step" mode with a distance of 100, to get gain as high as possible, but still have enough range on the damping to avoid ringing. I did this by increasing gain in increments, then reducing damping until ringing occurred on the rising and falling edges, the increase damping until the ring just goes away.
3) Use "move" mode with moves up to an inch or two, and fin-tune to minimize following error.

X, Y, and A went reasonably well. Though, oddly, I ended up with Gain at max, and damping rather low on all of them. However, X and Y now seem to work perfectly. On A, I had to reduce acceleration greatly (from 5in/sec2 to 2in/sec2), and reduce velocity significantly (from 50IPM to 30IPM). But, I still get faulting, very often at the end of a move, sometimes at the start. Oddly, it almost seems happier with a high Jerk value.

The quill, with the G320X did not go so well at all. It seems remarkably insensitive to the adjustments, and I could get a reasonable looking response with the Gain adjustment pretty much anywhere in its range. Even stranger, unless it is constantly dithering rather loudly, I see significant over-shoot and following error, and the sound of the drive is VERY different from what it's ever been in the past - much more of a "growl", where it used to have that nice servo-y "whine". I can't seem to find a setting where it is quiet when stopped that doesn't look massively under-damped. This also seems really strange to me, and opposite what I see with the G320s - I have to REDUCE damping to make it quieter. The higher the damping, the more it dithers at rest. I don't think I'm seeing faults, but it's too soon to be sure. The settings I ended up with seem really odd - gain is well under half-way up, Damping is full up, and I is less than one-quarter. But ringing and over-shoot are minimal, and following error seems generally quite small (20 or so in most cases).

So, I'm not sure how much I've improved things here. Any recommendations on how to really optimally tune these things?

Also, with a STEP/DIR drive, I'm really not clear on what the "step" mode is actually doing? I know the procedure Gecko recommends, which makes sense to me, is to feed a constant stream of Step pulses, and periodically toggle the Dir line, and see what the response looks like. I don't see any way to do that with KFlop.

Regards,
Ray L.
Group: DynoMotion Message: 3789 From: Tom Kerekes Date: 2/7/2012
Subject: Re: Servo Tuning Questions....
Hi Ray,
 
I don't know anything about tuning Geckos, but using the "Step" button isn't really useful for Step/Dir interfaces.  The result would be a burst of pulses at the Max Step/Dir Generator Rate of ~ 2.5MHz.
 
I don't know that it really makes sense but to do the equivalent of instant reversals set the Jerk and Acceleration to infinity and just command positive and negative Jogs.
 
Regards
TK

Group: DynoMotion Message: 3797 From: himykabibble Date: 2/7/2012
Subject: Re: Servo Tuning Questions....
Tom,

You've confirmed what I thought about the step mode. I did find it useful just to see what the response to an unreasonable stimulus was - it's the easiest way to clearly see whether there is ringing or rounding of the corners due to incorrect damping. But I'm not sure that's really useful in terms of overall servo performance.

I have at least gotten to the bottom of one problem - the knee on my machine is not well, and I suspect a significant part of the problem is the cold weather. It increases friction, and seems to make the gas struts that help counter-balance it less effective, both adding to the load on the servos. If I warm it up, it behaves better. So, looks like I have some mechanical work to do...

Regards,
Ray L.

--- In DynoMotion@yahoogroups.com, Tom Kerekes <tk@...> wrote:
>
> Hi Ray,
>  
> I don't know anything about tuning Geckos, but using the "Step" button isn't really useful for Step/Dir interfaces.  The result would be a burst of pulses at the Max Step/Dir Generator Rate of ~ 2.5MHz.
>  
> I don't know that it really makes sense but to do the equivalent of instant reversals set the Jerk and Acceleration to infinity and just command positive and negative Jogs.
>  
> Regards
> TK
>
> From: himykabibble <jagboy@...>
> To: DynoMotion@yahoogroups.com
> Sent: Tuesday, February 7, 2012 12:58 PM
> Subject: [DynoMotion] Servo Tuning Questions....
>
>
>  
> Today I attempted to do some servo tuning, by running the encoder signals from the Geckos into the KFlop, and using the Step Response dialog in KMotion. Results are rather mixed, which makes me wonder if I'm doing it wrong.
>
> First, I have four Geckos, three are G320s, the fourth is a G320X. The 320X is on the quill. All have been working for about 3 years with no problems whatsoever. But, with the KFlop, I'm seeing random faults, especially on the knee. Since I originally did the tuning entirely by "ear", it seems safe to assume there is room for improvement.
>
> So, I tried several things, but settled on this rough procedure:
>
> 1) Tune by ear to get it moving reliably.
> 2) Use the "step" mode with a distance of 100, to get gain as high as possible, but still have enough range on the damping to avoid ringing. I did this by increasing gain in increments, then reducing damping until ringing occurred on the rising and falling edges, the increase damping until the ring just goes away.
> 3) Use "move" mode with moves up to an inch or two, and fin-tune to minimize following error.
>
> X, Y, and A went reasonably well. Though, oddly, I ended up with Gain at max, and damping rather low on all of them. However, X and Y now seem to work perfectly. On A, I had to reduce acceleration greatly (from 5in/sec2 to 2in/sec2), and reduce velocity significantly (from 50IPM to 30IPM). But, I still get faulting, very often at the end of a move, sometimes at the start. Oddly, it almost seems happier with a high Jerk value.
>
> The quill, with the G320X did not go so well at all. It seems remarkably insensitive to the adjustments, and I could get a reasonable looking response with the Gain adjustment pretty much anywhere in its range. Even stranger, unless it is constantly dithering rather loudly, I see significant over-shoot and following error, and the sound of the drive is VERY different from what it's ever been in the past - much more of a "growl", where it used to have that nice servo-y "whine". I can't seem to find a setting where it is quiet when stopped that doesn't look massively under-damped. This also seems really strange to me, and opposite what I see with the G320s - I have to REDUCE damping to make it quieter. The higher the damping, the more it dithers at rest. I don't think I'm seeing faults, but it's too soon to be sure. The settings I ended up with seem really odd - gain is well under half-way up, Damping is full up, and I is less than one-quarter. But ringing
> and over-shoot are minimal, and following error seems generally quite small (20 or so in most cases).
>
> So, I'm not sure how much I've improved things here. Any recommendations on how to really optimally tune these things?
>
> Also, with a STEP/DIR drive, I'm really not clear on what the "step" mode is actually doing? I know the procedure Gecko recommends, which makes sense to me, is to feed a constant stream of Step pulses, and periodically toggle the Dir line, and see what the response looks like. I don't see any way to do that with KFlop.
>
> Regards,
> Ray L.
>