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Author Topic: New Build Suggestions  (Read 6060 times)

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Offline simpson36

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Re: New Build Suggestions
« Reply #10 on: September 24, 2013, 07:10:01 AM »
Quote
Oversizing a servo can result in all sorts of problems including making the system un-tunable, defeating built in auto tuining and/or having the motors break into uncontrolled self exciting harmonics (given an elastic connection to the load).
Interesting, I found the exact opposite on my Bridgeport when I swapped it over to servos. I had some 1.5Kw motors and they were extremely easy to tune, in fact they were the first ones I have had any real success with the Auto Tuning in the Allen Bradley drives.

Would I be correct in guessing that you have a big bridgeport with dovetail slides and the orignal leadscrews directly coupled to big low RPM servo motors with nice heavy armatures?

Offline Hood

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Re: New Build Suggestions
« Reply #11 on: September 24, 2013, 02:14:20 PM »
Steve, it was a Bridgeport Boss 6 series 1 CNC , so dovetails on short table, ground ballscrews and the motors I put on it were Allen Bradley MPL (Light inertia) motors with a rated RPM of 5000 and I kept the original gearing of 2.5 :1

Hood

Offline simpson36

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Re: New Build Suggestions
« Reply #12 on: September 25, 2013, 04:45:13 AM »
I kept the original gearing of 2.5 :1

Gears or rubber belts?

Offline Hood

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Re: New Build Suggestions
« Reply #13 on: September 25, 2013, 05:43:14 PM »
Timing belts, original Imperial sized ones.

Hood

Offline simpson36

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Re: New Build Suggestions
« Reply #14 on: September 25, 2013, 07:16:51 PM »

Mr. Hood, sometimes you just get lucky, I guess. Probably your proximity to Ireland . .  ;)

Offline simpson36

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Re: New Build Suggestions
« Reply #15 on: September 27, 2013, 03:24:34 AM »
This thread seems pretty much dead, but before I sign off, I want to mention a few caveats and clarify some terms in case there is confusion about the 'rule of thumb'.

*Stepper motors ratings and Servo motor ratings cannot be compared directly as they are different measurments made in different ways. For the same reason, industrial AC servo ratings cannot be directly comapred to DC servo ratings, particularly the typical hobby level DC servos and drives.

Offline simpson36

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Re: New Build Suggestions
« Reply #16 on: September 27, 2013, 04:30:01 AM »
In hindsight, I think a 'rule of thumb' is a bad idea for this topic, but I already stuck my foot in it, so this post is a final attempt to clarify some of it before I sign off the thread and hopefully it will prove genrally useful in selecting and purchasing Servos for new builds. 

UNDERSIZED:

*Stepper motors ratings and Servo motor ratings cannot be compared directly as they are different measurments made in different ways. For the same reason, industrial AC servo ratings cannot necessarily be directly compared to DC servo ratings, particularly the typical hobby level DC servos and drives.
  for example a Keling 90V 40A motor would theoretically be 90 x 40 = 3,600 watts peak. But paired with a Gecko drive which only puts out 20A max, that number gets cut in half. A typical linear power supply drops voltage as more power is drawn, so you will never see full voltage at full amps. Therefor take away for various losses and you might have 1,200 watts peak. Industrial AC servos are rated very conservatively and typically have several times their rated torque as a useable reserve.
 Bottom line is that a 400 watt industrial AC servo motos is roughly equivalent to the 3,600 watt hobby DC servo/Gecko combination . . . so direct comparisons are not meaningful.

* Given the vast difference in 'rated' vs real world performance, it should be clear that a 'rule of thumb' must be taken in context and not applied across the board. In the case of my rablings, they apply to Industrial AC servos.

OVERSIZED:

*Not all 'Auto Tuning' is created equal. Newer models can be vastly improved over previous drives and if you are buying 'whatever' brand used on Ebay for example, do not assume you will be getting the same level of capability as the setup you just read about on a foruim (unless you buy the same parts).
for example:  Mitsubishi J2S series servos are very susceptable to the harmonics issue that I described earlier (elastic connection of an oversized motor to a large load). The newer J3 series drives can measure the machine's resonance and has input filters and smoothing and can detect a variety of harmonic issues and compensate automatically.
 Bottom line is that the J3 is nearly immune to a common issue that will absolutely confound an earlier model drive. Don't assume that buying a 'Mistubishi' will erase a bunch of problems. Make sure you are getting the same model as are reading about.

*'Oversize' in the context of my 'rule of thumb' means that the motor falls outside the drives inertia ratio as published in the drive manual. This varies quite a bit between brands and between models and also with the types of motor.

*'Inability to tune' in conrtext means that the tuning can only be accomplished with the gain set very low, which is a typical bandaid for a host of problems. Prozac for Servo Drives. In particular this is one of the solutions to the harmonics problem I described. With the gain (responsiveness, frequency, etc) set low to overcome vibration issues, the overall performance envelope suffers and the super responsive 'stop-on-a-dime' behavior without overrun or underrun is not achievable.

*'defeating AutoTune' in context means that if the overall system responsiveness (gain, stiffness, etc) has to be set so far from the default (to counteract instability) that the auto-tune cannot achieve the performance as described above, then the Auto-Tuning has been defeated, in my opinion. 

Bottom line; tuning in general for ALL servo drives and Auto Tuning (which comes in m any flavors) are able to work withing a certain range of parameters. This 'window' is pretty much a guessing game with the hobby level drives, and you find out it won't work when it doesn't work  .  . usually after many frustrating hours of trial and error. However, on the commercial/industrial side, the envelope is well defined and published in the documentation.  Ultra low inertia motors are targeted at super-response applications like pick-and-place machines and similar ultra-resonsive, light-load robotic applications. Given a simiar ineria ratio, 'oversize' for these motors (moving the same load) would differ from the same size, same power 'standard' motor by a factor of 2.

That's a wrap and I'm outta here.



Either way, good luck with all projects and endeavors. That's a wrap. I'm outa here.

Offline Dan13

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Re: New Build Suggestions
« Reply #17 on: September 27, 2013, 01:20:11 PM »
Only times I had trouble with tuning a servo were when the servo was undersized.

Dan

Offline simpson36

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Re: New Build Suggestions
« Reply #18 on: September 27, 2013, 06:07:31 PM »
Only times I had trouble with tuning a servo were when the servo was undersized.

'trouble' is not a useful description. Anyone who knows how to tune servos can certainly be more specific than that.