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Old 18th August 2003, 05:13 AM   #1
zulu5d is offline zulu5d  Australia
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Default Stepper motors

I know this topic was touched on very briefly in:

AC Motor Power Supply Project

and basically was discounted because of the inherent cogging effect, but wondered if anyone's tried it? I'm pretty sure there is a TT manufacturer that does use them, but can't remember the name.

J.
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Old 27th August 2003, 11:39 PM   #2
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It has been done by the German company "Horstmann und Petter" on their model "Ulysses":

Click the image to open in full size.

I once had the chance to listen to that unbelieveable machine for a few weeks - at least in the bass region one of the best i have ever heard. Sadly they went out of business some years ago.
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Old 28th August 2003, 02:40 AM   #3
zulu5d is offline zulu5d  Australia
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Yes, thats the one I was thinking of!
Thanks for reminding me, time to drag out some steppers and give it a try.

J.
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Old 2nd September 2003, 06:13 AM   #4
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I have had some experience trying to make a stepper do an impression of a servo motor at work. The general idea was to take a cheap(ish) off the shelf stepper motor and make it able to operate with out much cogging and to keep the noise to a minimum. While doing that it was desired to keep the high torque constant found in most stepper motors. I was able to throw whatever technology I wanted (within reason) to solve this problem, as it was a feasibility study.

I used a fairly sophisticated digital control system for control and a high resolution (2e6 counts / rev) encoder for feedback as well as some high quality linear amplifiers for power.

I eventually gave up as:
- I could reduce noise, but not enough for it not to sound like a stepper.
- I could get the servo to settle to minute fractions of a step under load, but not quickly (10's msec), meaning that cogging was an issue while moving.
- Sometimes drastic reductions in cogging was possible, but very small changes in the motor (like replacing it with another motor of the same type), made a bunch of re-optimizing necessary.

The big problem I found was linearizing the motor's behavior. I could knock off some of the big non-linear behaviors by modeling the system, but some effects were more subtle, defying my attempts at linearization.
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Old 2nd September 2003, 08:44 AM   #5
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Quote:
Originally posted by zulu5d
Yes, thats the one I was thinking of!
Thanks for reminding me, time to drag out some steppers and give it a try.

J.

No no no!

You can’t use “some steppers”! You need special steppers designed for “micro stepping” AND drive them with micro stepping. This is not easy business to get it right, although it can be done.

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Old 2nd September 2003, 12:48 PM   #6
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"Micro Stepping" is not a matter of the motor used but of the controller.
The trick in using a stepper motor is somewhere else: You somehow have to "smooth" the steps of the motor. Normally, the stepper's windings are simply switched on and off with some kind of switchmode current control.
Smoothing the motor can be done by steering the motor with a sinusoidal current ramping. This gives definitely less torque, but that's not too important here.

I remember some detailed information on this matter in an ST application note, must be somewhere here: http://eu.st.com/stonline/books/toc/an/41.htm. Look for "Stepper motor control" and "L297".
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Old 2nd September 2003, 01:14 PM   #7
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True Holger,

But steppers of any kind are designed and intended for low cost position control. These are not designed for constant speed. Standard steppers have a large torque ripple and are noisy when driven, even with a sine. Steppers DESIGNED to be driven with micro stepping controllers run much more smoothly and are more expensive. Yes you can drive al steppers with a multiphase sine source of course. But in that case you are far better of with synchronous motors or with brushless DC motors.

The most silent running steppers I have seen were on the X-Y table of an automated microscope (of $200.000). They were running quite silent but incorporated a feedback tacho to get every thing right. The steppers were not as quiet as a DC motor + position/speed feedback system.

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Old 2nd September 2003, 01:52 PM   #8
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Some years ago I talked to a turntable distributor - I'm not sure, I think it was the SME distributor for Germany. He told me that SME (in the case I do remember correctly) had som attempts on stepper motors. It turned out that the biggest problem was the stepper's bearing, which was far too noisy.
They ended up with a complete motor reconstruction (slide instead of ball bearing). I have no idea if this has ever turned into a product.
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Old 2nd September 2003, 03:50 PM   #9
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Hello -

Just as another reference point, the (now discontinued) Versa Dynamics turntables used stepper motors, with excellent results. Would they have sounded better with another type of motor? Hard to say, although their designer was convinced that a stepper motor was the best solution.

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Charles Hansen
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Old 3rd September 2003, 06:31 PM   #10
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Default Easier Solution

Stepper motors make a whine/buzz no matter what ime.
An old video recorder direct drive capstan motor would be a much better choice imo.
With a suitable pulley fitted these would run quite silently.
They have servos on board, and require only a power supply and a DC speed control voltage.
One day I'll use one to power some vintage turntables to replace the rim drive setups.

Eric.
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