Stepper Motor Controller design problems

Hi,

I need help. I have been in electronics design for almost 20 years but this is getting me down.

I had to design a stepper controller for a linear actuator from Haydon. It has a worm screw and a nut and it is used to lift a panel to exact positions. I have a controller/driver chip like PBL3717 and I drive it from a Micdoprocessor . The controller chip uses a chopper drive with the 5V motor running off 40V @ I max=560mA Motor is 200 step/rev bipolar.

I am stepping in fullstep mode with 2 coild nergized at all times. The 4 step phases are: A1A2 B1B2

0 1 0 1 0 1 1 0 1 0 1 0 1 0 0 1

I am stepping batches of 400 steps at a time and I repeat this over and over.

I use a accelerate and decelerate routine as follows:

Step 0-23 = accelerate 0 to 635 stpes/sec Step 24 - 374 = run at 635 steps/sec Step 374 - 399 = decelerate to - and then stop.

Problem: Motor starts runs and stops correctly but as I repeat the batches of 400, suddenly, RANDOM (!) motor starts and runs at a higher (audible buzz) speed in the opposite direction. This is NOT caused by the drive sequence or anything in the pulses. In fact, there is no electrical difference at all in the driving. The motor just does his. The next batch will be ok again. Sometimes it works fine for 30 an dmore batches and suddely it does it again.

Has anyone come across a problem like this and how is it best solved?

I tell you, stepper motors are not simple when resonance, acceleration and exact position all come into it simultaneously!

Regards Bernt

Reply to
Ben
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Stepper motors don't do ANYTHING without the pulses. Have you got high frequencies on the pulse outputs, superimposed on the normal stepping sequence?

Reply to
Paul Burke

is

V @

This would seem to be excessive, typicaly a motor this size can reach speed in 5 steps. When you shorten the steps remember the acceleration profile should not be linear.

ed

Sounds like resonance, do you have the load connected? You must not try to run the system with the motor lying loose on the benchtop.

ct,

oes

Reply to
cbarn24050

Mechanical resonance will cause that - particularly at close to maximum torque - running backwards. Try to change the load or damping.

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Reply to
default

Do you know what the resonant frequencies for the motor are? Most of the motors I worked on has a first resonance - the product of the moment of interia of the rotor and the spring constant of the magnetic field - at around 100Hz, comfortably below your 635 steps a second. There is almost always a second resonance due to the spring constant of the shaft connecting rotor to to its load - here, your worm screw. In some linear actuators the rotor is the nut driving the worm screw, and the spring constant of the worm screw would then be the corresponding factor - complicated by its tendency to change as it feeds through the motor.

In one application, I ran into another interesting trap for the unwary

- the motor was being asked to run fast enough that the back-emf in the windings was almost as high as the drive voltage, which produced some very interesting coil current waveforms.

We identified the problem by driving the stepper motor shaft with a variable speed electric dril and monitored the coil voltages with an oscilliscope. Solving it was tolerably straightforward - the nominally

5V motor was replaced by the 2.5V version, and all the power tranistors in the drivers got changed for appreciably beefier parts ...

Good motors show the back-emf factor in the data sheet, and good designers read the whole data sheet - but "goodness" seems to be as rare in the data sheets as it is in the designers.

It might be worth checking to see if a 40V drive is high enouhg to drive your motor at 635 step/sec.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

On 06/10/2006 the venerable snipped-for-privacy@ieee.org etched in runes:

You could try half-stepping the motor or adding a damper to the shaft.

This is an excellent reference book for all stepper motor problems:

Rather long, I hope it wraps OK.

--
John B
Reply to
John B

this

@

speed

fact,

does

You sometimes have to identify where the resonance is and avoid stepping at that frequency, It might be that you are accelerating through the point of resonance too slowly.

Colin =^.^=

Reply to
colin

It

40V
400,

and

at

I was going to suggest that maybe he's ramping the motor up too quickly, but you have a good point. Ramping through the point of resonance more quickly has cured my troubles in the past. But I've never had a motr run backwards under these conditions. They've just stopped spinning and sat there buzzing. ... Johnny

Reply to
Johnny Boy

but

buzzing.

Yes it is a bit strange but possible, if its suddenly running backwards at twice speed it means it has plenty of power to spare to accelerate more quickly.

Colin =^.^=

Reply to
colin

I've had the worm-gear type steppers run backwards like that if they were lifting the load -- the upset of the step sequence on the coils (and possibly some harmonic of said sequence) would make the thing go essentially frictionless and fall down.

It was a consequence of trying to push the motor too fast, with too little time in the project to get things right.

--
Tim Wescott
Wescott Design Services
 Click to see the full signature
Reply to
Tim Wescott

stepping

too

quickly

backwards

at

yes step too fast too quickly and it becomes useless, however it seems as though his is actually 'stepping' backwards, theres a lot to be said for positional feedback servos.

Colin =^.^=

Reply to
colin
[snip]

I've not done any stepper controller designs, but about ten years ago I did a design for a 3-phase EC motor for Bosch (Mercedes A/C blower).

The squirrel cage would spin backwards when the outside vent was open and the motor was not driven. Thus it would run backwards when powered up under those conditions.

I came up with a cute way to cure that. I'll look through my notes. (I even have a video tape of me driving the cage backwards using a leaf blower, then applying power, and it slows down to a stop and starts running in the correct direction.)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

That's exactly what ours did -- because 'useless' in this case meant 'fall', and it was trying to lift.

Hear hear!

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Powering the motors from a current limited power supply solves that proplem. About twelve years ago I build something that required a 1.8 Ohm / 2.5 Amp stepper motor to run at 5000 steps/s. At top speed, the supply voltage was around 80V.

--
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Reply to
Nico Coesel

Hello Jim,

Haven't done much with steppers either but one thing I did to avoid back travel was to turn on one winding at a substantial DC current. That stopped and held it.

At Bosch? I didn't know they had those noisy leaf blowers in Germany. Well, maybe now they do.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Nah. This was in my office where I was _breadboarding_ the chip design. (Electric blower.)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

This ought to be obvious. The point I was making was that you have to make sure that the supply voltage can go high enough to exceed the back-emf from the motor - and even stepper motors have back-emf - as well as coping with the resistance and inductance of the coils.

It is a point that I've seen ostensibly professional engineers miss.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

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