Fast stepper motor x16

[Guest peepshow]

My stepper motor, for tracking runs at 60 RPM with a drive frequency of 200Hz

 

For rewinding back I need it to run at least x16 faster.

Until this morning no matter what I did ( changing drive volts  up to 35) and

trying various motors of different  resistances I could not make it run faster that 240RPM by switching in a faster driving frequency of 800Hz.

 

But today I knocked up a variable freq square wave generator and found that if I started the motor slow and increased the freq slowly it now runs at over 1000 RPM, giving me the x16 rewind.

 

Is this normal for stepper motors, to make them run fast, the drive frequency has to be increased slowly rather that suddenly switched in?

[Werisit]

Hi Richard

Interesting. You must be running some gearing then. I have built a simple tracking mount that runs at 1 rev per 186 seconds with a  0.9 step motor running 1/4 steps.

I guess accelerating the motor slowly gives it a chance to achieve what the signals are asking it to do.

[Guest peepshow]

Yes Alan, I have a 60/1 worm drive.  I have checked on other forums and it sems that for the higher speeds,

one has to start slow and then increase the speed.

My motor is 200 pulses per rev (1.8 degree) running at full steps.  I did wonder if going mini steps

and higher freq would improve matters but haven't tried that.

[Werisit]

Hi Richard. I'm puzzled as a motor running at 60 rpm with a 60:1 worm drive gives the mount 1 revolution per minute. I aim for 361 degrees per day as a target speed. I'm missing something aren't I.

[Guest peepshow]

I think we have different equipment. 

My stepper motor is running a barn door tracker where

a 20 TPI lead screw needs to rotate at 1 RPM for tracking.

[Werisit]

Ah, that explains it. So your barn door is, in effect another gear stage. My worm is held against the gear with a spring so I can disengage it to return it to the start. It's only a part gear and has to be returned to the start again after about 3 hours. 

Do you have any problems with periodic error ? 

[+Graham]

Yes Alan, I have a 60/1 worm drive.  I have checked on other forums and it sems that for the higher speeds,

one has to start slow and then increase the speed.

My motor is 200 pulses per rev (1.8 degree) running at full steps.  I did wonder if going mini steps

and higher freq would improve matters but haven't tried that.

 

You are doing the right thing now.

If you go to mini steps it will slow down.

You need to be at full steps to keep the speed up.

[Guest peepshow]

Ah, that explains it. So your barn door is, in effect another gear stage. My worm is held against the gear with a spring so I can disengage it to return it to the start. It's only a part gear and has to be returned to the start again after about 3 hours. 

Do you have any problems with periodic error ? 

Alan, the barn door has no gears as such.

The drive rod, in effect acts as a huge worm wheel which rotates the camera platform at

an effective rate of 1 rev per siderial day............except it only tracks about 2 hours of that day at a time.  Hence the rewind speed to return  to the start point. 

 

Periodic error ?   Barn doors are used for wide angle shots so many errors are hidden.  

 

Mine had hinges as the bearings, but has now been completely redesigned over the last 2 months and now has ground steel shafts and ball races.  The motor gimble and drive nut now have  ball race bearings too.  Soon to be road tested.

[Werisit]

Sounds like a great and evolving project Richard. A clever mechanism. It's easy to get addicted to these things and it doesn't have to cost the earth. Do you have any pictures on line anywhere I can see them?

You can see some of my pictures in the gallery on the Milton Keynes Astro Soc web site.