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Stepper Motor General/Basic Question

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Peter Bell25/04/2010 08:13:52
399 forum posts
167 photos

I want to run a small machine at 120 rpm using a stepper motor.
 
I could run it direct or by a pully to give a reduction so that the stepper motor runs faster.

Which will give the best torque to the machine from the motor? Direct drive on  1/4 step setting or full step and  say 3:1 reduction with a toothed belt drive? 
 
Thanks  Peter
 
Les Jones 125/04/2010 09:40:22
2292 forum posts
159 photos
Hi Peter,
                  I do not think there is a simple answer to your question. A lot depends on the design of the driver electronics. As you run a stepper motor faster the inductance of the windings has more effect which means that a higher supply voltage is required. This then means that a way is required to limit the current at lower speed or when the motor is stopped. This can be just series resistance but this is very inefficient. Pulse width modulation is normally used to control the current. From what I have read it is normally recommended to buy the stepper motor drivers rather than design your own. Here is a link to some good information on stepper motors.
Les.
 
Martin W25/04/2010 13:14:07
940 forum posts
30 photos
Peter
 
Stepper motors are exactly what it says on the tin. They actually revolve in a fixed number of steps per revolution which makes them ideal for shaft position controls and things like print head drivers where repeatability of shaft or linear position is important/critical and they then can actively hold this stationary position. I personally would not use one for a shaft drive unless you want these very particular features. That said this effect can be smoothed out a bit by choosing a stepper motor that has a high number of steps per revolution but the effect is still present.
 
If it is just to turn a shaft at an approximate speed then I would favour a brushless DC motor similar to those used in models with the relevant controller, in fact Sieg have moved to a brushless motor for their new release of their mini lathe replacing the brushed motor they used to employ.
 
I totally agree with Les regarding the driver issues.
 
Good luck
 
Martin
Peter Bell25/04/2010 13:46:09
399 forum posts
167 photos

Thanks for the replies,
I thought that there may have been some sort of rule which applied seeing that there is so much cnc about now, but perhaps that is too simple!
 
I want to run the motor for a specific number of revs as part of the process which really rules out a dc motor, got that part going using some electronics rather than a PC. I have never had anything to do with stepper motors so I just wanted to give it a try---without getting in too deep
 
Peter
Martin W25/04/2010 18:36:17
940 forum posts
30 photos
Peter
 
It really depends on what you are trying to achieve. If the load is fairly light then a small stepper motor could be utilised, with the provisos given in the above posts, and this could be directly driven from a dedicated stepper driver integrated circuit with minimal component count.
 
Either the number of steps could be counted and from that the number of shaft revolutions deduced or number of revolutions could be preset by preloading a countdown register with the required number of steps and counting this to zero. The only danger is that if the motor stalls and/or misses one or more steps then the final position is in error.
 
You could control it from a PIC chip via a driver IC and then Les would be your man for that. This way you could easily vary the number of steps, the step frequency/speed of the motor and make a closed loop system that would actively monitor the shaft rotation and true position.
 
Again good luck and have fun.
 
 
Regards
 
Martin
 
Les Jones 125/04/2010 19:36:46
2292 forum posts
159 photos
Hi Peter,
                 As Martin says a lot depends on your exact requirement. For example the torque requirement and how critical missing a pulse would be. The inertia of the system would also influence the design. Your requirement does not rule out using a DC motor but it would require using a position encoder. There is a design called the "UHU servo" that may fit your requirements.  Here is a link to it.  UHU Servo
Les.
Peter Bell25/04/2010 19:39:02
399 forum posts
167 photos
Martin,
 
Yes that what was I intended doing so that the machine will stop after a specific number of pulses which depends upon what step I am using, which is why I asked the question but it would be 20,000 for a full step.  Press reset and away it goes again. I amy resort to just using a position sensor to count the rev, depends how reliable it is.
 
That part is working, I have used a 555 as oscillator and 4017's as the pre settable counters because I had them in stock. I am using a driver frrom Model Engineers Digital Workshop which also seem fine.
 
Regards  Peter
 
John Haine25/04/2010 19:55:29
5563 forum posts
322 photos
If you want to run the motor fast but for an exact number of steps then you need to be careful about how it is accelerated and decelerated.  I remember being at Leeds University where a lot of the basic work on stepper motors was  done (see the classic textbook by Acarnley) - every day one would hear wailing like a WWII siren as motors were run up to high speed then down again to stop at an exact position.  The pulse frequency profile to do this is not trivial and you would be better using a computer to generate it I suspect.

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