I want to drive 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? Looking at previous posts the suggestion is that full step is best but how would direct drive on 1/4 step setting compare with full step and say 3:1 reduction with a toothed belt drive?
Are there any easy rules of thumb to apply?
Thanks Peter
You might find the thread "Sizing Stepper motors for CNC" in this group dated 4th to 6th april 10 useful Especially the comments made by Tony Jeffree.
I am taking my first 'steps' in using stepper motors so have lots to learn. You will certainly have to ramp up your drive frequency gently to accelerate the motor from a standstill to 120 rpm. I have used a voltage controlled oscillator, VCO, followed by a divided by 1, 2, 4, &8, so that I can play with microstepping on the driver module and still get the same motor speed eg divide by 2 and use 1/4 microstepping gives same speed as divide by 8 and full step. So far the vco control voltage is slugged with a capacitor to stop the motor stalling which is simple and effective.
This could all be done with a PIC but I don't have the software development environment set up at home so mixed analogue/digital design is more suitable for me at the moment.
Bob
Bob,
I am also taking my first steps but is is interesting to learn.
I want the machine to stop after a specific number of pulses which depends upon what step I am using from the motor, 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 may resort to just using a position sensor to count the revs, 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 seems fine.
I was thinking about using a VCO as it is difficult to ramp up a 555 so I was going to drive via a belt with a tensioner to take up some of the jolt.
Would be interested in seeing your VCO circuit if possible? it is around 2khz depending on the step I use.
Peter
I use a 4046 cmos phase lock loop chip but just the vco section and connect the phase comparator inputs to Vss.
Circuit is straight from the data sheet. The control voltage is slugged with series 330 and 10uF capacitor.
In your system you can still count the pluses and when you want to stop, arrange to short the capacitor to Vss and use the logic VCO enable to stop the oscillator.
Email me direct if you want to discuss further.
Bob
Thats a good idea Bob. Just ordered some 4046's to give it a try, never used one before but the gating facility sounds good. Was thinking of going the pic route but thats even more learing!
Thanks Peter
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