Hi All,

I am using the Nanotech ST5709S1208 (0.9 degree, 400 steps for a revolution) motor in bipolar series configuration to rotate a light mirror mounted on its shaft. I have constructed a driver circuit using a half bridge circuit and a PIC16 microcontroller.

I find that the maximum I can drive this motor is about 1.25 - 1.4 revolutions per second without the mirror loosing step. I have tried microstepping the motor with half/8/16 and 32 steps and though I can increase the stepping frequency as the number of microsteps increases, the speed of rotation of the motor is still about 1.25 rps. This is a bit too slow for my optical scanning application. I have a feeling that I may be doing something else wrong since I am new to stepper motors. To control the motor I use a chopping waveform of 20kHz and a linear velocity profile to attain max speed

I would like to know what any of your experience is regarding the maximum speed of this motor in particular and steppers in general and why do you think I might be getting such a low speed in spite of the fact that the torque I require is quite low (load = small, light mirror) ?

Thanks in advance!
Govind

 

You should calculate your inertia of the mirror and the system you use to mount the mirror (i.e. the load). Rotary Inertia increases dramatically with the radius of the load, so you need to keep this in mind. A rule of thumb is that your inertia should be less than or equal to that of the motor's own internal inertia.

The stepper should have some specs on what velocity profile is acceptable for different load inertia values.

KEJR

 

It sounds like you are hitting a resonant frequency, this is primarily a mechanical issue. Try damping the mechanical system with friction or add/subtract mass to change the system dynamics. You might be able to accelerate quickly through the critical speed or start stepping above the critical speed also.

Bill Sturm
Abbeytronics LLC

 

Do some googling on damping stepper motors. If you are just a little too slow, that will almost certainly help. You could try just adding a little mechanical braking with a friction pad or a cord wrap as a test.

Regards
cww

 

What voltage are you running the motor at? That is the primary speed limiter for stepper motors. You will get higher torque using full steps.

 

Since your load is light, it is mostly inertial. That means the profile you use to reach any given velocity is very important. You said you used a linear profile. I would decrease the ramp rate and make sure the ramp is smooth. How are you controlling the rate in the PIC16? Perhaps your speed profile has big steps in it. With very inertial loads, even a small discontinuity in the speed profile can upset the stepper motor. I have done lots of PIC16 programming, and some stepper motor control. There are tricks you can use to generate pulse rates that have non-integer relationships with your basic clock. Send me a PM for more info.

Robert Scott
Real-Time Specialties
Ypsilanti, Michigan
Embedded Systems Consulting

 

Sorry I had not noticed your post.

How do I contact you? my email ID is govind DOT mukundan AT gmail DOT com