stepper motor

a stepper motor is microstepped. when pulsed, the shaft moves by a
small step and then returns to its original position, ultimately
producing no revolution at all. this happens for the entire duration of
the pulse.
what could the reason be ?
the reply i got is :I presume you are using a small microstep size. The
magnetic pole at
the full step position you started from pulls the rotor back to that
full step position because there is not enough energy in the coils to
overcome the strong magnetic attraction at full step positions.
Microstepping at small resolutions is not a guarantee that the motor
will move a full "microstep" for each pulse issued by the controller.
Microstepping mearly provides smooth motion. You may need to issue a
few pulses before the rotor actually moves off a pole position.
Between the poles, you are more likely to see a single pulse result in
rotor movement because the energy from the pole magnets is equalized
and the stator currents are stronger. (Sine Cosine relationship is
about equal). Use larger microsteps if you want to guarantee rotor
position without an encoder. You will need to experiment on your
system to find the smallest step that will "hold" position. It may be
only in the order of 1/16 or 1/8. To hold position you need a closed
loop controller with encoder feedback. Using an encoder will also show
you how much the shaft actually moves for 1 microstep. For lower
resolutions, the step size will vary depending on how far you are from
a pole.
Hope this helps.
(If you really need to position very precisely and in fine increments,
you will need to use a servo or a high mechanical ratio, which will
severely cut down on your top speed.)
My response :
Thanks a lot for the detailed reply.. my application is actually
microstepping a stepper to 128 micro steps and then converting it into
linear motion, in the order of microns. The prob i actually experienced
was : while using a stepper of 0.65 peak current and 5V dc, 1.8 degree
step angle, i could feel a jerk when pulsed for one microstep. But when
i went in for a motor of .16 peak current and 5V same 1.8 step angle,
there was an oscillation as i mentioned in my earlier query.
From what you have said, does it mean it is impossible to microstep any
stepper to 128 steps or may be not above 16 microsteps ????? As my
entire application revolves around this microstepping by a very small
step, its of great concern. please help me out.
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You need to fit a gear reduction head to the motor, so that you can do full steps, yet get the fine resolution you need.
For instance with a 100 to 1 gear motor you can get 1/50th of a degree stable micro steps that will hold stable between pulses. that would be 17,000 steps within a 360 degree circle.. great leverage via the gear box for both the drive and resistance to forces that would drive the motor backwards.
Phil Scott
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Phil Scott
To answer your question, no, it's not impossible to step at 128 microsteps. The problem is that you may never get the same shaft angle movement for each step. High microstep resolution is primarily for smooth motion, not accurate positioning. The other reply has a good way to solve the problem if your budget can afford a gear box. Regarding your oscillation, you did not state if you were trying to make a continuous move, or a 1 step move. If you were only going one step, I can't explain why the motor would oscillate (you are running open loop, right?). If you were tyring to make a move from one point to another, you may have encountered mid-band resonance for the step angle and speed you were trying to run. Or mechanical resonance of the stage assembly. Another way to solve your problem, as I mentioned, would be to use an encoder on your motor with a closed loop controller capable of reading back the encoder counts to see if the move was made correctly. A servo would also be a good choice for moving small amounts accurately. My company makes servo amplifiers that are used in sub-micron positioning applications all the time. With the right controller, amplifier and stage, it's not a hard task.The tradeoff for the performance gain over the stepper is cost, of course :). Bob
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