Re: PWM stepper driver with external bridge

Hi,

There are two concepts here to consider, resolution and accuracy.

Resolution: Microstepping always increases resolution without limit.
A million microsteps per step drive will take 200,000,000 microsteps
per revolution.

Accuracy depends on 3 things:

1) Motor accuracy. Typically +/- 5% non-accumulative tolerance per
full step. This means the ultimate accuracy of the motor is one part
in 2,000 or 10 microsteps.

2) Drive accuracy: The drive must provide a "high-fidelity" sine and
cosine current to the motor.

3) Load: A step motor is a mass-spring system. It generates a
restoring torque proportional to error angle for small loads. What
this means is a 100 oz/in motor generates 100 oz/in of torque if its
shaft is displaced from its rest position by 1.8 degrees. A 10 oz/in
torque would move the shaft off-position by 0.18 degrees.

Any resolution in excess of accuracy is a waste unless:

1) The motor is run closed-loop with an encoder.
2) The motor must move smoothly (but not accurately) at very low
speeds.

Mariss

> This is a very good point that I think is often overlooked. It

seems that

> many people I have communicated with think that a microstepper will

give

> greater resolution.
>
> Tim
> [Denver, CO]
>
> >
> > FAQ: http://www.ktmarketing.com/faq.html