It’s a give and take kind of situation:

1) For the same peak current, a microstepped motor will have 71% (1/sqrt 2) the holding torque of a full-step drive. This is because motor torque is the vector sum of the phase currents. Advantage: Goes to full-steppers.

2) Most people want motors to turn, not just ‘hold’. As soon a full-step driven motor turns, its torque drops to 65% of its holding torque. Where did the missing torque go? To resonating the motor is where. Motor mfgs sometimes specify ‘dynamic torque’; this is specified at 5 full steps per second. It is always between 60 to 65% of holding torque. Not mentioned is the horrible racket the motor makes at 5 full steps per second.

Microstepped motors do not resonate at low speeds, so no torque is invested in resonance. Microstepped motors keep all their holding torque while turning slowly. 65% for full-steppers, 71% for microsteppers. Advantage: By a hair (6%), goes to microsteppers.

3) Things get a little dicy as speed increases. Microstepping ceases to have any benefit above 3 to 4 revolutions per second. The motor is now turning fast enough to not respond to the start-stop nature of full steps. You can say the step pulse rate is above the mechanical low-pass frequency limit (100Hz or so) of the motor. Motion becomes smooth either way.

Simple drives persist in microstepping anyway above this speed. This means they still try to make the motor phase currents sine and cosine past this speed. A little problem with that and it’s called ‘area under the curve’. The area under the sine function (0 to 180 degrees) is only 78% of a square wave (full-step). Advantage: Goes to full-step again.

More sophisticated drives transition from a sine-cosine currents to square-wave quadrature currents about then. Same as full-steppers. Advantage: Draw.

4) As peed increases even more, another really big problem crops up; mid-band resonance. This is the bane of full-steppers and microsteppers alike.

Maybe you have experienced it; the motor is turning 5 to 15 revs per second when you hear a descending growing sound from the motor and then it stalls for no good reason at all. Faster it’s OK, slower it’s OK, but not OK in that range. All you know is there is a big notch in the speed-torque curve. This mid-band instability, or parametric resonance.

Simple drives have no defense against this except to try not run the motor in this speed range. Better drives have circuitry to suppress this phenomena and it involves rate damping.

This is the equivalent of shock absorbers (rate dampers) on a car, without them a car bounces repeatedly. Imagine a washboard road surface in sync with this bounce; there would be sparks flying from the undercarriage in short order. With rate dampers the ‘bounce’ is suppressed to a single cycle. Mid-band compensation does the same with steppers.

5) More than any other type of motor, step motor performance is tied to the kind of drive connected to it. More than any other type motor, a stepper can be driven from very simple drives (full-step unipolar L/R) to very complex ones (microstepping full-bridge bipolar synchronous PWM mid-band compensated).

Motor performance will range from “Miserable, give me a servo, I’ll never use another stepper again” to “What is the big deal about servos anyway, this is just as good.”

It’s ALL in the drive.:-)

Mariss