Getting more accuracy out of a stepper motor

Ok after reading a little more I guess you wouldn't need any optical encoding with a stepper motor, that looks like a technique that only applies to dc motors.. does that sound correct? I've been reading a little about microstepping controllers, but have yet to determine if stepper motors are even capable of high speed with gear reduction used...

A stepper motor is just that. The variance may be in number of steps per revolution and the speed at which it makes them. Any required changes have to be in external mechanics, and those have to be spring loaded so there is no free play when changing directions. And there is NO such thing like 'half a step'. So be conservative and design your setup for worst case probability.

HTH

Stanislaw.

You can use microstepping to get better resolution. It requires a DAC (digital to analog converter) to vary the current through the windings, giving you resolutions of fractions of a step. Keep in mind that you lose torque when you microstep. See this device by Parker (Selectable resolution up to 50,800 steps/rev ):

Half-stepping drive is common, as is microstepping. You can buy a

1/256 step microstepping driver, and get very smooth motion with no perceptable stepping, but 1/256 step position accuracy probably isn't achievable.

John

I also agree that 1/256 step position accuracy would be pretty hard to do, and would move with any change in torque load on the motor. AC servo motors with optical encoder feedback are much better at maintaining really small increments, but are costly. They are basically a stepper with really large steps and optical position feedback. They also don't "cog" when turned manually.

I still find microstepping noisy and resonance prone. AC servo drives are really smooth and can be very quiet - often the high precision ball screws they drive and the high precision linear ball slides make more noise. Even large ones rated for 7.5 HP are quiet. The linear motors we have are however very loud, but they accelerate at around 4 G and move at 40 to 50" per second.

One of our solder paste printers uses microstepping, and achieves a 3 sigma accuracy (the new ones are 6 sigma!) of 0.001" with 1.8deg per step steppers driving ball screws with about 1/2" pitch. That's after accounting for the X,Y, Z, Theta table axis, in which the z axis moves about 4" or 10 cm after the vision processor camera reads the fiducials (which is not perfect), and the camera head moves in on linear motors (basically a long, flat stepper - electrically they are the same) with a slat pitch of about 1 mm (about

0.040"), which gives a stepping pitch of about 0.5 or 0.25 mm. I figure the steppers must achieve a repeatable ~1/20 step, and the linear motors 3 or 4 times better (the linear motors are sitting on air cushions). Other then the zero setting home position sensors, there is no feedback for mechanical position, which I think is quite bad in this precision application, especially when the motors get old or abused (like getting jammed up and the motor field continues to rotate, while the rotor stays put - we all know what strong alternating magnetic fields do to magnets) and start slipping from core demagnetization. Steppers in the range of 70 to 500 W are not cheap. Platens for linear motors are around $3k

For the original poster, take a look at floppy drives for the stepper actuated head, and look at modern high speed optical drive laser moving mechinisims.

Stepper motors have limitations on speed and accuracy. Speed is a function of the maximum steps per second the motor (and driver) can do. It is limited by the induction of the motor coils. There are tricks to force more current through the windings faster to get higher speed, but there are limits and steppers are not especially fast as motors go.

You'd think that stepper accuracy is limited to the number of steps per revolution. And for the most part it is. However, if you add a step between two given steps (by energizing both coils at once) you double the number of steps. which works pretty well. You can carry that further by what is termed "micro-stepping" Here you essential vary the voltage between the two simultaneously energized coils to produce fine variations between two steps. However, as someone else noted you lose torque. The bottom line is that with a simple set of driver currents (say set by series resistors) you can get maybe 16 microsteps. BUT don't expect any torque. And friction will kill most accuracy. I've got that working pretty well if you are driving a MIRROR on the end of the stepper shaft. But if there are gears or friction, you'll need a huge stepper to generate the torque.

Optical encoding (really an analog motor technique) can be combined with variable current half-stepping to create stable and repeatable micro-stepping. The accuracy is set by the encoder and the current drivers supply the torque. It's sort of a hybrid stepper and DC motor with optical encoder system.

Hope this helps.

Stepping motors are just synchronous motors. The motor coils generate a magnetic field that varies - more or less sinusoidally around the circumference of the motor, and the rotor lines itself up with that magnetic field.

If the currents through the two coils are held constant, and you try and rotate the rotor, you will have to apply a progressively increasing torque to move it out of alignment with that magnetic field, up to a certain angle, where the restoring force will start decreasing, falling to zero at twice that angle after which the rotor will experiece an increasing force in the opposite direction, driving it to the next stable alignment.

The peak restoring force is the drop-out torque of the stepper motor.

In a synchronous motor, you modulate the the currents through the two coils to create a rotating magnetic field that drags the rotor around with it. The resisitng torque that has to be overcome to allow the rotor to rotate causes the rotor to lag the field by an angle that depends on the ratio of the resisting torque to the drop-out torque.

When a synchronous motor is used as a stepping motor, the current through the coils is switched on and off to create square wave approximations the ideal sinusoidal modulation.

Microstepping drives generate stair-case approximations to the ideal sinusoids.

In practice, not all stepping motors produce smooth rotation when driven by sinusoidal fields - an early example of a 1024-microstep microstepping drive written up in the then Journal of Physics E: Scientific Instruments (now Mreasurment Science and Technology) ended up being used to generate some 800 not too evenly spaced microsteps,

ESCAP still seems to sell stepping motors designed for microstepping

Getting stepping motors to rotate fast involves modulating the current through the coils fast enough to rotate the magnetic field at the appropriate rate, despite the inductance of the coils. This means driving the coils with a voltage very much higher than the manufacturers ticket voltage, which is just the rated current through the coils multiplied by the resistance of the coils.

I've used 60V on a nominally 5V motor, and this wasn't exceptionally high.

-- Bill Sloman, Nijmegen

If you grab a typical classic Slo-Syn type stepper motor and connect it to a microstepper drive, the accuracy won't be good, because the tooth profiles were optimized for torque in full-step mode. If you program slow, smooth motion, you'll get nonlinear angular motion, maybe even a zero-velocity flat spot every 4 steps. Some people make steppers that are optimized for microstepping, but even these probably can't do 1/256 accurately. Not to mention static friction, which no open-loop system can deal with.

I once wrote an n/c compiler for a Whitney punch press. It would slam around 12-foot long steel sheets at impressive speeds and whack them with a hydraulic punch, a hole a second or so, and the building would shake when we did the bigger holes in 1/8 inch steel. It used dc drive motors and linear encoders and was rarely more than a couple of mils (that's thous! not mm) off.

John