Stepper motor control

'Ramping' is a technique to accelerate through the resonance region and enter the synchronous slewing region. When the magnetic field changes to take a step, the motor accelerates, passes the pole that it was attracted to due to momentum, then must decellerate, stop, backup and settle. If you take a second step right when the armature is even with the pole from the first step, it will accelerate to yet a faster speed, rather than beginning to slow. Repeat this process for a couple more steps and you are really moving. Obviously the acceleration of the rotating parts is a function of the mass, so the timing requires sub microsecond accuracy to get the acceleration profile right.

The drive circuitry and the motor proper also have a large effect on what you can do with it. Back emf can really mess up the operation of a stepper motor. The flywheel diodes are very important. Are you driving the motor with a squarewave or a sinewave? etc.

Bob

This is roughly right - with a rotary stepper motor, the moment of inertia is what matters, not the mass.

For the stepper motors where I did my calculations, the resonant frequency set by the moment of inertia of the unloaded rotor and the stiffness of the magetic field was around 100Hz, so millisecond timing was close close enough - if you want to microstep a stepper motor well above resonance you may want to go up to a 50MHz clock which isn't exactly rocket science these days, but single stepping and half- stepping are pretty rough driving regimes, and you can rely on the moment of intertia of the rotor to smooth out small irregularities in the acceleration profile.

As is mentioned in another response, you do have to pay attention to the back emf generated in the drive coils when you are spinning the motor fast - note that the back emf coefficient in volts per radian per second is numerically identical to the torque constant of the motor in newton metres per ampere. You also have to provide enough drive voltage to change the current in the coils within a step time despite the inductance of the coils, which tends to dominate the impedance when the motor is being spun fast.

-- Bill Sloman, Nijmegen

Hi All,

Thanks for your reply. I could understand that inertia of the motor and back emf causes this problem. Could you please help in finding the inertia of the motor and linkages. For my setup, the Max.force developed by the motor is 45N, and that required by the load is 20N. Mine is 12V motor having 3.8mH coil inductance, and 20Ohm resistance. Since mine is stepper based linear actuator the linear travel/step is 0.0254mm. I really do not know how to account the back emf and the inertia of the system from the above mentioned information. Could you please help me in arriving the same?

Also how is the resonant come in to this picture also how to estimate this resonance value hence we could avoid?

I am going to drive my motor with square wave only. But how the sine wave will drive a stepper motor?

Thanks & Regards, Nag.

Try reading the manufacturers data sheet. They usually list the moment of inertia of the rotor, the maximum torque and the step size - which is enough information to let you work out the first resonance of a bare or lightly loaded motor.

Many data sheets actually give the first resonance frequency, to make life a bit easier for the enumerate.

Driving a sine wave currents through the motor coils is a slightly better way of driving a stepper motor - which is, in fact a synchrononous alternating current motor - than driving with a square wave. For regular stepper motors, you want to keep to the two sine waves at the same frequency and ninety degrees out of phase, though there are three phase stepper motors with three coils where you need different phase shifts - try reading the manufacturer's application notes, or this classic tutorial.

-- Bill Sloman, Nijmegen

read this book: Stepping motors: a guide to modern theory & pratice by P.P.Acarnley, ISBN0 906048 75 3

Its extremely detailed, and goes into the practical aspects of steppers in a lot of detail. my copy cost about $10.

Cheers Terry