[MrWild]

I completely lack electronic credentials so what I'm putting out is only a thought. Steppers stall when they hit certain frequencies. The dreaded mid band stalls. Physics explain harmonics, in that a wave can increase or decrease depending on the frequencies and timing of input. Many of us have seen the bridge being destroyed due to a steady wind which caused destructive harmonics. Cadillacs have used this principal in their muffler designs.

In a PWM drive, power is induced as square wave that broadens the on time but keeps set frequency. It seems to me that if the frequency was changed in the trouble areas, you could induce a negative wave form negating the trouble spots. A person would need programming skills to control the areas of frequency change but it seems to make sense from a physics point of view.

[Al_The_Man]

From the past when I played with steppers I seem to remember the Lanchester damper was used to combat resonance?
See p14 of the DC steppers.
http://mcsupplyco.com/uploads/images.../SYNCGUIDE.pdf
Al.

[vger]

Good link Al,

Mid band resonance of a stepper is caused by a combination of the mechanical and magnetic properties of the system. The inertia of the complete mechanical system causes the motor rotor to slightly over-shoot the step and it "rings" at its resonant frequency with the magnetic circuit acting as the spring. When the step rate approches the resonant frequency, the motor can stall, skip steps, or even step backwards. There are some drives that compensate for this electronicly by sensing the motion of the rotor (feedback generated by the moving rotor) and applying current to the coils to dampen the resonance.

Steve