Torque Speed Curve

[Imbue]

I have a quick question about the torque speed curve charts from Keling at http://www.kelinginc.net/KL23H286-20-8BT.pdf (for example)

My question, what does pps stand for? (It's the unit along the X axis) I might assume that pps is pulses per second or something similar. How do I convert that into rpm.

I might guess that because it's a 200 step per revolution motor, I can just divide by 200 and then multiply by 60. To confuse me more, the chart says they are running the motors in half steps. Does this mean there are 400 steps per revolution?

One more thing. I understand that to half step a person alternates between energizing one coil, and energizing both coils. Does this mean that if I take full steps and energize only one coil at a time, I'll have less torque than the chart? If I take full steps and only energize both coils, I'll have more torque than the chart?

Thanks very much.

[a77]

hmmm... bump?

I'm also wondering about the half-steps. If you run full steps, what would the torque curve look like?

[Imbue]

It's nice to get a reply after three and a half years. I never did figure out what "pps" stood for, exactly. Or how torque changes with half/full steps.

Thanks for the bump.

[Mariss Freimanis]

pps stands for 'pulses per second'.

Classical half-stepping (the kind you describe) results in a weak step and strong step. The weak step gives you 70% the torque of a strong step. Above a few steps per second this averages out to 85% (70% + 100% divided by 2) the torque of a full-step drive. Because a full-step drive resonates so badly at low speeds, the reduced torque of half-stepping more than makes up for what's lost to resonance in a full-step drive.

A classic half-step drive takes a serious performance hit at higher speeds when compared to full-step drives because each winding is powered only 75% 0f the time (+++0---0+++0--- etc instead of ++--++-- etc). At these speeds it's best to use full stepping drive or use a microstepping drive that morphs to a full-step sequence at higher speeds. Microstepping gives the best low-speed performance because the motor doesn't resonate at all which means it can deliver all the available torque to the load. Resonance of any kind robs the motor of available torque; it takes energy to shake, rattle and roll.:-)

Mariss

[a77]

Thanks Mariss,

So for full-stepping, I can theoretically take a point on the torque curve and add up to 30% for torque and double the rpm?

Would a gecko g540 be able to microstep the keling motors at lower speeds and switch to full step for rapid travels?

[Mariss Freimanis]

That would be correct, a full-step drive gives the theoretical maximum possible torque at higher speeds (above 4 revolutions per second and faster). This advantage is about 20% to 30% over a half-step drive or a standard microstep drive for the reasons I mentioned. Of course full-step drives are miserable from zero to 2 revolutions per second because of severe vibration (low-speed resonance). This alone is enough to eliminate them from serious consideration despite their high speed advantage.

I class a half-step drive as microstepping, specifically a 2 microstep drive. Low-speed resonance decreases with the square of microsteps used so a half-step drive has only 25% the resonance of a full-step drive (1 divided by 2 squared is 1/4 or 25%). At 10 microsteps, resonance is only 1% of a full-step drive (1 / 10^2 is 1/100 or 1%). It would take an infinite microstep drive to eliminate that remaining 1%.:-)

I don't want to hijack this thread so I'm reluctant to talk about a particular manufacturer's drive performance (ours). I'll just say ours morphs to a full-step drive when the motor speed is high enough to no longer benifit from microstepping. It's a 'have your cake and eat it too' situation.

Mariss

[Tartan5]

Hi Mariss,

So, are you saying that at higher speeds, a full step drive would actually perform better than even pure sin/cosine coil current?

[Mariss Freimanis]

Absolutely. Look at the "area under the curve" for a sine-wave inscribed in a square-wave. Its area will be 78% (pi/4) of a square-wave meaning the motor power will be 78% of a full-step driven motor. This is why we morph our drives from sin-cos to quadrature phase currents.

Mariss