stepper acceleration rate - limits?

[dsc]

The motor is used in a slightly different scenario, it runs a shaft which acts a bit like a spindle driven via a pulley / belt at 2.67:1 ratio. The load is pure friction on the bottom of the 'spindle', this is variable really and it's really hard to define what the max / min is. So far I've done the following:

- had an 8Nm stepper mounted, running by mistake at 480RPM. This would sometimes stall at full speed (480RPM), a lot of hit and miss, sometimes it was stalling at ramp up. I've only just realised that I was generating 3200Hz to the drive, yet the drive was set to 400 microsteps which gives 480RPM, I was really aiming for 240RPM, so I increased the amount of microsteps to 800. That made the motor super quiet and will probably bump the torque at full speed (240RPM now) significantly, but maybe not enough to get it past stalling.

- switched to a 12Nm stepper for which I haven't got a torque curve so it's hard to judge what the torque is at 240RPM. I've ran some tests yesterday with very slow ramp ups (PR2 decreased by 1 every 100ms) and the motor was stalling before it got to 240RPM. I've lowered the max speed to 180RPM, it was still stalling, went down to 120RPM and it runs fine. Looking at some videos of the motor in action, I'm judging I can probably push it to 150RPM, but anything higher than that and the torque drops off really quickly.

The acceleration 'ramp' is not linear, as I do this via modifying the PR2 register, the RPM changes vary as the speed goes up ie. the lower the PR2 gets, the higher the speed, but the bigger the impact a 'PR2 - 1' operation has. For example going from PR2 = 155 to 154 means a change in speed from 30.05RPM to 30.24RPM, going from PR2 = 40 to 39 means a change in speed from 114RPM to 117RPM. Not sure if it affects anything, the bigger motor seems to stall at the same spot every single time, but I haven't yet tried a constant RPM delta for ramp up (say 1RPM per 0.025s or similar)

Does switching from 400 microsteps to 800 microsteps decrease torque?

Regards,
T.

[Jonathan]

From the numbers you've quoted, you've tried at a very low acceleration and the motor is still stalling at a low speed. There's something seriously wrong if the motor can't exceed 150rpm - perhaps it's resonant around that speed? How does it when the motor is close to stalling? Have you checked what the corner speed for the 12Nm motor is on the drive voltage you're using?

As irving said, you can easily get a good enough approximation for the inertia of the parts from assuming they're cylinders and using the following formula:
.
Where m is the mass of the cylinder and r is its radius. That will get the inertia of the pulley connected to the rotor of the motor, but you need to scale the inertia of the parts on the driven shaft by the square of the drive ratio, so in your case they will be 2.67^2=7.11 times lower than calculated, as I think you have the smaller pulley on the motor. The total inertia of the system is therefore motor rotor ineria+motor pulley inertia+1/7.11*(the rest). By all means work it out, but currently I think you're problem lies elsewhere, as with a very low acceleration the inertia will make little difference.