stepper torque interpretation

[janwan]

I am trying to read the torque carve for stepper (pulse frequency vs torque). The curve is for half step (micro-stepping by 2), we are micro-stepping by 10.
If my pulse frequency is 5000 do I check torque for 1000 (5000/5) or for 25 000 (5000 x 5). 10 microsteps by 2 microsteps = 5.

Appreciate your help and please do not lough if it is obvious for you.

 

[MikeHalloran]

The factory torque curves for a stepping motor are made with simple driver circuits (they should be in the catalog), and exercised with simple sequencers for full step, half step, etc.

Beware #1: The torque that you can actually develop from a stepper is highly dependent on both the load _and_ the driver circuit, including which transistors you use. If you design your own driver circuits, the factory torque curves won't apply; you'll have to make your own measurements.

Beware #2: Commercial microstepping driver circuits mostly work by controlling the coil current, not the voltage. They don't look at all like the circuits that the motor manufacturers use to measure motor torque. So the motor factory's torque curves will not correlate to what you'll get with a microstepping driver. The microstepping driver manufacturer should have torque curves that will be a good approximation of what you'll get, if they used a motor that's similar to the ones you're buying.

Beware #3: Stepping motors' performance is hugely influenced by the magnetic circuits within, which vary widely from one motor design to another, and more than you'd like from one lot of 'identical' motors to another. Your best chance of getting predictable torque from a microstepped motor is to buy both the motor and the driver from the same manufacturer.

Beware #4: If your microstepping driver circuits are of your own design, well, you're really on your own. Buy the Magtrol dynamometer.



Mike Halloran
Pembroke Pines, FL, USA

 

[janwan]

Mike,
Thank you for your replay. Just to give your more background. We are using vendor's driver. The motor size 42 is lifting 15 lb making very short moves 0.76 mm in 0.15s (almost continuously).
When I contacted vendor with the same questions they told me to lower the torque ( I am reading from the curve ) by 10% due to the fact we are microstepping by 10. The other rep told me to use "my frequency" on the curve multiplied by 5.
We also make few longer and faster moves. If I multiply the faster frequency I will be of the scale big time and I should not be able to use this motor that is not the case.

 

[MikeHalloran]

I find it's less confusing to normalize all the pulse rate mumbo-jumbo back to RPM.

I.e., when you feed a X10 microstep circuit with pulses at 5000Hz, the motor is rotating at a speed that you would get by full-stepping at 500Hz.




Mike Halloran
Pembroke Pines, FL, USA

 

[worker72]

Why are you microstepping? And why with a factor of 10? I've never heard of that, mostly microstepping is done by a power of 2 (half-step, quarterstepping, 1/8, 1/16, 1/32).

My experience is that quarterstepping helps reducing sound and vibrations from the steppermotor, the motor runs smoother. I don't think 1/8 step or more helps any extra.
And take care that while microstepping raises the resolution that you work with, it doesn't improve the precision which the motor runs with. The "stiffness" of the motor around the setpoint you give it depends on the current you send through the motor, and not on stepping mode.

Furthermore, I agree with postings above: always use your own torque measurements, never trust the ones from the manufacturer. At certain frequencies there can be big fluctuations in the pull-out curve, which the manufacturer never shows.

And while I'm at it: the mechanism you drive better not have any play. Steppers don't like sudden torque variations. And in certain situations it helps to mount the motor ont the frame via some flexible (but still stiff) element.

 

[janwan]

We microstep by 10 to get 1 µm resolution. I am just checking the numbers because we are asking the system to do more than it is design for.
Appreciate your comments.
If it would be up to me I would never use it.
After converting to RPM i actually answer my question, it needs to be divided by 5 to multiply.

 

[worker72]

And what stepperdriver-IC are you using? Just curious, because I've never seen 10 fold microstepping.

 

[janwan]

We are using vendor's driver but if I give you their name I would be saying they have incompetent technical staff.
Better not

..and the last part of my previous post should be

"divided by 5 not to multiplied"