Stepper Motor starting pulses 2

[zahid61]

Hi!
Would someone please answer what are starting pps(pulses per second?) parameter of a stepper motor. This parameter is different than that while it is running?
Thanks in advance,
Zahid

 

[Clyde38]

If I understand your question, you are referring to “pull in” or torque. And yes this is different than running torque. Pull in torque includes maximum load as well as minimum load capabilities. This takes into account response to friction torque, acceleration torque, and oscillation etc. and refers to the motors ability to start with out loosing synchronization.

 

[zahid61]

Thanks for the reply. This means for a given load condition the starting pulse rate will be slower than when running in normal condition. Secondly would you also please tell me if I can use a NEMA 23 size 7.5 deg/step which takes about 4 millseconds for a step? What parameter should I look for in a stepper to confirm this time and step angle? Similarly can I achieve five complete revolutions in about one second?
(this is another way of asking the same question of 4 msec. per step time)I shall be grateful for the reply.
Zahid

 

[cswilson]

I think this refers to something different. Stepper motor systems have a pretty nasty resonant-frequency problem, because the stepper motor's torque generation mechanism is effectively a virtually undamped spring, so you get a strong resonant action at a frequency of sqrt(k/J), where k is the effective spring constant, and J is the total moment of inertia of motor and load. (This occurs even with a perfectly stiff coupling, by the way. A compliant coupling can introduce a second resonant frequency.)

If you input a step pulse frequency to the drive at around this resonant frequency, the motor can oscillate pretty wildly and lose synchronism. This can even occur if you are ramping the pulse frequency through this resonant frequency.

For this reason, some stepper indexers and smart drives permit you to set a starting frequency, so you can start above the problem frequency. Rather than ramping to full speed from zero, as a servo system would typically do, there is a velocity step to this frequency followed by a ramp to the full frequency of the move.

Of course, the velocity step creates its own problems, as it is a request for infinite acceleration, so you have to be careful with this.

Curt Wilson
Delta Tau Data Systems

 

[zahid61]

Thanks dear Curt for the post. You say I start like this, the problem frequecy is say, 100 Hz, I apply 200 Hz then the ramping increase from 200 Hz to say 1000 Hz at full speed?
What effects the resonance of the motor have on the driver?
sincerely,
Zahid

 

[sreid]

My understanding of the starting PPS is that the limit is based on the initial angular acceleration without pulling out of step. The limit is determined by the step motor torque and inertia.

The full speed PPS is limited by the ability to drive current into the step motor phase inductances.

The spec numbers are usually no load numbers and often reflect the best case situation (salesman numbers). The performance in a system is usually )a lot) lower.

 

[itsmoked]

zahid61; No problems with ramping thru a region only operation in the region.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[cswilson]

Zahid -- You understand it as I understand it (but remember that I am a servo jock, not a stepper jock).

As to the effects of motor resonance on the drive itself, they should be minimal to non-existent. A stalled motor will have much less back EMF at the terminals, and you won't have a consistent pattern of inductance changes, but these won't really affect the drive.

The key issue is the motor's inability to follow command sequences when the pulse rate is near the resonant frequency.

Curt Wilson
Delta Tau Data Systems

 

[zahid61]

Thanks indeed Curt for continued discussion.
Zahid

 

[cswilson]

itsmoked: I believe the entire reason for a "starting pps" parameter is that you can get into trouble just ramping through a resonant frequency, so you set the starting pps frequency above the problem frequency.

Curt Wilson
Delta Tau Data Systems

 

[itsmoked]

I don't see how you will get into resonance if you aren't exciting a system in its resonance region but I would certainly differ to you Curt in this case.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[zahid61]

That's what I say, start above the resonance frequency so you donot ramp through, Curt?

 

[cswilson]

itsmoked: While I don't have that much direct experience with these types of low-end stepper systems, my understanding is that even the brief time you spend in the region of the resonant frequency as you ramp from lower to higher frequencies (or the other way) can get you in trouble. Remember that the resonant spike of the system has "width" on a frequency plot. Again, if this were not a real problem, I don't think there would be "starting pps" parameters in these drives.

Thinking of the issue in the time domain: Because there is almost no damping in the motor's response to a step, one period of the resonant frequency after the step command is given, you will be almost back to the starting position. In a full-step system, if you try to issue the next step at this time, you will have virtually zero torque. One step like this, and you could lose sync and not recover.

By the way, this is one of the key problems that microstepping was invented to solve. In my opinion, the fact that microstepping solves this problem is probably more significant for most applications that the increase in final positioning resolution.

Curt Wilson
Delta Tau Data Systems

 

[sreid]

I undrstanding is that if you slew through the resonance reasonably fast, mis-stepping is usually not a problem.

At one time I worked on a floppy drive tester. One test was to do track seeks in which you could specify how many tracks to jump and how many times. All drives would mis-step if you found the found the correct frequency and did it long enough. And the tester had a Random Mode just to find the combination to make this happen.

 

[MikeHalloran]

An ideal stepper would be able to start any load at its maximum slewing speed. There ain't no such animal.

The starting pps setting on a controller is there so you can start the motor moving at a speed at which it will reliably move the load without losing steps, and ramp up smoothly from there. For a size 23 stepper and a load you can hold in your hand 100pps works okay as a starting point.

With a controller you can make from something primitive like an 8048, you can ramp an average load by changing the timing by about 3 pct per step, and not much more. With dynamic analysis of the _exact_ load you intend to move, much faster ramp rates are possible. (The analysis basically uses the ringing from step N as the start of step N+1). Google Taft, Gauthier, University of New Hampshire)

There's not much damping in the motor or mechanical parts of the drive, but there's often damping in the circuit that drives the motor coils... that circuit has a huge effect on motor performance, so motor comparisons should all be made with the same circuit.

The other reasons for starting at a (mechanically high) pps like 100Hz are to save time, and to avoid exciting the building you're in. Ramp a stepper up at 3pct per step from 1 Hz, and you'll excite the building as you pass near 4 Hz, getting everyone's attention. Don't believe me; you'll be locally famous for a while.



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

Shades of Tesla.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com