unable to set a specific velocity to a stepper with 7342

[Giana Nicolini]

Hi all,

my system is a PCI-7342 driving two stepper motors via an UNI-7774 in open loop.

The stepper(s) moved at the first try without problem, but...

Last week I was tuning the system with MAX, trying to find a specific velocity I need around 70 step/s.

With my surpise I noticed that it is not possible to vary at wish the velocity.

I was varying the velocity of few step/s but the effective velocity did not change with continuity.

In fact, the only two real speeds I've obtained are 63 step/s (for vel set between 50 and 70) and 79 step/s (when set above 71 step/s).

Is this a limitation of my setup or something I can solve by tuning some contrlol parameter?

Some info about the system:

OS: Win XP Home

Ni- Motion 7.4.0.3002

Max 4.1.0.3001

Board: PCI-7342 (but in max it says 7340)

F/W: 68331: 7.31.3003

    DSP: 7.20.3001

   FPGA1: 7.00.3001

   FPGA2: 7.00.3001

Thanks in advance,

G.

[Brian_Beal]

What drive and motor are you using, and how are you measuring velocity?

[Giana Nicolini]

I'm using an RS motor (440-464) driven with a Superior Electric "SLO-SYN 230 T" translator drive modulein step/dir mode.

It is the motor of the alpha axis of a telescope.

I've made a brand new telescope control system  with labview but I had to keep the existant motors and encoders.

I get the feedback about the velocity in two ways: with a star (qualitatively) and via max. I cannot use the telescope encoder because there is a ratio of about 60 beween the motor steps and encoder counts.

I've noticed the problem because with a velocity of 79 I was a bit faster than the sidereal motion and even decreasing via Labview the speed it remained faster. Keeping to decrease the velocity in small amount, the telescope became discontinuosly slower (velocity of 63) than the star i the.

Then I switched to max and I observed the same behaviour.

G.

[Brian_Beal]

Are you saying that if you run this from MAX, and you set a certain speed, that it shows it moving at a different speed?  If you are running open loop, then MAX is just reporting the steps it is (or thinks it is) sending out.  The reported velocity in MAX can will only be from the motor encoder if you are running closed loop.

[Jochen]

When reading this thread another question came to my mind: How do you load the velocity value? In RPM or on counts/s? Have you tried both methods? The velocity resolution should be better if you load the value in RPM.

By the way, you should be able to use the encoder, even if it has a resolution ratio of 60:1 to the stepper resolution. You just need to set the correct values of stepper steps/rev and encoder counts/rev in MAX, but in terms of velocity control this shouldn't make a difference.

I'm not really sure, what's going on here. At such low velocitis the resolution should be better than 1 step/s, but just as an idea: Is there a way to increase the microstepping value of your drive?

Regards,

Jochen Klier

National Instruments

[Giana Nicolini]

First of all, thanks for the replays.

I try to answer to your questions.

@Brian: With MAX in the 1-D interactive panel, I set the movement mode in velocity, I load the desired steps/s and start the motion. Then, in the same panel, I read the velocity.

In open loop this is, I think but I'd like to have it confirmed, the frequency of the step/colck wavevorm produced by the controller, still at digital level. Is it correct?

@Jochen:

- Up to now I've used only counts/s. But from your question I understand that while I cannot set 45.5 counts/s, I can set a 3.35 RPM achieving thus a better ( < 1step/s) velocity resolution. Again, is it correct?

- About the encoders, at such a speed, I get about 1 count/s. I'm working on an "hystorical" telescope and the mechanics+encoder system presents some periodic errors which are probably larger than just having the motor in open loop. And I wuold be happy, in this phase of the work, simply have a fine tunable open loop system.

- Microstepping: the drive is not easily accessible but by harth i don't remember a microstepping control. I've to check

BTW, should I try to measure the step signal frequency directly at UMI-7774 level with and without motor? Update max and motion to a more recent version? I other words do you think I should investigate firts the digital or the analogic side?

Thanks again,

G.

[Jochen]

Giana,

you are right. If you load the velocity in RPM, you could set velocities with fractional counts/s values. Still there are of course physical limits concerning the velocity resolution (please refer to the NI-Motion documentation).

I'm not aware of a major change in the step generation algorithm between the current NI-Motion version and the version that you are using, but if it turns out, that we need R&D involvement for your request, the first suggestion will be to upgrade to the latest version of NI-Motion, so this is recommended.

If you want to measure the step frequency directly, you can do this at the UMI, but you don't need to connect the motor drive during the measurement, as in open loop mode it doesn't make a difference for the step generation, if the drive is connected or not.

Jochen

[Giana Nicolini]

Just an update:

I mange to measure the clock frequency with a scope and it is possible to fine tune the stepper velocity.

But max still shows the false not continous value in the velocity box.

I'm going to update NI-motion and max as well

Thanks for te moment,

G.

[Jochen]

Giana,

what you see is probably expected behavior. Please refer to this thread (last post) for more information about quantization errors when reading the velocity with NI-Motion.

Jochen