Unable to set a specific velocity to a stepper with 7342

Question

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.

Answer 1

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.