7332 velocity accuracy

[ben64]

I'm controlling a stepper motor in open loop using a PCI-7332 controller. Axis 1 is set to velocity mode (pps).

When I set the velocity to 100 pps and start the motor (using the MAX interface) the indicated trajectory speed displayed is 93 pps. Now when I set the velocity to -100 pps the displayed trajectory speed is -125 pps (25% error!).

I know that the output clock is probably derived from an oscillator and that the accuracy of a specified pulse rate is depending on this oscillator frequency and the dividers used but should'nt I expect the same accuracy for CW and CCW directions? (and something better than a 25% error).

Ben

[chris.b]

Is there any chance we could see a screen shot of your 1-D interactive window when the motor is running?

[ben64]

Hi Chris, attached are the screen shots of the 1-D interactive window.

Ben

[Jochen]

Hi Ben,

I'm pretty sure, that the axis moves with the correct velocity. What you see is probably just a quantization error in the calculated velocity that is displayed in MAX.

As there is no continuous feedback signal for the velocity, it has to be calculated from the encoder counts (are you using an encoder?) or the step signal. As the calcualtions and the movement of the axis are not synchronized, you may see a significant deviation between the physical velocity and the calculated and displayed velocity.

In the Trajectory Settings in MAX you can affect the behavior of the velocity calculations by adjusting the Velocity Filter Settings (please refer to the online help for more information).

I hope that helps,

Jochen