softmotion trajectory velocity seems incorrect

[bscout]

What am I doing incorrect here?  I have been having a real problem getting the trajectory generator to behave as it should.  In order to trouble shoot this problem, I made a simple FPGA vi that is simply an IRQ, which sets the host loop rate.  The rest pretty much follows shipped examples.  I have a timer which times the completion of a straight line move.  This time is both inaccurate as well as imprecise.  Can someone look at my code and tell me if there is something wrong with it?

Bscout

PS I've also tried using a timed loop with no real improvement

[bscout]

oops, I guess the timed loop does work properly, but not the IRQ method.  This is the method used in the Softmotion Position Control example.

I thought that the FPGA timing would be very stable and accurate??

Bscout

[Brandon M]

Hi Bscout,

I took a look at your code, and the first thing that occurs to me is that there is no target loop phase offset.  You need this to mitigate errors caused by real-time jitter.  You should take a look at (and run if you get a chance) the example code StepperGeneration.  In the realtime code associated with that project, the start of the Target Loop is offset by ((Host Loop Rate / 2) + RT response time).

Because your code lacks this, it is likely that RT jitter is causing the loop rate to appear to be inaccurate.

Let me know if this helps!