PCI-7340 won't achieve commanded speed

[crweiss]

I am trying to command a speed of 3700 RPM at a rate of 60rpm/sec. The speed seems to max out at 500 RPM. I can reach higher rpms at faster ramp rates.

Max speed seems to be related to the acceleration rate.

I have replaced the motion board and I have the same problem.

NI Software : LabVIEW version 8.2

NI Hardware : Motion Control device PCI-7340

Driver Version : 1.11.1f0

OS : Windows XP Customer Information :

[Jochen]

Before I can give you an advise I need to know if you are using a stepper or a servo motor and how many axes you are using of your 7340. Is it a 2 or 4-axes board?

Thanks,

Jochen Klier
National Instruments

[crweiss]

It is a servo motor.  The 7340 is a 4-axis board and I am using 3 out of the 4 axis.

Thanks

[Jochen]

Ok, I see. So with this setup there is no good option to trick around this issue. Here you can find a good explanation for the effect that you are seeing.
Please note, that you could double the maximum velocity that you could reach by doubling the PID period (e. g. by increasing the PID period from 250 µs to 500 µs). You will need to be very careful with this parameter, as changing the PID period results in a significantly different behavior of the control algorithm and you will have to retune the PID controller.

A better option to increase the maximum velocity dramatically is to increase the deceleration value moderately as described in the knowledge base document that I have linked above.

I hope that helps,

Jochen

[crweiss]

Jochen,

Thanks for looking onto this.  I have tried changing the PID period, and successfully doubled the maximum velocity, by retuning the PID controller.  Unfortunately, this still doesn't get me where I need to be.  How do you decrease the deceleration value?  I haven't been able to find it?

[Jochen]

I'm referring to this formula:

velocity < (32,767 x deceleration) - acceleration

As you can see, the value for the deceleration has a 32,767 times higher impact on the maximum velocity than the acceleration value. Load Acceleration/Deceleration.flx in LabVIEW or flex_load_acceleration() in text based programming languages allow you to set different values for acceleration and deceleration seperately.
So you could leave the value for the acceleration as it is and e. g. almost double the maximum velocity by increasing (not decreasing!) the deceleration value by the factor of 2.

I hope this solution works for you.

Jochen