How is the NI-9505 'current sense' measured?

[BruceAmmons]

I thought people following this thread might be interested in an update.

My original plan was to do an onboard PID on the FPGA side that used current feedback to control the torque provided by the motor.  The idea was to provide a torque (current) setpoint on the RT side, then a fast loop in the FPGA would adjust the PWM value every cycle to generate the desired torque (current).  I implemented this idea and was disappointed with the results.  The torque (current) was in no way linear to the actual load on the motor.  There were offsets that depended on direction (probably friction), and the current measurement was so noisy that the control just couldn't be very fast.

I ended up abandoning current measurement altogether.  Now I just use a 1 msec loop on the RT side to output the desired PWM.  My feedback is position and speed measured using an encoder.  I found that after offsetting input PWM values to account for the dead zone around zero, it works very,  very well.  My system does not respond fast enough to benefit from a PID loop on the FPGA side.

So, after doing all this work to analyze and understand the current measurement, I went in a different direction.  It was interesting to examine the current measurement, though.  Hopefully my analysis will help somebody else that needs to accurately measure current.

Bruce

[Eric_R]

Hi Bruce

Thank you for your feedback, it was a pleasure discussing the current sense on the NI 9505 with you. I'm sorry that you encountered noise on the line, from what I can remember on our discusion the motor used was a smaller inductance than mine which would have lead to more sensitivity. Just for the record, based on the feedback of this thread I am planing on updating the PWM generation on the shipping example for the NI 9505 to simplify some of the timing adjustments and cut back on the required gates. I am happy to hear you got your system working and this does provide a work around for those with sensitivity issues on the current.

[BruceAmmons]

Perhaps I oversimplified.  It wasn't just noise in the current measurement.  There was also noise in my speed measurements, which didn't really help.  Also the fact that my system just didn't respond that fast to PWM changes.  The biggest thing was probably that the current I measured was not proportional to torque, force, acceleration, etc.  When I ran my cart in a simple sine motion, I expected the current to be a sine wave to match the acceleration.  Instead, it was a funky curve (kind of a straight line ramp) with jumps whenever the direction changed.

Once again, Eric, I appreciated the information you provided.  Without understanding the current measurement circuit I couldn't have figured out a good way to measure it.

Bruce

[dawe02]

Apology for resuscitating very old thread but I noted here people having lot of practical experience with NI-9505. I have several questions which are not explicitly answered by NI9505 datasheet.

Appreciate any help.

1. Value of RdsON of used H-Bridge (transistors) ??
2. Is there a practical LOW boundary of the DC motor PWM signal frequency? Need to use 1Khz which  is far away from default 20kHz value. Any measureble performance degradation except loud acousting noise from switching?
3. Datasheet mentiones Impedance of 600uH in the current sense section. What exactly does it mean? Is it a minimum impedance of the rotor coils + choke coils required in order to avoid ringing? Or any other reason? It is probably very simple but I can't see that yet.
4. Performance of the current sensing. Is there experience from life long term industrial usage? Especially current feedback accuracy vs. module temperature and current feedback noise figures.

Application background: it is about long term cycling under load test of automotive actuators containing small DC motor (<20W avg) . Position control only but current monitoring  as important parameter and external transducer difficult.

Thanks for any Hint. Dawe