@Venkat wrote:
Hi,
I am using PXI7340 Motion Control card with RT Controller. I am using servo motor with 35:1 gear to increase the output torque. I have two encoders (One at the motor side and the other at the load side). If I want to control motor position based on the second encoder feedback then what is the procedure for that.
Now I connected Motor side encoder as Primary feedback and load side encoder as secondary feedback. But there is no option to enter gear ratio in MAX.
Hello Venkat,
Thank you for contacting National Instruments. It is advised to use secondary feedback in situations like yours to overcome backlash due to gearing. Backlash is the delay there can be in the motion from the motor to transfer over to the load. If the encoder connected to the load is used to close the velocity loop it can lead to a delayed response in terms of correcting the velocity. Secondary feedback is used to compensate for the backlash and is used to complete the velocity loop. Secondary feedback comes from the encoder connected to the motor. Primary feedback in this case is used to correct position errors and needs to come from an encoder connected to the load.
When using secondary feedback the Kd term of the PID is set to 0 and the Kv term is used instead. You will not need to set the gearing ratio or the encoder counts per revolution for the secondary feedback since the velocity loop is only interested in the difference in steps. More information about this procedure can be found in the NI-Motion User Manual installed on your machine with the NI-Motion drivers. You can find it under Start>>Programs>>National Instruments>>NI-Motion>>Documentation>>NI-Motion User Manual or "C:\Program Files\National Instruments\Motion\Documentation\NI-Motion User Manual.pdf". Dual Loop Feedback on page 4-30 goes into more details about this procedure. Hope this helps your out. Have a nice day!
Regards,
Nipun M.
Applications Engineer
National Instruments
