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Tuning servo motor problems
Synder
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02-04-2004 01:55 PM
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I have a PCI 7344 connected to a UMI and a Ecodrive amplifier, with a linear modul and servo. Feedback is a incremental encoder 16404counts/rev (400counts/mm). When i try to autotune, the servo motor goes berserk, so I have to manually tune it with "Step response".
I am able to get a nice response curve according to "Understanding servotune" but the problem is, curve starts at -50mS? and at 50mS it seems stable, but settling time is indicated to be 500 - 600 mS, and when i use "wait for move complete.vi" it waits about the same period before it returns "move complete". I was expecting/hoping for 50 - 100mS settling time. How to tune this? I run position mode and there is no problem regarding velocity, acceleration and position feedback, the only
thing is that settling time is to long. I have no following error. Is there any other way to get a signal exactly when a position is reached.
I am able to get a nice response curve according to "Understanding servotune" but the problem is, curve starts at -50mS? and at 50mS it seems stable, but settling time is indicated to be 500 - 600 mS, and when i use "wait for move complete.vi" it waits about the same period before it returns "move complete". I was expecting/hoping for 50 - 100mS settling time. How to tune this? I run position mode and there is no problem regarding velocity, acceleration and position feedback, the only
thing is that settling time is to long. I have no following error. Is there any other way to get a signal exactly when a position is reached.
JuliaD
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02-05-2004 06:28 PM
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Hello,
To address the issue of settling time, I would try to play around with the Kp, Kd and Ki parameters. First off, set Ki and Kd both to 0 and set Kp to a small value (e.g. 20). What you should see is the response plot will go up and flat line below 0 (undesirable). Keep increasing the Kp value a little at a time until you reach the threshold value (i.e. when the response is oscillating). For example, let's say this happens at a Kp value of 100. At this point, keep the Kp value the same and then change your Kd value. You should start out with a Kd value that is at least two times the Kp value (in this example you would start out with Kd=200). At this point, you want to keep increasing the Kd value a little at a time until you reach the desired settling ti
me.
NOTE!
Make sure that after each change you test the step response by clicking on "Apply" (NOT "Apply All" - this will apply the changes to ALL axes) and click on the "Step Response" button (to actually obtain the step response for the current settings). These can be found under the "Calibration" folder for your 7344 in MAX.
I think this should help you out. Have a great day!
Regards,
Julia D.
Applications Engineer
National Instruments
To address the issue of settling time, I would try to play around with the Kp, Kd and Ki parameters. First off, set Ki and Kd both to 0 and set Kp to a small value (e.g. 20). What you should see is the response plot will go up and flat line below 0 (undesirable). Keep increasing the Kp value a little at a time until you reach the threshold value (i.e. when the response is oscillating). For example, let's say this happens at a Kp value of 100. At this point, keep the Kp value the same and then change your Kd value. You should start out with a Kd value that is at least two times the Kp value (in this example you would start out with Kd=200). At this point, you want to keep increasing the Kd value a little at a time until you reach the desired settling ti
me.
NOTE!
Make sure that after each change you test the step response by clicking on "Apply" (NOT "Apply All" - this will apply the changes to ALL axes) and click on the "Step Response" button (to actually obtain the step response for the current settings). These can be found under the "Calibration" folder for your 7344 in MAX.
I think this should help you out. Have a great day!
Regards,
Julia D.
Applications Engineer
National Instruments
02-06-2004 03:14 PM
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Hi, thank you for your answer.
I still have problems with this, so here is some more questions.
1. I am getting a perfect curve on the step response, similar to attached picture (1), only with shorter time scale. The step response report under the chart that settling time is 500-600mS, which is settling time (<100mS) and steady state (>400-500mS) together. This period seems to be the same that i am experience with "move complete.vi".
I have Kp=7 Kd=14 and Ki=1, and i have tried to adjust these values but this is the best i get.
Does the loop need that time to determine that move is complete?
2. The move i want to do in my application is a movement about 20000 counts (50mm) in 130mS, wait 100mS and return to start in 130mS, this is possible if I use
software delay and start return move before first move is settled according to "move complete.vi" But this generate a increasing stop position error each move if I loop this sequense with 100mS rest time. These timings should be possible with the hardware I am using. Any suggestions?
3. I have set the driver/amplifier in velocity mode (incremental encoder feedback), how is that compared to torque mode in the driver in this case?
4. It is possible to get a second feedback from the drive (torque, velocity or position from an analog output) but this is 8 bit and less accurate than encoder. I also have a second external encoder for precision measurement and a force cell, but they are only active parts of the range to the servo (under the desired move) is there any other way i could set this up?
Regards
I still have problems with this, so here is some more questions.
1. I am getting a perfect curve on the step response, similar to attached picture (1), only with shorter time scale. The step response report under the chart that settling time is 500-600mS, which is settling time (<100mS) and steady state (>400-500mS) together. This period seems to be the same that i am experience with "move complete.vi".
I have Kp=7 Kd=14 and Ki=1, and i have tried to adjust these values but this is the best i get.
Does the loop need that time to determine that move is complete?
2. The move i want to do in my application is a movement about 20000 counts (50mm) in 130mS, wait 100mS and return to start in 130mS, this is possible if I use
software delay and start return move before first move is settled according to "move complete.vi" But this generate a increasing stop position error each move if I loop this sequense with 100mS rest time. These timings should be possible with the hardware I am using. Any suggestions?
3. I have set the driver/amplifier in velocity mode (incremental encoder feedback), how is that compared to torque mode in the driver in this case?
4. It is possible to get a second feedback from the drive (torque, velocity or position from an analog output) but this is 8 bit and less accurate than encoder. I also have a second external encoder for precision measurement and a force cell, but they are only active parts of the range to the servo (under the desired move) is there any other way i could set this up?
Regards
JuliaD
Member
02-09-2004 02:57 PM
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To answer your questions, I think it would be better to know more about what you are trying to achieve with this application and how you have things set up. As such, it would be best to go over these questions via the phone. To do so, you can Request support online via the following:
Request Support
Note: you can access the Request Support link from the main ni.com page (the last link in the grey "Support" section on the left-hand side of the page).
Be sure to select "Phone NI" as your "Communication Method". If you reach another Applications Engineer in the Motion group, just reference this Developer Zone activity and my name and I will work with you. Have a great afternoon!
Regards,
Julia D.
Applications Engineer
National Instruments
Request Support
Note: you can access the Request Support link from the main ni.com page (the last link in the grey "Support" section on the left-hand side of the page).
Be sure to select "Phone NI" as your "Communication Method". If you reach another Applications Engineer in the Motion group, just reference this Developer Zone activity and my name and I will work with you. Have a great afternoon!
Regards,
Julia D.
Applications Engineer
National Instruments
