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Thread: Setting up Closed Loop Stepper Mode

  1. #13
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    Default Re: Setting up Closed Loop Stepper Mode

    Tom,

    Well after your note last night I was almost ready to throw in the towel. Seems I take two steps forward and four steps backwards. LOL

    I setup a dial indicator to see if when I issued a move it was getting back to where it started and indeed it was getting back to zero. I also adjusted the gain to .001 as you recommended and here is the latest plot.

    Not sure why the destination shows -1, but it was set to that when I started up and enabled the axis.

    Setting up Closed Loop Stepper Mode-test2-jpg

    The commanded position and the actual position are virtually right now top of each other. The green adjustment line also looks much simpler, not sure I understand exactly how to read the green line.

    Russ



  2. #14
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    Default Re: Setting up Closed Loop Stepper Mode

    Hi Russ,

    Heh heh. Don't give up you are making progress.

    The Plot now looks more stable. But it is still saturating as the output (green) is still going to 200 (right side scale) and remaining clamped there. The Output is how much correction in Motor Steps the servo is applying to try to correct the error (difference in commanded Step Position and measured Encoder Position). Do you understand this part? In other words there is an error of ~300 Steps (300 / 13970 steps/in = 0.02 inches). But with your Max Output setting of 200 you are only allowing the servo to correct 200 steps so it stops correcting the error. Maybe increase that to 500.

    I see that after the move is complete the output is at about -150. This means the servo is make a correction of 150 steps to the motor position to get the encoder to read 0 (or -1 since that is where you started). That's about 0.01 inches. Do you think your system could have that much mechanical slop?

    I'm assuming when you used the dial gauge you were running the closed loop move in the plot? That shows the closed loop correction is working as it returns to the same place even with so much mechanical slop.

    Like I asked you to do several times now I always prefer to start with open loop moves (Step Dir mode or PID gains zero) to first understand the issues with your hardware and mechanics. In open loop mode the encoder is only used as an observer so you can get insight into issues with your system. You are sort of jumping to the final result which makes it more difficult to understand as the feedback tends to hide the real mechanical issues.

    Its hard to see errors on that scale. You can zoom in or plot errors to see small errors better. Please save and attach the raw data file. That allows us to display the move data however we want.

    Hang in there you'll get it
    Regards

    TK
    http://dynomotion.com


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    Default Re: Setting up Closed Loop Stepper Mode

    Tom,
    Thanks for your feedback. Yeah, apparently these HD Harmonic drive units are generating about 10 thousands of backlash when connected to my 40mm ground ballscrews. Yes, I understand the correction was limited to 200, that must have been the default since I did not type that into the configuration tool. I have been trying to figure out a different approach as I don't like that much backlash, just very disappointing to buy those expensive HD units with 11:1 reduction and get that kind of result. I was looking to see if I had some high quality belt and gears to setup one axis using the reduction using a belt reduction. My guess is the root of all my issues started with using such rapid ballscrews 40mm pitch, these were used in expensive placement machines, but trying to use them on a CNC machine was probably not my smartest move, but ground screws in perfect condition at a reasonable price. Yes, I used the dial indicator with the correction turned on, understand your recommendation to do several open loop plots to get to the source of backlash, which I am pretty sure is the HD reduction gears.

    Russ



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    Default Re: Setting up Closed Loop Stepper Mode

    Tom,

    After two days of frustration all ready to pack it in, I came to the conclusion the Harmonic Drives must just be bad, even though they are new. This afternoon I decided to take of the Y axis drive and just spin it with no load to see if it was behaving correctly. Right as I was pulling it off I noticed the end with the coupling started to spin, which is really strange since there is a 11:1 gear reduction. Upon close inspection when they assembled this unit they forgot to install (8) 4mm x .7 screws that attach the face with the shaft coupling to the front of the harmonic drive itself. It was just a friction fit so when the motor would turn it would move the ballscrew sometimes and then it would start to slip. This explain why it was taking so much correction that never really worked. This evening I will remount the unit with the (8) new screws installed and see if it moves things better now. I have three of these units and have never got one with the screws missing, unreal. Well like you said something is WRONG. LOL

    Russ


    Setting up Closed Loop Stepper Mode-hd-img_1506-jpg



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    Default Re: Setting up Closed Loop Stepper Mode

    Tom,
    After I installed the missing 8 screws on the HD unit, I reinstalled and ran a quick test. Here is the plot.

    Setting up Closed Loop Stepper Mode-hd-fix-jpg

    If I am reading the green line correctly at the end it had to correct 52 steps is that correct?

    Russ



  6. #18
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    Default Re: Setting up Closed Loop Stepper Mode

    Hi Russ,

    Good catch. That's 3~6X better! Yes the final Output correction is ~52 steps or ~ 0.0035 inches.

    Regards

    TK
    http://dynomotion.com


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Setting up Closed Loop Stepper Mode
Setting up Closed Loop Stepper Mode