Unexplained positioning in KmotionCNC 4.34H


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Thread: Unexplained positioning in KmotionCNC 4.34H

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    Default Unexplained positioning in KmotionCNC 4.34H

    Tom,

    I have a very strange problem on my machine, which is running 4.34H and has been flashed accordingly. I was running a drilling program the other day and noticed the holes were being placed in the wrong spots. The simple program listed below drills 9 holes from the front of the machine towards the back of the machine. Then it moves over .1" and drills those holes coming back towards the front of the machine, then goes back the other way drilling nine more holes, and finally comes to the front of the machine and goes home.

    Photo-P1 shows the four holes at the first position
    Photo-P6 shows what the four holes look like at P2,P3,P4,P5,P6,P7,P8
    Photo-P9 shows the four holes at the last stop and they are all perfectly in line which is what was supposed to happen at each position.

    My machine has a Panasonic Servo driving a Harmonic Drive with 11:1 reduction, using a ground ballscrew on the Y axis. I have the exact same setup on the X axis. I am having the Panasonic Amplifiers close the loop with the Servos and they have been tuned with the Panasonic software. At first I figured I must have some form of backlash, but I have checked the harmonic drive and can turn those 80 turns under Kmotion and with a reference line marked after rotating both direction they always come to rest at the reference line as they should. I then check my ground ballscrew and nut and can not detect any backlash with a dial indicator. The next thing I did was adjust the Velocity and Acceleration and slowed things up thinking some how I am maybe losing steps. Well no errors on the amplifier and I attached the step response curve. Next I modified the program listed below and changed the G0 command on line N106 to G1 F30, thinking perhaps at G0 speed something is getting screwed up. The exact same results.

    I have also attached my init.c program, any clue what might be wrong? I am pulling out my hair I can not find anything wrong with the machine.

    Russ



    Unexplained positioning in KmotionCNC 4.34H-p1-pngUnexplained positioning in KmotionCNC 4.34H-p6-pngUnexplained positioning in KmotionCNC 4.34H-p9-png

    Unexplained positioning in KmotionCNC 4.34H-ys-png

    Unexplained positioning in KmotionCNC 4.34H-tajectory-jpg



    %
    O0000(CNC-SUPPORT-COLUMN-3)
    ( T1 | Very small drill bit for testing | H1 )
    N100 G20
    N102 G0 G17 G40 G49 G80 G90
    N104 T1 M6
    N106 G0 G90 G54 X.1 Y1. S6000 M3
    N108 G43 H1 Z.1
    N110 G99 G81 Z-.035 R.1 F5.
    N112 Y6.2813
    N114 Y11.5625
    N116 Y16.8438
    N118 Y22.125
    N120 Y27.4063
    N122 Y32.6875
    N124 Y37.9688
    N126 Y43.25
    N128 X.2
    N130 Y37.9688
    N132 Y32.6875
    N134 Y27.4063
    N136 Y22.125
    N138 Y16.8438
    N140 Y11.5625
    N142 Y6.2813
    N144 Y1.
    N146 X.3
    N148 Y6.2813
    N150 Y11.5625
    N152 Y16.8438
    N154 Y22.125
    N156 Y27.4063
    N158 Y32.6875
    N160 Y37.9688
    N162 Y43.25
    N164 X.4
    N166 Y37.9688
    N168 Y32.6875
    N170 Y27.4063
    N172 Y22.125
    N174 Y16.8438
    N176 Y11.5625
    N178 Y6.2813
    N180 Y1.
    N182 G80
    N184 M5
    N186 G91 G28 Z0.
    N188 G28 X0. Y0.
    N190 M30
    %

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  2. #2
    Gold Member TomKerekes's Avatar
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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Hi Russ,

    If I follow what you are doing correctly and the starting Y position is less than 1.0 then the pattern would be exactly like what would be caused by backlash.

    Where and how did you check the backlash? I didn't understand what the 80 turns and reference mark were referring to.

    Could the drive have a deadband setting?

    I don't see where you attached your Init.c program.

    What are your KFLOP Axis settings?

    I notice crazy Velocity settings in the Trajectory Planner 250 inches/sec. But as long as low feed rates are used that shouldn't matter.

    Acceleration Settings are also quite high. Higher than what is specified in KFLOP.

    You might consider connecting the encoder to KFLOP so you can observe and plot what the Drive is doing.

    Regards

    TK
    http://dynomotion.com


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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom,
    The 80 turns was the servo motor driven harmonic drive. I put a reference mark on a table bolted the assembly to the table put a piece of tape on the harmonic drive lined up with the reference line on the table. Then using kmotion told it to move80000 or whatever it would spin manyt turns left and then the same number right when it was done it land on the reference line on the table. I figured that was an indication the HD was not the issue.

    Tonight I put a 24 inch digital scale on my table and had my spindle push the scale. I told kmotioncnc to move the y axis 20 inches repeatedly at different free rates. The scale shows anywhere from 19.991 inches to 20.013 inches. The scale was not physically attached to spindle but this seems to also indicate backlash. These HD drives are supposed to have next to no backlash.

    Right now kflop is just sending step and direction to the servo amps. I could reprogram and put it in velocity mode and have kflop close the loop but if the HD units have backlash that will not help

    Russ



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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom the settings in velocity planner were probably the default setting that came in kmotioncnc, not sure if any of the setting are helping compound the issue.

    Russ



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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom,
    Here is the init file, I thought it attached early guess not.

    Russ

    Attached Files Attached Files


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    Gold Member TomKerekes's Avatar
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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Hi Russ,

    Sorry I still don't understand your Index Mark 80 rev test. Maybe a picture would help. Are you saying you have an 880X reduction from the motor to the leadscrew? What is the pitch of the leadscrew? How many steps/rev is the Amplifier?

    To check backlash you would need to approach a position from the left and later from the right. Your description only describes going left and then back to the right. That wouldn't necessarily test backlash as we don't know what direction it came from originally.

    What is the scale of the photos? Approximately how much is the backlash indicated by the drilled holes?

    The init file seems to be similar but conflict with the screen shot settings you sent.

    I don't think the default values in KMotionCNC are 250 inches/sec

    Regards

    TK
    http://dynomotion.com


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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom,
    Sorry I was trying to type this stuff on an ipad while watching TV. LOL

    Let me attempt to explain my poor man test of the servo and harmonic drive assembly.

    The components:
    Panasonic 200W servo motor, MSM021A1E max speed 3000 RPM
    Panasonic 200W servo amp, MSMA021A1E
    Harmonic Drive 15:1 reduction HD CP-20-11
    Ground Ballscrew: Pitch 40mm

    Calculations:
    Servo motor has an encoder that will produce 2500 p/r, however I have electronic gearing enabled so it is now 2000 P/R

    2000 P/R * 11:1 Reduction = 22,000 Pulses/Revolution

    40mm/22000 = 25.4/X Means it takes 13,970 Pulses/Inch

    What was my poor man test? If you take a servo motor and let kmotion run a step response, and give it a step size of 80,000 over 3 seconds, the motor will spins many times to the right and then many times to the left. What I did was clamp the servo/HD assembly to the bench, and laid a piece of blue tape on the bench with a straight black line on the tape. Now I added another piece of blue tape to the end of the harmonic drive with another black line. These two black lines are lined up exactly prior to the test. If you spin the motor with 22,000 pulses the Gearhead will turn exactly one revolution. Now if the gearhead had a good deal of backlash when you spin it in the opposite way it would most likely not line up on the black line again. Now with the step response situation you are spinning the motor many times in each direction and it always comes back to the black reference line on the tape on the table. This means it is always find the same start position.

    I understand backlash, if you move an axis with ballscrew and motor and move it 15 inches, then reverse it should come back exactly 15 inches, not short or long. My test above did not test the ballscrew only the motor harmonic drive assembly.

    I slept on this last night and actually seems to me this is really acting like overshoot and undershoot, based on my 24" digital scale test last night.

    The amplifier can self tune the motor and close the loop and I have been running this on my other machine for 15 years and can hold tolerances of .001. The holes in the picture are off very close to .100 which is a huge amount for backlash. My ballscrews are ground and when I hook up a dial indicator you can detect no backlash.

    I am wondering since I did not set anything except steps/unit in the trajectory my velocity and acceleration are set so high that it can't stop in time? My gantry weight close to 300 pounds with the 7.5HP spindle motor, with huge linear bearings THK SHS45

    I will play some more today.

    Russ

    Last edited by CNCMAN172; 03-24-2017 at 07:56 AM. Reason: typo


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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom,
    Well I ran a test this morning, you are correct I had set the velocity and acceleration in KmotionCNC on the trajectory tab. Where I screwed up is I did read read closely and figured that would be in Inches/Minute not Inches/Sec, same for acceleration. I set the Velocity to 10 in/sec and Acceleration to 3 in/sec2. Then I ran the 24" digital scale test, the distance was always within 1 or 2 thousands at various feed rates. I figured wow this is the answer, but then I ran the drilling test and got the exact same results. This seems to suggest the HD is causing backlash. The next test is to attach a servo motor directly to the shaft the same way I have my machines down stairs, without a harmonic drive. The reason I ordered the harmonic drives is I have ballscrews with a fast pitch 40mm, if connected to the servo motor directly the motor will need to run pretty slow, since the machine is only 5'x5'

    Wow this is really disappointing the HD brand harmonic drives are supposed to have like 2 arc/sec of backlash next to nothing.

    More to follow:

    Russ



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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom,
    Another question, on the motion profile. The following values is what is populated in my init.c This is position units per second based on the definition on the dynomotion site. What are the position units? Is this pulses allowed per second? Trying to make sure I have this set correctly.

    Russ


    ch1->Vel=95000;
    ch1->Accel=400000;
    ch1->Jerk=4e+006;



    The Motion Profile parameters set the maximum allowed velocity (in position units per second), the maximum allowed acceleration (in position units per second2), and the maximum allowed jerk (in position units per second3). These parameters will be utilized for any independent (non coordinated motion) move command for the axis. The acceleration and jerk also apply to jog commands (move at continuous velocity) for the axis.



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    Gold Member TomKerekes's Avatar
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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Hi Russ,

    What was my poor man test? If you take a servo motor and let kmotion run a step response, and give it a step size of 80,000 over 3 seconds, the motor will spins many times to the right and then many times to the left. What I did was clamp the servo/HD assembly to the bench, and laid a piece of blue tape on the bench with a straight black line on the tape. Now I added another piece of blue tape to the end of the harmonic drive with another black line. These two black lines are lined up exactly prior to the test. If you spin the motor with 22,000 pulses the Gearhead will turn exactly one revolution. Now if the gearhead had a good deal of backlash when you spin it in the opposite way it would most likely not line up on the black line again. Now with the step response situation you are spinning the motor many times in each direction and it always comes back to the black reference line on the tape on the table. This means it is always find the same start position.
    I think I understand. This results in position zero being approached going negative and position 22,000 going positive so backlash would show two different angles. This is a pretty good test but I would have performed a move to some negative position and returned to zero and then to some positive position and returned to zero. This method eliminates gearing, scaling and so forth influencing the result. But I still think your test is conclusive.

    Wow this is really disappointing the HD brand harmonic drives are supposed to have like 2 arc/sec of backlash next to nothing.
    Yes that seems very strange. To get 0.1inch backlash on a 40mm pitch leadscrew would require like +/-11 degrees of slop. Hundreds of times more than that.

    ch1->Vel=95000;
    ch1->Accel=400000;
    ch1->Jerk=4e+006;

    The Motion Profile parameters set the maximum allowed velocity (in position units per second), the maximum allowed acceleration (in position units per second2), and the maximum allowed jerk (in position units per second3). These parameters will be utilized for any independent (non coordinated motion) move command for the axis. The acceleration and jerk also apply to jog commands (move at continuous velocity) for the axis.
    So divide by your 13,970 Pulses/Inch to convert to inches

    Vel = 95000 Pulses/sec / 13,970 Pulses/Inch = 6.8 inches/sec
    Accel = 400000 Pulses/sec2 / 13,970 Pulses/Inch = 28.6 inches/sec2
    Jerk = 4e+006 Pulses/sec3 / 13,970 Pulses/Inch = 286 inches/sec3

    HTH
    Regards

    TK
    http://dynomotion.com


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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    Tom,

    Thanks for the feedback. I actually used a digital scale which is 24" long and after adjusting the velocity and acceleration in the trajectory tab, I can move the Y axis 20 inches and it was with one or two thousands at most. When I returned it always came back to zero. This was a second pretty good test that I should not be dealing with backlash? I then tried to attach a brand new Panasonic 400W Servo and Amp directly to the ballscrew to see how the gcode would run. The problem I ran into was jerky movement due to the massive 300+ pound gantry. With the fast 40mm ballscrew it would move but was kind of jerky on start until it got the load moving and then it would continue. Clearly I will need a gear head or different ballscrews which both cost a fortune.

    I am looking elsewhere to see if I have something else set wrong, the gcode program simulates perfect in kmotioncnc and also in Mach4, so the program appears to be correct.

    I am really scratching my head on this issue.

    Russ



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    Default Re: Unexplained positioning in KmotionCNC 4.34H

    With the servo enabled and stationary, have you tried to turn the ballscrew? You may want to try with a suitable spanner/pair of pliers, and see if you can get any movement.
    11 degrees is a lot of movement, and if it was the HD, it should be obvious.

    You have gone back to basics, and checked for any other mechanical issues?
    Given everything good push/pull to make sure nothing is broken/loose, checked any couplers are in good condition and tight on their respective shafts?



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Unexplained positioning in KmotionCNC 4.34H
Unexplained positioning in KmotionCNC 4.34H