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Procedure for Tuning axes to reduce Following Error


Raghav
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Dear All,

I'm facing situation in which I'm unable to achieve a particular machining surface output. We are using PMAC Turbo2 for our diamond turning machine. The contour toolpath is fine and the achieved surface is acceptable. But in the case of Flat facing, there are surface irregularities which causes rainbow pattern in the surface. From the surface measurement, it is found that there are deep machining marks on the surface. Apparently it is evident that it could be caused due to the following error of the axes. The error value is close to 20 cts, which is around 100nm for RAMP test. Can anyone suggest how to overcome this issue? Can a user-servo be implemented to eradicate this issue? Thanks in advance.

 

Attached are the motor gather data for the two axes. The FE (time based and FFT) at the static position and while homing from the extreme positions are captured. Similarly the servo cmd is also captured for the same. Kindly provide me some solution to solve this/ understand the following error plot. Any document to explain this will be appreciated.

 

Regards,

Raghav

X_axis_static.JPG.0c9f48a9b4dd824a3f584f20ce66d849.JPG

X_home_time.jpg.0554ad427971ec8c67e72b9870fd9046.jpg

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Z_home_time.thumb.jpg.a5465b1b755c5fa13f1762d60720f422.jpg

Z_home_FFT.thumb.jpg.dc189f48607efb030be2fef99e0a9445.jpg

Z_Static_FE_Servo.jpg.2f8c86fdd46fbd69ea6c9077905bef69.jpg

Z_Static_FE_Servo_FFT.thumb.jpg.6d52c89a87fb0ea2ae494338ab5e1440.jpg

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Unfortunately this is a very broad question that must first be addressed at the system level. As a very generic answer, your system has inadequate loop gain at the frequencies where the disturbances are showing up in the FFT's above. This could be because the gain of the transfer function of the controller is lower than it could be (i.e. system bandwidth is unnecessarily low), or it could be that the disturbances (i.e. vibration, electrical noise, etc.) are of such an amplitude that they result in unacceptable performance. Without knowing about the architecture of the machine one could only guess what is limiting performance.

 

I do see that you have large errors at approximately 7.5 Hz, which is in a frequency range that could be vibration due to ground motion which is being transmitted into the base of the machine. You may be able to improve this with increased loop gain at 7.5 Hz, which you might be able to achieve by increasing the proportional gain (assuming that the stability limit of the loop has not already been reached).

 

Given that he problems are at reasonably low frequencies, I seriously doubt you need a more complicated (i.e. more poles and zeros) controller, you likely just need to increase the bandwidth of the one you have. State of the art diamond turning machines achieve nanometer level surface finish with simple PID controllers routinely.

 

If you could give a more complete description of the machine elements then I could be more helpful.

 

Lastly, it takes a very good machine to do quality diamond machining. The controller can not overcome inadequate performance of the electromechanical systems.

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I agree with Brian in regards of the low frequency characteristic. What is the natural frequency of each of the axis? Usually shown, in the tuning software, under the step response statistics.

 

Can you list Ixx30 through Ixx39? Also clock settings, and segmentation time Isx13? Are you using lookahead?

 

Now that you have a spec on the actual following error (say +/-20 counts), what would be an acceptable flat facing machining error? Were the axes tuned with such tolerance in mind?

 

What kind of feedback devices do you have? and what kind of mechanical couplings?

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As Brian said, the 7.5 Hz points to floor noise. At these low frequencies, increased integral gain is also effective. You don't mention feedback, but interpolation errors can also be a disturbance source. To test this raise or lower feedrate by 10% and look at the FFT again.

If Delta Tau has a 30 day trial version of the Servo Analyzer you can get a better idea of the loop by looking at the Bode plots.

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As Brian said, the 7.5 Hz points to floor noise. At these low frequencies, increased integral gain is also effective. You don't mention feedback, but interpolation errors can also be a disturbance source. To test this raise or lower feedrate by 10% and look at the FFT again.

If Delta Tau has a 30 day trial version of the Servo Analyzer you can get a better idea of the loop by looking at the Bode plots.

 

Thank you Jeff. The response seems to be same.

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I agree with Brian in regards of the low frequency characteristic. What is the natural frequency of each of the axis? Usually shown, in the tuning software, under the step response statistics.

 

Can you list Ixx30 through Ixx39? Also clock settings, and segmentation time Isx13? Are you using lookahead?

 

Now that you have a spec on the actual following error (say +/-20 counts), what would be an acceptable flat facing machining error? Were the axes tuned with such tolerance in mind?

 

What kind of feedback devices do you have? and what kind of mechanical couplings?

 

Hi Richard,

The following are the basic servo configuration

I130=6000

I131=11550

I132=12100

I133=3000

I134=1

I135=-700

I136=0

I137=0

I138=0

I139=0

 

I5113=5

I5120=23

 

&1 DEFINE LOOKAHEAD 1000,500

 

These are the machine parameters. The surface finish we achieve is within the specifications but there is a non-periodic circular marks which spoils the aesthetics of the machined components. So we are trying to get rid of this. Since there could be many machining parameters which affect this, we also try tweaking them. But FE is one of the important factor for non-periodic surface normal errors we wanted to fine tune the axes to minimize its effect.

 

Hope with these information, you could suggest some fine tuning of FE

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Another thing that would be helpful for me would be to tell us what the current Ixx30 settings are, and then tell me how high you can raise Ixx30 before the loops start to go unstable.

 

The maximum Ixx30 which I could increase to is around 7200 after which there is a strong vibration sound at certain locations in the axes.

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Please accept that I would be trying to tune the axis "blind" as I don't know what frequency the slide resonates at when it goes unstable, and I don't know anything about the mechanics, but assuming that you have high resolution feedback, linear motors, and either aerostatic or hydrostatic slides, the I131 and I132 seem "high" relative to I130, and I133 seems "low". You might try looking at cutting the I131 and I132 in half and increasing I133 substantially, perhaps as much as 10X. Try doubling it and then doubling it again etc. It is possible that your proportional gain is being limited by the high I131 gain "buzzing" with quantization noise, and your I133 is also low, both of which is limiting disturbance rejection at 7.5 Hz. I am totally guessing here.
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