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Commanded velocity fluctuation


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Firmware version:

Controller: CK3M

Servo update frequency 4KHz.


In my machine B axis is defined as

#5-> 1#5->5965233B

Coord[1].FRAxes = $1C0  //XYZ

I am commanding as per the following program.

G01 B0 F1000

dwell 500


dwell 500


When I plot graph for commanded velovity and acceleration, it shows velocity is oscillates after  acceleration is complete. Due to these fluctuations, acceleration feed forward is also fluctuating and creating noise in the motor.

Can you suggest what should the cause?




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  • 2 weeks later...
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  • 5 weeks later...

I have conducted following two test on other motor

1. When the acceleration is restricted to 60mm/sec^2, the commanded feed of 2000mm/min only reaches a maximum value of 1741mm/min with ripples.

2. However, If a feed of 1400mm/min is set instead, the velocity reaches the commanded value without any additional fluctuations.  

I could not understand why in case 1 motor does not reach to commanded feed.




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If I increase the acceleration from 60mm/sec^2 to 100mm/sec^2, then it is able to reach to 2000mm/min ( I have set Coord[x].Ta=0 ,Coord[x].Td=0 ,Coord[x].Ts=0.02 with segmentation enabled). Acceleration is changed with Motor[x].InvaMax.  It looks like that some other parameter is limiting the maximum speed other then Motor[x].InvaMax.

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  • 4 weeks later...

Why do you say that it looks like some other parameter is limiting the maximum speed, other than Motor[x].InvaMax?  If increasing Motor[x].InvaMax from 60 to 100  mm/s^2 allows the max speed to be reached, isn't that the parameter that was limiting the speed?  By the way, being that this is "inverse of maximum acceleration" increasing from 60 to 100 is affected by changing the value from 0.017 to 0.01 (1/60 to 1/100), right?

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  • 2 weeks later...

1. As I could not see any logical reason for INVAMax limiting the maximum speed. It can only increase the acceleration time. 

2. We have axis assignment as #1->#1->1677722X, and calculated INVAMax for 60mm/sec^2 and 100mm^sec^2 based on this. 

I request you to simulate at your end



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Too much acceleration limiting (big acceleration times or low acceleration values) will also limit motor speed. The motor does not have enough “time” to reach programmed velocity.


See the “Power PMAC User’s Manual” in the subsection “Acceleration Specification” (677) under section “Linear Move Mode” starting on page 672.



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I could understand this topic. Case  explained in the manual is with less distance   where motor must start to decelerate to reach and stop at programed point. But in this case, there is large travel distance is programed so that axis can reach to programed velocity.  See the graph below. I have programed the velocity of 2000mm/min but it could only reach up to 1741mm/min and it continues with that velocity for quite some time before starting to decelerate. Also velocity is not constant at 1741mm/min but have lot of oscillations. 

Only one motor is programed in this case and no coordinated movement with other axis.



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