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Why Is My Velocity Trace Resolution Inaccurate In Sysmac Studio?


The Actual Velocity of a motion axis (like GX encoder or G5 servo) is not sent by the slave. It is calculated by the NJ Motion Control engine using the position counts between two control cycles. Because Actual Velocity is always reported in units per second, the calculation is multiplied by a factor to scale it.
This means that:
  • - Speed feedback for an axis is just a calculation
  • - The quicker the cycle time, the larger the scaling factor use for the calculation to scale to units per second
  • - The lower encoder resolution, the more inaccuracy of each position pulse
Example:
Drive: G5 Servo
Units of display: degree

Command pulse count
per motor rotation:

1048576 pulse/rev
Work travel distance: 36

This means that:

resolution => (36 deg/rev) / (1048576 pulse /rev) = 0.00003433 deg/pulse

Let's analyse how the monitoring speed is changing when using different cycles, 1ms and 2ms:

The speed is changing by1 pulse:

(0.00003433 deg/pulse) x 1 pulse / 1 ms = + 0.03433 deg/s (pink)
(0.00003433 deg/pulse) x 1 pulse / 2 ms = + 0.01716 deg/s (green)

So the smallest change in speed can appear much larger that the resolution of the position counts.

 

 

Additionally a common mistake is to enter the encoder resolution as it is, without taking into account that the GX counts 4 rise/falling edges per pulse by default.

If the encoder manual states 2000 incr/revolution, GX-EC02XX could count 4 x 2000 = 8000.
8000 pulses/rev and is the correct value for the encoder setting.
Setting 2000 pulses/rev will result in showing the speed value which is 4 times less than the real one.
 

Solution

To minimize this effect, a monitoring filter on axis configuration can be used to calculate a running average. For instance if you set on MC_AX000

filter = 0   (blue)

filter = 4. (green)

 




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Created 2013-05-22
Modified 2013-05-23
Views 4010

 

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