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# PMAC command position deviation for G02/03

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when we run G02/03 circular path on a clipper, we found clipper command positions are slightly different from mathmatical positions of G02/03. With increased feedrate, this position difference is getting bigger.

We dont have any kerf turned on when doing this test.

I can provide data collected if needed.

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Lukewang,

Please see the section "Interpolation Errors" described under Turbo PMAC User Manual.

Interpolation Errors

The cubic-spline interpolation technique that Turbo PMAC uses to connect the intermediate segment

points is very accurate, but it does create small errors. These errors can be calculated as:

Error = V^2 * T^2 / (6*R)

where V is the vector velocity along the path, T is the segmentation time (watch the units!), and R is the

local radius of curvature of the path. For example, if the speed is 100 mm/sec (~4 in/sec), the

segmentation time is 0.01 sec (Isx13 = 10 msec), and the minimum radius at this speed is 50 mm (~2 in),

then the worst-case interpolation error can be calculated as:

Error = 100^2 mm^2/sec^2 * 0.01^2 sec^2 / (6* 50mm) = 0.003mm=3um

Regards,

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