Industrial Servo Motor 2.8A Yaskawa Sigma 2 AC SERVO MOTOR 400W SGMAH-04A1A21

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What Is Required to Maintain Accuracy During Coordinated Motions?
The magnitude of the error really does not matter if the path being followed is a single axis move. The
axis will trail the moving command, but will catch up when the endpoint is reached. One could not detect,
by observing the cut, that an error ever existed. When two axes are moved simultaneously to generate a
sloping straight cut, large errors can develop. Figure 2 shows a two axis move along a 45° slope where
both X and Y are being commanded at the same velocity. The gain of the X axis is twice that of the Y
axis, so the X axis error (EX) is half that of the Y axis error (EY). The resulting path is offset from the
commanded one depending on direction, velocity, gains and angle of slope. If the gains of the two axes in
the example were identical, EX and EY would be identical and the machine would lag the moving
command, but it would be precisely on the desired path. It would catch up when the command stops at the
endpoint. Once the gains are precisely matched, the direction, velocity and angle of slope no longer
matter. As long as the commanded path remains on a straight line, the axes will always lag, but precisely
on that line. Maintaining accuracy for linear moves becomes an exercise in matching gains. This will
require detuning the more responsive axes to match the poorest performing one. Many systems allow
gains to be set digitally (and thereby precisely). Often the gain will be a potentiometer or digital register
adjustment. This adjustment is made by commanding each axis at the same medium range value and
adjusting the potentiometers to achieve equal errors.

Circular moves, where the commanded path is generated by circular interpolation, is another story. Again,
the axes gains must be matched or one will be cutting eggs instead of circles. With matched gains, circles
will always result, but not necessarily of the commanded size. With low velocities and high circle radii,
errors are negligible, however, as the ratio of velocity to circle radius increases, the error in the circle size
increases. This raises the question: Will the resultant circle be larger or smaller than the commanded one?
(Think about this before reading on.)

There will be servo lag errors, so the machine will lag behind command. As the velocity increases or the
radius decreases, will the lagging point move outside the circle or inside? Many people will say that the
lagging point moves outside the circle resulting in too large of a circle. This is because they are viewing it
like centrifugal force, which it is not. For example, if you hooked a short rubber band with a weight on it
to a pencil and drew a circle, the weight would fall farther and farther inside the circle as the rubber band
stretched (which is what occurs at higher velocities).