G-code G1 move command is not consistent with MM114 get motor position command

Question

I am using the G1 move command to rotate aa stepper motor with a Duet3D stepper controller board. I just realised by using the M114 command, which returns the actual motor position, that there is an error margin between the 2 commands, but I don't understand why as I directly ask a distance to move, which should be added to the relative motor position saved on the board.

1. sending get position command: M114
   returns: X:0.000

2. sending move command: M120 G91 G1 S1 X-28.897 F600 M121

3. sending get position command after move is done: M114

   returns: -28.934

Why is M114 returning a slightly higher value than what I asked the motor to move?

Thank you!

Answer 1

I think the minor correction of 0.5mm is because of auto calibration or homing. When the device is homed there is a minor correction applied for all the axis with respect to the end stops.

Another point I would check would be the step calculation for the motor. sometimes the mm/steps of motor calculation may not be perfect and has to be manually fine tuned. You can find the steps defined in the config.g file in the command M92 . You can vary in steps of 0.005 for each axis and verify the movement.

The steps/mm latency is not visible in small machines but this latency increases with the size of machine.