Resolution/Step size (serial commands)

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    I’m having fun learning how to control the AxiDraw V3 via serial port commands (from Max/MSP). I can’t figure out the relationship between steps and distance, however – specifically using the EM command with different arguments as specified in the EBB documentation. The Axidraw manual says the default resolution is 2870 DPI (113 steps per mm). However trying a move with XM,5000,1130,0 on Full Step gives me a 226 mm travel. Decreasing motor steps ends up halving this distance so that 1130 steps with 1/16 ends up with a line 1.4125 cm long, as expected.

    But I can’t figure out the relationship – shouldn’t 1130 steps at 1/16th step setting yield a line 1 cm long? Or is it that 113 steps at Full Step equal a line 1 cm long? Is it a matter of calibrating the steppers?


    Windell Oskay

    The resolution for any device with discrete subdivisions is always given along the native axes of resolution. For example, if you have a monitor with dimensions X by Y pixels and R pixels per mm, that value R will always be the resolution as measured along the X or Y axis of the monitor.

    For the AxiDraw (V2 and newer), the native axes, accessed through the SM command, are at directions (X+Y) and (X-Y). The resolution is measured along *those* axes. So, if you set up with 1/16 microsteps (EM,1,1) and perform a move along the native axes of SM,1000,0,2870, you’ll end up with a movement 1 inch long and SM,1000,1130,0 will give you a 1 cm movement. This is the resolution described in the user guide, and you should be able to check and verify it with direct commands. No calibration is needed– it should be correct.

    If you use the XM command, pay attention to what it actually does. From the documentation, XM,duration,AxisStepsA,AxisStepsB is an alias to
    SM,duration,AxisStepsA + AxisStepsB, AxisStepsA - AxisStepsB.

    So, XM,5000,1130,0 is an alias to SM,5000,1130,1130. The total distance travelled by the carriage is 1130 steps each, along the native axes of the machine. You should expect the distance travelled to be sqrt(2) * 1 cm.


    Thanks so much for the clarification and prompt help, Windell. I should be using the SM command. Hah, makes sense my measurement was around sqrt(2) * 1 cm :)

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