Duet v0.8.5 - Problems with adjusting the motor current using M906

Hi everyone,

I am using the Duet v0.8.5 with RepRap Firmware (dc42 fork: 1.09k) for controlling a delta printer.

The gcode command M906 is used to adjust the motor current. Unfortunately the chosen motor current differs a lot from the true motor current.


Example:

M906 X1600 Y1600 Z1600 E300 I60

X,Y,Z-Motors should run with 1600mA (4800mA in total) at 12V right?

If I meassure the actual current supplied there are only 1680mA in total. Minus 130mA for the board = 1550mA. This means around 500mA per motor.


I guess it doesn't work correctly because the coil resistance of my stepper motors is different to the resistance of the motors normally used. (I have 1,65 Ohms per coil)

Any other ideas?
What do I have to change in the firmware to get the correct current?


cheers!

You have overlooked the fact that the motors require much less than 12V, and the Duet (like all other 3D printer electronics) uses chopper-type drivers to reduce the 12V (or whatever) supply voltage to the actual voltage used by the motors. If your motors need 3V @ 1600mA, then if the driver were 100% efficient, the current drawn from 12V would be (3/12) * 1600mA = 400mA per motor when the motor is in a full-step position, and sqrt(2) times that when in a half-step position. In practice the drivers are less than 100% efficient, so the 12V current draw is a little higher.

If you want to measure the actual motor current, you will need to connect your multimeter in series with one of the motor coils. Always remove the power to the electronics before connecting or disconnecting motors.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Ah okay makes sense. Can you also tell me how the drivers "know" with how much voltage they need to supply the motors? (only curious)


thanks a lot! :-)

There are two 0.1 ohm current sense resistors connected to each driver. The motor current flows through them some of the time, and the driver chip monitors the voltage drop across them to work out the current flowing.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].