Torque mode on expansion boards for RRF 3.4

[Ravs99]

Any update or details on torque mode on the expansion boards for DUET3?

@dc42 will this "torque mode" be similar to the torque mode in O-drives that are used in the Hangprinter v4?

[dc42]

@ravs99 open loop stepper motors cannot be operated in torque mode in the way that DC motors and BLDC motors can. So we can't implement it in open loop expansion boards designed to drive stepper motors.

You can achieve a somewhat similar affect by reducing the motor current; but this will cause the motor to skip backwards by 4 full steps at a time whenever the motor current does not supply the torque demanded by the load.

It may be possible for us to implement something resembling torque mode on the EXP1HC board and future closed-loop expansion boards in future.

[Ravs99]

@dc42 Ah i see. Yes i was referring to those closed loop expansion boards. In the g-code dictionary for M569.4, theres planned torque mode support for RRf 3.4. Was just curious about that as it may offer an opportunity for me to implement Tobbens semi-autocalibration which uses encoder readings from moving the hangprinter to known positions in torque mode on the drives and BLDCs. (but instead using stepper motor and encoders)

[dc42]

@ravs99 I guess it would be possible to implement torque mode on the 1HCL, by keeping the motor phase currents 90deg ahead of the current position, and controlling the current to give the required torque.