Using stepper currrent to detect a tool is loaded

GeneRisi

Is there any way to ask the stepper controller on the Duet2 how much current was last used to move the stepper? I am wondering whether I can use it as a way to compare free rotation (no tool loaded on the e3d tool changer) vs rotation with friction (tool loaded)?
Thanks!

zapta

@generisi, I believe that the current is the same regardless of the mechanical load.

dc42

@zapta said in Using stepper currrent to detect a tool is loaded:

@generisi, I believe that the current is the same regardless of the mechanical load.

Correct, the drivers are constant current. However, the drivers do contain a means to measure load. The M122 report includes the load measurement, it's the SG values in the driver reports. The SG values get reset each time M122 is run.

GeneRisi

@dc42 Are the SG values available in the object model ( if that's the right terminology )? I am running RRF 3.3b3 right now.

dc42

@generisi said in Using stepper currrent to detect a tool is loaded:

@dc42 Are the SG values available in the object model ( if that's the right terminology )? I am running RRF 3.3b3 right now.

No they are not.

rzi

@dc42 CNC user here. Is it possible to add. I'm using two free drivers for a motorised vise. Would be super to have a mean of applying same force on each part I'm machining.

dc42

@rzi I suggest you check whether the stallGuard values (as reported by M122) are useful first. I doubt that the instantaneous values would be useful. The values reported by M122 are the min and max values since M122 was last run. If we did add them to the OM then we would need to decide what period of time to sample them over, and whether to report an average or the min/max values over that period.

fma

@rzi said in Using stepper currrent to detect a tool is loaded:

@dc42 CNC user here. Is it possible to add. I'm using two free drivers for a motorised vise. Would be super to have a mean of applying same force on each part I'm machining.

You could set the current of the vise motor to set the wanted force (torque). Then, just turn the motor until it stalls (using stallGuard feature, through a homing procedure, for example), and you should get roughly the same force each time.

GeneRisi

@dc42 In my case the SG values are useful (for the E3D tool changer coupler stepper). If I had a gcode command that would clear any min and max values in the object model and begin sampling and another gcode command would set the min and max values in the object model, then I could do whatever I want to measure between the two gcode commands. This behavior is almost identical to the M122 behavior. It might also be useful to include a "start sampling" time stamp and an "end sampling" time stamp.

What do you think ?

rzi

@fma said in Using stepper currrent to detect a tool is loaded:

@rzi said in Using stepper currrent to detect a tool is loaded:

@dc42 CNC user here. Is it possible to add. I'm using two free drivers for a motorised vise. Would be super to have a mean of applying same force on each part I'm machining.

You could set the current of the vise motor to set the wanted force (torque). Then, just turn the motor until it stalls (using stallGuard feature, through a homing procedure, for example), and you should get roughly the same force each time.

Ah yes, going to test that. A much more straight forward approach. Thanx