servo support question and AC/brushed motors

[bschwand]

Hi all,

In various parts of the documentation I have seen that "servo motors" are supported through M10 codes

• what kind of servo are we talking about ? are those like AC servo motors, or hobby-type RC servo controlled actuators that take a position as duty cycle of a 50Hz signal ?
• where is the documentation for the servo M10 code ? I could not find any. Marlin, grbl, LinuxCNC do not have an M10 code or they mean something else.

I see quadrature encoders are supported also

• are those currently meant to only do closed-loop position control of stepper motors ?
• is there any plan to add brushless closed loop position control with quadrature encoders and PID control of power through PWM ? I mean by that: instead of having an external controller like this https://github.com/ottoragam/Tarocco, uCNC could do the motor control directly instead of an external interface and STEP/DIR pins. It just needs a regular encoder, a PWM and a DIR pin.

How hard would it be to implement 2) ? I imagine there is some point in the code where uCNC keeps the desired (and probably real) position for each stepper, before toggling the step pin. This could be decoupled from the step pin toggling and instead used periodically as input to a PID controller that configures a PWM to modulate the power output to the brushed motor according to the error between the desired position and the current encoder position.

All answers and feedback welcome, cheers !

[Paciente8159]

Hi there.

About servos:

what kind of servo are we talking about ? are those like AC servo motors, or hobby-type RC servo controlled actuators that take a position as duty cycle of a 50Hz signal ?

About that, the awnser is actually in the readme file in https://github.com/Paciente8159/uCNC?tab=readme-ov-file#current-%C2%B5cnc-status under changes of version 1.4. It's 50Hz 1ms to 2ms pulse width.

where is the documentation for the servo M10 code ? I could not find any. Marlin, grbl, LinuxCNC do not have an M10 code or they mean something else.

About M10 the info is in the wiki here https://github.com/Paciente8159/uCNC/wiki/Basic-user-guide#%C2%B5cnc-gcode-commands

To make SERVO0 go to 50% of it's maximum position you need to to M10 P0 S127 (range is from 0 to 255)

About encoders:

are those currently meant to only do closed-loop position control of stepper motors ?

They are meant to be general purpose encoders and are assignable.

is there any plan to add brushless closed loop position control with quadrature encoders and PID control of power through PWM ? I mean by that: instead of having an external controller like this https://github.com/ottoragam/Tarocco, uCNC could do the motor control directly instead of an external interface and STEP/DIR pins. It just needs a regular encoder, a PWM and a DIR pin.

Yes I would like to have that available. It has been left behind for other stuff. I would need some good and very cheap hardware to start laying the stones to do that. Do you have any recommendations?

How hard would it be to implement 2) ? I imagine there is some point in the code where uCNC keeps the desired (and probably real) position for each stepper, before toggling the step pin. This could be decoupled from the step pin toggling and instead used periodically as input to a PID controller that configures a PWM to modulate the power output to the brushed motor according to the error between the desired position and the current encoder position.

Hopefully not that hard. All the fundamental building blocks are there..including PID controller, separate counters for the desired position and encoders position..etc.. It's a matter of combining the building blocks and tune it (I believe)

[bschwand]

About that, the awnser is actually in the readme file in https://github.com/Paciente8159/uCNC?tab=readme-ov-file#current-%C2%B5cnc-status under changes of version 1.4. It's 50Hz 1ms to 2ms pulse width.

I saw this
new servo PIN type that generates a 50Hz with TOn - 1~2ms needed to control servo type motors.

I am sorry if I am obtuse but the description could also describe a regular servo/or brushed DC motor with a PWM input for power, and that the 50Hz is the regulation frequency (PID loop). Thus my question. Maybe mentioning specifically that this is about R/C type servos would make it less ambiguous.

About M10 the info is in the wiki here https://github.com/Paciente8159/uCNC/wiki/Basic-user-guide#%C2%B5cnc-gcode-commands

Ahh sorry I totally missed that although I really searched for it ! thanks.

is there any plan to add brushless closed loop position control with quadrature encoders and PID control of power through PWM ? I mean by that: instead of having an external controller like this https://github.com/ottoragam/Tarocco, uCNC could do the motor control directly instead of an external interface and STEP/DIR pins. It just needs a regular encoder, a PWM and a DIR pin.

Yes I would like to have that available. It has been left behind for other stuff. I would need some good and very cheap hardware to start laying the stones to do that. Do you have any recommendations?

Would be a great feature to have !

I do not have really any recommendation, but I have some L298 dual hbridge boards like this https://www.aliexpress.com/item/1005007219197315.html
and some L9110S based boards
https://www.aliexpress.com/item/1005005469263941.html

they both work fine and are very cheap.

As for the encoders, I got some of these MT6701 absolute magnetic encoders
https://www.aliexpress.com/item/1005007961159153.html
and corresponding magnets
https://www.aliexpress.com/item/1005004814383575.html
the MT6701 can do 14 bit absolute position up to 50000 rpm, over SPI or I2C and also is a 1024 lines ABZ or UVW classic quadrature encoder, just depends how you configure the hardware. Those MT6701 boards above let you use all the different modes.

I am planning to use the MT6701 over SPI for a different applications with BLDC and FOC control as it gives the highest precision and speed, and it would have been great to use the same stuff with uCNC and brushed motors.

As far as cheap, this is my current choice, along some regular RS550 brushed DC motors. I think for experimenting it certainly beats custom motors with integrated encoders, particularly because the MT6701 has multiple interfaces so it's easier to test different input kinds with the same hardware.

Hopefully not that hard. All the fundamental building blocks are there..including PID controller, separate counters for the desired position and encoders position..etc.. It's a matter of combining the building blocks and tune it (I believe)

That's great, I had that impression too !

[Paciente8159]

Thanks. I'll see what I already have and order the rest. I should have all the necessary parts in 3 or 4 weeks.