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Adding Step and Direction (CNC) control to Cheetah style motors #Robotics @buildlog

Bart Dring posts about the motors designed for builds like Cheetah robots and modding them for CNC use:

The proliferation of dog like robots has spawned a new generation of servo motors. These motors combine Brushless DC motors from the RC/Drone world with encoders, motor controllers and gearboxes in one small package. They are very compact, super strong and are in an easy to use package. Some of these are open source and are now being made cheaply in China.

To make them compatible with basic CNC machines you need to have step and direction signals, similar to what stepper drivers use. My initial goal was to add these inputs without changing anything to the physical motor. I will only touch the firmware.

Fortunately the motors Bart has are open source and the firmware is shared. The device uses the STM32F446 microcontroller.

There are 5 connectors on the controller board that are exposed through the side.

  • 24VDC In This is the primary motor power.
  • 5VDC in/out This can be used to power the board during programming. The 24VDC will create 5V on this if present.
  • CAN Interface
  • UART. This presents a menu that can be used to setup and calibrate the motor.
  • SWD. This is the programming interface.

I decided to use the UART connector. I would allow it to be optionally used as a UART for setup or for step and direction. I made an interface connection with some 1k resistors in line. This would save me if I screwed up. This would limit any current to a couple of milliamps. Any delays due to capacitance seem to be negligible. The UART still works fine at 921600 baud.

See the results and documentation in the blog post here.


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