Custom Servo

Hardware
#1

Hi all,

we see that most people make use of the Dynamixel motors for their robots. My team is also using Dynamixel motors but due to budget constraints we want to make our own motors. we are considering taking JX hobby servo (comparable specs with mx64) and hacking the circuitry to make it communicate on Dynamixel protocol. Has anyone tried doing something similar and have any advice on the above idea?

Thanks
Nam Nguyen
UTRA Team Lead

#2

Hi,
we (the Hamburg Bit-Bots) also investigated this idea a few years back. There were multiple issues that we had:

  1. The PWM servos we found were not strong enough (can you provide a link to the one you want to use?)
  2. Most PWM servos can’t be turned full 360° (I think due to using a potentiometer instead of a hall sensor for sensing the position)
  3. It is actually some work required to make them daisy-chainable like the Dynamixel servos.

A PhD student from Berlin called Matthias Kubisch did some work on this while he created a 4-legged robot. He build a electric board that you can add to a PWM servo and control it via an RS485 bus. Not using the Dynamixel protocol, but reprogramming the firmware a bit to use another protocol is easy.
You can find more resources here:
Talk (english translation) - https://www.youtube.com/watch?v=UACttkydAfU
Github - https://github.com/suprememachines/sensorimotor

There are some things you might want to improve on this project. Especially, that the bus control only works with 1MBaud if I remember correctly.
I can also recommend to read the paper that we presented on the last RoboCup about communication with Dynamixel protocol https://robocup.informatik.uni-hamburg.de/wp-content/uploads/2019/06/dxl_paper-4.pdf

Best regards,
Marc Bestmann

#3

Hi Marc,

Thanks for the thorough reply! I will have a look at the resources you pointed to.

We bought a bunch of these motors and they seem promising: 7.4V, ~60RPM, 180 degrees, 62kg*cm. Pretty good specs for ~$50CAD

Currently our plan is to replace some of the motors in the legs with JX hobby servos to evaluate their performance. If they perform well then we will think about implementing Dynamixel protocol to better integrate into our system

Regards,
Nam (UTRA)

#4

Hi,
the specs look comparable to a MX-64 and in the legs you don’t need more than 180°, so it might work.

I would be very interested in your results. Having a cheaper alternative to Dynamixel would really make life easier for the humanoid league.

Best regards,
Marc