The Darkpaw is a hobbyist quadruped kit that I’ve been playing around with for some time now. The goal is to learn how to model, control and program articulated robots for fun and (maybe one day) profit. This post will give a bit of an overview of how i got started on this, where I’m at, and lay some groundwork for future posts.

Into the web of legged robots.

Legged robots are pretty awesome (see this video by Zenta)

(As a aside – I showed my partner this video and now she wants one to keep her company on jogs so as to light her path and further intimidate the locals…)

or this one by Matt Denton…

Anyway, not long after the start of lockdowns during the COVID-19 pandemic in Melbourne, I found myself looking for a new project. I had been working as a post-doc at the Aero-Mech flight lab, and had been learning about underactuated robotics via Russ Tedrakes excellent course from MIT (very highly recommended). Additionally, I had been working mostly in Python (and ROS – ugh – but more on that another time) and developing/re-discovering my C chops. Upon this backdrop, and inspired by a random incursion into the youtube hexapod rabbit hole, I figured it’d would be fun to actually learn how to program such a robot.

Enter the Darkpaw

After a bit of rummaging around ye-olde-internets, I found the Adeept Darkpaw. It seemed to suit my needs; had most of the parts including motor driver hat, servos, camera and light – all that was needed was a Raspberry Pi 3b+ and some batteries. The ‘code’ was all python, as is the trend in hobbyist robotics, so it would be trivial to reverse-engineer to do whatever i wanted with it. So i got it and then got to work.

Two issues were immediately apparent. Firstly; neither my power adapter nor the two 18650 batteries were sufficient to keep the Pi from going under-voltage and shutting the thing down. (I’ve recently upgraded my power supply to a higher power, and i still get undervoltages, but at least the Pi doesn’t reset).

Secondly. Well. The shipped code was… lacking, and we’ll leave it at that.
Of course, this did not really bother me, (other than the general feeling of distaste when looking at it) since the plan was to re-write my own implementation.

The assembly was pretty straight-forward, and there were plenty of spares just-in-case. I wish it was easier to get the Pi in and out, or at least to access the SD card, but one can’t ask too much of such a cheap kit.

The biggest learning curve has been getting to grips with kinematics in practice. Given my background, one might expect me to find rigid body modelling ‘trivial’. The reality is that it took me a number of attempts to get it right – partly due to the fact that the system has algebraic constraints. This is a consequence of the design, as the motors do not drive the leg joints directly, they instead translate force through levers and links. I’ll save the maths for a later post.

Current State

Currently Implemented:

• Motor / LED drivers (via user-space i2c).
• Forward and inverse kinematics.
• A basic sweep/reset walk cycle for testing.

Next goal is a footstep planner!