Sunday, July 16, 2023

Mojo5 - Minimal Viable Mojo


Lately, I've been on a bit of a hiatus from robot building, due to the stresses of life and my role as the technical lead in a startup. Yet my thoughts have never strayed from the topic, frequently returning to many different types of robots. But quadrupeds – they have a special place in my heart. You always go back to your first love.

In recent times, there have been numerous discussions about the prerequisites for a good, affordable quadruped. Since embarking on this journey, I've seen a few open-source designs come into existence, such as the Stanford Puppers. To me, the real thrill lies in creating my own, even though I do keep an eye on the work of others in this field.

Looking back, the primary challenges with my previous designs stemmed from the strength of the servos and the overall weight of the robot. To be successful, you need 25+ kg/cm servos with speeds of less than 0.2 seconds for 60 degrees. These servos typically cost $20-25 each (when on sale), and a basic 8-degree-of-freedom quadruped would need eight of them. Before I take the plunge and invest in these high-end servos, I want to try one more design to see if it might be possible to work with the 11 kg/cm servos I currently have.

Enter the Minimal Viable Mojo - Mojo5.

For my Mojo5 design, I aim to strip everything down to the bare minimum necessary to create a walking quadruped.

Minimal parts:

  • 4 leg assemblies
  • 4 quadrant servo mounts
  • 8x MG995 servos (60g each) 480g
  • Light weight wooden dowels connecting the parts
  • Arduino nano pro
  • 7.4v battery for servos
  • 9v battery for controller

This MVM - minimal viable mojo will be a mini-mini quadruped.

Moving forward with this reduced configuration, we have to address several fundamental challenges, the foremost being power optimization. Lighter and less powerful servos mean we have to be extra diligent about weight distribution and energy efficiency.

We're dealing with MG995 servos that are a bit less powerful than the ideal 25+ kg/cm servos, hence the need to maintain a weight that falls within their operating parameters. Every gram of weight matters in our design, hence the decision to use lightweight wooden dowels as linkages.

The Arduino Nano Pro will be the brain of the Mojo5, controlling the servos and coordinating their movement to achieve walking and possible turning. It's a cost-effective and well-documented controller, which makes it ideal for this type of application. However, there will be some limitations due to the lower processing power compared to higher-end controllers.

Power-wise, we're going with a 7.4v battery for the servos and a separate 9v battery for the controller. This dual power source approach is necessary to avoid a brownout scenario when the servos draw a significant amount of current, which could lead to a temporary loss of control.

One innovative aspect of the Mojo5's design will be its cam, pushrod-controlled lower leg. This type of mechanism can provide the torque needed for locomotion without adding much weight, but it does introduce some complexity into the design and programming.

For the leg assemblies, we're starting with 70cm sections. This is a reasonable starting point, but it's likely that we'll have to iterate and adjust these dimensions as we move forward with the project and start testing the prototype. It's going to be a careful balancing act to ensure the legs are long enough for effective locomotion, yet not so long as to place too much strain on our MG995 servos.

Overall, the Mojo5 design will be a journey of continuous iteration and improvement. Each step will likely present new challenges, from structural tweaks to software adjustments, but that's part of the excitement of building our own quadruped from scratch. Stay tuned as we start prototyping and begin to bring Mojo5 to life. We're looking forward to sharing this journey with you.

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