On the Optimal Design of Cantilever Beams

The Problem Let’s say we need to design a structural section of a robot arm, as shown. We’ve been given a few requirements: The mass of the component must be minimized at all costs. The length of the link must be 300 mm. The maximum allowed deflection (caused by the component) is 1 mm. The combined mass of the maximum payload and end-effector is 30 kg. The combined center of mass of the payload and end-effector is 50 mm from the joint....

April 17, 2023 · 8 min · Ben Bokser

The Parts Are In!

The new REx Hopper leg parts (which I hinted at in an earlier post) are finally in, and they look great. I was worried they’d be expensive, but the total cost for all three components shown here (including shipping from China) was only $312. As for the other components, they’ve been 3D-printed using Windform XT 2.0, which is supposed to have excellent mechanical properties. Anyway, I’ve started my new job in Boston, whereas work on the REx Hopper is supposed to continue in Pittsburgh....

April 8, 2023 · 1 min · Ben Bokser

A Lesson in Mechanical Resonance

The REx Hopper is now capable of balancing! However, it suffers from extreme oscillation at a relatively low frequency (about 8 Hz). This looks like a controller issue–perhaps even a simple matter of gain tuning–until you look closer and slow the footage down. It’s a clear case of mechanical resonance–notice how much the leg flexes back and forth. Unfortunately, I designed this leg with excessive focus on machining cost and ease of assembly, which led to the use of cantilevered joints and the use of, in retrospect, tiny bearings and shoulder screws....

February 17, 2023 · 2 min · Ben Bokser