Mechanical on Ben Bokser's Blog

On the Optimal Design of Cantilever Beams

Mon, 17 Apr 2023 00:00:00 +0000

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.
Both the base and end-effector joints can rotate in any axis.

We are allowed to assume that the arm does not move dynamically, so that our analysis can be static-only.

The Parts Are In!

Sat, 08 Apr 2023 00:00:00 +0000

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. As such, I’ll be directing my design modifications from afar. The project is gradually being transferred into the hands of my former labmates at the REx Lab, who will be running control experiments on the robot.

A Lesson in Mechanical Resonance

Fri, 17 Feb 2023 00:00:00 +0000

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.