Stompy simulations, full scale test leg out for quote!

Ladies, Gentlemen, Undecided and Robots –

It’s been two weeks since our last update and the team has been cranking away…

We’ve been iterating through a design spiral trying to hammer out a leg design. Starting with rough estimates of masses and link lengths, the controls team figures out what kind of joint torques and range of motion would be required to meet the design goals and passes this information back the the mechanical team. The mechanical team takes this data to come up with a leg design, which they then run through FEA to determine the viability of the design. If they think it’s a viable design, they pass its critical parameters (masses, force/torque limits, actuator placements) back to the controls group, who plug the new parameters in to the simulation and give feedback back to the mechanical group, who modify the leg design, etc.

This is a labor intensive cycle that we’ve run through several times in the past 2 weeks and have come up with leg designs that we think will work. Without further ado, our first glimpse of Stompy walking…

These legs are designed to be cut from sheet steel on a water jet. The waterjet cuts slots and tabs so the pieces of the structural elements will slot together like Ikea furniture. We will then weld over all the tabs to make extremely strong leg pieces with minimal machining on our part.

This is what our full-size prototype leg will look like extended.

The torques on these legs is tremendous… at the hip the torque is on the order of 14,000 foot pounds. The torques decrease in magnitude as you go further out on the leg, allowing for lighter construction. The yaw link (the little chunk of metal between the body and the thigh) is made of thicker sheet steel than the rest and ends up weighing about 70lbs. The thigh link is around 200lbs without the actuator attached. One of the comprimises we had to make in the design spiral was to shorten the legs… we simply couldn’t achieve the safety factors we wanted with the legs spread out as far as they were in the original concept art.

The prototype leg is out for quote at local machine shops, as is the next version of electronics, about which there will be a post soon. Also expect to see a Gimpy update!


Controls Assignment 2 Solution

The controls team pulled it together at the last minute and submitted a solution to the problem of pushing the leg cart in a smooth, controlled fashion.  “Controlled” in this context means that it can’t lift itself off the ground and foot slippage has to be minimal.

The control code here is 100% student written, from the joint controllers to the kinematics to the trajectory generation.  Go team!

For reference, the dots are 1m apart.  The simulation makes it clear that we really need to watch out for reaction torques about the yaw axis (note that the whole cart twists itself… this is with a relatively realistic friction coefficient on the wheels).  We will probably want to look at this more closely before putting anything on hardware.

Beware the Barge

I built a model of the leg cart/leg test stand in our simulation environment.  Leg dimensions, masses, ranges of motion and actuator force limits are all in the model.

On the meatspace implementation the inputs to the system will be valve commands, which roughly correlate to flow rates.  In an attempt to model this, we present actuator linear rates as the inputs to the model.

This week we assigned the controls team the task of controlling the cart and making it row itself along in a straight, controlled line.  We’ve got groups working on joint level control, forward kinematics, inverse kinematics, trajectory generation and the nebulous “high level”.  As a starting point I put together a little demo with the leg cart rowing itself along very poorly:


For reference, the cart weighs about 600 pounds and the pins weigh about 200 pounds and are 6 feet tall. When this thing exists in real life, we will have to be very careful…  More videos next week when the student solution starts coming together!