In my last post about trotbots, I mentioned I’d be incorporating the Lego Mindstorms EV3 brick into new versions. I have now posted instructions for two walkers designed to handle the weight of the EV3 brick: Klann’s robust Mechanical Spider and TrotBot Ver 2 with retractable toes. The latter increases its foot-contact with the ground and reduces the robot’s power requirements to bear the weight.
Building functional walkers with the heavy EV3 brick is a good test of a walking mechanism’s capabilities. It can reveal weaknesses in designs similar to a partial scale-up.
TrotBot, Ver 2
Background on TrotBot’s Feet
A big focus of our TrotBot project has been designing active feet to improve gait and reduce the number of legs required. We discovered that when TrotBot had only eight legs, a lot of torque was required to keep the walker’s gait smooth and drive the robot up from the gait’s low point. We did not want to start over from scratch by re-building a 12-legged version, so instead we explored ideas for active feet that could smooth TrotBot’s 8-legged gait.
Using a galloping horse for inspiration, we sought to add some sort of second foot to each leg that would mimic how a horse’s rear then front legs land in pairs. This resulted in what we call TrotBot’s “heel.” It increased TrotBot’s foot-contact with the ground by about 10%, reduced how much the feet skidded, and increased the step-height of TrotBot’s rear legs. Shown below is a video comparing TrotBot and its “heels” to a galloping horse.
Next, we explored adding some sort of active toes that would push down on the ground as the foot begins to lift, just like how humans use their toes to walk. We installed one of these toe ideas on our larger TrotBot. They gave the robot a smoother gait, but since they were attached to the legs at a fixed angle, they tended to catch on obstacles. Catching on obstacles occasionally locked the linkage and caused the gears to grind or break. Sadly, this toe compromised our main goal of creating a mechanism that could walk on rough terrain!
Once again looking to a galloping horse for inspiration, we started to experiment with linkage configurations that mimicked how horses paw their hooves backward, and then keep them folded back as they lift their legs to strike the ground again. We discovered a few options that mimicked this action, and they increased TrotBot’s foot-contact by another 10% while maintaining its high foot-path. TrotBot ver 2 in the below video uses a LEGO approximation of one of these toe options.
Klann’s Mechanical Spider
The Klann Linkage, designed by Joe Klann, is one of the more popular and functional walking linkages in the engineering world, and a few huge versions occasionally visit Burning Man.
There were a number of challenges in completing this build:
1. It was difficult to approximate Klann’s linkage in Lego using the images that Klann provided. I found the posted patent details a bit confusing, so instead I took the coordinates at each end of the mechanism’s bars in Klann’s images, calculated the bar lengths with the Pythagorean theorem, and chose Lego beams that most closely approximated them. My resulting foot-path looks pretty close to Joe Klann’s, and as you can see in my video below, it is a beast!
2. Walkers require stronger frames than wheeled vehicles (especially with the heavy EV3 brick), so finding ways to incorporate triangles into their frames was crucial for success. However, neither Klann’s nor TrotBot’s linkage dimensions work with integer-based right triangles like the 3,4,5 or 6,8,10, and Lego’s integer length beams did not seem to be viable options for frame triangles. So, I used the Pythagorean theorem and found approximate right triangles that worked with their linkage’s dimensions.
3. Since the inner and outer leg pairs are always 180° out of phase, these walkers don’t have a differential to prevent feet from skidding during turns, and therefore should be built as narrowly as possible. I reduced Klann’s width by mounting the EV3 brick on its side, and reduced TrotBot’s width by mounting the EV3 brick underneath the frame.
4. My Klann build requires an 11-hole beam for the lower leg section. If unsupported, this beam is at risk of bending or breaking under the weight of the EV3 frame. My solution was to make the lower leg section as a triangle as shown in the photo below: