Post by Ben
Boris Ingram has been creating some remarkable walkers!
Inspired by Theo Jansen’s Strandbeest, Ingram set out to create his own highly functional versions in the early 2000’s. His initial attempt is shown below and was constructed out of plastic straws due to their inexpensiveness and excellent performance under tension and compression forces.
However, like us, Ingram wanted to go bigger. To that end he wrote some linkage simulation software that would allow him to optimize the Strandbeest linkage for:
With this new linkage configuration Ingram was able to construct the absolutely insane 6-legged machine below:
After completing this machine and getting a PhD in mechanical engineering, Boris took a 7 year hiatus from building. During that time he began to brainstorm ways to improve upon his already impressive work.
Boris’ latest design is an octopedal walker. He chose to move from 6 to 8 legs because with only 6 legs there were times where an individual leg was taking a disproportionately large amount of the weight of the machine, somewhat compromising its structural stability. The octopedal machine has the following features:
1. Steerable with variable stride lengths. This allows the legs on the inside of a turn to have a shorter stride length and cover less horizontal distance with each crank rotation. This is important because the outer legs need to travel faster to keep up with the inner legs, as you can see in this classic video on how differentials work:
2. Feet with auto reset in case they meet obstacles during foot placement, a key feature for going over complex kinds of rough terrain.
3. A newly optimized set of linkage dimensions which produce a foot-path that is almost totally flat during the walking phase. This allows the machine to walk more smoothly which in turn reduces the force loads on the legs.
4. Non-constant rotational speed on crank which eliminates deviations in longitudinal velocity (foot-speed).
5. Welded steel construction and ball bearings for low friction. The machine weighs in at about 500 kg with a Kawasaki 200ccc motorbike engine.
6. An additional reduction box incorporating reverse gear and ability to drive inner or outer legs for tank like steering
7. And the ability to ride it!
The CAD model of it can be seen below:
While Boris has done the lion's share of the work, he could use some help from aspiring engineering interns!
If anyone has any interest in helping Boris in the construction of this machine you can contact him at: email@example.com
As you can see in the following images, Klann's foot-speed slows down significantly at each corner of its foot-path:
This causes robots using the Klann linkage to have a halting gait, which can be a problem on higher-friction terrain at higher speeds. One solution is to add feet that passively rotate as Klann's speed varies, like Strandbeest uses.
As you can in the following video, rotating feet also reduce how much the feet drag during turns, since the inner and outer feet can rotate at different speeds and function somewhat like a differential.
Strandbeest's foot-speed also slows at each corner of its foot-path, but less so than Klann's:
This may be part of the reason that Theo added rotating feet to Strandbeest? ( Jeez that thing is cool!)
Similar to Klann robots, adding rotating feet to LEGO Strandbeests smooths their speed:
When two linked bars are nearly parallel their connecting joint can easily flip orientations and cause the linkage to lock. This phenomena is known as a "Dead Point" or "toggle point". Here's an example of what Klann Ver 1 experienced, since it used a configuration of the linkage where the "knee" joint came close to being straight :
The below right picture is near the Dead Point, where two bars highlighted in red are nearly parallel:
Due to the force on the foot, the joint can hyper-extend and "flip" as shown below, which causes the linkage to lock::
Like how human joint hyper-extension is prevented by structures like ligaments, linkage joint hyper-extension can be prevented by adding structure. Below are some examples of "hyper-extension blockers" in LEGO.
The following solution for Klann Ver 1 blocked the joint from flipping with the addition of a 2-hole red LEGO beam:
As shown below, Strandbeest's knee joint can also flip:
Strider's knee joint also has a dead point that needs to be managed. As you can see in the following simulation, Strider's knee joint hyper-extends inward during the weight-bearing phase of the crank's rotation:
Adding weight to Strider robots can cause the knee joint to hyper-extend further, which can either reduce the height of the leg, resulting in a bumpier gait, or cause the joint to flip.
As shown in the image below, Strider Ver 3 uses blue LEGO pins to limit knee hyper-extension, which work well at LEGO-scale weights, but when heavy loads are added to Strider, these blue pins bend and the gait gets bumpier.
In the following experiment we added stronger "hyper-extension blockers" to a Strider robot using Linkage Variation #6, which we tested with a 25 pound load. The blue pins were still used, but an additional part was added to the front of the knee that presses against the shin if the knee hyper-extends too far.
Below is a video of the test. Notice when the weight is initially placed onto Strider, the inner knee on the right side hyper-extends slightly, but not enough to prevent Strider from lumbering under the 25 pound load:
Note: before performing this test, the plastic LEGO axles were replaced with steel axles to handle the torque. Other than that, and the 2 steel support rods, all of the parts are plastic LEGO parts connected by LEGO pins (no glue).
Below shows how we managed Strider Ver 2's dead point:
Below is another possible solution that uses an additional linkage on the inside of the joint that allows the knee joint to bend toward the robot, but prevents bending away from the robot.
Below is another idea for a Strider robot using linkage variation #4:
Welcome to DIYWalkers! I'm Ben Vagle, and I've been building mechanical walkers since I was 11 years old, both big and small. I started this blog to share what I've learned, and to collaborate with you. Let's see if we can take walkers to the next level!