Posted by Wade Vagle
Just like how we appreciate Nike's when running on concrete, large scale walkers benefit from shock-absorbing feet. By increasing the spring's travel, shock-absorbing feet can also increase the percentage of ground-contact of each leg, which can smooth the gaits of high-stepping walkers like TrotBot and Strider. And of course, this can create new problems to be solved!
First, here's the Mondo Spider's feet in action, which provide some shock absorption, and they also slide on the smooth concrete, which helps with turning and with smoothing Klann's speed:
It walks amazingly fluidly considering how Klann's foot-path comes to a stop at each end, and the springs probably smooth the transition between feet somewhat:
Watching the video raises some questions, like:
Implementing Klann's linkage without shock-absorbing feet that slide results in a more halting gait, as can be seen in this version of the Walking Beast:
Next, here are some shock-absorbing feet ideas from Mechanical Walker pioneer, Professor Joseph Shigley:
Feet with such springs extend the feet toward the ground. So, in addition to absorbing shocks, the springs also increase the percentage of ground contact of each leg. For example, feet with very long springs could theoretically increase a 12-legged Strider's ground-contact of each leg to 50% of the crank's rotation, like is required by 8-legged Striders graphed in red below, but do so without causing the robot to drop at the two ends of the foot-path (where the red dots curl up).
However, a few of the (probably numerous) issues of long-spring feet are:
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: firstname.lastname@example.org
Post by Wade Vagle
If you are looking for ideas on how to create highly functional mechanical walkers, Professor Joseph Shigley's 1960 feasibility study for the army is a great resource. It combines engineering rigor and sound reasoning to illuminate many of the challenges, and potential capabilities of mechanical walkers.
To meet the requirements of walking tanks, Shigley sought a mechanism that could:
To meet the rugged terrain and speed requirements of tanks, much of Shigley's study focused on the foot-paths of mechanisms. Below is Shigley's diagram of foot-path types, with type E representing his ideal type for rugged terrain and fast speeds.
Shigley's study helped us to develop Strider's mechanism
In his feasibility study, Shigley advised investigating 4-bar linkages as a first step, and referenced Hrones-Nelson's atlas of hundreds of 4-bar linkage coupler curves, which must have been a vital resource for mechanical engineers in the days before personal computers. Below is about the best 4-bar linkage configuration Dr. Shigley found, but it doesn't step high enough for rugged terrain.
Shigley showed how the mechanism could be improved by allowing a joint to slide along a cam groove, but we wanted a mechanism that could be prototyped in LEGO. So, instead we experimented with pairing two 4-bar linkages into a combined 10-bar linkage with front and back legs, such that the rear leg lifted the front foot and vice versa. This increased the step-height significantly, as shown below.
You may have noticed that Strider shares a characteristic with TrotBot - we designed both mechanisms' "hamstrings" to lift high to increase step-height for rugged terrain performance:
Below is this simulated version prototyped in LEGO. To replicate a wheel, the speed of the feet when in contact with the ground should be constant. Strider has nearly constant foot-speed, resulting in a very efficient gait, and is the only mechanism that we've tested which can walk with a 1:1 gear ratio without the LEGO motors stalling:
Strider's linkage dimensions can be found here, and other configurations of Strider can be found here where you can also create your own customized Strider linkage.
Below is Prof Shigley's feasibility study, a 1960 Popular Science article discussing it, and the Hrones Nelson Atlas of four bar linkage coupler curves.
Michael Frey, an enthusiast who makes technical animations of mechanisms, has created some wonderful animations of TrotBot's Linkage.
We've already used one of Michael's Klann GIFs on the site, so we're very honored that he's directed his considerable talents towards animating TrotBot!
Some of his work is shown below:
You can see more of Michael's creations here, like his animations of engines - so cool!
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?
Similar to Klann robots, adding rotating feet to LEGO Strandbeests smooths their speed:
This summer I created a Walking Machine Curriculum for middle schoolers. I am in the final stages of its development and am piloting it at a local middle school this winter...
If you'd like to receive an early copy of it, just email me at email@example.com.
TrotBots with 8 legs balance by having 4 feet in contact with the ground, one at each corner of the robot. If one of these feet were removed, then TrotBot would tip, similar to what would happen if you took one wheel off of a car.
For a tripod gait to be balanced, the feet need to be arranged like an equilateral triangle, so we removed the two outer pairs of legs, and added a pair of legs to the center of TrotBot, inside the frame:
Also, we needed to adjust the timing of TrotBot's front and rear feet. As shown in the image below, hexapod robots with tripod gaits transition from one tripod to another as they walk, which requires TrotBot's front and rear feet to be 180 degrees out of phase:
However, orienting TrotBot's front and rear cranks 180 degrees out of phase won't put the feet 180 degrees out of phase, because the location of the two leg's upper frame connections relative to the cranks is in the opposite direction. Looking from the side of the robot, the left leg's upper frame connection is 49.4 degrees to the left, and the right leg's is 49.4 degrees to the right. Here's a diagram of the left leg's frame connection relative to the crank:
So, in order to have the left and right feet touching the ground at the same time the right crank would need to be rotated clockwise by 49.4 degrees x 2, or 98.8 degrees. For the foot contact to be 180 degrees out of phase, the right crank would need to be rotated a further 180 degrees, or 278.8 degrees in total, as shown in the image below.
Here's a simulation of TrotBot's legs with this 278.8 degree phase shift of the right crank:
Also, I added 10 pounds to a toe-less TrotBot that used LEGO's plastic axles, but its bumpier gait required more torque than the plastic axles could handle. Those axles twisted so much that TrotBot could barely walk, so I replaced them with steel axles before filming this test. I should have included a clip of the plastic axle version to better show how heavy walkers with bumpy gaits may need LEGO's plastic axles replaced with steel axles to handle the torque.
TrotBot's lower leg pins tend to come out when the legs experience sideways forces, as can happen when turning TrotBot on terrain with a lot of friction (like on thick carpeting).
If the legs aren't snapped back in place, then friction on the pin's lips will wear them down, and the pins will no longer join with a sharp "snap", causing them to pull out more easily. Ideally, joints should be 3 beams wide and symmetrical like the red chain of beams below, which prevents pins from pulling out or bending sideways when bearing weight:
However, using LEGO's parts to sandwich TrotBot's leg joints inline like the red beams above would add a lot of width to the robot. Instead, I sandwiched the leg joints by attaching an additional 3x5 L-shaped beam to the outside of the legs, which is a bit off center but still works well with LEGO's high strength-to-weight ratios. I tested these new attachments by turning TrotBot on some thick carpeting, which would usually cause a few of the leg's pins to pull out. Below the video are some pictures of how I added the parts, and I used these attachments in my TrotBot version 3 builds.
I've got a few other ideas to test over the next few weeks, and then I'll post some new TrotBot instructions with the improvements. UPDATE: Here are the new instructions with a part list.
Welcome to DIYWalkers! My name is Ben Vagle, I'm 18 years old and I've been building mechanical walkers since I was 11. 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!