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.
When Creating New Mechanisms, Start with a 4-Bar Linkage
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 step-height could be increased 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 allowed us to increase the step-height significantly, as shown below.
We found that Strider's boat-shaped foot-path mitigates this problem, since the foot's horizontal speed during the lift and lower phases is similar to the foot-speed of the drive phase. This can be seen in the above GIF of Strider walking across the screen. Notice how the foot's horizontal position doesn't change much during the bottom halves of its lift and lower phases. So, when a foot steps on an obstacle halfway down its lower phase, it won't skid or cause the robot's forward motion to stop while stepping up onto the obstacle.
The impact of the foot-path's boat shape can be exaggerated by reducing the number of legs from 12 to 8, causing the feet to contact the ground well above the bottom drive phase of a 12 leg Strider:
You can see this in action in the below video of an 8-legged Strider walking on rocky terrain. Strider's feet often contact the rocks well above the bottom drive phase, yet the robot's speed remains fairly consistent as it walks across the rocks, and it doesn't come to a stop with each step like our 8-legged Klann walkers do on such terrain.
We also optimized TrotBot's linkage for this characteristic, although not as effectively as we did for Strider's linkage:
Strider's high-stepping, boat-shaped foot-path allows it to meet Shigley's rugged terrain requirements fairly well - at least at LEGO-scale. How about the ability to carry heavy loads and the speed requirements of tanks?
High Speed Walking
As Shigley described, an ideal walking mechanism for a tank should have the constant horizontal speed of a wheel, while also being able to step over obstacles that block wheels. Strider has relatively constant foot-speed, resulting in an 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:
High Speed Vibration
Shigley also described how the vertical and horizontal inertia forces of the mechanism should be balanced for a tank to walk fast without vibrating itself to death. Such vibration isn't much of a problem at LEGO-scales, but it gets worse as scale increases. If you've ever driven a vehicle where the steering wheel shakes due to its wheels being out of balance, and seen how it's corrected by attaching small metal weights to the wheels, then you can probably imagine how much worse the vibration problems could be for a vehicle with large mechanical legs running at high RPMs.
Strider's mechanism would almost certainly need to be modified in order for a large-scale version to walk at high RPMs with limited vibration. If curious, Shigley's study below shows how to mathematically analyze inertia forces, and gives some suggestions for balancing them.
Load Bearing and the Linkage's Bar Count
The more bars a linkage has, the more joints it needs. Each joint adds friction. Furthermore, the joints will always have some play, which can cause long legs with many joints to bend under loads, especially when turning them tank-style. Strider's paired-leg, 10-bar linkage has relatively few bars per leg (5 bars/leg) versus TrotBot's and Strandbeest's 8 bars/leg, and Klann's 6 bars/leg. If implemented well, this implies that Strider's linkage should be able to handle loads relatively well.
Below tests linkage variation #6 with a 25 pound load. The plastic parts bend and shift somewhat under the load, increasing the difficulty of carrying the load, but the LEGO motors didn't stall. Also, we were planning on uploading a video with Strider self-destructing at the end by attempting to turn it while carrying 25 pounds. Some of our other walkers self-destructed while carrying much less weight, so we were a bit surprised that Strider survived with its legs intact. Strider's low bar count per leg does appear to help with carrying heavy loads.
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).
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.
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!