Biomimetic mechanisms
Biomimetics is copying a solution from nature. Not the look of an animal — the mechanism. Wheels are a human invention and a very good one, but nothing in nature has them, and there are places a walking machine goes that a wheeled one cannot.
The idea that has to land first
Nothing in a walking robot walks. There is a motor going round and round at a perfectly steady rate — the same motion that drives a car — and a linkage: a set of bars pinned together whose shape turns that circle into a footstep.
Push down, lift, swing forward, put down. No sensor decides when to do each part, and no block in the program says step. The bars do it, because of how long they are and where they are pinned.
The faint circle is the crank pin. The green outline is where the foot goes. They are driven by the same axle and they are not remotely the same shape — that difference is the linkage.
The two paths on that stage are the whole module. The crank pin travels a perfect circle. The foot travels something flat along the bottom and arched over the top — and they are driven by the same axle at the same speed. Everything between them is link lengths.
Why the foot path matters
- Flat along the bottom. While the foot is on the ground it must travel in a straight line at a steady rate, or the body bounces and the robot shuffles instead of walking.
- Lifted on the return. High enough to clear the ground and whatever is on it. A higher arch climbs better and wastes more effort.
- Legs out of phase. With legs half a turn apart there is always a foot down, so the body never drops. Four or six legs in pairs is steadier still — which is why insects keep three feet down at a time and are almost impossible to knock over.
The famous designs are worth looking up by name: Jansen linkages (the strandbeest walkers), Klann linkages, and the four-bar “Chebyshev” walker. All of them are the same trick with different numbers.
Other things worth copying
| In nature | As a mechanism |
|---|---|
| A caterpillar or inchworm | Anchor one end, contract, anchor the other, extend. Slow, and it climbs anything. |
| A fish tail or a snake | A chain of segments swung out of phase — a travelling wave that pushes backwards. |
| A bird wing | A four-bar linkage that flaps and feathers at once, from one rotation. |
| A hand or a claw | Linked fingers driven from a single input, so they close around a shape rather than onto a fixed position — a lever chain that adapts by itself. |
Walking versus wheels
Be honest about the trade, because it is the engineering content of the whole topic. Wheels are far more efficient, far simpler and far faster on anything flat. Legs are worth their complexity when the ground is not flat: steps, rubble, soft sand, gaps. A walking machine chooses where to put its feet; a wheel has to roll over everything in between.
Building one out of LEGO
- Link lengths are everything, and a stud out is a broken gait. Build one leg, get its foot path right, and only then copy it.
- Legs need to be built in mirrored pairs on a common axle, half a turn apart. Getting that phase wrong is the usual reason a walker rocks in place.
- Walking is hard work — expect to gear down heavily, and expect it to be slow.
- A tall walker tips easily. Keep the Brick low and the stance wide — see Centre of gravity.
Why it matters
Robots that copy animals are how machines reach places wheels cannot: search and rescue in collapsed buildings, inspection inside pipes, planetary rovers over loose ground. Studying the mechanism instead of the appearance is what turns an interesting animal into a working machine.
More mechanics tutorials
- Mechanisms without motors — How to investigate a build with no electronics in it, and where each idea lives.
- Gearing up and gearing down — Trade turns for force, or force for turns — you cannot have both.
- Changing the direction of a turn — Reverse a turn, restore it with an idler, or send it round a 90° corner.
- Pulley systems — Fixed, movable and combined — how rope pulled trades against force needed.
- Levers — effort, load and pivot — Where the pivot sits decides the force you need and the distance you get.
- Elastic and stored energy — Stretch a band, store energy, let it go — and find where more stops helping.
- Scissor mechanisms — Crossed links that extend and retract, and why the last bit is the hardest.
- Oscillatory motion — Turning a rotation into a back-and-forth, and what sets the rhythm.
- Centre of gravity — Why a robot tips, and how to build one that does not.