Quadruped robot

A quadruped robot has four legs. Four legs is the easiest legged form to balance (the robot is stable when standing still — three legs is enough to keep upright) but the most complex for high-speed running (a galloping quadruped has all four legs off the ground simultaneously). It's where most of the real-world deployed legged robots live today.

Why four legs is the sweet spot

Wheels are cheap and efficient on flat ground but useless on stairs, in rubble, or on rough terrain. Two legs is dramatic but unstable. Four legs is the engineering middle ground:

• Static stability when standing still
• Stable enough to walk slowly with no fancy control
• Capable of dynamic gaits (trot, pace, gallop) for speed
• Can climb stairs, traverse uneven ground, and recover from a kick

This is why dogs, horses, and cats are excellent at moving through the world — and why robotic quadrupeds work as inspection robots in environments humans would rather not visit.

Who makes them

Boston Dynamics Spot — the famous one. Yellow, dog-shaped, ~$75-150k depending on configuration. Used in oil rig inspection, nuclear plant monitoring, construction site walking, and a thousand viral YouTube clips.

Unitree Go2 — the cheap-and-cheerful Chinese alternative. Around ₹2-5 lakh ($2.5-6k). Sells in vastly higher volumes than Spot to research labs, universities, and increasingly to industrial customers.

ANYbotics ANYmal — Swiss-made, industrial inspection focus.

Deep Robotics — Chinese, X20 and X30 quadrupeds, big in Asian markets.

There are 20+ smaller players, mostly building on top of the Unitree Go family as a research base.

What they do

The five biggest current applications:

1. Industrial inspection — oil and gas, power plants, chemical plants, mining
2. Construction site monitoring — walk the site, take photos, compare to plans
3. Defence and security — perimeter patrols (controversial)
4. Research and education — pretty much every robotics PhD program uses one
5. Entertainment — events, films, art installations

Notably not yet a use case: pets or companions. Spot exists and people love it, but it's too expensive and too short-lived (1.5h battery) to be a real household robot.

The current frontier

Two open problems are driving research:

• Stairs going down. Stairs going up are largely solved. Going down — where a misstep means a faceplant — is harder. Most current quadrupeds descend stairs slowly and gingerly.
• Manipulation. A robot dog with no hands is limited. Adding an arm to the back (Boston Dynamics' "Spot Arm" and various Unitree add-ons) is the current move.

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The mathematical breakthrough that made quadrupeds work is the same one Marc Raibert bet his career on in the 1980s — dynamic balance.