Centre of gravity
Every object behaves, as far as balancing is concerned, as though all its weight were concentrated at a single point: its centre of gravity. Find that point and you can predict whether a robot stands up or falls over, without knowing anything else about it.
The rule
Drop a vertical line from the centre of gravity. If it lands inside the base — the patch of floor between the parts touching the ground — the object stands. If it lands outside, it tips.
That is the whole of it. Everything else — wide wheelbases, low batteries, counterweights — is a way of keeping that line inside that patch.
The red line is the pull of gravity through the centre of the whole robot. It tips the moment that line lands outside the wheels.
It stands. Raising the load costs nothing while the robot is level, and costs everything the moment it tilts: this one goes over at about 42°.
Two ways to make a robot harder to tip, and they are both in the numbers above: put the heavy parts low so the line barely moves when it tilts, and make the wheelbase wide so there is more floor for the line to land on.
Height only matters once it tilts
This is the part worth being deliberate about, because it is why a robot passes every test on the bench and falls over in the competition.
Standing level, a load carried high is exactly as stable as the same load carried low — the line comes down in the same place either way. Tilt the robot, though, and the line swings sideways by an amount proportional to how high the centre of gravity is. A tall robot needs only a small tilt to push the line past its wheels; a low one can be tipped a long way and still come back.
A robot does not have to be on a ramp to be tilted, either. Accelerating hard, braking hard and turning sharply all lean the effective pull of gravity to one side, and a tall robot finds that out at the first corner.
Making a robot hard to tip
- Put the heavy things low. The Brick and its batteries are much the heaviest part of an EV3 robot — mounting them low is the single most effective change you can make.
- Make the base wide. More floor for the line to land on. A wide wheelbase resists tipping sideways; a long one resists tipping forwards under braking.
- Keep the load in. A gripper holding something out at arm’s length moves the centre of gravity out with it. Carry things tucked in and only extend to place them.
- Counterweight the other side. If an arm must reach out, a weight behind the wheels pulls the centre of gravity back where you want it.
There is a cost to a very low, very wide robot: it cannot climb over anything and it will not fit through gaps. Like everything else in mechanics, it is a trade.
Finding it on a real build
Balance the robot on one finger, or on the edge of a ruler, until it sits level. The centre of gravity is directly above where you are supporting it. Do it from two directions — front to back, then side to side — and you have located it well enough to build around.
Then test it properly: raise one side of a board until the robot goes over, and measure the angle. Change one thing, do it again, and record both. That is the same one-thing-at-a-time discipline Mechanisms without motors asks for everywhere else.
Why it matters
Racing cars are built low and wide for exactly this reason, and double-decker buses are tested by tilting them until they fall. A crane carries huge counterweights so that the line from its centre of gravity stays inside its tracks even with a load out at full reach.
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.
- Biomimetic mechanisms — Linkages that copy how animals move — turning a rotation into a step.