Structures and stability

Learn

What you’ll learn

• Why some shapes are strong and others wobble
• What the centre of gravity is
• How to keep a robot from tipping over

Strong shapes

Push the corner of a square frame made from four sticks and it folds into a diamond. Add one diagonal and it can’t move. A triangle is the only shape whose angles can’t change while its sides stay the same length, so engineers use triangles everywhere: bridges, cranes, bike frames, robot arms.

• Brace rectangular frames with a diagonal.
• Make beams taller rather than wider: a ruler on its edge bends far less than one lying flat.
• Fold or flange sheet material: a flat piece of card bends easily; the same card folded into an L or U shape is stiff.

Centre of gravity

Every object behaves as if all its weight acts at one point: its centre of gravity (CoG). For a robot, it’s where the heavy parts are, usually the batteries and motors.

An object stays upright as long as its CoG is above its base of support (the area between the wheels or feet that touch the ground). If the CoG moves outside that area, it tips.

   stable                    tipping
  ┌──────┐                 ┌──────┐
  │  ●   │  CoG             │      │●  CoG outside the wheels
  └o────o┘                 └o────o┘
   ↑ base ↑

Making robots stable

• Put heavy parts low, especially batteries.
• Keep heavy parts near the centre, between the wheels.
• Make the wheelbase wide enough for the robot’s height.
• A robot arm that reaches out moves the CoG forward: add a counterweight or a wider base.
• Fast robots that brake hard tip forwards: keep them low.

Tip

A quick test: gently tilt your robot. The further it can lean before falling, the more stable it is.

Examples

Example: the top-heavy robot car

A student mounts the battery pack on a tall bracket above the Arduino. The car flips when it brakes. Moving the batteries under the chassis plate, between the wheels, lowers the CoG and fixes it with no other changes.

Example: bracing a tower

Design	Result with a 200 g weight on top
Four straws in a square, taped at corners	Leans and folds
Same, plus one diagonal per side	Stands straight
Triangle base (three straws) with diagonals	Stiffest, uses the fewest straws

Example: a robot arm reaching out

An arm holding a 100 g object 20 cm in front of the base pulls the CoG forward. A 100 g counterweight 20 cm behind the base balances it, because weight × distance is equal on both sides (100 × 20 = 100 × 20).

Practice

Spot the trianglesEasy

Find three triangles in structures around you (furniture, bridges, bikes, scaffolding). What would happen without them?

Paper bridgeMedium

Make a bridge from one A4 sheet that spans 20 cm between two books. How many coins can it hold? Fold it into a different shape and test again. Try folding it into a U shape or a zig-zag, like corrugated cardboard. A flat sheet holds almost nothing. A zig-zag or U-channel holds many more coins, because folding makes the material taller in the direction it bends.

Balance the armHard

A robot arm holds 150 g at 30 cm from its base. Where would you place a 450 g battery to balance it? Balance happens when weight × distance is equal on both sides. 150 × 30 = 4500, and 4500 ÷ 450 = 10, so put the battery 10 cm behind the base.

Quick check

1. Why do engineers use triangles in frames?

They look modernA triangle cannot change shape without its sides changing lengthThey use less glueThey are lighter than squares

2. Where should a robot’s battery usually go?

As high as possibleLow and near the centreHanging off the backIt doesn’t matter

3. A robot tips over when…

Its centre of gravity moves outside its base of supportIt has four wheelsIts battery is fullIt drives slowly

Check my answers