Name: ______________________________
Date: ____________________
Week 2
Building a Robot That Moves
Turn a motor's spin into steady rolling, and design a chassis that moves without tipping.
65-80 minutes
Key ideas
From a spinning motor to a rolling robot
A motor spins when it gets power, but a spinning motor by itself does not go anywhere. To move, the robot connects the motor to an axle, and the axle holds the wheels. When the motor turns the axle, the wheels turn, and the robot rolls.
Gears trade speed for torque
Gears are toothed wheels that lock together so one can turn the other. When a small gear turns a big gear, the big gear spins slower but with more turning power. When a big gear turns a small gear, the small gear spins faster but with less power.
Friction and traction: how wheels grip
Friction is the rubbing force between two surfaces that touch. Without friction, a spinning wheel would just slip in place and the robot would go nowhere - like tires on ice.
Balance, stability, and center of mass
The center of mass is the average spot where a robot's weight is centered. A robot tips over when its center of mass leans out past its wheels.
Chassis design: the robot's frame
The chassis is the frame that holds the motors, axles, wheels, and battery together. A good chassis keeps the wheels lined up straight, holds the motors firmly, and puts the heavy parts low.
Differential drive: steering by speed
Most small robots steer with differential drive: two wheels, one on each side, each turned by its own motor. When both wheels spin at the same speed, the robot goes straight.
Words to know
- Motor:
- A part that spins when it gets power, giving the robot the movement it needs to roll.
- Axle:
- A rod that a wheel is attached to, so when the axle turns, the wheel turns with it.
- Wheel:
- A round part that rolls the robot along the ground when its axle spins.
- Gear:
- A toothed wheel that locks into another gear to pass along spinning motion and change its speed or power.
- Speed:
- How fast the robot moves. A small gear driving a big gear makes the wheels spin slower.
- Torque:
- The turning power a motor or gear has. More torque means more force to move a heavy robot or climb.
- Friction:
- The rubbing force between two surfaces that touch. It slows sliding and lets wheels grip instead of spinning in place.
- Traction:
- How well a wheel grips the ground. Good traction means the wheel pushes the robot forward instead of slipping.
- Balance:
- Keeping the robot's weight spread so it stays upright and does not tip over.
- Stability:
- How hard it is to tip a robot over. A low, wide base is more stable than a tall, narrow one.
- Center of mass:
- The average spot where a robot's weight is centered. A lower center of mass makes the robot harder to tip.
- Chassis:
- The frame or base of the robot that holds the motors, wheels, and everything else together.
- Differential drive:
- A way to steer by driving the two side wheels at different speeds: the robot turns toward the slower wheel.
Your activity
Virtual chassis investigation
Goal: Explore how wheel size, gearing, and base shape change how a rolling base moves, grips, and stays upright.
- You will change one thing at a time - wheel size, gears, base width, or weight - and watch how the base moves. Changing one thing at a time is how you tell which change did what.
- For each change, note what happened to speed, grip, and stability. Write down which setup rolled straight and fast, and which one stayed the steadiest.
Write what happened:
Rolling-base challenge
Goal: Build a rolling base that drives straight, stays upright, and can steer with differential drive.
- Build a base with two driven wheels, one on each side, plus a support at the front or back so it does not tip. Keep the heavy parts low and the wheels lined up straight.
- Test three things: does it roll straight, can it steer by driving the wheels at different speeds, and does it stay upright when it stops and turns? Fix one thing at a time until all three work.
Write what happened:
Predict
Write your guess before you test.
Before the tip test: which base do you think stays upright longest on the ramp - the tall narrow one or the low wide one? Why?
Before swapping gears: do you think lower gearing will make the base faster or stronger at climbing? Predict what it trades away.
Test and record
Rolling-base speed and straightness test
Mark a start line and a finish line one meter apart. Drive the base from the start and record how many seconds it takes and how far off the line it ends up. Run it three times.
Record: The time to cover the distance and how far the base drifted from a straight line
| Run | Time to finish (seconds) | How far off the straight line (cm) | Notes |
|---|---|---|---|
Tip-over stability test
Place the base on the ramp and slowly raise the ramp until the base tips over. Record the tipping angle (or the number of books) for a low, wide setup and a tall, narrow setup.
Record: The angle or ramp height at which each setup tips over
| Setup | Tips at how many books / what angle | Stayed upright? (Y/N) |
|---|---|---|
Knowledge check
Answer each question. For choice questions, circle the best answer.
Diagnose the design problem.
A student's robot is tall and narrow with the battery mounted on top. It drives straight fine, but it tips over every time it makes a fast turn.
- A.The center of mass is too high
- B.The wheels have too much traction
- C.The motor has too little torque
- D.The axles are too long
How does a spinning motor make a robot roll across the floor?
- A.The motor spins an axle, and the axle turns the wheels
- B.The motor blows air backward to push the robot
- C.The motor makes the robot lighter
- D.The wheels roll on their own without the motor
You gear a robot down so its wheels turn slower. What do you gain, and what do you give up?
- A.You gain speed and give up torque
- B.You gain torque (turning power) and give up speed
- C.You gain both speed and torque
- D.You give up both speed and torque
A robot's wheels spin fast but it barely moves on a smooth, slippery floor. What is the problem?
- A.Too much torque
- B.The motor is off
- C.Low traction - the wheels are slipping instead of gripping
- D.The center of mass is too low
Which robot base is the hardest to tip over?
- A.A tall, narrow base with the heavy parts up high
- B.A low, wide base with the heavy parts near the bottom
- C.A base with the biggest wheels, no matter its shape
- D.The most colorful base
In differential drive, how does a two-wheeled robot turn left?
- A.It uses a steering wheel like a car
- B.The left wheel turns slower than the right wheel
- C.Both wheels speed up together
- D.The motor tilts sideways
Reflect
What mechanical change made the biggest difference in your robot's movement, and why?
What change made your base the most stable, and why did it help?
Describe how you would steer your robot left using only its two wheels.