Hold a ruler at one end and wave it. The far end moves much farther than your hand. That is a lever, and it is how every robot arm works.
Key idea
A lever turns around a pivot. A point far from the pivot moves farther than a point close to it. But it pushes with less force. You cannot get both for free.
Why it works
Robot arms are chains of levers. Long arms reach far but cannot lift much. Strong arms are short or have big motors. Engineers always trade reach for strength.
Let's think it through
Worked example: How far does it reach?
You push a lever 2 cm from its pivot and move it down 1 cm. The other end is 6 cm from the pivot. How far does it move up?
- The far end is 3 times as far from the pivot (6 ÷ 2 = 3).
- So it moves 3 times as far: 3 × 1 cm.
Answer: 3 cm.
Common mistake
Thinking a long arm is both farther-reaching and stronger. Each step out along the arm gives more reach and less strength.
Hands-on project 1: a cardboard arm
Parts (mostly free)
- Cardboard from a box, a ruler and a pencil
- Brass paper fasteners for the joints
- Clothespin for the gripper
- Optional: plastic syringes and tubing (a few dollars) to make the joints move with water pressure
- Cut three cardboard strips: a base, an upper arm and a forearm.
- Join them with paper fasteners at the shoulder and elbow so they bend.
- Glue a clothespin at the end as the hand. Move the joints by hand and pick up a paper cup.
- Measure: how far can the hand reach? What can it lift without bending?
Hands-on project 2: a bristlebot
Parts (about \$10 for several)
- A toothbrush head (or a brush with stiff bristles)
- A small vibration motor, sold for hobby projects
- A 3-volt coin cell and double-sided foam tape
Tape the motor and coin cell on top of the brush head, and touch the motor's wires to the battery. The motor shakes, the bristles bend, and the bot scoots. Try bending the bristles one way to make it go straighter. It is not a robot (it does not sense), but it is a first machine you built yourself.
Safety.
- A grown-up does the cutting with a craft knife, on a cutting mat. Kids can use safety scissors.
- Use a low-temperature glue gun with a grown-up, or plain white glue.
- Coin cells: a grown-up hands them out and puts them away; they are dangerous if swallowed.