Arm and Claw Together

Try It — Be the Robot

A claw is an effector — the part of the robot that does work on the world. In Project 6 your effector was the front of the robot, and it could only shove. A claw can hold.

Two servos, two jobs

Now act it out

Put a cube on the table in front of you. Pick it up with one hand, move it, and set it back down.

Do it again — slowly this time — and write down every separate movement, in order. Do not skip anything, even the parts that feel too obvious to write.

Try swapping any two of your steps. Pick a pair and describe what would go wrong.

Learn It — Five Numbers and an Order

Write down all five positions

Put them at the top of every program, in the name = number format:

// Your positions will be different --- do not copy these!
// arm        = 0
// up         = 230
// horizontal = 1000
// down       = 1234
//
// claw       = 3
// open       = 1246
// closed     = 340

Notice the first line of each group is the port, not a position. You now have seven numbers to keep straight, and every one of them is buried somewhere in your code.

Preset both servos

Project 7 taught you the preset trick for one servo. With two, it matters twice as much — an arm that slams down and a claw that snaps shut is a good way to break something.

// Arm up
set_servo_position(0, 230);
// Claw open
set_servo_position(3, 1246);
enable_servos();
msleep(1000);

Arm up, claw open, then enable. The robot wakes up in a known, safe shape every single time.

The grab sequence

Every single grab in this project is that sequence. You will type it four times today. Remember how that feels — Project 12 is about fixing it.

Live judged or final position — this changes your strategy

Look at how a mission is scored, not just what it asks for. Two missions can want the same thing and mean very different things by it.

Setting an object down is harder than picking it up

Opening the claw is easy. Opening the claw without dragging, tipping, or flicking the object is the actual skill.

• Lower the arm before you open, so the object has a short drop, not a long one.
• Open, then pause, then back away. Backing off while still opening drags the object with you.
• Back straight away, not on a curve. A curve sweeps the claw sideways through what you just placed.

Do It — Grab, Carry, Stack

Step 1 — Find your claw positions

Servo Widget again. Your arm numbers are already on your Project 7 card — now add the claw.

A pom squashes, a cube does not. That is why they get different numbers.

Step 2 — Grab it

Create a new project called Grab. No driving at all — place a cube inside the open claw by hand.

Write the pseudocode first. Two rules: the arm starts up and the claw starts open.

// 1. Preset arm up and claw open, then enable
// 2. Move the arm down
// 3. Close the claw
// 4. Move the arm up
// 5. Disable servos

• The cube comes up off the table when the arm rises

Did you need an msleep() between every step, or only some of them? How did you decide?

Step 3 — Go grab

Create a new project called Go Grab. Now add driving — the cube sits a short distance away.

// 1. Preset arm up, claw open, enable
// 2. Drive forward to the cube
// 3. Stop --- ao()
// 4. Arm down
// 5. Close claw
// 6. Arm up
// 7. Disable servos

• My robot drives to the cube and lifts it
• It does not knock the cube over on the way in

What happened the first time? Most robots either stop too early, stop too late, or push the cube away before the claw gets down.

Step 4 — Put it down without wrecking it

Add to the end of your program: carry the cube somewhere, set it down, and back away leaving it standing.

• The cube stays upright after I let go
• Backing away does not drag or knock it

What did you have to change to stop dragging the cube when you backed off?

Step 5 — Mission 13 Bonus — Your First Stack for 7 points

• One Unstraight Cube is ON TOP OF another

Step 6 — Mission 12 — Stack and Grab for 16 points

• Two spilled cubes are stacked
• The stack stands on its own with the lower cube on the black line

Read the “does not score” list again. Why does the lower cube have to be the one touching the line?

Step 7 — Mission 3 — Mixed Freight for 7 points

• I have one mixed-colour stack

Step 8 — Mission 11 — Hazard Disposal for 7 points

• An orange pom is IN a basket and stays there

Step 9 — Run the whole thing five times

Chain all four missions into one program, same as Project 6 — comment banners, one section at a time.

Which grab was least reliable, and what made that one harder than the others?

Score It — Checkpoint

My best run

My full servo card

Seven numbers now. Copy them somewhere you will not lose them.

Live or final?

Say how each one is judged, and what that means for whether your claw can still be holding it.

Can you do it again?

• I can find and record open and closed positions for different objects
• I preset both servos before enabling, so the robot always starts in a safe shape
• I can write the grab sequence in the right order without looking it up
• I can set an object down and back away without knocking it over
• I can stack one cube on another
• I can tell a live-judged mission from a final-position one and change my plan to match
• I never close the claw past the point where the object stops it

Think about it

Count how many times the six-step grab sequence appears in your program. Now imagine your claw’s closed position changes by 30. How many separate places would you have to edit?

A judge tells your team the Mission 12 stack scored the base but not the bonus. Your robot did everything you programmed. What most likely happened between the judge seeing it and the end of the match?

Your robot can now drive, turn, grab, carry, stack, and release. Everything it does is still measured in time — how long to push, how long to turn. What would change if it could measure in distance instead?

Next

You are carrying seven numbers around in comments, retyping them everywhere, and one rebuild would break all of it.

In Project 9 — Names for Your Numbers, those comments become real. You give each number a name once, and the whole program starts using it.

KIPR · Botball Explorer — Discovery Projects · © KISS Institute for Practical Robotics 1997–2026