Your Robot's Arm

Try It — A Different Kind of Job

Your arm motor is the exact same kind of motor as your wheels. What changes today is not the motor — it is the job.

Remember the number from Project 3 — the one that changed when you turned a wheel by hand? The hub is counting every motor’s position, all the time. Today that counting stops being a curiosity and becomes the whole point: “up” is a place, not a push.

Feel the places

Arm motor in port C. Connect the app and open the view that shows the connected devices, so you can see the arm motor’s live number.

Move the arm gently with your hand, from one end of its swing to the other, and watch the number.

• My arm motor is in port C and shows up in the app
• I moved the arm by hand and watched its number change

In your own words: what is the difference between telling a motor how far to roll and telling it where to be?

Learn It — Positions, Not Spins

One block does today’s job. It needs three pieces of information: which motor, which way round, and which place.

Positions live on a circle

A motor’s positions run from 0 to 359 degrees, like the face of a clock. And 0 is not random: look closely at the motor and you will find two little marks on its case. When the marks line up, the motor is at position 0. That is its home, and it is the same home every time — which is exactly what an arm needs.

This block finishes the trip

A go to position block does not start the arm and let go. It moves the arm all the way to the place, and only then does the next block run — just like your move-for blocks on the wheels.

Where you are headed

Seven points for one touch. It is the best points-per-effort deal on the field — as long as you actually make contact.

Do It — Reach and Touch

Step 1 — Find your arm’s home

New project called Arm — names in the project name. Build just this, and run it:

Watch where the arm ends up. That is your arm’s position 0 — its home. Everything else gets measured from here.

Step 2 — Find your three positions

Now move the arm by hand, slowly, and read its live number in the app at each pose. These three numbers are about to run your whole season.

Step 3 — The Wave

Make the arm visit all three positions, holding each one long enough to see. Plan it as pseudocode first — speed, then up, pause, horizontal, pause, down, pause, back up — then build it:

• My arm moves to all three positions with a pause between each

The position block already finishes the trip before the next block runs. So what are the wait blocks for?

Step 4 — Prove the speed matters

Change the set speed block to 100% and run the Wave again. Stand back a little.

Put it back to a gentle number. That is the arm’s speed from now on.

• The speed is back to gentle

Step 5 — Prove the direction matters

Pick one position block and change shortest path to the arrow that goes the long way round. Run it — with your hand ready near the stop button.

What happened, and what could that do on a real field?

Set it back. From now on, the direction dropdown is a decision, not a default.

• Every position block is back to a route I chose on purpose

Step 6 — Drive and touch

New project called Reach. Practice away from the field first: put any object a short drive away, then drive to it and touch it with the arm.

Two rules, borrowed from how the mission works:

• The arm must start up and move down to touch. No driving around with the arm already out front.
• Touch it with the arm only — not with the robot’s body.

• I can drive to an object and touch it with the arm

Step 7 — Mission 9 — touch Botguy7 pts

Onto the field. New project called Botguy.

Botguy is inside the enclosure. Your robot has to reach in and make contact — direct contact, arm to Botguy.

• My arm makes direct contact with Botguy
• I did not knock the enclosure out of place getting there

Step 8 — Run it five times

Score It — Checkpoint

My score

My arm card

Keep these next to your driving numbers. Everything from here uses them.

Can you do it again?

• I can point to the little marks that show a motor’s position 0
• I know which port my arm is in, and my blocks all say the same port
• I know my arm’s real ends and never command a place past them
• I can build a go-to-position block with the right port, direction, and degrees
• I set a gentle speed before the first arm move, so the arm never slams
• I know where my arm points at position 0
• I can combine driving and arm movement in one run

Think about it

A move block tells the wheels how far to roll from wherever they are. A position block tells the arm exactly where to end up. Why does the arm need the second kind?

Your position numbers are scattered through your program as bare numbers. If you rebuilt your arm slightly differently tomorrow, how many places would you have to change?

You can touch Botguy but you cannot get him out of the enclosure. What exactly can a claw do that a single arm cannot?

Next

In Project 8 — Arm and Claw Together, a second motor joins the first. Two motors working as one system means you can finally grab, carry, and release — and a whole set of missions opens up at once.