Your Robot’s Arm

Try It — A Different Kind of Motor

Hold a wheel motor in one hand and a servo in the other. They look similar. They are not.

Plug it in — the wire order matters

Plug your servo into port 0, brown wire toward the minus side.

• My servo is in port 0 with the wires the right way round
• A servo horn is attached — washer first, then horn, then the small screw

Try the widget before you write anything

On the Wombat’s Home Screen, tap Servos. Enable port 0 and drag the slider slowly.

Now let go of the slider and try to push the arm with your finger. What happens?

Learn It — Positions, Not Powers

Notice that we are counting from zero again — same as the motor ports.

Position 1024 is the middle. That is where a servo goes by default.

The four commands

The gotcha that catches everybody

When you call enable_servos(), every servo immediately jumps to whatever position it was in last time. Which might be anywhere.

If your arm was down when you last ran a program, it will slam down again the instant your new program enables the servos — possibly into the field, or into a cube, or into your hand.

// Risky: arm snaps to last position
enable_servos();
set_servo_position(0, 524);

// Better: arm starts at 524
set_servo_position(0, 524);
enable_servos();

Write your positions down

Position numbers mean nothing on their own. set_servo_position(0, 1746); tells you nothing about what the arm is doing. Comment every one:

// up         = 524
// horizontal = 1566
// down       = 1746

Use the format name = number. There is a reason for that — in Project 9 those names stop being comments and become part of the program.

Where you are headed

Do It — Reach and Touch

Step 1 — Center the horn first

• Unscrew the servo horn.
• In the Servo Widget, enable the servo and set it to 1024.
• Put the horn back on pointing at the middle of the range you actually want.
• Check both directions with the widget. Screw it back down when it is right.

• My horn is centered and screwed down

Step 2 — Find your three positions

Using the widget, find the number for each of these and write it down. Stay inside 150–1900.

Step 3 — The Wave

Create a new project called Wave. Move the arm to all three positions with a one-second pause between each. Put your position comments at the top.

#include <kipr/wombat.h>

int main ()
{
	// up         = 524
	// horizontal = 1566
	// down       = 1746

	// Preset to down
	set_servo_position(0, 1746);
	enable_servos();
	msleep(1000);

	// Horizontal
	set_servo_position(0, 1566);
	msleep(1000);

	// Up
	set_servo_position(0, 524);
	msleep(1000);

	disable_servos();
	return 0;
}

Use your numbers, not these. Compile and run.

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

Step 4 — Prove the msleep matters

Delete all three msleep() lines. Compile. Run. Watch closely.

What did the arm do?

Put them back.

Step 5 — Prove the preset trick matters

Run your Wave program so the arm finishes up. Now swap lines 9 and 10 so enable_servos() comes first, and run it again.

What did the arm do the instant the program started?

Why could that be a problem in the middle of a match?

Put it back the safe way round.

Step 6 — Drive and touch

Create a 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.

// 1. Arm starts up
// 2. Drive to the object
// 3. Stop
// 4. Lower the arm to touch it
// 5. Raise the arm again

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

Step 7 — Mission 9 — touch Botguy for 7 points

Onto the field. Create a 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 servo card

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

Can you do it again?

• I can tell a servo from a wheel motor by looking at it
• I can plug a servo in with the wires the right way round
• I know the safe range is 150 to 1900 and I never leave it
• I can write all four servo commands in the right order
• I set a position before enabling, so the arm never snaps somewhere unexpected
• I can center a servo horn using the widget
• I can combine driving and arm movement in one run

Think about it

A wheel motor takes a power. A servo takes a position. Why does an arm need a position and a wheel does not?

Your position numbers are scattered through your program as bare numbers with comments next to them. 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 servo joins the first. Two servos working as one system means you can finally grab, carry, and release — and a whole set of missions opens up at once.

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