Motors and Ports

Try It — Which Way Does It Go?

Think about walking across the room. Your brain does not send one signal to “your body.” It sends a signal to your left leg and a separate signal to your right leg.

The Wombat works the same way. Two motors, two wheels, two separate commands.

Find the ports

Look at the Wombat. There are four ports for motors. Two on the left, two on the right.

0

Left side

1

Left side

2

Right side

3

Right side

Find your motors and plug them in

A motor has a two-prong plug and a double wire — one red, one black.

Plug one motor into port 0 and the other into port 3.

• I found all four motor ports and can point to 0, 1, 2, and 3
• One motor is in port 0 and one is in port 3
• My robot is on a block with the wheels in the air

The plug goes in two ways — and it matters

There is no mark on the motor plug telling you which way is right. You can put it in either way around, and the two ways do opposite things.

A motor spins in whichever direction the electricity flows through it. Flip the plug, and the wheel spins the other way.

Green

— that direction is forward (+)

Red

— that direction is reverse (−)

Spin both wheels the direction you want the robot to drive forward. Record what you see.

• Both LEDs are green when I spin the wheels forward

Why does a robot spin in a circle when one motor runs forward and the other runs backward?

Learn It — The motor() Command

One command runs a motor. It needs two pieces of information: which motor, and how hard.

motor(0, 50);
      ↑   ↑
   port  power

Each piece of information inside the parentheses is called an parameter. printf() took one parameter. motor() takes two, port and power, separated by a comma.

An argument is the actual value you provide to a parameter when you call a function. For example, in motor(0, 50), 0 and 50 are arguments.

Power runs from −100 to 100

A minus sign in front of the power number reverses that motor. This is how you back up, and later, how you turn.

Which end is the front?

Your robot has two driven wheels and one small free-rolling wheel called a caster. Before you send it anywhere, decide which end is the front — because that decides what motor(0, 100) actually does.

Set your robot up so that the green power LED and the ports face forward — the same end the arm will eventually go on. That end is the front, and everything you write from now on assumes it.

Turning motors off

ao() stands for all off. It stops every motor at once and takes no arguments — just empty parentheses.

A stop is not instant either

ao() cuts the power. It does not grab the wheels and hold them — the robot is still moving when that line finishes, and it coasts a little further before it truly stops.

So give it a moment to settle before you do anything else:

motor(0, 50);
motor(3, 50);
msleep(2000);
ao();
msleep(30);
// Let it come to rest
msleep(30);

Thirty milliseconds is not long enough to notice, and it is long enough to matter. Without it, your next command starts while the robot is still drifting — and a turn that begins mid-drift ends up somewhere you did not ask for.

From here on, every ao() in this curriculum is followed by msleep(30). Get in the habit now.

Why you always need msleep()

Here is the thing that surprises everyone. Look at this program:

motor(0, 50);
motor(3, 50);
ao();
msleep(30);

The wheels will not move. Not even a twitch.

The controller turns both motors on, and then — faster than you can blink — reads the next line and turns them off again. Remember from Project 2: it moves through lines far quicker than your eye can follow.

Turning a motor on does not mean “run for a while.” It means “run, starting now, until something tells you to stop.” The msleep() is what gives it that while.

In your own words: what is msleep() actually doing while the motors run?

Do It — Make It Turn

Step 1 — Drive the motors by hand first

Find the red line for a port and drag it left and right with your finger.

Remember that number. You will use it in a much later project to drive exact distances.

Step 2 — Write your first motor program

Make a new project called Motors. Do not forget your attribution comments at the top.

#include <kipr/wombat.h>

int main ()
{
	// Turn motor 0 on at 50% power
	motor(0, 50);
	// Turn motor 3 on at 50% power
	motor(3, 50);
	// Keep both motors running for 2 seconds
	msleep(2000);
	// Turn all motors off
	ao();
	msleep(30);
	return 0;
}

Compile and run it. Watch the wheels.

• Both wheels spun the same direction for about 2 seconds

Did both wheels spin the same way? If not, what does that tell you about your plugs?

Step 3 — Prove that msleep matters

Delete the msleep(2000); line. Compile. Run.

Now put it back.

• msleep is back and the program works again

Step 4 — Prove that ao() matters

Now delete the ao(); line instead. Compile. Run. Watch carefully.

Put it back. Leaving motors running with no ao() is one of the most common bugs in Botball — and on a real field it means a robot that will not stop.

• ao() is back

Step 5 — Explore power

Change both power numbers, run, and record what you notice. Keep msleep(2000) the same every time so it is a fair test.

Is power 100 exactly twice as fast as power 50? What makes you say that?

Step 6 — Go backward

Put a minus sign in front of both power numbers:

motor(0, -50);
motor(3, -50);

Run it and watch the LEDs by the ports as well as the wheels.

• Both wheels spun backward

Step 7 — Make it disagree with itself

Now give the two motors opposite powers:

motor(0, 50);
motor(3, -50);

Run it. The wheels are in the air, so watch what would happen on the ground.

If this robot were on the floor, what would it do?

Hold on to this. It is exactly how you will make the robot turn in Project 5.

Step 8 — Write three of your own

Write a command for each of these. Do not run them yet — just write them.

• I tested one of my own commands and it did what I expected

Score It — Checkpoint

No field mission yet — that starts in Project 4. This checkpoint is about whether your motors are set up correctly and whether you can control them on purpose.

My robot’s setup

Write this down. Every program you write from now on depends on it.

Read the code

What will this program do? Write it out before you run it.

motor(0, 100);
motor(3, 100);
msleep(1000);
motor(0, -100);
motor(3, -100);
msleep(1000);
ao();
msleep(30);

Now run it. Were you right? If not, what did you miss?

Can you do it again?

• I can name all four motor ports and know counting starts at 0
• I can use the wheel-spin trick to check a motor’s direction without running a program
• I can fix a backward motor by flipping the plug
• I can write motor() with the right port and power
• I can make a motor run backward using a negative power
• I can explain why a motor program needs msleep()
• I always finish a motor program with ao()

Think about it

A teammate says their robot “just doesn’t work” — they run the program and nothing moves. Name two things you would check first, and why.

Nothing in this project told the robot how far to go — only how hard to push and for how long. Do you think that is enough to hit a target on the field? Why or why not?

Next

In Project 4 — Out and Back, the robot comes off the block and onto the field. You will drive out of the starting box, park IN THE ZONE, and drive back — and that scores a real mission.

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