Counting Wheel Ticks

Try It — The Counter Was Always There

Back in Project 3 you dragged the slider on the Motors screen and noticed a number changing next to each port. You were told to remember it. Here it is.

Open the Motors screen on the Wombat. Do not run anything — just turn a wheel with your hand and watch.

Find your ticks per inch

Ticks are the motor’s own unit. To use them you need to know how many make an inch on your robot.

• Put the robot on the floor and mark where the front edge sits.
• Clear the counter on the Motors screen.
• Push the robot straight forward exactly 24 inches. Slowly, and in a straight line.
• Read the counter.

Your three numbers are not identical. Why not — and which one should you use?

Learn It — A Number Instead of a Switch

A tick is one small step of the motor shaft. There are about 1820 ticks in one full revolution of the shaft.

Two commands

Both names do the same thing. The short ones are quicker to type and you will see both in other people’s code.

The same loop you already know

Look at these two side by side. Only the condition changed.

Project 10 — a switch

while (digital(bump) == 0)
{
	motor(left, 50);
	motor(right, 50);
}
motor(0, 0);
motor(3, 0);
msleep(30);

Project 11 — a number

while (gmpc(left) < 4000)
{
	motor(left, 50);
	motor(right, 50);
}
motor(0, 0);
motor(3, 0);
msleep(30);

Read the new one out loud: “While the left motor has turned fewer than 4000 ticks, keep driving. The moment it reaches 4000, stop.”

Always clear before you count:

// Start counting from zero
cmpc(left);

while (gmpc(left) < 4000)
{
	motor(left, 50);
	motor(right, 50);
}

motor(0, 0);
motor(3, 0);
msleep(30);
// Let it settle
msleep(500);

It will go too far. Every time.

Run that program with a target of 4000 and then check the Motors screen. You will not see 4000. You will see something like 4310.

Nothing is broken. Your robot has inertia — it is moving, and when the loop shuts the motors off it keeps coasting.

Going backward makes the number go down

Driving in reverse decreases the counter, and it will go negative. That gives you two ways to come home.

Do not clear — count back to 0

cmpc(left);
// Drive Out
while (gmpc(left) < 4000)
{
	motor(0, 0);
	motor(3, 0);
	msleep(500);
}

// Drive Home
while (gmpc(left) > 0)
{
	motor(0, 0);
	motor(3, 0);
	msleep(30);
}

Clear again — count to −4000

cmpc(left);
// Drive Out
while (gmpc(left) < 4000)
{
	motor(0, 0);
	motor(3, 0);
	msleep(500);
}

cmpc(left);
// Drive Home
while (gmpc(left) > -4000)
{
	motor(0, 0);
	motor(3, 0);
	msleep(30);
}

Both work. The first one always returns to where it started, no matter how far out it went. The second one always travels the same distance back. Those are not the same thing, and one day the difference will matter.

Do It — Drive by the Numbers

Step 1 — Measure your overshoot

Create a new project called Ticks. Clear, drive to 4000 ticks, stop. Then check the Motors screen for what you actually got.

What is the relationship between power and overshoot?

Step 2 — Correct for it

Subtract your overshoot from your target and run again. Keep adjusting until the robot lands on 4000.

• My robot lands within about 50 ticks of where I asked

Step 3 — Drive a real distance

Use your ticks-per-inch from Try It. Work out the ticks for each distance, then test it with a ruler.

Which distance was least accurate? Why do you think that is?

Step 4 — Out and back

Drive out 4000 ticks, stop and settle, then return to exactly where you started. Use whichever of the two methods from Learn It you prefer.

• My robot returns to its starting mark

Step 5 — Turn by ticks

Create a new project called Right Turn. Zero radius turn — one wheel forward, one back, same speed — but this time stop it by counting ticks instead of by time.

cmpc(left);

while (gmpc(left) < turnTicks)
{
	motor(left, 50);
	motor(right, -50);
}
motor(0, 0);
motor(3, 0);
msleep(30);

Step 6 — The square, again

In Project 5 you drove a square with four timed turns and it did not close. Do it again — same square, but every distance and every turn counted in ticks.

Your timed square drifted a little more with every corner. Why does a tick-counted square not do that?

Step 7 — Mission 9 Bonus — get Botguy out for 9 points

• Botguy is out of the enclosure and touching the floor

Step 8 — Mission 18 Base — take him to the loading zone for 11 points

Mission 18 is final position, so Botguy has to still be IN THE ZONE when the match ends. Set him down properly and back straight away.

• Botguy is IN the Loading Zone and stays there

Step 9 — Mission 8 — deliver the red cube for 20 points

• Pallet and Large Red Cube are both on the Loading Dock
• A Small Red Cube is on top of the Large Red Cube up there

Step 10 — Run it five times

Score It — Checkpoint

My score

My tick card

These replace every timing number you have been carrying since Project 4.

Write the condition

Can you do it again?

• I can read a motor’s position counter on the Wombat screen
• I can work out my robot’s ticks per inch and convert a distance into ticks
• I always clear the counter before I start counting
• I measure my overshoot and subtract it from my target
• I use < going forward and > going backward, and I check it before running
• I can make a repeatable 90° turn using ticks instead of time
• My square closes

Think about it

Your battery is half flat, so the robot drives slower. What happens to a run built on msleep()? What happens to a run built on ticks?

Ticks tell you how far the wheels turned — not how far the robot moved. Name a situation where those two are different.

You have now written the drive-a-set-distance code and the turn-90° code several times each, in several projects. What would you rather do than keep retyping them?

Next

You have a robot that can measure. You also have a program that says the same twenty lines over and over.

In Project 12 — Teaching Your Robot New Moves, you give those twenty lines a name — and then you just say the name.

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