The Balancer

1. Clear a big table and protect it with an old piece of cardboard.
2. Lay out every part from the shopping list in front of you.
3. Put the small parts (screws, screw eye, paper clip) in a bowl so they don't get lost.
4. Ask a grown-up to install the Arduino IDE software on the computer.

Tip

Take a photo of all your parts before you start. It will be the first page of your robot-builder's logbook!

1. Cut a 12 cm piece from the batten: that's the post.
2. Cut a second piece of 6 cm: that will hold the motor.
3. Glue the post standing up, 3 cm from the left edge of the board, right in the middle of its width.
4. Let it dry for 30 minutes. Meanwhile, move on to step 3!

What do you think?

Why does the post need to be perfectly straight? Hint: if the rail already tilts at the start, who will have to work harder?

1. Lay the two dowels on the strip, nicely parallel, about 2.5 cm apart.
2. Test with a ball: it must roll on the dowels without touching the strip.
3. When the gap is right, glue the dowels and hold them with tape while they dry.
4. Glue a small piece of cardboard on the right end: it's a stopper so the ball doesn't fly off.

Tip

Draw two pencil lines on the strip before gluing. The dowels will stay nice and straight.

1. Screw one half of the hinge on top of the post.
2. Screw the other half under the rail, about 2 cm from its left end.
3. Lift the right end of the rail with your hand: it should go up and down easily, like a seesaw.

Careful

If the hinge rubs or squeaks, the motor will struggle. Loosen the screws just a tiny bit.

Wires are like roads: each colour goes to one exact place. Use the breadboard so the motor and the sensor share the 5V and the GND.

Golden rule

We plug and unplug wires only with the USB cable unplugged. And we check twice before plugging it back in.

1. Connect the Arduino to the computer with the USB cable.
2. In Arduino IDE, choose Tools → Board → Arduino Uno and the right Port.
3. Copy this little program, then click the Upload arrow.

#include <Servo.h>

Servo motor;

void setup() {
  motor.attach(9);    // the orange wire is on pin 9
  motor.write(90);    // 90 degrees = the middle position
}

void loop() {
  // nothing: the motor stays in the middle
}

4. The motor moves a little, then stops: it's now exactly in the middle.
5. Push the little plastic arm (the horn) onto the shaft, perfectly level.

Why code first?

If you fix the arm first, you don't know where the motor's "middle" is. The rail might never be flat.

1. Screw the screw eye under the rail, 35 cm from the hinge.
2. Fix the servo onto its wooden block (hot glue or cable ties), shaft facing forward.
3. Place the block on the board, right under the screw eye. Don't glue it yet!
4. Unfold the paper clip into a straight rod and bend a small hook at each end with pliers.
5. Hook one end into the outer hole of the horn, the other into the screw eye.
6. Slide the block until the rail is perfectly flat. Check with a ball: it must not roll. Now you can glue the block.

Stuck?

If the rail is too high or too low, change the length of the rod instead of forcing the motor.

1. Glue the sensor at the left end of the rail, on the hinge side, its two "eyes" facing along the rail.
2. The eyes should be at the height of the middle of the ball.
3. Upload this program, then open Tools → Serial Monitor.

const int PIN_TRIG = 8;
const int PIN_ECHO = 7;

void setup() {
  Serial.begin(9600);
  pinMode(PIN_TRIG, OUTPUT);
  pinMode(PIN_ECHO, INPUT);
}

void loop() {
  digitalWrite(PIN_TRIG, LOW);  delayMicroseconds(2);
  digitalWrite(PIN_TRIG, HIGH); delayMicroseconds(10);  // the shout!
  digitalWrite(PIN_TRIG, LOW);
  long duration = pulseIn(PIN_ECHO, HIGH, 25000);  // listen for the echo
  float cm = duration * 0.0343 / 2;               // there and back again
  Serial.print("Ball at ");
  Serial.print(cm);
  Serial.println(" cm");
  delay(100);
}

4. Roll the ball gently with your finger. Do the numbers change? Well done, your robot can see!

Did you know?

Sound travels at 343 metres per second. We divide by 2 because the sound goes to the ball and comes back.

Here's the complete brain. Read the green comments: they explain every line. Upload it, put the ball on the rail and watch!

#include <Servo.h>

// ----- The wiring -----
const int PIN_SERVO = 9;
const int PIN_TRIG  = 8;
const int PIN_ECHO  = 7;

// ----- Lili's settings -----
float TARGET     = 18.0;  // where the ball should stop (cm from the sensor)
int   FLAT_ANGLE = 90;    // motor angle when the rail is flat
int   DIRECTION  = 1;     // set to -1 if the ball runs away instead of coming back
float Kp = 2.0;   // P: Push
float Ki = 0.02;  // I: Insist
float Kd = 0.8;   // D: Dampen

Servo motor;
float distance = TARGET;
float lastError = 0;
float sum = 0;
unsigned long lastTime = 0;

float measure() {
  digitalWrite(PIN_TRIG, LOW);  delayMicroseconds(2);
  digitalWrite(PIN_TRIG, HIGH); delayMicroseconds(10);
  digitalWrite(PIN_TRIG, LOW);
  long duration = pulseIn(PIN_ECHO, HIGH, 25000);
  if (duration == 0) return -1;           // no echo: ignore it
  return duration * 0.0343 / 2;
}

void setup() {
  Serial.begin(9600);
  pinMode(PIN_TRIG, OUTPUT);
  pinMode(PIN_ECHO, INPUT);
  motor.attach(PIN_SERVO);
  motor.write(FLAT_ANGLE);
  delay(1000);
  lastTime = millis();
}

void loop() {
  // 1. The eyes look
  float d = measure();
  if (d > 2 && d < 40) {
    distance = 0.7 * distance + 0.3 * d;   // smooth out jumpy readings
  }

  // 2. The brain thinks
  unsigned long now = millis();
  float dt = (now - lastTime) / 1000.0;
  lastTime = now;

  float error = distance - TARGET;                      // how far from the goal?
  sum = constrain(sum + error * dt, -50, 50);           // I
  float speed = (error - lastError) / dt;                // D
  lastError = error;

  float correction = Kp * error + Ki * sum + Kd * speed;
  correction = constrain(correction, -30, 30);         // no more than 30 degrees

  // 3. The muscle moves
  motor.write(FLAT_ANGLE + DIRECTION * correction);

  // For the Serial Plotter: the ball and the target
  Serial.print(distance); Serial.print(" "); Serial.println(TARGET);
  delay(40);
}

Ball running away?

Don't panic, that's normal the first time! The motor may be mounted the other way round. Change DIRECTION = 1 to DIRECTION = -1 and upload again.

Open Tools → Serial Plotter: you'll see two lines, the ball and the target. Your goal: get the ball's line to meet the target and stay there. Change only one number at a time, upload, watch.

Logbook

Write down every try in a notebook: "Kp = 2, Kd = 0.8 → the ball swings 3 times". That's exactly what engineers do!

Motor shaking and the Arduino restarting?

The motor is asking for too much power from the USB cable. Ask a grown-up to add a 4×AA battery holder for the motor, with its "minus" connected to the Arduino's GND.