Lab 1 — Four-Motor Car Bring-Up and Motor Mismatch¶

Duration90 minutes

PlatformESP32 + dual H-bridge + 4-motor car

MilestoneWorking car + PWM table + measured drift

BringLaptop with Arduino IDE

1. Lab objective¶

In this first laboratory you will connect and test the four DC motors of the robot car using an ESP32 and an H-bridge motor driver.

The main experiment investigates a simple question:

The question this lab is built around

If the same PWM command is sent to the motors on both sides of the car, will the car travel straight?

Hold onto your answer. Week 7 is where you fix whatever you find today.

2. Hardware required¶

Each group requires:

Four-motor robot car chassis
ESP32 development board
Dual H-bridge motor driver
Battery
Jumper wires
USB cable
Computer with Arduino IDE
Measuring tape

Before you power anything

Lift the car so all four wheels spin free before the first run. A car that starts unexpectedly on the bench will drive itself onto the floor, and a trailing USB cable will take your laptop with it.

3. Electrical architecture¶

                   ESP32
                     │
              Control Signals
                     │
                     ▼
              ┌──────────────┐
Battery ─────►│   H-BRIDGE   │
              │ MOTOR DRIVER │
              └──────┬───────┘
                     │
             ┌───────┴───────┐
             │               │
             ▼               ▼
        Left Motors      Right Motors

The ESP32 controls the H-bridge. The H-bridge supplies the current required by the motors.

The ESP32 never powers the motors

Motor current comes from the battery, through the H-bridge. The ESP32 only sends signals. If you try to run motors off the ESP32's 3.3 V pin you will brown out the board and it will reset the moment the wheels load up.

Record your connections¶

Before writing the program, complete this table. Keep it — every later lab assumes these pins.

Function	ESP32 pin
Left IN1	`______`
Left IN2	`______`
Left PWM / Enable	`______`
Right IN1	`______`
Right IN2	`______`
Right PWM / Enable	`______`

4. Starting program¶

Complete the missing parts.

ME222_Lab1.ino

// ======================================
// ME 222 - LAB 1
// Four-Motor Robot Car
// ======================================


// --------------------------------------
// Enter your ESP32 pin numbers
// --------------------------------------

#define LEFT_IN1   ___
#define LEFT_IN2   ___
#define LEFT_PWM   ___

#define RIGHT_IN1  ___
#define RIGHT_IN2  ___
#define RIGHT_PWM  ___


// --------------------------------------
// Initial motor commands
// SAME command for both sides
// --------------------------------------

int leftSpeed  = 180;
int rightSpeed = 180;


// ======================================
// SETUP
// ======================================

void setup()
{
  pinMode(LEFT_IN1, OUTPUT);
  pinMode(LEFT_IN2, OUTPUT);
  pinMode(LEFT_PWM, OUTPUT);

  pinMode(RIGHT_IN1, OUTPUT);
  pinMode(RIGHT_IN2, OUTPUT);
  pinMode(RIGHT_PWM, OUTPUT);

  stopCar();
}


// ======================================
// MOVE FORWARD
//
// Fill in HIGH and LOW.
//
// You may need to change the direction
// depending on your motor wiring.
// ======================================

void forward()
{
  // LEFT SIDE

  digitalWrite(LEFT_IN1, ___);
  digitalWrite(LEFT_IN2, ___);


  // RIGHT SIDE

  digitalWrite(RIGHT_IN1, ___);
  digitalWrite(RIGHT_IN2, ___);


  // Apply motor speed

  analogWrite(LEFT_PWM, leftSpeed);
  analogWrite(RIGHT_PWM, rightSpeed);
}


// ======================================
// STOP
// ======================================

void stopCar()
{
  digitalWrite(LEFT_IN1, LOW);
  digitalWrite(LEFT_IN2, LOW);

  digitalWrite(RIGHT_IN1, LOW);
  digitalWrite(RIGHT_IN2, LOW);

  analogWrite(LEFT_PWM, 0);
  analogWrite(RIGHT_PWM, 0);
}


// ======================================
// MAIN TEST
// ======================================

void loop()
{
  forward();

  // Long enough for the car
  // to travel several metres.

  delay(8000);

  stopCar();

  // Do not repeat automatically.

  while(1);
}

Why while(1) at the end

Without it, loop() restarts and the car drives off again the moment you catch it. The program runs once per reset, deliberately.

5. Task 1 — Test the four motors¶

Lift the car so that all four wheels are free to rotate.
Run the program.
Check each motor turns:
Left-front motor rotates
Left-rear motor rotates
Right-front motor rotates
Right-rear motor rotates

If one motor rotates in the wrong direction¶

You may correct it by:

reversing the two wires connected to that motor, or
correcting the direction in the motor-control logic.

For this first lab, use the simplest reliable solution.

Checkpoint 1 — all four turning, correct direction

Get a TA before putting the car on the floor. Four motors turning the right way is the whole prerequisite for everything below.

6. Task 2 — Verify motion¶

Place the car on the floor.
Use:
```
leftSpeed  = 180;
rightSpeed = 180;
```
Run the car for approximately 1–2 metres.

Your goal at this stage is only to confirm that the entire vehicle moves forward.

Stop and fix wiring first

If the car rotates on the spot or one side moves backward, correct the wiring before continuing. Do not proceed to the drift measurement with a vehicle that isn't driving forward — you'd be measuring the wrong thing.

7. Prepare the test track¶

Use a straight section of floor approximately 4 metres long. A longer track may be used if space allows.

Mark:

START position
Intended straight reference line
FINISH position

START
  │
  │
  │
  │
  │     Intended Path
  │
  │
  │
  ▼
FINISH

Place the centre of the vehicle on the reference line. Align the vehicle carefully before every run — a sloppy start looks exactly like motor mismatch and will ruin your data.

Run the car 5 times. Observe its trajectory. At the end of each run, measure the lateral drift — the sideways distance from the reference line to the centre of the vehicle.

Run	Left PWM	Right PWM	Distance	Drift	Direction
1	180	180	`____` m	`____` cm	`______`
2	180	180	`____` m	`____` cm	`______`
3	180	180	`____` m	`____` cm	`______`
4	180	180	`____` m	`____` cm	`______`
5	180	180	`____` m	`____` cm	`______`

Discuss what you observed¶

Answer:

Does the vehicle travel perfectly straight?
Does it tend to move repeatedly toward the same side?
Is the error exactly the same on every run?

Question 3 is the important one

A drift that is consistent points at a fixed physical difference between the two sides. A drift that is random points at something else — surface, alignment, battery state. Which one you have determines whether a fixed correction could ever work.

8. Write your problem statement¶

Complete the following.

Observation

When we applied the same PWM command of ______ to both sides of the vehicle, the car _________________________________________________.

After travelling approximately ______ metres, the vehicle had a lateral error of approximately ______ cm.

Engineering problem

Although the left and right motors receive the same electrical command, _________________________________________________.

Consequence

Because of this difference, the vehicle _________________________________________________.

Deliverable¶

Working car + PWM table + measured straight-line drift.

Submit:

Your completed pin connection table.
Your 5-run drift table.
Your completed problem statement.

Carry this into Lab 2

You have just measured a problem you cannot yet fix, because you have no way to know how fast each wheel is actually turning — only what you commanded. Lab 2 gives you that measurement.