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IMU

IMU Overview

The Inertial Measurement Unit (IMU) provides three-axis acceleration, angular velocity, and magnetic field measurements for attitude determination and motion analysis.

Communication Interfaces

  • Accelerometer: I2C or SPI
  • Gyroscope: I2C or SPI
  • Magnetometer: I2C or SPI
  • Update Rate: Typically 1-100+ Hz (configurable)

Data Structure

typedef struct {
    // Acceleration (m/s² or g-units)
    float accel[3];            // [X, Y, Z] acceleration
    
    // Angular Velocity (rad/s or °/s)
    float gyro[3];             // [X, Y, Z] rotation rate
    
    // Magnetic Field (Gauss or µT)
    float mag[3];              // [X, Y, Z] magnetic field
    
    // Timestamps
    uint32_t timestamp;        // Current reading timestamp
    uint32_t last_timestamp;   // Previous reading timestamp
} imu_data_t;

Initialization & Usage

Initialize IMU

imu_data_t imu_data;
imu_sensor_init(&imu_data);

Poll IMU Data

result_t imu_result = poll_imu_sensor(&imu_data);

if (imu_result == RESULT_OK) {
    // Acceleration
    float accel_x = imu_data.accel[0];
    float accel_y = imu_data.accel[1];
    float accel_z = imu_data.accel[2];
    
    // Angular velocity
    float gyro_x = imu_data.gyro[0];
    float gyro_y = imu_data.gyro[1];
    float gyro_z = imu_data.gyro[2];
    
    // Magnetic field
    float mag_x = imu_data.mag[0];
    float mag_y = imu_data.mag[1];
    float mag_z = imu_data.mag[2];
}

Advanced Functions

Update with Raw Values

result_t imu_sensor_update(
    imu_data_t *imu,
    float ax, float ay, float az,  // Accelerometer values
    float gx, float gy, float gz,  // Gyroscope values
    float mx, float my, float mz,  // Magnetometer values
    uint32_t timestamp
);

Calculate Acceleration Magnitude

float acceleration_magnitude = imu_get_acceleration_magnitude(&imu_data);
// Useful for impact detection and free-fall detection

Thread-Safe Data Reading

imu_data_t imu_copy;
imu_sensor_read(&imu_data, &imu_copy);
// Use imu_copy in another thread without locking

Typical Sensor Ranges

Measurement

Typical Range

Units

Acceleration

±16

g

Angular Velocity

±2000

°/s

Magnetic Field

±4800

µT

Conversion Reference

From

To

Factor

g

m/s²

× 9.81

°/s

rad/s

× π/180

Gauss

µT

× 100

Validation Functions

// Validate single accelerometer value
// Typical range: -50 to +50 m/s2  
result_t validate_accelerometer_value(float accel_value);

// Validate all three acceleration axes
result_t validate_imu_data(
    float accel_x,
    float accel_y,
    float accel_z
);

Protobuf Message Format

message SensorBoardIMUInfo {
    float accel_x;
    float accel_y;
    float accel_z;
    float gyro_x;
    float gyro_y;
    float gyro_z;
    SensorState state;
}

Common Applications

Impact Detection

float mag = imu_get_acceleration_magnitude(&imu_data);
if (mag > IMPACT_THRESHOLD) {
    // High acceleration detected
}

Tilt Detection

// Calculate tilt angle from acceleration
float tilt_angle = atan2(imu_data.accel[0], imu_data.accel[2]);

Motion Classification

// Static vs dynamic based on gyro magnitude
float gyro_mag = sqrt(gyro_x*gyro_x + gyro_y*gyro_y + gyro_z*gyro_z);

Integration Notes

  • Single IMU instance with 3-axis sensors (dual planned)
  • All three axes transmitted as independent fields
  • Acceleration magnitude available for impact detection
  • Timestamp tracking enables dead reckoning applications
  • Typical IMU update rate: 10-100 Hz
  • Filter algorithms can be applied to raw data for smoothing
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