Introduction

The myAHRS+ is a low cost high performance AHRS(Attitude Heading Reference System). Its attitude output is more stable to acceleration and magnetic disturbances then other low cost AHRS/IMU products. Communication and configuration are enabled via UART/USB interface for user applications. And I2C interface is available for embedded application like arduino projects. The GUI(myAHRS+ Monitor) is available, which allows users configure all myAHRS+ settings, view attitude of myAHRS+ and IMU(Inertial Measurement Unit) data in realtime and save sensor data in a text file. Custom user software may be developed using the myAHRS+ SDK.

Features

• Sensors
  • Triple axis 16-bit gyroscope : ± 2,000 dps
  • Triple axis 16-bit accelerometer : ± 16 g
  • Triple axis 13-bit magnetometer : ± 1200 μT
• On board software
  • Extended Kalman filter
  • max 100 Hz output rate
    • Attitude : Euler angle, Quaternion
    • sensor : acceleration, rotation rate, magnetic field
• Connectivity
  • USB : Virtual COM PORT
  • UART : Standard baud rates up to 460800 bps
  • I2C : up to 1kHz
• GUI(myAHRS+ Monitor)
  • display attitude and sensor data from myAHRS+ on various viewers
  • Configuration
  • magnetometer calibration

Performance

Performance comparison between myAHRS+ and Razor 9 axis IMU.

Performance comparison between myAHRS+ and MPU9150 MotionFit.

Pin map

Pin	Name	Type	Description
J3-1	INT	O	DATA READY interrupt output
J3-2	SLEEP	I	Sleep mode selection input: L-sleep mode, H-normal mode. (Normal mode If not connected)
J3-3	I2C_SCL	I	I2C clock input
J3-4	I2C_SDA	I/O	I2C data input and output
J3-5	USB_DM	I/O	USB D-
J3-6	USB_DP	I/O	USB D+
J3-7	NC		Do Not Connect
J3-8	NC		Do Not Connect
J3-9	NC		Do Not Connect
J3-10	NC		Do Not Connect
J4-1	VDD	PWR	+5V power input
J4-2	nRST	I	Reset input. L-reset, H-normal
J4-3	NC		Do Not Connect
J4-4	UART_TX	O	UART transmit
J4-5	UART_RX	I	UART receive
J4-6	NC		Do Not Connect
J4-7	NC		Do Not Connect
J4-8	NC		Do Not Connect
J4-9	NC		Do Not Connect
J4-10	GND	PWR	Power ground. 0V

Testing the myAHRS+

First, connect myAHRS+ to your PC via USB. And then, check LEDs on myAHRS+. The blinking yellow LED means that myAHRS+ is in normal state. And the red one means that myAHRS+ is connected to PC via USB successfuly.

The following video shows the tesing procedure of myAHRS+ using GUI(myAHRS+ Monitor).

Configuration

Using the myAHRS+ Monitor, user can change various properties of myAHRS+ for user application. Some properties are saved in non-volatile memory(flash memory), but others are not. Therefore, the volatile properties should be re-initialized to appropriate values when the user application starts.

• Non-volatile properties
• Sensor ID
• Baudrate of UART interface
• Sensor calibration parameters
• Volatile properties
• Divider
• Protocol
• Transfer mode
• ASCII output format
• BINARY output format

To update the myAHRS+ settings(properties), run myAHRS+ Monitor. And then open the 'Configuration' dialog window.

This video shows the setting procedure of myAHRS+ using myAHRS+ Monitor.

Calibration

The magnetometer(compass) used to estimate yaw angle of AHRS. To get better measurment of yaw angle, the magnetometer of myAHRS+ should be calibrated by the user at the very place where it'll be used. The following description explains why to do so.

Measurement of magnetic field will be subjected to distortion. There are two categories of these distortions: the hard iron distortions and the soft iron distortions. The hard iron errors refer to the presence of magnetic fields around the sensor such as magnets, power supply wires are related to the measurement offset errors, while the soft iron errors refer to the presence of the ferromagnetic materials around the sensor which skew the density of the Earth's magnetic field locally, are related to the scaling offset errors. (http://diydrones.com/profiles/blogs/advanced-hard-and-soft-iron-magnetometer-calibration-for-dummies)

This video shows the calibration procedure of myAHRS+ using myAHRS+ Monitor.

I2C Interface

###1. I2C Interface

The myAHRS+ operates as I2C slave and the I2C bus requires the pull-up register.
Normally 4.7kΩ register is used but 1kΩ ~ 10kΩ register can be used depends on the situation.

The I2C port operates in 3.3V voltage level and can be used directly with a MCU with 5V IO without an extra circuit.

• I2C Slave address: 7bit, 0x20
• Data bit: 8bit
• I2C clock speed: Normal mode(100KHz), Fast mode(400KHz)

I2C interface of the myAHRS+ supports four sequences shown below.

###2. I2C Register description

Register Name	Attributes	Address	Default value	Description
WHO_AM_I	R	0x01	0xB1	Sensor status
REV_ID_MAJOR	R	0x02	-	-
REV_ID_MINOR	R	0x03	-	-
STATUS	R	0x04	0x80	-
I_ACC_X_LOW	R	0x10	DATA	Acceleration raw data
I_ACC_X_HIGH	R	0x11	DATA	Acceleration raw data
I_ACC_Y_LOW	R	0x12	DATA	Acceleration raw data
I_ACC_Y_HIGH	R	0x13	DATA	Acceleration raw data
I_ACC_Z_LOW	R	0x14	DATA	Acceleration raw data
I_ACC_Z_HIGH	R	0x15	DATA	Acceleration raw data
I_GYRO_X_LOW	R	0x16	DATA	Gyroscope raw data
I_GYRO_X_HIGH	R	0x17	DATA	Gyroscope raw data
I_GYRO_Y_LOW	R	0x18	DATA	Gyroscope raw data
I_GYRO_Y_HIGH	R	0x19	DATA	Gyroscope raw data
I_GYRO_Z_LOW	R	0x1A	DATA	Gyroscope raw data
I_GYRO_Z_HIGH	R	0x1B	DATA	Gyroscope raw data
I_MAGNET_X_LOW	R	0x1C	DATA	Magnetometer raw data
I_MAGNET_X_HIGH	R	0x1D	DATA	Magnetometer raw data
I_MAGNET_Y_LOW	R	0x1E	DATA	Magnetometer raw data
I_MAGNET_Y_HIGH	R	0x1F	DATA	Magnetometer raw data
I_MAGNET_Z_LOW	R	0x20	DATA	Magnetometer raw data
I_MAGNET_Z_HIGH	R	0x21	DATA	Magnetometer raw data
C_ACC_X_LOW	R	0x22	DATA	Calibrated acceleration data
C_ACC_X_HIGH	R	0x23	DATA	Calibrated acceleration data
C_ACC_Y_LOW	R	0x24	DATA	Calibrated acceleration data
C_ACC_Y_HIGH	R	0x25	DATA	Calibrated acceleration data
C_ACC_Z_LOW	R	0x26	DATA	Calibrated acceleration data
C_ACC_Z_HIGH	R	0x27	DATA	Calibrated acceleration data
C_GYRO_X_LOW	R	0x28	DATA	Calibrated gyroscope data
C_GYRO_X_HIGH	R	0x29	DATA	Calibrated gyroscope data
C_GYRO_Y_LOW	R	0x2A	DATA	Calibrated gyroscope data
C_GYRO_Y_HIGH	R	0x2B	DATA	Calibrated gyroscope data
C_GYRO_Z_LOW	R	0x2C	DATA	Calibrated gyroscope data
C_GYRO_Z_HIGH	R	0x2D	DATA	Calibrated gyroscope data
C_MAGNET_X_LOW	R	0x2E	DATA	Calibrated magnetometer data
C_MAGNET_X_HIGH	R	0x2F	DATA	Calibrated magnetometer data
C_MAGNET_Y_LOW	R	0x30	DATA	Calibrated magnetometer data
C_MAGNET_Y_HIGH	R	0x31	DATA	Calibrated magnetometer data
C_MAGNET_Z_LOW	R	0x32	DATA	Calibrated magnetometer data
C_MAGNET_Z_HIGH	R	0x33	DATA	Calibrated magnetometer data
TEMPERATURE_LOW	R	0x34	DATA	Temperature data
TEMPERATURE_HIGH	R	0x35	DATA	Temperature data
ROLL_LOW	R	0x36	DATA	Euler angle
ROLL_HIGH	R	0x37	DATA	Euler angle
PITCH_LOW	R	0x38	DATA	Euler angle
PITCH_HIGH	R	0x39	DATA	Euler angle
YAW_LOW	R	0x3A	DATA	Euler angle
YAW_HIGH	R	0x3B	DATA	Euler angle
QUATERNION_X_LOW	R	0x3C	DATA	Quaternion
QUATERNION_X_HIGH	R	0x3D	DATA	Quaternion
QUATERNION_Y_LOW	R	0x3E	DATA	Quaternion
QUATERNION_Y_HIGH	R	0x3F	DATA	Quaternion
QUATERNION_Z_LOW	R	0x40	DATA	Quaternion
QUATERNION_Z_HIGH	R	0x41	DATA	Quaternion
QUATERNION_W_LOW	R	0x42	DATA	Quaternion
QUATERNION_W_HIGH	R	0x43	DATA	Quaternion

###3. Data transform formula

• I_ACC_X_LOW ~ I_MAGNET_Z_HIGH

• These registers store the sensor output value that is not compensated by the calibration parameter. Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. Since the value is originally Integer, there is no need to convert it into a Real number.

• C_ACC_X_LOW ~ C_ACC_Z_HIGH

• These registers store the acceleration output value (signed 16bit) that is compensated by the calibration parameter. Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. Multiplying a scale factor (16 / 32767) converts the unit of acceleration value into the (g) unit.

• Acceleration(g) = C_ACC × 16 / 32767

• C_GYRO_X_LOW ~ C_ GYRO _Z_HIGH

• These registers store the gyroscope output value (signed 16bit) that is compensated by the calibration parameter. Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. Multiplying a scale factor (2000 / 32767) converts the unit of gyroscope value into the (dps / degree per second) unit.

• Gyroscope(dps) = C_GYRO × 2000 / 32767

• C_MAGNET_X_LOW ~ C_ MAGNET _Z_HIGH

• These registers store the magnetometer output value (signed 16bit) that is compensated by the calibration parameter. Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. The output value of a calibrated magnetometer sensor is not required to convert into a Real number, because there is no physical unit.

• If you need magnetic field measurement, multiply the scale factor 0.3 to not-calibrated magnetometer output (I_MAGNET_X_LOW ~ I_MAGNET_Z_HIGH) then you can get the magnetic field measurement in uT.

• Magnetometer(uT) = I_MAGNET × 0.3

• TEMPERATURE_LOW ~ TEMPERATURE_HIGH

• These registers store the temperature output value (signed 16bit). Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. Multiplying a scale factor (200 / 32767) converts the unit of the temperature value into the (℃) unit.

• Temperature(℃) = TEMPERATURE_N × 200 / 32767

• ROLL_LOW ~ YAW_HIGH

• These registers store the Euler angle output value (signed 16bit). Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. Multiplying a scale factor (180 / 32767) converts the unit of the angle value into the (°) unit.

• Euler angle(°) = ROLL(PITCH, YAW) × 180 / 32767

• QUATERNION_X_LOW ~ QUATERNION_W_HIGH

• These registers store the quaternion output value (signed 16bit). Lower 8bit is saved in a LOW register and higher 8bit register is saved in a HIGH register. Multiplying a scale factor (1 / 32767) converts the value into the Real number.

• Quaternion = X(Y, Z, W) / 32767

UART Protocol

Examples

Some useful examples for using the myAHRS+ are available for various platforms.

• myAHRS+ SDK.
• python.
• odroid XU3.
• odroid C1.
• I2C Examples
• myCortex-STM32F4.
• UART Examples