driver_mpu9250

Overview

This package includes ROS driver software for the InvenSense MPU9250 9DoF IMU. A ROS node publishes data from the IMU, and permits configuration through the ROS Parameter Server. A Raspberry Pi driver is included, but users may also easily create other device-specific drivers by extending the base driver class.

License: MIT

Author: Paul D'Angio

Contents

1. Installation: Instructions for installing this ROS package.
2. Usage: How to configure and use the driver_mpu9250 ROS node.
3. Extending: How to easily add your own custom device driver.
4. Additional Information: Other important information relevant to this package.

1: Installation

1.1: Building from Source

1.1.1: Dependencies

• Robot Operating System (ROS) (middleware for robotics)
• sensor_msgs_ext (ROS extended sensor messages)
• pigpio (Raspberry PI I/O)

1.1.2: Building

To build from source, clone the latest version from this repository into your catkin workspace and compile the package using

# Switch to your catkin workspace directory.
cd catkin_workspace

# Clone the package into the workspace src directory.
git clone https://github.com/pcdangio/ros-driver_mpu9250.git src/driver_mpu9250

# Build the package.
catkin_make

2: Usage

This package includes a ROS node that publishes data from the IMU and permits configuration through the ROS Parameter Server.

2.1: Configuration

The table below describes parameters may be set on the ROS Parameter Server to configure the MPU9250. These parameters must be set before the ROS node is run. Recall that ~ is the node's private namespace.

Parameter	Type	Default	Description
~/i2c_bus	int	1	The I2C bus number to communicate with the MPU9250 over.
~/i2c_address	int	0x68	The I2C address of the MPU9250.
~/interrupt_gpio_pin	int	0	The GPIO input pin connected to the MPU9250's interrupt pin. NOTE: You must use GPIO numbers, not pin numbers.
~/gyro_dlpf_frequency	int	0	An enum value representing the digital low pass filter (DLPF) cutoff frequency for the gyroscopes and temperature sensor. See table for enumerated DLPF values to select from.
~/accel_dlpf_frequency	int	0	An enum value representing the digital low pass filter (DLPF) cutoff frequency for the accelerometers. See table for enumerated DLPF values to select from.
~/gyro_fsr	int	0	The full scale range (FSR) of the gyroscopes. See table for enumerated FSR values to select from.
~/accel_fsr	int	0	The full scale range (FSR) of the accelerometers. See table for enumerated FSR values to select from.
~/max_data_rate	float	8000	The maximum allowable sensor data rate, in Hz. The sensor's data rate is normally calculated from the DLPF frequencies, but this parameter may be used to set a lower cap on the data rate.
~/calibration/accelerometer	float[16]	empty	The 4x4 calibration matrix for the accelerometer in row-major order. See the calibration section for more information.
~/calibration/magnetometer	float[16]	empty	The 4x4 calibration matrix for the magnetometer in row-major order. See the calibration section for more information.

2.2: Starting the Driver Node

Run any of the driver nodes with (where xxx is the driver type):

rosrun driver_mpu9250 driver_mpu9250_xxx

To use the driver node for a Raspberry Pi:

# Make sure that the linux user has been added to the "gpio" group.

# In one terminal, start up the PiGPIO Daemon.
sudo pigpiod

# In a second terminal, start up the driver.
rosrun driver_mpu9250 driver_mpu9250_rpi

2.3: Topics

Topic	Message Type	Description
imu/accelerometer	sensor_msgs_ext/accelerometer	The 3D acceleration measurements from the MPU9250.
imu/gyroscope	sensor_msgs_ext/gyroscope	The 3D gyroscope measurements from the MPU9250.
imu/magnetometer	sensor_msgs_ext/magnetometer	The 3D magnetic field measurements from the onboard AK8963 compass.
imu/temperature	sensor_msgs_ext/temperature	The die temperature of the MPU9250 sensor

2.4: Services

Service	Service Type	Description
imu/calibrate_gyroscope	sensor_msgs_ext/calibrate_gyroscope	Calibrates the gyroscope by calculating and removing gyroscope bias. The bias is calculated by averaging gyroscope data over an averaging period. See the calibration section below for more detail.

3: Extending

This package uses polymorphism with a base driver class to enable drivers to be easily built for a variety of hardware interfaces. To create your own device-specific driver:

• Create a new class for your device driver that extends the base driver class in src/driver.h
• In your derived driver class, implement the several pure abstract functions of the base driver class
• Create a new main.cpp that instantiates your derived class and passes it to the src/ros_node.h instance through polymorphism before running the node.
• Add the new main.cpp to your CMakeLists.txt as a new node executable.

You can refer to the following files in this package as an example for creating your custom device driver:

src/rpi_driver.h
src/rpi_driver.cpp
src/main_rpi.cpp
CMakeLists.txt

If you have created and tested a driver for a device not already covered in this package, please feel free to submit a pull request!

4: Additional Information

4.1: Full-Scale Ranges

The "full-scale range", or FSR, of a sensor specifies the range of measurements that must be sensed. Smaller ranges will measure at higher resolution, while larger ranges will measure at lower resolution. The MPU9250 has several full-scale ranges to select from, as shown in the table below. The Enumerated Value is the integer that should be used in the ~/gyro_fsr and ~/accel_fsr ROS parameters during configuration.

Enumerated Value	Gyroscope Range	Accelerometer Range
0	+/- 250 deg/sec	+/- 2g
1	+/- 500 deg/sec	+/- 4g
2	+/- 1000 deg/sec	+/- 8g
3	+/- 2000 deg/sec	+/- 16g

4.2: Digital Low-Pass Filters

The MPU9250 has internal Digital Low-Pass Filters (DLPF) that can be used to filter out unwanted noise from the gyroscope, accelerometer, and temperature measurements. The table below lists the available cutoff frequencies that can be chosen. Lower cutoff frequencies will filter out more noise, but will increase the lag (delay) of the measurements. The cutoff frequency selected for the gyroscopes is also applied to the temperature sensor. The Enumerated Value is the integer that should be used in the ~/gyro_dlpf_frequency and ~/accel_dlpf_frequency ROS parameters during configuration.

Enumerated Value	Gyroscope Cutoff	Gyroscope Delay	Accelerometer Cutoff	Accelerometer Delay
0	250 Hz	0.97ms	218.1 Hz	1.88ms
1	184 Hz	2.9ms	-	-
2	92 Hz	3.9ms	99 Hz	2.88ms
3	41 Hz	5.9ms	44.4 Hz	4.88ms
4	20 Hz	9.9ms	21.2 Hz	8.87ms
5	10 Hz	17.85ms	10.2 Hz	16.83ms
6	5 Hz	33.48ms	5.05 Hz	32.48ms

4.3: Data Rates

The data rate is the frequency at which the sensor makes measurements and publishes them through ROS. This driver automatically calculates the appropriate data rate based on the cutoff frequencies of the DLPFs of the acceleremeters and gyroscopes. The driver finds which DLPF has the highest cutoff frequency, and multiplies that frequency by 2.5 to calculate a data rate with a proper Nyquist sampling rate. The calculated data rate is output to the node's ROS log at startup.

The ~/max_data_rate parameter can be used to override the calculated data rate to a lower frequency.

4.4: Calibration

Accelerometers, gyroscopes, and magnetometers all require calibration to provide accurate measurements:

• Accelerometers suffer from axis misalignment and bias.
• Gyroscopes suffer from bias and bias drift.
• Magnetometers suffer from hard iron and soft iron distortion.

This driver handles calibrations differently for each sensor:

Accelerometer and Magnetometer

The MPU9250's 3-axis accelerometer and 3-axis magnetometer have relatively stable bias (assuming no changes in sensor mounting and metals/magnets surrounding the magnetometer). Thus, the driver uses static calibration matrices to make adjustments to sensor data before it is published through ROS. These matrices can be calculated using the calibration_imu package, and set as parameters for the driver to load and use. NOTE: You should recalibrate the accelerometer and magnetometer any time you make hardware changes to your platform.

Gyroscope

The MPU9250's 3-axis gyroscope, like all MEMs gyroscopes, suffers from continuous bias drift. Thus, it is necessary to regularly recalibrate the gyroscope when it is known that the IMU is not in motion. This calibration routine can be called through a service, which calculates the current bias of the gyroscope and subtracts it from future measurements.