IMU

Github Repo	C Header	C source	JS source
mongoose-os-libs/imu	mgos_imu.h

This library provides a simple API that describes inertial measurement units. It implements various popular I2C and SPI IMUs behind that API. Callers of the library can rely on the API returning consistently typed data regardless of the choice of sensor.

Anatomy

This library provides an API to expose various implementations of gyroscopes, accelerometers, and magnetometers. It offers a suitable abstraction that presents the sensor data in a consistent way, so application authors need not worry about the implementation details of the sensors.

Users create a struct mgos_imu object, to which they add sensors (gyroscope, acellerometer and magnetometer) either via I2C or SPI. Some chips have all three sensor types, some have only two (typically accelerometer and gyroscope) and some have only one type. After successfully adding the sensors to the mgos_imu object, reads can be performed, mostly by performing mgos_imu_*_get() calls.

All implementations offer the calls described below in the IMU API. Some implementations offer chip-specific addendums, mostly setting sensitivity and range options.

IMU API

IMU primitives

struct mgos_imu *mgos_imu_create() -- This call creates a new (opaque) object which represents the IMU device. Upon success, a pointer to the object will be returned. If the creation fails, NULL is returned.

void mgos_imu_destroy() -- This cleans up all resources associated with with the IMU device. The caller passes a pointer to the object pointer. If any sensors are attached (gyroscope, accelerometer, magnetometer), they will be destroyed in turn using mgos_imu_*_destroy() calls.

bool mgos_imu_read() -- This call schedules all sensors attached to the IMU to be read from. It is not generally necessary to call this method directly, as the mgos_imu_*_get() calls internally schedule reads from the sensors as well.

bool mgos_imu_accelerometer_present() -- This returns true if the IMU has an attached accelerometer sensor, or false otherwise.

bool mgos_imu_gyroscope_present() -- This returns true if the IMU has an attached gyroscope sensor, or false otherwise.

bool mgos_imu_magnetometer_present() -- This returns true if the IMU has an attached magnetometer, or false otherwise.

IMU Sensor primitives

For each of accelerometer, gyroscope and magnetometer, the following primitives exist:

bool mgos_imu_*_create_i2c() -- This attaches a sensor to the IMU based on the type enum given, using the i2c bus and specified i2caddr address. The function will return true upon success and false in case either detection of the sensor, or creation of it failed.

bool mgos_imu_*_create_spi() -- This attaches a sensor to the IMU based on the type enum given, using the spi bus and a specified cable select in cs_gpio. The function will return true upon success and false in case either detection of the sensor or creation of it failed.

bool mgos_imu_*_destroy() -- This detaches a sensor from the IMU if it exists. It takes care of cleaning up all resources associated with the sensor, and detaches it from the i2c or spi bus. The higher level mgos_imu_destroy() call uses these lower level calls to clean up sensors as well.

bool mgos_imu_*_get() -- This call returns sensor data after polling the sensor for the data. It returns true if the read succeeded, in which case the floats pointed to by x, y and z are filled in. If an error occurred, false is returned and the contents of x, y and z are unmodified. Note the units of the return values:

• magnetometer returns units of Gauss.
• accelerometer returns units of G.
• gyroscope returns units of degrees per second.

const char *mgos_imu_*_get_name() -- This returns a symbolic name of the attached sensor, which is guaranteed to be less than or equal to 10 characters and always exist. If there is no sensor of this type attached, VOID will be returned. If the sensor is not known, UNKNOWN will be returned. Otherwise, the chip manufacturer / type will be returned, for example MPU9250 or ADXL345 or MAG3110.

bool mgos_imu_*_set_orientation() and bool mgos_imu_*_get_orientation() -- Often times a 9DOF sensor will have multiple chips, whose axes do not line up correctly. Even within a single chip the accelerometer, gyroscope and magnetometer axes might not line up (for an example of this, see MPU9250 chapter 9.1). To ensure that the x, y, and z axes on all sensors are pointed in the same direction, we can set the orientation on the gyroscope and magnetometer. See mgos_imu.h for more details and an example of how to do this.

Supported devices

Accelerometer

• MPU9250 and MPU9255
• ADXL345
• LSM303D and LSM303DLM
• MMA8451
• LSM9DS1
• LSM6DSL
• MPU6000 and MPU6050
• ICM20948

Gyroscope

• MPU9250 and MPU9255
• L3GD20 and L3GD20H
• ITG3205
• LSM9DS1
• LSM6DSL
• MPU6000 and MPU6050
• ICM20948

Magnetometer

• MAG3110
• AK8963 (as found in MPU9250/MPU9255)
• AK8975
• LSM303D and LSM303DLM
• HMC5883L
• LSM9DS1
• ICM20948

Adding devices

This is a fairly straight forward process, taking the ADXL345 accelerometer as example:

1. Add a new driver header and C file, say src/mgos_imu_adxl345.[ch]. The header file exposes all of the #define's for registers and constants.
2. In the header file, declare a detect, create, destroy and read function.
   • bool mgos_imu_adxl345_detect() -- this function optionally attempts to detect the chip, often times by reading a register or set of registers which uniquely identifies it. Not all chips can actually be detected, so it's OK to not define this function at all.
   • bool mgos_imu_adxl345_create() -- this function has to perform the initialization of the chip, typically by setting the right registers and possibly creating a driver-specific memory structure (for, say, coefficients or some such). If used, that memory structure is attached to the user_data pointer, and if so, the implementation of the _destroy() function must clean up and free this memory again.
   • bool mgos_imu_adxl345_read() -- this function performs the chip specific read functionality. This will be called whenever the user asks for data, either by calling mgos_imu_read() or by calling mgos_imu_*_get().
   • bool mgos_imu_adxl345_destroy() -- this function deinitializes the chip, and optionally clears and frees the driver-specific memory structure in user_data. Not all chips need additional memory structures or deinitialization code, in which case it's OK to not define this function at all.
3. Implement the detect, create, destroy and read functions in the source code file src/mgos_imu_adxl345.c.
4. Add the device to one of the enum mgos_imu_*_type in include/mgos_imu.h. In the example case of adxl345 (which is an accelerometer), add it to enum mgos_imu_acc_type.
5. Add a string version of this to function mgos_imu_*_get_name() so that callers can resolve the sensor to a human readable format. Make sure that the string name is not greater than 10 characters.
6. Add the type to the switch() in mgos_imu_*_create_i2c() (or _spi()) functions.
7. Update this document to add the driver to the list of supported drivers.
8. Test code on a working sample, and send a PR using the guidelines laid out in contributing.

It is important to note a few implementation rules when adding drivers:

• New drivers MUST NOT change any semantics of the abstraction layer (mgos_imu and mgos_imu_* members) nor the mgos_imu_*_get() functions.
• Implementations of drivers MUST provide bias, scaling and other normalization in the driver itself. What this means, in practice, is that the correct units must be produced (m/s/s, deg/s and uT).
• Pull Requests MUST NOT mix driver and abstraction changes. Separate them.
• Changes to the abstraction layer MUST be proven to work on all existing drivers, and will generally be scrutinized.

Example driver (AK8975)

Here's an example, for the magnetometer chip AK8975, showing a set of commits for each of the steps above (and honoring the driver implementation rules).

1. Add src/mgos_imu_ak8975.[ch] commit
2. Extend enum mgos_imu_mag_type in include/mgos_imu.h commit
3. Add to mgos_imu_magnetometer_get_name() in src/mgos_imu_magnetometer.c commit
4. Add to mgos_imu_magnetometer_create_i2c() in src/mgos_imu_magnetometer.c commit

Example Code

An example program that creates an IMU of type MPU9250 (which has an accelerometer, a gyroscope and a magnetometer all in one tiny package):

#include "mgos.h"
#include "mgos_i2c.h"
#include "mgos_imu.h"

static void imu_cb(void *user_data) {
  struct mgos_imu *imu = (struct mgos_imu *)user_data;
  float ax, ay, az;
  float gx, gy, gz;
  float mx, my, mz;

  if (!imu) return;

  if (mgos_imu_accelerometer_get(imu, &ax, &ay, &az))
    LOG(LL_INFO, ("type=%-10s Accel X=%.2f Y=%.2f Z=%.2f",
                  mgos_imu_accelerometer_get_name(imu), ax, ay, az));
  if (mgos_imu_gyroscope_get(imu, &gx, &gy, &gz))
    LOG(LL_INFO, ("type=%-10s Gyro  X=%.2f Y=%.2f Z=%.2f",
                  mgos_imu_gyroscope_get_name(imu), gx, gy, gz));
  if (mgos_imu_magnetometer_get(imu, &mx, &my, &mz))
    LOG(LL_INFO, ("type=%-10s Mag   X=%.2f Y=%.2f Z=%.2f",
                  mgos_imu_magnetometer_get_name(imu), mx, my, mz));
}

enum mgos_app_init_result mgos_app_init(void) {
  struct mgos_i2c *i2c = mgos_i2c_get_global();
  struct mgos_imu *imu = mgos_imu_create();
  struct mgos_imu_acc_opts acc_opts;
  struct mgos_imu_gyro_opts gyro_opts;
  struct mgos_imu_mag_opts mag_opts;

  if (!i2c) {
    LOG(LL_ERROR, ("I2C bus missing, set i2c.enable=true in mos.yml"));
    return false;
  }

  if (!imu) {
    LOG(LL_ERROR, ("Cannot create IMU"));
    return false;
  }

  acc_opts.type = ACC_MPU9250;
  acc_opts.scale = 16.0; // G
  acc_opts.odr = 100;    // Hz
  if (!mgos_imu_accelerometer_create_i2c(imu, i2c, 0x68, &acc_opts))
    LOG(LL_ERROR, ("Cannot create accelerometer on IMU"));

  gyro_opts.type = GYRO_MPU9250;
  gyro_opts.scale = 2000; // deg/sec
  gyro_opts.odr = 100;    // Hz
  if (!mgos_imu_gyroscope_create_i2c(imu, i2c, 0x68, &gyro_opts))
    LOG(LL_ERROR, ("Cannot create gyroscope on IMU"));

  mag_opts.type = MAG_AK8963;
  mag_opts.scale = 12.0; // Gauss
  mag_opts.odr = 10;     // Hz
  if (!mgos_imu_magnetometer_create_i2c(imu, i2c, 0x0C, &mag_opts))
    LOG(LL_ERROR, ("Cannot create magnetometer on IMU"));

  mgos_set_timer(1000, true, imu_cb, imu);
  return true;
}

Demo Code

For a cool demo, take a look at my Demo Apps:

Disclaimer

This project is not an official Google project. It is not supported by Google and Google specifically disclaims all warranties as to its quality, merchantability, or fitness for a particular purpose.

---

set_odr

float                   odr;    // Data rate, in Hz. See doc for set_odr().
  float                   scale;  // Scale. See doc for set_scale().
  bool                    no_rst; // Do not perform reset of the device when configuring.
};

Gyroscope type.

set_odr

float                  odr;    // Data rate, in Hz. See doc for set_odr().
  float                  scale;  // Scale. See doc for set_scale().
  bool                   no_rst; // Do not perform reset of the device when configuring.
};

Accelerometer type.

set_odr

float                  odr;    // Data rate, in Hz. See doc for set_odr().
  float                  scale;  // Scale. See doc for set_scale().
  bool                   no_rst; // Do not perform reset of the device when configuring.
};

Magnetometer type.

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