FreeIMU, how to get gyro rates and accelerometer reading

Hi,

I am using ArduIMU V3 with FreeIMU library. So far, it is working just great. However, I need to obtain Gyro rates(in deg/s or rad/s) and accelerometer( in m/s^2). I'm calling getValues() function but I'm not sure what are the units of the returned data.

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Please help!!! I really need to obtain those values!!

Khalid, I loaded the FreeIMUraw.pde and took a look.  I'd say it depends on how FreeIMU sets up the sensors so I took a look in the library MPU60X0.cpp file and the header file.  FreeIMU.cpp is using the

#define MPU60X0_GYRO_FS_2000        0x03

from the MPU60X0.h file to set the gyro at +/- 2000 degrees/sec

I haven't been able to find how FreeIMU sets the accel but I'd guess it's a default and looking at the raw output it looks like +/- 2g.  That's based on the raw numbers that are about 18-21 so it'd be 2 * 9.8 m/s/s.

 

Okay, do I need to scale the values I get from getValue()? The values for the accelerometer are in the order of 1000s

Hi Khalid,

I've been working with a FreeIMU V3 for some time trying to get a reasonable scaling on the sensor axis(using micro.net). I eventually thought my IMU was faulty. I then downloaded MultiWII software and had a look at all the drivers developed in the project. I hooked up my FreeIMU, modified the configuration to look for my IMU on the I2C port and dumped the program on a Arduiono Mini ATmega328, hooked up the IMU and powered up the system. MultiWII has a GUI that allows calibration of the sensors and a nice visual tool as well. The source is easy to read and, I would imagine, well tested. They have done a great job. I have multiple 'low cost' IMU's im using in prototyping and the MultiWII guy(s) have taken away a lot of the development pain with their sensor drivers.

Have a look at their methodology - I'm sure you will find your answers there.

Good luck,

Antonie

What happens if you divide the accel values by 4096?  Do they then look like +/- 2g values?

If I divide one of the values by 4096, I get values between -4 and 4. What does that mean?

Okay, after some digging in the source code, just like you suggested, I found that they set the full-scale range to 0, and according to the datasheet this corresponds to 16,384 LSB/g. So, I divided the values that I get by 16,384 and multiplied them by 9.81. The readings, as expected, are correct. I'm getting ~9.5 m/s^2 for z-axis when I hold the IMU horizontally. 

Now, the question is do I need to do the same for gyro readings? how do I check If they are correct or not?

I found this line. 

values[i] = ((float) accgyroval[i]) / 16.4f; // NOTE: this depends on the sensitivity chosen

16.4f is the same resolution indicated by the datasheet for +/-2000 deg/s. I conjecture that the readings for the gyro are already scaled and they need no scaling from my side of code. Correct me if i'm wrong!

Good job!  I found this comment in the MPU60X0.cpp

 * FS_SEL | Full Scale Range   | LSB Sensitivity  * -------+--------------------+----------------
 * 0      | +/- 250 degrees/s | 131 LSB/deg/s
 * 1      | +/- 500 degrees/s | 65.5 LSB/deg/s
 * 2      | +/- 1000 degrees/s | 32.8 LSB/deg/s
 * 3      | +/- 2000 degrees/s | 16.4 LSB/deg/s

Looks like 16.4f applies to the gyros too. 

I added this line to FreeIMU.cpp :

accgyro.setFullScaleAccelRange(MPU60X0_ACCEL_FS_4); 

The idea was to set the accel to +/- 4g, so that It would match how the ArduIMU 1.9.8 sets the accels.  I then used FreeIMU to initialize an MPU6050 using i2c instead of MPU6000 and spi.  I used the FreeIMU: my3IMU.getRawValues(raw_values) to replace the MPU6000_read().  I had to play with the sensor orientation and sensor signs in the ArduIMU sketch but it works. 

I did it to see if I could use the DCM algorithm on a different board than ArduIMU and I can.  FreeIMU helped simplify it for me at least.

   

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