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.
Replies
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
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.
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
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.
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?