Thursday, 7 November 2013

Reading data from the MPU-6050 on the Raspberry Pi

In a previous post I showed how to connect an Accelerometer & Gyro sensor to the Raspberry Pi, in this post I'll show some simple Python code to read the data it offers.

To be able to read from the I2C using Python bus we need to install the smbus module
sudo apt-get install python-smbus 
Now to some code, this is just simple test code to make sure the sensor is working
When you run the code you will see output similar to this
gyro data
gyro_xout:  -92  scaled:  -1
gyro_yout:  294  scaled:  2
gyro_zout:  -104 scaled:  -1

accelerometer data
accel_xout:  -3772  scaled:  -0.230224609375
accel_yout:  -52    scaled:  -0.003173828125
accel_zout:  15408  scaled:  0.9404296875
x rotation:  -13.7558411667
y rotation:  -0.187818934829

Accelerometer data

Let's have a look at the code in more detail.
These three lines read the raw X,Y & Z accelerometer values, the parameter in each call is the register within the sensor that holds the data.  The sensor has a number of registers which have different functionality as documented in this datasheet.  The registers we are interested in for the acceleromter data are 0x3b, 0x3d, 0x3f and these hold the raw data in 16 bit two's complement format.

The following code reads a word (16 bits) from a given register and converts it from two's complement
Once we have the raw data we need to scale it and then convert it into something useful like a rotation angle. Again from the data sheet we can see the default scaling we need to apply to the raw accelerometer values is 16384, so we divide the raw data by this value.
Now we have the values that gravity is exerting on the sensor in each of the three dimensions, from this we can calculate the rotations in the X & Y axes.
Here is an excellent article showing the details behind the maths for this.  What this gives us is the rotation angle in degrees for both the X & Y axes and is shown in the output.
x rotation: -13.755841166
y rotation: -0.187818934829
So in this instance the sensor is rotated by -13.7o around X and -0.1o around Y.

Gyroscope data

In a similar manner we can read the data from the Gyroscope part of the sensor. This is done in the following code
So we read the values from the registers 0x43, 0x45 & 0x47, again we can see from the datasheet that these hold the raw gyro data. To scale these we divide by 131 to give the degrees per second rotation value.
gyro_xout:  -92  scaled:  -1
gyro_yout:  294  scaled:  2
gyro_zout:  -104 scaled:  -1
The output in my case show the gyro wasn't moving when I took reading.

Final thoughts

The code I present here is very basic and should be extended to handle errors and allow the sensor to be configured with different sensitivity levels. I've done this in my application and embedded it into a web server. This allows me to make a simple http request to the Raspberry Pi and get a reading from the sensor.

To help me test and visualise the data better I've written some simple OpenGL code to graphically represent the sensor's orientation in 3D space.

This OpenGL code runs on my Linux desktop machine and queries the Pi periodically to get the data and renders the above image. See this post for details how

In the next article I'll show how to combine the accelerometer and gyroscope data together to get a more accurate reading and help reduce noise.