Recall from the previous article the brief introduction to Inertial Measurement Units (IMU)and Microelectromechanical Systems (MEMS). The IMU provides us with the information we need to track the position of the bat in space.

For our applications we need a small low power circuit. Our IMU circuit consists of 1 - ADXL330 Accelerometer from Analog Devices, and 1 - IDG-300 Gyroscope from Invensense. The ADXL330 is a triple axis accelerometer that has a measurement range of +/- 3g. The IDG-300 is an integrated dual axis gyroscope. The combination of these 2 circuits gives us Roll, Pitch, X, Y, and Z information. Now that we have our MEMS devices, we next need a way to get the information from them. Our microcontroller handles for us.

There is a huge selection of microcontrollers that work with this project. If you are used to working with a paticular microcontroller, you should have no problem integrating the devices above. The microcontroller used in our bat is the PIC16F88 by Microchip. Microchip manufactures affordable 8-bit microcontrollers that work well with popular IDE's (i.e. software application used to program the device) such as MPLAB. The microcontroller has Analog to Digital (A/D) pins that are used to connect our IMU device. The accelerometer and gyroscope output different voltages based on their movement and position. This voltage gets converted into a digital value via the A/D pins on the microcontroller.

The microcontroller receives the values from the accelerometer and gyroscope, at a rate based on the internal frequency set on the microcontroller. Depending on your microcontroller this can be adjusted to a rate of up to 8MHz. This allows you to process hundreds of values per second. Once the microcontroller gets the data, it buffers it, does minor processing to filter out noise, and sends it to the transiever. We will discuss the communications part of the bat in the next article.

There are a few thing to note about data received from the accelerometer. The accelerometer outputs acceleration values. This does not tell you right away the position or displacement of the bat. A double integration with respect to time is required to get x,y, and z values. Recall from your physics course that velocity is the integration of acceleration with respect to time. If you just want the velocity of the bat, you simply integrate the values you receive from the accelerometer within your measured time frame. You can then plot the change in speed over time, and get an idea of how fast the bat was swung. The graph below shows a sample plot of acceleration, velocity and position received from a single axis of the accelerometer.

For more information on how these values are calculated, read Implementing Positioning Algorithms Using Accelerometers by Freescale Semiconductor.

In the next article we will discuss comunication from the Bat to the PC.

Posted by Guru Lloyd | tagged: Lloyd