As I explained in a previous post, I want the plane to be able to track its attitude. The receiver is already doing a time critical function in software (namely the servo updates), so it would be quite a strain on it also to read all the different sensors and to filter the data. I thus ended up building these functions on a separate board, which interfaces with the receiver through UART.
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| The IMU module I ended up using |
The
IMU sensor board I'll be using has a gyroscope, a magnetometer and an accelerometer. The sensors are all read through I2C, which is nice as the microcontroller I thought of using (Atmega328) has built in I2C hardware support. However, in addition to these three I2C sensors, I need also to connect a GPS receiver. This presents a problem, because the microcontroller only has one hardware UART, and that is already in use for communication with the receiver. I could change my choice of microcontroller, to something that has two hardware UART ports. On the other hand, I could go with a software UART as I already have several Atmega328s in my components bin. I decided on the cheaper option, and went with an Atmega328 and software UART. I was fairly certain that this choice would bite me in the ass later on (which it did, in the form of timing issues in the code).
I ordered the IMU board from ebay and was so eager to get this thing built as fast as possible that I took measurements of the board dimensions from the photo on ebay and started the design. The board is very straightforward and there is nothing special about it what-so-ever, as can be seen in the schematics and the layout.
Unfortunately I screwed up measuring the dimensions from the photo, and as a result the mounting hole of the IMU board doesn't match up with the hole on my board. It doesn't really matter, as now I just hold the board steady with double sided tape :-).
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| Top side of the board after etching and drilling. |
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| Bottom side of the board after connecting the vias |
I have recently investigated the possibility of doing through hole plating for the vias, but it seems to be quite a bit more involved than what I'm currently doing. I guess I'll have to continue soldering short pins through the vias to connect them.
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| Top side almost completed |
The hardware UART port is used to communicate with the RC receiver whereas the software UART port connects to the GPS receiver. I put in the external I2C port for the possibility to add a barometric pressure sensor later on, although now you can buy cheap sensor boards that already contain a pressure sensor.
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| Bottom side almost completed |
While assembling the board, it turned out I had run out of 20 MHz crystals. I picked up some the next day, but I didn't take new photos of the board.
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