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Archive for the Shameless plugs for potential corporate sponsors Category

Guts

8. May 2009 by Sean.

I really like that picture of me in the track pants and golf shirt below. Very flattering.

The eletronics guts are coming together. Here is a 30,000 ft view of what’s happening:

The main board reads sensors from a number of sources - GPS module, compass sensor, light sensor, battery voltage sensor and temperature sensor. It retrieves GPS coordinates from an eeprom and logs data to another eeprom. In the off chance that we get our boat back after its big trip - we can see how hot it was, what the battery levels were, etc.

The main PIC microprocessor is programmed to calculate a target bearing from its current lat/long coordinates and the next lat/long coordinate. It compares the target bearing to the compass (actual) bearing and then adjusts the motors using differential steering and pulse width.

The target bearing calculation is surprisingly tricky to do on a microprocessor. Most of the issues are around precision when you’ve only got a maximum of 32 bits of precision and some complex calculations involving PI and arc-tangent. Note how I use those terms like I know what they mean. It’s all a ruse.

Thank goodness the fine people at HI-TECH have a C compiler which has trigonometric math functions and does floating point calculations. Often when working with microprocessors, floating point calculations are a luxury.

The GPS module (home grown) offloads the work of parsing the GPS serial stream and serves it up as an I2C message to the main board. Otherwise, this process alone would suck up about half of the microprocessor program and data space.

The Compass is a Devantech Magnetic Compass Module CMPS03

There is a real time clock chip on board to support the timed operations. We would use the GPS time but we need to turn the GPS module off when it’s not being used. That’s 50 milliamps we can’t afford.

Here’s what it looks like: