Posted by Faisal Shah on October 28, 2009 at 10:22pm
Yet Another DIY Autopilot :)I've decided to go about making my own AutoPilot board from the ground up. Well, I won't exactly be starting from scratch - hopefully I will be able to incorporate the collective wisdom of this community :). I know there are several boards out there that may fit my needs/wants closely. However, I just wanted to go through the whole process of doing it myself.My goal is to order a batch of PCB's by the end of this year, probably from Gold Phoenix PCB.For some reason I'm bent upon using a RTOS. Actually, there are very good reasons.FreeRTOS is free, popular, and well supported. So as far as processor selection, I want to choose a supported device.FreeRTOS Supported Deviceshttp://www.freertos.org/a00090.htmlI would like the device to be powerful enough to run an AHRS algorithm (extended kalman/DCM - possible both in parallel to compare them), navigation, telemetry, image compression (for images through wireless xbee or whatever data link), input filtering (some inexpensive FIR filters), and whatever else I can think of. Also, the availability of free development tools and debuggers for the processor is a must.As far as sensor inputs - versatility is the name of the game.* 3 axis gyro* 3 axis accel* 3 axis magnetometer* 3 axis thermopiles* differential pressure sensor (speed)* barometer (altitude)* GPS* (optional) camera (simple image processing, horizon detection or something)* Ultra sonic/laser for range finding (used for landing)So, something that can handle all of the above if possible.Another factor in deciding what hardware to use are the packaging of the various parts. For example, while BGA's and LGA's are possible to solder at home using a toaster oven reflow technique ... I would want to avoid that - unless many of you have had positive experiences with it. Quad flat packs are do-able by (my) hand ... Still considering if I want to make a mini project out of soldering grid-array type parts. I'm reading up on how easy or difficult it is currently - as it will effect my choice of parts.For the 3 axis accel and gyro, should I use the Sparkfun Razor 6DOF board? I heard that there are some significantly better performing gyro's in the same flat configuration [I really really want a flat board .. no funny things sticking out of it *cough* ArduIMU *cough* UAVDevBoard :) ]. Is that true? Part numbers? If there isn't much benefit in switching devices, I would rather just buy the SparkFun module just to stay away from having to solder BGA/LGA chips which the gyro/accel's usually come in. I would like the IMU board to be separate anyways, in order to mount it as close as possible to the CoG of the plane. Also, I would like the magnetometer to be able to be remotely located - so as to be able to position it away from EMI.Anywhoo, here's a thread - let's toss some ideas around. I'll be putting schematics together in the next few days and weeks - and will perhaps get an eval kit for whatever processor is chosen.
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Another capability that I would like to have on board:
Data logging to an SD flash card. This would be an invaluable resource - being able to be obscene and log as many parameters as I want to without worrying about available space in EEPROM/whatever :) .
I like your proposed approach. I'm considering something similar, although I'm probably going to use Arduino (on a Mega - with all sorts of modularity and the ability to leverage all of the work already done on ArduPilot), but I am curious about the FreeRTOS possibility. I wonder how much simpler that would make the project. I think the bulk of the challenge is in the math with respect to navigation and stabilization. Everything else (even the hardware) seems simple compared to that. It sounds like you are just looking for an opportunity to dive into RTOS programming - or do you have some benefits in mind for RTOS? BTW, it looks like that RTOS will run on a Mega. I'm not sure if the Mega can handle image manipulation (in fact I doubt it), but it would handle everything else all right.
I use the ADXRS300's which are BGA. I have soldered over 15-20 of these by hand and have had 100% success rate except for the time that I picked up the board when it was still hot and the part fell back off. So I'd say with the right techniques BGA are actually really easy to do. Kinda of scary to screw up soldering a one time only part that costs 50$ each but like I said....Haven't messed up one yet knock on wood!
Replies
Data logging to an SD flash card. This would be an invaluable resource - being able to be obscene and log as many parameters as I want to without worrying about available space in EEPROM/whatever :) .
Good luck.
Ryan