Sounds like a complex project - good luck with it!
twafky:
Should we include a solenoid powered off switch for manual shutdown?
You should include a SPST NO (normally open) continuous-current contactor relay directly after your main battery (+) line, right after your main fuse (which should be as close to the positive terminal of the battery as practical and possible - a fusible link in the cable would be best).
You should have an emergency switch on the robot itself that can open the line as well (not de-activate the relay). De-activation of the relay should be possible via a tag/key on the robot, as well as remotely. You want the relay to be de-activated in the event of an issue - not activated. These are fail-safe mechanisms. The key/tag is to grab and pull to activate the relay (in case the radio fails). The button is for the same, but should be activated when slammed/hit - and stay set (they make these for heavy equipment and industry - look into it).
twafky:
Should we use a Raspberry Pi for the video feeds (enabling us to run it over a webserver and be viewed around the scene of the accident/contamination) or should we stick with an Ardupilot for that?
You can do either - just realize the latency issues and such involved with the IP camera route.
twafky:
Anybody have any tips for using a motor controller? I'm solely a software/computers type and am not too familiar with currents and all that math-y stuff.
You need to know the current draw of the motors, and the voltage involved. Likely - in the case of these motors - they are 24 volts DC, but amperage is unknown (I've seen wheelchair motors draw anything from 5 amps to 50 amps - it just depends on the motor used). Measuring these motors is best via measuring the winding resistance, and using Ohm's law to figure it out (because you can't really easily pony-brake such a motor to determine it empirically). Take your measurement, add 10-15% - and call it good. Use that to size/pick your motor controller.
Depending on what you need - for these kinds of motors, you'll generally want something fairly high-power capable. Vantec and Roboteq are the "goto" guys for such controllers, if you want very high quality, no-hassle hookup, easy control, and money isn't a big issue (considering you are planning on FLiR - that shouldn't be a problem). Otherwise - if your needs or budget are more modest, and the current needs are low enough - there are other options out there (Sabertooth 25A controller - also some things by Pololu might work out - then there are the various Ebay offerings from China - and finally, if you can find one, old brushed-DC motor controllers for RC cars can work).
twafky:
Any regular tips or things we should watch for during installation of all of these parts and such?
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Always disconnect the battery before working on the electrical - believe me, I have seen more than enough of my share of "impromptu arc-welders" to know the truth of that!
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Before "ground-testing" your motor control code, test the system "off-ground" first! Put the machine up on a stand or something (Harbor Freight or similar tool places sell a very nifty floor jack lift for motorcycles that works great for wheelchair bases - if you are keeping the base).
Number 2 is especially important - if you have an "out-of-control" robot (for any number of reasons) - with the base, motors, and batteries alone weighing in over 100 lbs or more - you won't be able to stop it by hand, and you don't want to get run over by it. It can easily maim you, go thru drywall, bust bricks, etc. So - elevate it and test it first. You might even wire in a hardware PWM system to the motor controller (if the controller has PWM inputs, for instance - stick an AND gate in front, put your hardware PWM on one side, set for a low speed, and your controller PWM input to the other side of the gate).
Finally - don't attempt to hack the wheelchair's motor controller and/or joystick controller - you likely won't be too successful, because these devices have a ton of fail-safes built in that are difficult to bypass or trick in some cases (considering their original purpose - they have to be as safe as can be designed).
Just to let you know - I work with a guy where we refurbish and donate out electric wheelchairs and mobility devices - we also build custom chairs and other contraptions from old parts we can't donate. A big thing I have learned from these machines is that they aren't toys - you need to treat them with respect, and even then they can surprise you (ok - I'll be honest - if you re-program a quality chair for high-speed velocity control - it can be fun to ride).