Originally Posted by IKE (Post 1487708)

So, I do not know that I would recommend suspension and a lot of complication.

With such a small wheel-base, CG will be important, and in general, you should be able to drive around most troublesome ground.

I would suggest looking at self propelled lawnmowers for possible architectures and thoughts on structure.
http://www.blackanddecker.com/produc...mower/spcm1936

A concern I have is actually footprint vs. weight vs. soil (terramechanics). Think about it from a bike tire standpoint. A lot of mountain bikes operate with roughly 30 PSI tires and thus ground pressure around 30 psi. This is fine for paved, gravel road and hard pack, but soft sand and/or dirt or mud can be difficult.
Comparatively, many tractors or off road machinery tend to operate below 15 PSI for ground pressure. If you are trying to traverse Mud, you may need to go as low as 11 PSI ground pressure.
Thus for 200 pounds, you would need about 18 inches of srface area in contact with the ground.
To get down to these low levels, you typically need either balloon tires or tracks.
IE a 4 wheel solution would need over 4 in^2 per tire. 6 wheel would be 3 in^2 per tire.

A 6WD chassis with these wheels:
http://www.andymark.com/product-p/am-0970.htm
would be pretty reasonable to implement. They even have a base chassis at that website as well as adapters that should be close to meeting your needs. Similar to this one:
http://www.andymark.com/product-p/am-2289.htm

I think you could make either of these concepts work relatively well. If you are OK with a person manipulating controls, I personally would start with the self propelled mower and double the width of the wheels as well as ad more robust axle attachments to handle the 200 lbs vs the 90 lbs it was initially design for.
With it being a walk behind, the human would also add stabalizing properties when maneuvering which I suspect will be critical (with a base of 24x26, you center of gravity will need to be below about 12" of height assuming it is relatively centered. This is pretty low for anything that heavy, and also often with solar installations, a little height is very helpful towards avoiding shadows).

Originally Posted by gblake (Post 1487715)

I'll bet that driving over real terrain and obstacles, and getting enough endurance from your battery(s) (affected by battery storage, motor efficiency, payload power needs, and misc) will dominate your physics problems, if you have to traverse non-trivial distances or stay on-station 24x7. But they won't be your biggest problem (see last paragraph).

So,

• Build miniature prototypes out of wood, VEX/Tetrix parts, etc.
• Build a full-sized, full-weight prototype out of 2x4s and your chosen wheels/tracks (see Ike's post).
• Drive (tow, push, whatever) the prototypes outside.
• Use easily-revised, modular framing for the real thing. It will need to be adjusted as you iteratively design/integrate/test

To solve any other problems, if you burrow deeply enough into the archives here, you can probably find a workable solution.

Also, it's a good rule of thumb to estimate project size/complexity by the number of interfaces (mechanical, electrical, software) you have to design and/or implement, instead of counting the number of subsystems that exist. It's good to pay attention to both, but it's the interfaces that will eat you alive (in part by causing both your time spent searching for or choosing parts, and your resulting parts-count to explode).

Also, everything takes about 4 times as long as you think it will/should; and simple 10-minute tasks often balloon to take 2-4 hours by the time you are finished discussing them, setting-up for them, doing them, testing them, and cleaning up after them. The clock is your biggest enemy, not robot-physics.

Blake

Originally Posted by gblake (Post 1487781)

So, maybe what you want to do is buy one or two motorized wheel chairs and/or Rascal scooters; cut them up, weld the resulting parts into a 24"x36" frame, adapt the control and power systems to the new configuration, and call it a day?
;)