I really like your latest post with the tank.png picture. This thing is really looking good.
If I may, I would like to offer a few thoughts, not as criticism because I don't know enough to be critical:
- I notice that this configuration places a great deal of tread on the ground where it could create a lot of friction to overcome and possibly some tread wear.
- If your "ankle" servos offer enough rotation, you may be able relieve most of the friction and stress most of the time by rotating the "toe" ends of the treads further upward so that ground contact is concentrated on the road wheels currently pointed to by the ends of the femurs. This would still allow you to use full tread contact in dusty, soft, or slippery surfaces.
I certainly am not suggesting any sort of redesign because I love what you've done. But since you are working with a tracked vehicle, I thought you might be interested in an exchange I had in March 2009 on the Lets Make Robots Forum
with a former tank driver signed Cwignell. On that forum I was TechnoBuff until my brother envisioned me doing technology in the buff.
My very best wishes to you, RoboTed
Everything that follows is from the LMR Forum
. Snailkeeper correct me if I'm doing this wrong.
Track design for tracked robots
March 11, 2009
I am brand new to the robot hobby with questions about track profiles.
The high sided triangular track profile of Wall-E or Johnny 5 looks really good, but seems to make inefficient use of track links. Other than good looks, is there a good design reason for this profile?
Of the other general profiles available (parallelogram - WWI tanks), (rounded rectangle - Christie type WWII tanks), (inverted trapezoid - short base down), what are the relative merits in terms of efficient use of links vs. object climbing ability?
Of these, which tend to cause the most stress on the links, and which cause the least?
Thank you for your assistance.
@ Wed, 2009-03-11 10:04
You will see the Wall-E arrangement on BullDozers, it has the advantage the it puts the final drives closer to the middle of the vehicle and having the drive sprocket higher means the track tend to self clear rocks and crap which you can pick up when turning; this reduces thrown tracks.
The rombod tanks from WW1 were designed around the minimum radius wheel which would cross the trenches then in use.
The convential approach with the drive at one end at a similar level to the idlers puts the powerplant and transmission at one end of the vehicle (usually the rear) and leaves a big crew space (well it's not, it's very snug because there is a lot of stuff in there with you.). This also serves to lower the profile, handy went people are shooting at you. Picking up rocks is less of an issue with the modern tank but if you look closely at some of them they have plow like arrangements to clear the tracks before the sprockets.
As for climbing, the convential approach is the best normally you can get up verticle steps a little bigger than the height from the ground to the front idler/sprocket. The max gradent depends on a lot of things, Hp, transmission, traction, surface and so on.
The Wall-E arrangement will have a poor step performance, and step performance is not a big issue for bulldozers.
Oh if your wondering, I drove tanks for a living once.
@ Fri, 2009-03-13 23:43
Excellent answers concisely stated. Wall-E design now makes sense where CG considerations require driven sproket near center and climbing is minor design factor. Your comment on climbing is very encouraging. I had expected top sprocket would have to loom over step height rather than possibly being slightly below. A clarification question: When you described "conventional approach", I envision a Christie-like design but with the front and rear end wheels raised somewhat above the level of the ground contact base wheels and idlers guiding the tread on the return direction. This would give a little more climbing ability than an absolutely flat arrangement like some cranes and road paving equipment. Did I understand you correctly?
Re you last sentence, since you answered my inquiry, I assume "living" was the operative word. Good!
@ Sat, 2009-03-14 02:20
Yes when if refer to to convertional approach I mean where the leading wheel is raised and the the rear wheel is raised and all of the Roadwheels sit on the ground, note, the leading and read wheel can be either the driven (sprocket) or an Idler. The trend with moden tanks is the rears are the drives as it allows all of the engine and transmission to be incapsulated at the rear, but there are some front drive tanks around. Speed and mobility on the modern battle field are important, climbing ramparts (steps) and trenches is not so important any more.
To be strictly accurate Christie like design refers to a convential approach as above, but uses large diameter roadwheels on a swing arm suspension, the Russion T-34 and the English Crusaider were early examples of this. The robot I am building at the moment is basically a Christie design. Christies were optimised for speed and some of them could be driven with out tracks. Track wear is still an expensive issue with tracked machines.
Regarding getting up steps, the actual height you can climb depends on the geometry of the step, the type of surface before, available traction, torque available and if the driver is willing to break the vechicle! One upper limit to step type obsticles is the Height of the Center of gravity vs wheelbase vs step height. Tracked vehicles can assum alarming attitudes.