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6WD with suspension? (Robomagellan)

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Posted: 2012-03-21 13:52 
If you use the E-Maxx differentials, then you need to use Ackerman (or similar) steering, which really changes the whole idea of DPA's drive.

I'm just not ready at this time to invest the time and $$ necessary to develop my copy of DPA's 'bot. I did get a low-cost 4WD chassis a little like the E-Maxx at the end of last year, but I haven't done anything with it.

I am still perusing the navigation aspect of the RoboMagellan project, I have a 9DOF IMU to play with, and a new GPS module. However, I'm now leaning towards implementing them in a helicopter. Might be 'quad, but after seeing a real Hobby-Helicopter fly (and getting one!), I'm more anxious to make it autonomous (after I can fly it!).

Thanks for the comments.

Alan KM6VV

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Posted: 2012-06-29 12:26 
I've committed to a Robomagellan 'bot for next April.

Early work on Ebay has netted me the wheels and suspension parts I was after. Traxxas E-Maxx parts.

Lower chassis plate is fiberglass, the next two decks (below and above the fiberglass bulkheads) will also be of 1/8" stock.

Into each of the grey bulkheads will go a machined round aluminum plate to take a ball bearing race. There will be two motors each driving the three wheels on its side via 1/5" pitch timing belts and pulleys. a series of six drill-rod shafts will drive the CV joints to the wheels. The opposite ends of the shafts will be supported by bearing blocks.



A 15 A RoboClaw will drive the motors from a 24 V battery.

More on electronics later.

Alan KM6VV

Ha!
We need to expand the heading from:

"2WD, 4WD, Carpet Rover, and other rover robots."
to:
"2WD, 6WD, Carpet Rover, and other rover robots."

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Posted: 2012-06-29 19:01 
Neat idea! I'd love to see it under power.
Why don't you let me machine a chassis for your suspension?


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Posted: 2012-06-29 20:14 
Sure!

It's the length that's stopping me from easily machining it. For this stage of the chassis, it's a 24" x 6" piece of 1/8" fiberglass. Two more decks go on top, middle about 5" wide (top 7" wide), length to span the three red bulkheads. Bulkheads on 8" centers. So far, I've simply drilled the bottom chassis on the drill press. Next I've got to mount the bearing blocks for the pulleys, and the belt tensioners. More holes!

A little while before power, I forgot to order two drive belts for the motors. And I still have to finish making the bearing blocks, the tensioning blocks, and the motor mounting bulkheads!

Alan KM6VV

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Posted: 2012-06-29 21:48 
Give me a drawing of what you're looking for and we'll see what we can come up with.


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Posted: 2012-06-30 01:10 
Thanks I'll work on it this weekend.

Alan KM6VV

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Posted: 2012-06-30 22:44 
Good project idea.
Why not 6 motors ?

Look here you may find some parts or project start. Look for rollers
http://www.rcgroups.com/cars-trucks-and-parts-fs-w-564/

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Posted: 2012-07-01 07:30 
DiaLFonZo,
By ganging three wheels to each motor you only need two encoders, one for each motor, to be able to determine wheel speed and distance travelled. In other words all three wheels on each side are turning at the same speed and go relatively the same distance. No two motors turn the same speed for a given voltage. The spec is usually +/- 10%. If you were strictly using dead reckoning for navigation, that kind of error would be deadly.
Just a thought.
Regards,
TCIII

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Posted: 2012-07-01 14:07 
Quote:
By ganging three wheels to each motor you only need two encoders, one for each motor, to be able to determine wheel speed and distance travelled.


Encoders are usually only accurate when there is minimal or no slippage. Using encoders on the wheels of a vehicle like this would probably provide accurate distance measurementonly for limited straight line travel under ideal conditions.

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Posted: 2012-07-01 18:01 
@zoomkat,
Encoder (odometry) correction is a real art. Here is how it is done even when the rover does not move in a straight line from beginning to end: http://geology.heroy.smu.edu/~dpa-www/r ... r/imu_odo/
Regards,
TCIII

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Posted: 2012-07-01 19:19 
You could use six motors, but it would make the design more complected. Putting wheels and motors in a single line across the width of the 'bot makes for a wide bot. Or you could drive them with timing belts (like DPA, and mine!), but then you've still got a lot more work.

Driving three wheels by one motor on each side still results in a slip-steer, but I think it can be managed. I can even add Ackerman steering on both front and end! It can rotate in its own wheel base, so that's not bad.

Navigation will not be limited to Odometry; I intend to use a compass, GPS and inertial references as well (big undertaking).

Bob in the HBRobotics club IS using six motors, and the motors are pivoted from the rear to give some ground compliance. Large piano wires give some suspension.

Alan KM6VV

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Posted: 2012-07-31 16:43 
Some recent pix:









new motor bulkheads, motors, batteries.

Alan KM6VV

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Posted: 2012-07-31 16:56 
That you Alan?

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Posted: 2012-07-31 17:58 
Can't be me, that guy looks O-L-D.

Alan KM6VV

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Posted: 2012-08-01 06:40 
Nice work Alan. Looks really good!
Regards,
TCIII

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