Idea: Robot with powered caster wheels, two independently powered wheels per c
Dennis Ferron
Posts: 480
I know some researchers at the University of Oklahoma who work on a six-wheeled robot chassis similar in design to the Mars rovers. (Six powered wheels, each wheel being able to turn left/right 360 degrees so the robot can crab sideways, spin in place, or steer in any direction.) It takes a lot of torque to turn those wheels left/right though. The first time they tried the robot they burned out all the servos pretty much instantly because they couldn't handle the torque needed to steer the wheels. Even if they manage to find servos big enough to do it, I'm not sure the 1/4 inch aluminum input shafts won't shear under the stress!
That got me thinking: rather than try to engineer something that can take the stress of steering those wheels, why not make it so that the drive motors can also steer?
What I'm talking about is use double caster wheels (imagine the way the front landing gear wheel for a jumbo jet looks) and make it so that each side is independently powered. By turning the wheels on each side of the caster in opposite directions, you could make the assembly turn on its own. And you'd want to offset it slightly (caster wheel, like the front wheels on a shopping cart) so that when the wheels are pulling the robot they'll tend to self-align with all the casters facing forward, and if a motor goes dead it will tend to self-align with that caster being dragged.
I notice Pololu has double gearboxes on sale for ~$9, so it wouldn't be very expensive to build. I was thinking of 3 casters to start out (would make a tripod robot) but then again if I want to be able to demonstrate it pulling a dead wheel I might want more total casters than 3...
With the gearboxes so cheap I would say the biggest expense would be the motor controller and HERE's where the self-aligning caster concept can shine: once all the casters are pointed in roughly the same direction, you should be able to drive all the motors together on the same controller. And you only need to align them one at a time. So if you use MOSFETs to selectively break the circuit between all but one of the casters' motors at a time, you could use a single dual motor controller to align all the casters, and then once you're happy with the positions use a single high current driver to drive all the motors together in the forward direction.
That got me thinking: rather than try to engineer something that can take the stress of steering those wheels, why not make it so that the drive motors can also steer?
What I'm talking about is use double caster wheels (imagine the way the front landing gear wheel for a jumbo jet looks) and make it so that each side is independently powered. By turning the wheels on each side of the caster in opposite directions, you could make the assembly turn on its own. And you'd want to offset it slightly (caster wheel, like the front wheels on a shopping cart) so that when the wheels are pulling the robot they'll tend to self-align with all the casters facing forward, and if a motor goes dead it will tend to self-align with that caster being dragged.
I notice Pololu has double gearboxes on sale for ~$9, so it wouldn't be very expensive to build. I was thinking of 3 casters to start out (would make a tripod robot) but then again if I want to be able to demonstrate it pulling a dead wheel I might want more total casters than 3...
With the gearboxes so cheap I would say the biggest expense would be the motor controller and HERE's where the self-aligning caster concept can shine: once all the casters are pointed in roughly the same direction, you should be able to drive all the motors together on the same controller. And you only need to align them one at a time. So if you use MOSFETs to selectively break the circuit between all but one of the casters' motors at a time, you could use a single dual motor controller to align all the casters, and then once you're happy with the positions use a single high current driver to drive all the motors together in the forward direction.
Comments
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PG
Post Edited (Pi Guy) : 11/6/2009 10:12:55 PM GMT
But coincidently OU *is* actually building some large robots. There was a video posted to hackaday.com several weeks ago of a large gasoline-engine robot someone's working on. I'm not personally acquainted with the members of the group doing that robot, but I recognized having seen some of the parts in the lab, before they became part of a robot.
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·"If you build it, they will come."
You ought to show me the link to that gas-powered one, for some reason I'm strangely interested in that. I've been to Norman many times and have seen robotics articles from that school but never saw anything larger than a foot long, so I guess that's why I came to that conclusion. I guess if they have anything good enough I might even go there after high school, but I'm not worried about the future right now.
Back to the subject, there is one other way past differential drive, or an addition if you wish. What if the wheels were offset from their upper mounting and attached by a pivoting joint. This joint would have a brake on it, but can be released at times, allowing that wheel to rotate as it turns. Once at the desired point, it could once again lock and keep it in place. A bit tedious, but it could possibly become an advantage on the robot for moving around obstacles. Just an idea of many for the pool, but I'm sure every one of them is good in their own way. It just depends on what tasks the robot is required to do.
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PG
Here's the link to the gas powered robot at OU:
hackaday.com/2009/10/06/atv-brings-skynet-closer/
Whereabouts in Oklahoma are you from, Pi Guy? I live in Moore.
PG, that's an excellent idea on the locking pivot. It would work for some of the powerwheels robots I build. I read a whitepaper somewhere about a kind of adaptation of differential/skid steering where a large four-wheeled robot uses servos to steer the wheels slightly inward while executing turns to get a more favorable angle. (Otherwise with four wheels they skid while you're doing differential steering.) I thought, why use powered steering to do that? Why not, as you say, offset the wheels somewhat and allow them to pivot freely within some range. But of course the problem with that would be that the wheels would wobble while the robot tries to drive straight. But if you could lock the pivot when you want then that's a moot point. I don't think it would even be something you have to stop the robot and wait on: if we're just talking about pivoting the wheels within a very limited angle, like 30 degrees, you could get the wheels around to the most favorable angle for the turn you're going to make almost simultaneously to executing the turn, by giving some wheels more or less power, and then lock in place.
For the pivot locking mechanism, I'd wrap steel aircraft cable 8 or 10 turns around the pivot axle and use a servo to pull it tight or relax it.
As for the locking mechanism, I was actually thinking about the possibility of a rubber pad pressed down on a plate to stop it from rotating, but that won't necessarily have too high of locking capabilities. Probably for the highest of strength and efficiency, a notched plate could be used, with electromagnetic lock able to slide a bar into the notches to hold position. This may also increase accuracy as the notches will hold more precise positions than something without limits, and possibly able to slide unintentionally as well.
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PG
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·"If you build it, they will come."