Subject:
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The Differential and One-Motor Turning
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 19 Jan 2000 21:28:48 GMT
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Original-From:
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kennep <KENNEP@MAIL.ihatespamALUM.RPI.EDU>
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Reply-To:
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<KENNEP@MAIL.ALUM.ihatespamRPI.EDU>
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Viewed:
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680 times
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In response to several posts regarding driving and turning with only one motor...
(Please excuse my very basic mechanical engineering knowledge!)
Having only the two motors included with the RIS, I have also experimented with vehicles that are driven with just one motor. My first attempt consisted of a car with one back axle connected to the two "Big Wheels" and a pivot wheel in the front which was similar to the one found on the Constructopedia's "Acrobat."
It was my hope that such a robot would drive around, propelled by a single motor, and would turn whenever the pivot wheel was moved. My goal was to build a "mechanical" sensor/AI to avoid obstacles by connecting feelers directly to the pivot wheel such that the robot would turn away from anything it bumped.
However, I found that the car resisted turning because the two back wheels were forced to spin in the same direction at the same speed. Even when I tried holding the pivot wheel at an angle, the car would only turn slightly and force was applied to straighten the pivot wheel.
I struggled for a while trying to think of a way that I could drive both wheels with one motor, but still allow them to turn at different speed if they had to. If I could just do this, then a simple ratchet mechanism would allow me to lock one wheel from turning backwards, and I could even turn to one side in reverse with the single motor.
Being more a computer scientist than a mechanical engineer, I had no idea what that funny looking dark gray double-gear gadget was. But when I read about what the differential was for (in Dave Baum's book), I suddenly realized that you COULD use just one drive motor and still turn. Combining this with a ratchet would let a robot turn one way in reverse, or perhaps a mechanism like I mentioned before with the pivot wheel could be used. Of course, a second motor could be employed to operate the turning
in many ways.
For those who are not familiar with the differential, I will try to explain:
First, three of those small gears with the teeth at a 45 deg angle fit inside the assembly. Separate axles connect to the two outer gears, and the inner gear fits into the peg on the inside of the differential's "box." The result of this assembly is that the speed of the two axles must AVERAGE the speed of the gearbox itself. If a gear is used to spin the differential, then normally both axles will spin at the same speed. But if one axle is prevented from spinning, or slowed a little, then the other
axle will speed up to compensate.
This will allow your robot to drive both wheels forward, but when it enters a turn, the wheels are also allowed to spin at different speeds; so the resistance to turning that I encountered in my first single-axle attempt will be avoided. In the case of ratcheting one wheel in reverse, the other wheel will spin at TWICE the differentials speed (so that the average is maintained) and the robot will pivot around the still wheel.
I have only been playing with small assemblies, and have not yet constructed a full robot that uses this principle- but I plan to try it out soon.
Wish me luck!
-kennep
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Message has 2 Replies:
 | | Re: The Differential and One-Motor Turning
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| Of course the differential is very cool, but you can also get away with only EVER driving one wheel (i.e. the way the L3GO droid from the DDK works) if you have a pivot wheel that snaps straight when moved forward and rotates slightly when moved (...) (25 years ago, 20-Jan-00, to lugnet.robotics)
| | | Re: The Differential and One-Motor Turning
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| (...) turn. Combining this with a ratchet would let a robot turn one way in reverse, or perhaps a mechanism like I mentioned before with the pivot wheel could be used. Of course, a second motor could be employed to operate the turning (...) (...) (25 years ago, 20-Jan-00, to lugnet.robotics)
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