Subject:
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Re: 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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Thu, 20 Jan 2000 01:54:52 GMT
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Viewed:
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536 times
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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
backward.
Maybe I just have a mental block, but all the single-motor drive designs
seem so
*limited* I wish I could find a way to turn in either direction from a
single motor,
changing, perhaps, the power setting or some such to determine whether to
turn or
go straight... I always hated it when I'd get an RC car when I was a kid
and it only
turned in reverse...
-Peter
"kennep" <lego-robotics@crynwr.com> wrote in message
news:200001191628.AA3342602@mail.alum.rpi.edu...
> 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 is in Reply To:
 | | The Differential and One-Motor Turning
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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 (...) (24 years ago, 19-Jan-00, to lugnet.robotics)
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