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Posted by Miem on April 12, 2008, 11:12 am
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Hello,
I want to ask expert advise if someone on this group can explain (and/
or provide related URL) how the Mars rover's mobility system
(particularly the differential mechanism) works?
Following is a list of URLs showing some level of details about the
rovers mobility system. However, on these URLs, it is not clear to
figure out how the differential mechanism works.
http://robotics.wsu.edu/magellan/mechanical/
http://robotics.wsu.edu/magellan/mechanical/2005-06-09/3.jpg
http://www.geocities.com/SiliconValley/Circuit/2300/hund.htm
http://www-robotics.jpl.nasa.gov/projects/MSL.cfm?Project=3
Thank you,
Miem Chan
miemchan @ gmail . com
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Posted by Curt Welch on April 12, 2008, 12:36 pm
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> Hello,
>
> I want to ask expert advise if someone on this group can explain (and/
> or provide related URL) how the Mars rover's mobility system
> (particularly the differential mechanism) works?
>
> Following is a list of URLs showing some level of details about the
> rovers mobility system. However, on these URLs, it is not clear to
> figure out how the differential mechanism works.
>
> http://robotics.wsu.edu/magellan/mechanical/
> http://robotics.wsu.edu/magellan/mechanical/2005-06-09/3.jpg
> http://www.geocities.com/SiliconValley/Circuit/2300/hund.htm
> http://www-robotics.jpl.nasa.gov/projects/MSL.cfm?Project=3
>
> Thank you,
>
> Miem Chan
>
> miemchan @ gmail . com
I don't know anything about what the Mars rover did, but looking at that
WSU design, and the fact it's called a "differential" I can guess what it's
doing.
The idea seems to be that the wheel assemblies attach to the body with a
single shaft on each side of the body. When the shaft needs to rotate on
one side by not the other, the body rotates at only 1/2 the speed. That is
the right wheel assembly would rotate up 10 degrees, and the body would
rotate up only 5 degrees. This can be made to work with a standard
differential gear arrangement like this view from the front or the back:
body
|
\---//
| | |
-------| | |-----------
| | |
//---\
|
That drawing is 4 gears on 3 shafts. The center shaft is free to rotate,
but is attached to the body. The right and left gears are attached to the
shafts running to the wheel bogies. If the left shaft rotates, the right
shaft rotates the same amount, but in the opposite direction.
This is a normal differential drive as used to power two wheels from a
single shaft, except the drive shaft is clamped to the body instead of
being free to rotate.
At first, I couldn't figure out what the WSU design was doing, but I think
I see it now. You just use two normal gears (not beveled) in a row
(shown here as the two gears on the left column:
-
|
| |
---|---|--
| |
| +
- + belt drive
| +
| |
Left --------| |--------- right
| |
|
-
Then you attach the top right gear to the bottom right gear with a belt or
chain.
The net result is that you want the right shaft to rotate the same as the
left shaft, but in opposite directions.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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Posted by Miem on April 14, 2008, 7:26 am
Please log in for more thread options > Curt Welch http://CurtWelch.Com/
> c...@kcwc.com http://NewsReader.Com/
Thank you for your reply.
There is also some photos of a differential mechanism on the following
URLs
But, I could not understood how these two wheels can turn in the same
direction in the following configuration
http://www.vexforum.com/gallery/showimage.php?i=667&c=
http://www.vexforum.com/gallery/showimage.php?i=671&c=
http://www.vexforum.com/gallery/showimage.php?i=782&c=4
http://www.vexforum.com/gallery/showimage.php?i=743&c=4
No differential
http://www.vexrobotics.com/images/vex-robots/mars-rover.jpg
Miem Chan
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Posted by Curt Welch on April 14, 2008, 8:58 am
Please log in for more thread options > > Curt Welch
> > http://CurtWelch.Com/ c...@kcwc.com
> > http://NewsReader.Com/
>
> Thank you for your reply.
>
> There is also some photos of a differential mechanism on the following
> URLs
>
> But, I could not understood how these two wheels can turn in the same
> direction in the following configuration
> http://www.vexforum.com/gallery/showimage.php?i=667&c=
> http://www.vexforum.com/gallery/showimage.php?i=671&c=
> http://www.vexforum.com/gallery/showimage.php?i=782&c=4
> http://www.vexforum.com/gallery/showimage.php?i=743&c=4
You would understand it quickly if you could see a movie or you had one you
could play with.
The large gear in those pictures is attached to the plastic frame that
holds the small idler gear. When the large gear is driven, the frame
rotates, along with the idler gear. The idler gear doesn't stay where it
is. It flips end over end as the cage it is in rotates.
When the vehicle is moving forward (not turning), both wheel gears turn at
the same speed, and the idler gear doesn't spin at all, it just goes around
and around in the cage and the whole unit acts the same as if it were just
a solid axle with no gears. Or, it acts the same as it would if you just
glued all the gears together in that configuration.
When the vehicle needs to turn, then one wheel will need to spin slightly
faster than the other. At that point, the idler gear will spin allowing
that to happen, while at the same time it is driving both wheel gears by
rotating in the cage end over end.
When the drive motor is still, and you pick the unit off the ground, you
can spin one wheel, and the other wheel will spin in the opposite
direction. That's the behavior which is taken advantage of in these
suspension designs.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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Posted by John Nagle on April 14, 2008, 12:13 pm
Please log in for more thread options Curt Welch wrote:
>>> Curt Welch
>>> http://CurtWelch.Com/ c...@kcwc.com
>>> http://NewsReader.Com/
>> Thank you for your reply.
>>
>> There is also some photos of a differential mechanism on the following
>> URLs
>>
>> But, I could not understood how these two wheels can turn in the same
>> direction in the following configuration
>> http://www.vexforum.com/gallery/showimage.php?i=667&c=
>> http://www.vexforum.com/gallery/showimage.php?i=671&c=
>> http://www.vexforum.com/gallery/showimage.php?i=782&c=4
>> http://www.vexforum.com/gallery/showimage.php?i=743&c=4
This is the movie you need to see:
http://www.archive.org/details/Aroundth1937
"Around the Corner", produced for Chevrolet in 1937 by
the Jam Handy organization, makes it very, very clear how a
differential works.
John Nagle
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>
> I want to ask expert advise if someone on this group can explain (and/
> or provide related URL) how the Mars rover's mobility system
> (particularly the differential mechanism) works?
>
> Following is a list of URLs showing some level of details about the
> rovers mobility system. However, on these URLs, it is not clear to
> figure out how the differential mechanism works.
>
> http://robotics.wsu.edu/magellan/mechanical/
> http://robotics.wsu.edu/magellan/mechanical/2005-06-09/3.jpg
> http://www.geocities.com/SiliconValley/Circuit/2300/hund.htm
> http://www-robotics.jpl.nasa.gov/projects/MSL.cfm?Project=3
>
> Thank you,
>
> Miem Chan
>
> miemchan @ gmail . com