Mechanical design of Mars rover mobility subsystem

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/

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?if7&c=
http://www.vexforum.com/gallery/showimage.php?ig1&c=
http://www.vexforum.com/gallery/showimage.php?ix2&c=4
http://www.vexforum.com/gallery/showimage.php?it3&c=4

No differential
http://www.vexrobotics.com/images/vex-robots/mars-rover.jpg

Miem Chan

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/

     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

Thank you John!
That URL is fantastic!.. Yes, you are right I now understood how the
differential works!.
Thanks a lot!.

Miem Chan

Man, that IS a great video explaining how a differential works! Even after
building a couple of model ones using Lego, this makes
things clearer ... at least for me. Great find!
JCD

There was a *lot* of disinformation being spouted about this element of
the Mars Sojourner including by NASA at the time. On the whole, they
were *very* good at keeping the truth secret. I've no idea why, or why
they patented it (or even *if* they patented it!), as the whole idea is
basically just a "wiffle tree", a load-spreading device that was amply
studied in the 1950's. Some of the earlier Rocky robots used various
differential arrangements that were, I believe, not the same as what
Sojourner used.

Anyhow, one of the NASA labs (not JPL) released a very high-res shot
taken from an angle not otherwise seen anywhere, where you can see the
electronics box high up under the solar panel above the APXS mounting.
There's a simple lever that runs abeam on a vertical pivot, so that the
port and starboard ends more fore and aft in opposition. The ends of
these are connected by rods to cranks on the main axle, and it's this
linkage that produces the fore-aft differential motion. I doubt the
picture exists online any more, and the ones I still have only show the
linkages to the lever, not the lever itself.

If you see my Lego models at <http://cjh.polyplex.org/lego/> , the
"newer" model uses a geared differential. and the "old" one uses a
crank on a shaft. Both work, and neither is what Sojourner used. I
couldn't work out how to make the actual mechanism with the Lego I
had at the time. Perhaps with these pictures and my description,
you'll work it out. It is an amazing mechanism, and I started building
a full-sized model at the time, but didn't get past making a few bits.

Dear  Cliffordö

Thank you for your reply.
As you also pointed out that, unfortunately it is hard to find
detailed information about the mobility subsystem of the Mars rovers,
particularly details of the differential.

The following web site presents Sparky rover. Especially photos give
some idea about the mobility system. Probably this is something
similar to what you referred in your message.

http://claraty.jpl.nasa.gov/man/test_bed/test_platforms/sparky/


Regards,

Miem Chan

Sure, enough.  The linkage which creates the differential motion as
described by Clifford is on the outside of the box and easily visible here:

http://claraty.jpl.nasa.gov/man/test_bed/test_platforms/sparky/images/IMGP1
439.JPG

An obvious advantage to using the linkage instead of gears is that it will
limit maximum movement between the two wheel bogies.


--
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

Great - I hadn't seen that site, but that's exactly the linkage I described.
The same linkage wasn't used on Rocky 5 or 7, which might have used the
much-touted tube differential (I forget whose name this arrangement bore).

It's also apparent to me that a linear driver that can move the pivot point
towards/away from the body of the rover could effect the stand-up mechanism.
I've no idea whether they did that, but it would be very efficient. There's
a one-way latch that locks the fore and aft legs into position during stand-up.


The software does that too, but a safeguard is a good idea.

Clifford Heath.

IF they patented it, they would have had to disclose all the things you
talked about. So if they are still being secretive, and fully
descriptive photos like the one you mentioned are being censored (for
whatever reason), you can bet it's not been patented. (There would be
two reasons for that: it lacks novelty or other merit, or they decided
it's better to treat it as a trade secret.)

I wasn't really aware of the secretiveness, but if it is to this extent,
it's probably not legal. Just no one has called them on it yet. At least
in the U.S. I used to know, when public taxpayer dollars pay for
something, the government discloses the technology within a reasonable
time-frame - unless it's national security of course. This isn't. (There
are a few other exceptions to this, but I have a hard time seeing how
these rovers would fit into those exceptions.)

-- Gordon

Numerous people asked to see the designs for Sojourner and NASA
basically never let them out.  This is strange given what a public
relations coup Sojourner turned out to be.  My theory is that
Sojourner was done on such a shoe string budget, that they didn't
actually have a true set of plans -- just a bunch of 3D models sitting
in SolidWorks or some such.  Thus, when people asked, the answer
that came back was "we don't have any, sorry!".  By the way,
I have absolutely no evidence to support this theory.  My theory
is not as fun as a conspiracy theory, tho'.

-Wayne

Except to Mattel. That company's character might explain the secrecy.

Clifford Heath.

    JPL has a design patent (D0437255) on the thing as a decorative object.
That's presumably to cover usage as a toy.  There are reasonably decent
drawings in that patent.  That patent doesn't cover building a similar
mechanism, just building something that looks like Sojourner.

    There's an analysis of the suspension in "Ground mobility systems
for planetary exploration", Fiorini, P., Robotics and Automation, 2000. Proc.
IEEE International Conference on Robotics and Automation.

    It's not like this stuff is hard to find.

                    John Nagle
                    Animats

I have looked at the paper you referred [1]. Unfortunately, it does
not have much details of the mobility system and very little about the
differential.

Reference:
[1] P. Fiorini, "Ground mobility systems for planetary exploration,"
in Robotics and Automation, 2000. Proceedings. ICRA '00. IEEE
International Conference on, 2000, pp. 908-913 vol.1.


Regards,

Miem Chan

miemchan @ gmail . com

John:

To the best of my knowledge, NASA never released dimensioned drawings
of the Sojourner.  Lot's of people asked.  Please prove me wrong by
finding a paper/web site that has such drawings.

-Wayne

On my soapbox...

Some folks (not Wayne) are forgetting that just because there is a
picture of the mechanism somewhere that it's not the same as sharing or
disclosing the technology. NASA is largely funded by program dollars
that the US has an interest in recouping. It does this by selling rights
to the technology, assuming there's something to sell.

If the technology is not being shared and/or licensed there needs to be
a reason why, because the US taxpayers deserve to have their dollars go
as far as possible. In fact, it's the law. This is the fundamental
concept of tech re-use, and it's how the US government is able to offer
the grants that it does. Without the lure of tech licensing, the grants
become much more expensive; the more expensive the grants, the fewer
there can be. I think everyone can see the financial logic in this.
(And if grants are given under the false pretense of the value of the
technology if licensed, and no licensing is attempted, then that's
fraud. Not saying JPL did this, just reminding folks that receiving
money from the gobment brings with it a bunch of responsibilities.)

The Reagan administration actively discouraged tech sharing (Richard
Pearle had a lot to do with this), ostensibly for reasons of national
defense. The thing is once, something is demonstrated, most smart folks
can figure out how it works, and re-create it. Lacking a patent (where
disclosure is required) the US is left with nothing but an "invented
here first" attitude. Lots of good that does.

It would be a shame if under the Bush administration we had had a return
to this thinking. Hopefully the next president will be smarter about
technology, and the role the US in creating and using it to the
country's best advantage.

-- Gordon

/2300/hund.htmhttp://www-robotics.jpl.nasa.gov/projects/MSL.cfm?Project=3

I don't see the problem here. First of all this is not a
"differential" in the normal sense. It has nothing to do with drive
axles. Looking at the picture it is obvious what it does. It ties the
bogies together in a way that when one drops (or turns in a direction)
the other turns in the opposite direction. The two large gears drive
the upper belt pulley. The belt then drives the lower pulley driving
the opposing bogie shaft in the opposite direction. This has the
affect of keeping the body relatively level.

Why does everyone complicate such a simple question with multiple
examples of a true differentials and rantings about NASA withholding
information? What great scientific knowledge is NASA withholding?

None - and that was my point. They appeared to be pointlessly withholding
something that was commonly known.

Pathfinder didn't use belts and pulleys. It didn't use any exterior
flexible polymers in fact, because they would have cracked and/or
evaporated.