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.

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 , 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 Clifford=F6

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.

Regards,

Miem Chan

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

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

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

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.

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.

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

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

?i=671&c=?i=743&c=4 No differential 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.

This is the movie you need to see:

"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

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

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

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.