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Tracked or Wheeled?

Posted by: Andy Hill - Sat May 28, 2005 8:46 am
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Tracked or Wheeled? 
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Post Tracked or Wheeled?   Posted on: Sat May 28, 2005 8:46 am
Why are most rovers fitted with wheels rather than tracks?

The last few rovers sent to Mars have all had 6 wheels, is this the optimal method of locomotion or would tracks be better?

The recent problem that Opportunity had would probably have been avoided if it had had tracks whose larger footprint exert much less ground pressure. They also make it possible to climb much steeper inclines and it would have been much easier for it to climb out of the crater it landed in. Small surface cracks can be crossed rather than steered around and coming to the edge of a feature with potential unstable ground would not be so hazardous.

The downside is that they are not as efficient and require more energy, due to loses and the number of moving parts, to move them. But the extra manouverability might have been a fair trade off making it less likely the rovers would get bogged down and giving them access to much more difficult locations.

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Post    Posted on: Sun May 29, 2005 3:33 am
Tracked vehicles require a lot of maintenance. The track stretches and gets jammed easily. Tank crews sometimes have to remove a link from their stretched tracks to keep them from getting too loose. That is why tanks are shipped to the battlefield by train or wheeled vehicles instead of being driven there under their own power.


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Post    Posted on: Sun May 29, 2005 8:17 am
campbelp2002 wrote:
Tracked vehicles require a lot of maintenance. The track stretches and gets jammed easily. Tank crews sometimes have to remove a link from their stretched tracks to keep them from getting too loose. That is why tanks are shipped to the battlefield by train or wheeled vehicles instead of being driven there under their own power.


I used to EMC test millitary vehicles and in the mid-eighties I was sent on a driving course with the army to learn how to drive and maintain main battle tanks (Chieftan and Challenger) so have quite a lot of experience of tracked vehicles.

Tracks stretch because they are effectively rolled and squeezed by the heavy tank (a challenger weighs over 60 ton when fully loaded) running on top of it, this would not be a problem for a much lighter rover. Any slack in the track could be removed with a self adjusting tensioner, this is done on a tank manually before it is necessary to remove a link.

Tanks are normally shipped by other vehicles because of fuel costs or the damage their tracks would do to the road surface. A Challenger consumes something like 4 gallons of diesel to the mile and would probably need refuelling when it reached its destination. No point arriving at a battlefield with an empty tank, you would need to have supply depots every 200 mile just to get where you want to go. Also tank crews would be exhausted before they even started to fight.

All of the above would not apply to a Mars or Lunar rover which would be much lighter and electrically powered. My main concern would be dust which might clog the drive mechanism, one of the current rovers had a problem with a wheel because of this, but this should be able to be solved.

While it is true that tracks normally need more maintenance, the relatively short distances travelled by a rover should mean that this is not an issue.

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Post    Posted on: Sun May 29, 2005 1:07 pm
Wheeled: If you have a problem with one of the wheels (gets jammed or something), you still have the other 5 wheels. Keep going.

Tracked: If you have a problem with one of the drive wheels (gets jammed or something), you directly impede all the other drive wheels on that track. Keep going in circles?

That's too simplified I know, but that would be my understanding of why one might not go for the tracked approach.

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Post    Posted on: Sun May 29, 2005 4:37 pm
To be honest its not to likely that a track would get jammed, that is one of the reasons they are used for the roughest terrain. If a stone or rock got caught in a track and stopped it turning you can always reverse the track to free itself. They can also take a lot of punishment before sufferring damage, a main battle tank at 60 mph actually leaves the ground and "flies" over the dips between sand dunes in the dessert and snow mobiles often leave the ground when speeding across uneven ground.

There is also no reason you cant have more than one drive wheel per track each with the ability to disengage or free wheel should it stop working.

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Post    Posted on: Mon May 30, 2005 3:57 am
I seem to recall reading that tracks were considered too complex and unreliable for a Mars rover, but I could be wrong about that. I am sure an internet search would turn up something on the subject. Now one of us just has to do the search!

Aren't tracks used mainly to distribute a heavy load over a larger area of soft ground? Maybe this is not needed for a small and light weight rover.


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Post    Posted on: Mon May 30, 2005 7:24 am
campbelp2002 wrote:
Aren't tracks used mainly to distribute a heavy load over a larger area of soft ground? Maybe this is not needed for a small and light weight rover.


Yes, they increase the vehicle's foot print so that even a 60 ton tank exerts less force per square foot than a small car because its weight is spread across the length and width of each track rather than the small area on each tyre in contact with the road.

There are lots of examples of small vehicles with tracks, a snow mobile uses a single track which allows it to traverse soft snow. The army's remote bomb disposal equipment is tracked and building firms often use small excavators (approx 0.5 ton) to get into tight places.

I read that the next generation of rovers due to visit Mars will be a lot more capability and heavier than the current ones (they will be nuclear powered rather than rely on solar power partly because of this), having tracks would stop them sinking. The problem that Opportunity had would probably not have happened with a tracked vehicle 10 times as heavy.

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Post    Posted on: Mon May 30, 2005 7:25 am
campbelp2002 wrote:
Aren't tracks used mainly to distribute a heavy load over a larger area of soft ground? Maybe this is not needed for a small and light weight rover.

In that case I bet Opportunity could probably handle its current dilemma a little better with tracks.

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Post    Posted on: Mon May 30, 2005 7:36 am
Andy Hill wrote:
There is also no reason you cant have more than one drive wheel per track each with the ability to disengage or free wheel should it stop working.

I dunno, it still seems to be better if you have six un-linked drive wheels, rather than a single tracked drive wheel ... if your single tracked drive wheel has a problem there's no redundancy to pick up the slack.

EDIT: I just reread what you wrote ... ignore my above sentence.

While I believe you that a given design of track is less likely to jam than a given design of wheel (it's not hard to imagine engineering solutions for each), I would be concerned about other ways that the system could be interrupted/jammed (electronic glitch, lubricant failure, I really don't know) and that because one part of the system is connected to the other parts with the track that might cause a more serious problem than in the case of isolated wheels.

I know you mentioned being able to remove the problematic wheel from the track ... but that sort of back-up contingency engineering would begin to further complicates the system. With an un-tracked wheel, the worst is that it drags (also a problem, but at least it doesn't directly impede the other wheels).

I'm just trying to understand why the current and previous mars rovers did not opt for track. There must be a good reason.

DKH

P.S. did any rovers (moon rovers) use track?

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Post    Posted on: Mon May 30, 2005 7:58 am
I was thinking about something like having the current wheeled rover but adding a track around each set of 3 wheels to spread the weight, that way there would be 3 drive wheels to each track and they could be individually disengaged. Additional free running wheels could be added to spread the load further. You could even make it possible to jetison the track if it became a hinderance and allow the vehicle to act in a wheeled mode.

Any glitches with the drive wheels should not effect the vehicle anymore than they would on the current rovers.

I think some of the early Russian designs for lunar rovers were tracked but I'm not sure if any of them made it there. I think that Mars rovers have used wheels because their weight and the distances they travel has not been a problem until now, Opportunity has shown that this approach may not be suitable for heavier rovers designed to travel longer distances.

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Post    Posted on: Mon May 30, 2005 8:41 am
It may be stability that could be the issue in the choice between tracked and un-tracked ... in an environment where there are multiple rocks a tracked vehicle would be spending a lot of time "balancing" as it tracks over them ... with wheels on free bogeys then you have more contact with the actual surface as the wheels are far enough apart to straddle the rock being passaged over. Less jolting ride?

How does that sound? Did it make any sense to you?

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Post    Posted on: Mon May 30, 2005 8:55 am
I finally "tracked" an authoritative sounding document (someone's thesis "Motion control for a Planetary Rover with Six Steerable Wheels") which suggests why the current rovers don't use tracks ... turns out to be issues of "energy consumption" and "localization". The first issue might be profitably dealt with using a fissile energy source. I'm not incredibly clear about the second issue though.

Here's the document in question. It's a pdf file.

DKH

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Post    Posted on: Mon May 30, 2005 9:09 am
As far as I remeber one of Spirit's wheels has got problems because a little stone or/and a little bit soil keeps it from working.

What if such stones or soil gets on the track and this way to the wheels? Could this increase the danger that a wheel gets such problems one of Spirit's wheels has got? Wouldn't that be a reason to jettison the track?

Would it be possible to construct tracks that avoid such problems? ...



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Post    Posted on: Mon May 30, 2005 4:00 pm
Interesting document Dr Keith.

The power issue ties up with what I said earlier about the fuel costs when moving tanks that track vehicles trade efficiency for traction. This may be the reason that given the limited amount of power available wheels have been chosen for past rovers but as you say this might not be a problem if a nuclear power source is used. Then again power diverted to the drive system means less for the instruments and I guess it comes down to the question whether a much heavier rover can use the same system without sinking, I would say that Oppotunity's current situation would indicate not.

Skimming through the document (it was to large to read all of it) the six wheel variant does seem to be the optimum drive system for wheeled rovers due to redundancy and stability issues which explains why it has been used on the rovers to date.

A tracked vehicle is more stable than a wheeled one, that is why large artillary pieces are normally mounted on tracked chassis. It is also more manouverable, being able to turn on its axis by making the tracks go in opposite directions.

Check out this company who make small tracked vehicles for the military (I have tested one of their Pakbots and you would not believe how fast and manouvable it is).

http://www.irobot.com/governmentindustr ... ?prodid=30

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Post    Posted on: Tue May 31, 2005 6:40 am
Nice link, if a little arresting. The concept of military robots scares the bejesus out of me ... mostly because of Hoolywud's terminators I guess.

So it really does look like tracked rovers are going to be the way to go when the power issues are solved.

DKH

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