PostHeaderIcon Mars Rover Rocker-Bogie Differential

Today’s is a guest post, brought to you by my father, engineer, physicist, computer programmer, and Lego, Mars, and animation enthusiast: Keith Enevoldsen. If you’re not caught by the title of the post, scroll down and look at the pictures… then scroll back up and read the rest of the article because it is fascinating stuff! (P.S. He has a website full of awesome science stuff too!)

A teaser for what you have to look forward to…

~ A l i c e !

The Rocker-Bogie

All the Mars rovers have six wheels and use a rocker-bogie suspension system to drive smoothly over bumpy ground. The rocker-bogies are easy to see in pictures of the rovers (see pictures below). There is one rocker-bogie assembly on each side of the rover. The rocker is the larger link that connects to the rover body (the chassis) in the middle (at the rocker pivot), has a wheel on the front, and connects to the bogie in the back. The bogie is the smaller link that connects to the rocker in the middle (at the bogie pivot), and has wheels at both ends. Each of the six wheels has its own motor.

The Differential

It is not so easy to see and understand how the rocker-bogie mechanism keeps the body level. What prevents the rover body from tipping all the way forward or backward around the rocker pivots? If you build a model rover and you attach the rockers to the body with an axle or two pivot pins, the body will tip forward or backward until it hits the ground! In the real rovers the two rockers connect to each other and to the body through a mechanism called a differential. The differential is what keeps the body level. Relative to the body, when one rocker goes up, the other rocker goes down. Relative to the ground, the body angle is halfway between the angles of the two rockers. That’s cool, but how does it work? The different rovers use different mechanisms: a differential gearbox or a differential bar.

Differential Gearbox

The Mars Pathfinder (Sojourner) and Mars Exploration Rovers (Spirit and Opportunity) use differential gearboxes.

MER’s Differential
Image and Model © 2012 Keith Enevoldsen

The gearbox is inside the rover body, so you never see it. No wonder it is hard to figure out how it works! In my Lego model rover shown here, I use a simple three-gear differential. Two gears connect to the two rockers and the third (middle) gear connects to the body. If you hold the model rover body steady in midair and tilt one rocker up, the gears will turn and the other rocker will tilt down (see the animations below).

The real Mars Exploration Rovers use more complicated gearboxes with more gears but they are functionally equivalent to this simple three-gear differential.

Differential Bar

The Mars Science Laboratory (Curiosity) uses a differential bar. This is the big black bar that you see across the deck of the rover.

Curiosity’s Differential
Image and Model © 2012 Keith Enevoldsen

The middle of the bar is connected to the body with a pivot and the two ends are connected to the two rockers through some short links. If you hold the model rover body steady in midair and tilt one rocker up, one end of the bar will go back, the other end will go forward, and the other rocker will tilt down (see the animations below).

The Mars Exploration Rovers did not use a differential bar because it would interfere with the solar panels. But the Mars Science Laboratory does not have that problem because it is nuclear powered and has no solar panels.

Animations

Animations of a Rocker-Bogie with a Differential Gearbox

Mars Exploration Rover (Spirit and Opportunity)

Body held steady in midair:

 

Wheels on the ground:

 

Animations of a Rocker-Bogie with a Differential Bar

Mars Science Laboratory (Curiosity)
Body held steady in midair:

Wheels on the ground:

 

No Springs

The rocker-bogie mechanism has no springs. The absence of springs helps it keep all six wheels on the ground with approximately equal pressure on each wheel. This is a good thing when you are driving on sand!

Want More? (and References)

“Mars Exploration Rover Mobility Assembly Design, Test and Performance”, JPL

“A Capable and Temporary Test Facility on a Shoestring Budget: The MSL Touchdown Test Facility”, JPL

 

I hope you enjoyed my dad’s post. He sure had fun figuring out those mechanisms and creating them out of Lego!

I’ll be posting a Curiosity-landing-”watching” event here very soon.

~ A l i c e !

16 Responses to “Mars Rover Rocker-Bogie Differential”

  • Keith says:

    I’d like to thank Emily Lakdawalla for her help. I had emailed her a photo of one of my earlier Lego rovers and said I knew something was missing from my rocker bogie mechanism, but I couldn’t figure out what. She supplied me with the key word “differential”.

  • Paul says:

    this made getting it so much easire! thank you

  • jumpjack says:

    Actually your differential does not look so “simplified”!
    http://library.iyte.edu.tr/tezler/master/makinamuh/t000341.pdf

  • Pluto says:

    Thanks for gathering this information, LEGO is a great way to show how the suspension works!

    You mention that Sojourner used a differential gearbox but it looks to me like it used a differential bar, see this image: http://er.jsc.nasa.gov/seh/marspathrov.jpg

    The only mention of the differential I’ve found for Sojourner is in this text (http://www.mit.edu/~ykuroda/research/iSAIRAS99-M5SUS.pdf) where they suggest it used a differential bar.

    • Alice says:

      The image you sent shows the rocker-bogie system, but not the differential. The link you send is for MER (Opprtunity and Spirit), Sojourner was before that. I still think that Sojourner had a gearbox — but my research was a LONG time ago.

      • Pluto says:

        In the picture if you look at the front of the rover there is a bar with a pivot in the centre just underneath the solar panel. This bar then extends around both sides of the rover and connects to an arm protruding from the top of the rotary joint where the rocker attaches to the body of the rover. This looks a lot like a differential arm to me but perhaps that’s not the actual flight model Sojourner…?

        In the PDF I linked to, under section 3.3 it states:

        “Rocker-bogie suspension [2]-[4][12][13] is the
        system to rove on Martian surface in the mission
        Mars Pathfinder”

        which is of course the mission to which Sojourner belongs. It then states:

        “A pair of lockers and the
        body are connected via a differential bar in order to
        stay pitching angle of the body at the middle of two
        lockers”

        However that text is about “Micro5″ (not MER?) and, while I’ve read most of their references, I can’t find any other mention of Sojourners differential.

        • Alice says:

          Hmm..

          Okay, this is going to sound a bit like a cop-out, but let me get my dad to take a look at it, since he’s the one who built the LEGO models and did the research for this post.

          I see the bar you’re talking about, but I’m not yet convinced that it is the differential. I should find some citations myself, eh?

          Are you in agreement that MER used a differential gearbox or not that either?

          Hang on, let’s see what Keith says.

          -Alice

    • Keith says:

      Pluto is correct and I was incorrect. The Pathfinder rover photo http://er.jsc.nasa.gov/seh/marspathrov.jpg clearly shows a thin differential bar pivoted under the solar panel.

      Alice, please correct the post in two places. First, strikeout “Mars Pathfinder (Sojourner)” from the sentence about gearboxes. Second, correct the caption on the photo of my Lego rover with a gearbox. It’s not Pathfinder (Sojourner), it’s MER (Spirit and Opportunity). It looks like Pathfinder because I removed the solar wings for this photo so you can see both the rocker-bogie and the (simplified) gearbox.

  • Pluto says:

    Yes MER definitely used differential gears, there’s plenty of documents around describing how their suspension works, like this one: http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37604/1/05-1950.pdf

    And just to make it more confusing “Rocky 7″, I think the last development prototype rover before Sojourner was built, used differential gears too rather than a diff bar, at least according to this: https://www-robotics.jpl.nasa.gov/publications/Richard_Volpe/iros96.pdf

    Hugh.

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