Micro Sawppy Beta 1 Kicking Up Its Middle Wheels

Modifying Micro Sawppy Beta 1 (MSB1) suspension geometry to be front-back symmetric didn’t seem to cause any problems. Or at least, it didn’t seem to add any new ones. One trait I observed while running my little rover around is a tendency to kick up its middle wheel. When MSB1 encounters an obstacle it cannot climb, the front wheel stops moving but the rear wheel tries to push forward. As a result the suspension folds up, lifting the middle wheel. This is something that can be traced back to features inherited from big rovers.

Here are two side views for comparison between MSB1 and Curiosity rover. In an effort to draw up a cute little baby rover, I compressed overall length (front-back distance). This gives us a stubby little rover, but it also meant the rear wheel now has better leverage for lifting up the middle wheel.

I see this behavior occasionally on Sawppy V1, but not as frequently. Part of this is because Sawppy V1 proportions are faithful to Curiosity proportions and not squashed for cuteness, but also because of its heavier weight. In order to lift the middle wheel, we also need to lift a portion of the rover’s weight, and MSB1 is proportionally far lighter.

This lighter weight is a natural property of all scale models. MSB1 is roughly 1/3 scale of Sawppy V1, meaning it is about 1/3 as long. Which means it occupies about 1/9 the floor space, and occupies roughly 1/27 of the volume. All else being equal (they weren’t, but just for the sake of this simplified explanation) we would expect MSB1 to weight about 1/27 as much as Sawppy V1, making wheel lifts much more likely.

This propensity to lift a wheel is puzzling when we look at the forces involved and notice what happens when the rover is traveling backwards: If it encounters an obstacle it cannot climb, the rear wheel stops and the middle wheel will try to push backwards. In this case, the push will help lift the rear wheel so it could climb the obstacle, making the wheel lift a feature and not a bug. I was able to experimentally verify this property with MSB1: it climbs better running backwards than forwards.

Why would JPL engineers design a Mars rover that has superior climbing ability traversing backwards than forwards? It makes sense when we consider the operating environment: there are no roads or roadside assistance on Mars. Curiosity is designed so that it can back out of whatever situation it might get into, because the nearest tow truck is over 54 million kilometers away.

But this means we have to make a decision for little rovers running on earth, where we have the option to walk over and pick it up if it gets stuck. Do we flip around the suspension geometry so it climbs better? Or do we maintain geometry faithful to Mars rovers? For Sawppy V1 I chose to copy Curiosity, but I might need to take the other path for Micro Sawppy rover given its shorter limbs and light weight. That’s a decision I haven’t made for MSB2 but if I do so in the future at least I have precedent. JPL’s own Open Source Rover has its bogies up front instead of back.

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