After more than two years of building Sawppy and publishing information for others to build their own, I’ve collected quite a list of things I’d like to change for future revision. I want to make Sawppy more affordable to buy, easier to build, and smoother to run. However, there are a few things I would not give up to accomplish those goals.
Chief among them is the six wheel drive, four wheel steering vehicle chassis employing the rocker-bogie suspension geometry of JPL Mars rovers Curiosity and Perseverance. This includes copying all degrees of articulation including the differential bar across the top of the chassis. A lot of other rover models sacrificed various parts of suspension fidelity to become cheaper and/or simpler. They might have reduced amount of articulation, or used fewer wheels, or driving fewer of them, or not steering them. These are valid tradeoffs but I’m not willing to make them for my project. Sawppy is unnecessary complex for flat ground and does not travel fast. But as a motorized model of Mars rovers, Sawppy will continue to maintain fidelity to all of those traits.
For steering the rover’s four corner wheels, I also want to keep the axis of steering rotation aligned with wheel center. This ensures any steering movement would pivot the wheel in place. It is much easier to build steering mechanisms with an axis of rotation offset from the wheel, but steering with such a mechanism drags the wheel through an arc instead of pivoting in place and I prefer not to do that.
On the construction side, I want to avoid risk of suspension joints binding up, which means ball bearings at as many axes of rotation as possible. I know I can keep using ball bearings for all suspension members, I’m pretty confident I can maintain ball bearings for the four steering axes, but as I simplify and reduce cost, I may have to give up ball bearings on wheel axles.
On the subject of wheel axle design, I still have a preference to maintain Sawppy’s excellent ground clearance, but I am more flexible about this topic as people don’t seem very fussed about compromising it as they modify their rovers. Another item of frequent discussion is the fact Sawppy’s wheels are not well suited to human-friendly floors but again they are accurate to their inspiration so I’ll probably keep them that way on my own creations. Fortunately wheels are relatively easy to modify or replace should other rover builders desire more traction.
I feel pretty good about these two lists: the things I want to change, and the things I don’t. However, putting all of these thoughts down also makes one thing clear: one rover size will not fit all.
[Title image: the Meeting of Rovers at Maker Faire Bay Area 2019]