The third revision of my little rover prototypes, Micro Sawppy Beta 3 (MSB3), uses TT gear motors and matching wheels for the six-wheel-drive aspect of its rover suspension. Using commodity components solves a lot of problems, but now I have to integrate them. The starting point is a rectangular bracket that bolts onto two top mounting points of a TT gearbox, and angles over the wheel to a hole for a bearing that is aligned with where the wheel touches the ground. (“Contact patch”) This alignment is necessary in order for the wheel to pivot in place. If the rotational axis of the bearing is not aligned, then any rotation would drag the wheel across an arc instead of pivoting on the axis of rotation.

Sawppy V1 had the same steering axis alignment requirement, and I used 8mm shafts running through the center of 608 bearings. For MSB3 I’ve turned that design around: instead of transmitting the rotational force through the center of the bearing, now the bearing center remains static while steering forces are transmitted around it. M3 fasteners bolt a pair of bearings to the front end of the suspension rocker, one top and one bottom. The steering mechanism takes the general shape of a C holding above and below this pair of bearings.

MSB1 and MSB2 ran M3 screw through the center of bearings in their suspension members, but that used the M3 fastener as a rotational axle much like how Sawppy V1 used 8mm shafts. This meant the smoothness of the rotation is sensitive to how tightly the fastener was torqued down. Too loosely, and it rattles. Too tightly, and the fastener would cause the bearings to bind up. Which defeats the purpose of using ball bearings to begin with! I had a few other ideas on how to address these problems, but I decided to try this one first. Inverting the roles was less dependent on 3D printer precision and hence easier to build.

Part of making micro Sawppy easier to build is to avoid similar looking parts that are not interchangeable. For MSB3 I designed the two parts of this steering C-shaped assembly so it can be used on all four corners. That is to say, a builder will print four copies of the same design. Rather than Sawppy V1 which had distinct front-left, front-right, rear-left, and rear-right corner steering components people might (and did) inadvertently swap causing confusion. This is something I wanted to mitigate by stamping reference numbers on parts, but it’s better to design so the user doesn’t have to squint to make out part numbers at all.

In order to make a part usable in multiple directions, it is symmetric front-back as well as side-to-side. Adding provisions to allow the parts to be attached in multiple directions also added unnecessary bulk, and I haven’t made up my mind whether the tradeoff is worthwhile. Clever designers know how to design parts so they are clearly unambiguous, I might want to tackle that as a challenge instead. In the meantime I’ll leave this as-is and proceed to steering servo installation and associated adjustment.

My little rover Micro Sawppy Beta 2 (MSB2) has a mock RTG in the back, and in the front I installed a mockup for a rover robot arm. Even as a non-motorized mockup, it was more than Sawppy V1 ever got! A Sawppy arm was always on the to-do list but I never dedicated the time to make it happen. In the absence of my own efforts, I’m happy to see some other Sawppy builders have added robot arms to their rovers. Like CJ’s arm documented here on Hackaday.io and on YouTube.

And this rover flexing its arm on Twitter.

Both of these robot arm projects followed the mechanical geometry of real Mars rover arms, which makes me happy in a “These are my kind of people!” kind of way. I haven’t studied the arm as much as these other Sawppy builders had, so I decided to start simple and small with MSB2. I designed and printed a very simple arm as practice. I didn’t have any kind of actuators on hand that would work at such a small scale, so this arm is limited to manual articulation. (I could pose it like an action figure, but no remote control or autonomy.)

Still, it was instructive to see what kind of motion is possible with this mechanism. I knew two things from the start: it needed to go into the stowed position and it needed to extend out to act like it is retrieving a Mars sample. Once I started playing with it, though, I learned two other things.

The first lesson is that this draft had only a subset of range of motion as the real thing. I couldn’t replicate the sample storage movement sequence shown in a NASA tweet illustrating how Perseverance will transfer core samples to the carousel handling tools and sample tubes. The second lesson is that I had a vague idea to build a rover that can be friendly and wave “Hi” to people, but this arm could not bend the way I wanted because it would collide with the left front wheel steering servo. I’m glad I learned these lessons on a simple test arm, before I spent the effort to motorize my arm. What I could use for arm actuator is a problem itself, as I encountered a problem with the micro-servos I’ve been using.

The equipment bay for my little rover Micro Sawppy Beta 2 (MSB2) had enough space for the temporary setup of Raspberry Pi 3 and Adafruit PWM/Servo HAT along with all wires and a battery. However, I’m trying to think ahead to more elaborate setups and how I might accommodate their needs. One obvious concern is that bigger computers would need more space and bigger batteries, both competing for room inside the equipment bay.

Mars rover engineers at JPL had already encountered this problem and we can see their solution: Curiosity and Perseverance draw their electrical power from a radioisotope thermoelectric generator (RTG). Unlike their predecessors, who drew electrical power from solar panels that imposed a lot of operational limitations. Mars-bound RTG are large and bulky devices bolted prominently to the rear of those two rovers, giving them a very distinctive rump. Since MSB2 tries to look like the big rovers, it will also have a similar-looking attachment to the back, so we might as well make it serve the same purpose! (And to be clear I meant power supply, not a radioactive generator.)

Since MSB2 could fit its battery inside the main body, I didn’t need to make this faux RTG functional just yet. For MSB2 it is only a test run mockup without any paths for power wiring. I found that if I made it according to proper scale, the mock RTG would be (just barely) large enough to accommodate a single 18650 lithium-ion cell. It also looks pretty easy to make the chamber easily accessible, so the battery could be swapped out.

But with that success, my ambition grew: what if I could find a little more room? I could accommodate a USB power bank built around a single 18650 cell. If micro Sawppy could be built to run off a commodity USB power bank, that would make it a lot more beginner friendly. First, the construction would be simpler and second, beginners would be spared the learning curve of working directly with infamously temperamental lithium ion battery cells.

So that’s another research project: what are the power requirements for a micro rover, and would it fit within the power delivery capabilities of a USB power bank? I know this specific USB power bank in my picture doesn’t have the grunt to keep a Pi 3 running, so it is another motivation to scale down from the current electronics configuration to make room in the power budget for other component like a robot arm.

My little rover prototype Micro Sawppy Beta 2 (MSB2) had a few suspension changes relative to its predecessor MSB1, but the real attraction is the rover body. Whereas MSB1 had a minimalist box, MSB2 makes an effort to look like Mars rover Perseverance.

The minimalist box of MSB1 is, in fact, a little too minimalist. I thought I made it large enough to accommodate a Raspberry Pi 3 and Adafruit PWM/Servo HAT but I made some mistakes and it was too small, leaving its brain dangling outside the body by wires. Not exactly a robust approach. MSB2 has a much larger body with a correspondingly larger enclosed equipment bay for electronics.

The major structural points of a Curiosity/Perseverance-like rover are where the differential pivot attaches plus the two rocker attachment points. These three attachment points carry the entire weight of the rover so I wanted it to be a strong single piece, but printing it as one piece added a lot of complication elsewhere. MSB2 body was printed upside down so they could be together, but that meant I couldn’t print a bottom since it would be an unsupported top surface while printing. And while I could theoretically seal off the top of the box (since it’s bottom and facing the print bed when printed upside down) I didn’t want to do so for two reasons: One, I wanted to print some surface features to resemble Perseverance, and I couldn’t do that if it’s flat against the print bed. And second, I wanted the equipment bay to be accessible while the rover is standing on its wheels right side up. With all these conflicting desires, the main body box ended up with too many separate pieces. I plan to play with other ideas in future iterations, the next thing I’ll try is to abandon the desire to print all three structural attachment points with a single piece. At this scale, a few M3 fasteners are strong enough to hold things together.

One thing I did like about this box was the volume, which is modest but enough for a Raspberry Pi and Adafruit HAT. I have ambition to build smaller and/or simpler electronics for future iterations of micro Sawppy, but those plans have not yet solidified. I think leaving enough room for Pi and a hat leaves a good upgrade path, but there’s always a question of how much to plan for upgrades. I think it’ll fit some Ardupilot control units, but I don’t know for sure since I lack experience with them. This body is definitely not big enough for something like an Intel NUC, though perhaps it’s enough for a Jetson Nano. However, the sealed box would present cooling challenges for those power-hungry devices, in addition to their need for big batteries.

My little rover prototype Micro Sawppy Beta 2 (MSB2) has a few suspension changes relative to MSB1. The change to its differential linkage system is not as significant as the changes to steering mechanism, more of a small evolution.

While driving MSB1 around I watched the linkage move and thought it was worth an experiment to see if I could eliminate all the metal components. (Bearing and associated screw and nut.) The linkages became two 3D-printed living hinges that are designed to flex on axis perpendicular to each other. Ideally it would allow each hinge to accommodate most of motion along one axis letting the other one handle the rest. In practice this only partially worked and the hinges were too stiff. The loads didn’t distribute as nicely as I had imagined in my head. Real world is like that sometimes! (Actually most of the time, if I’m being honest.) The end result is that these differential link hinges hindered weight distribution mechanism of the rocker-bogie suspension.

I could make these hinges more flexible by printing thinner plastic, but then we increase risk of fatigue and breakage. PETG is more ductile and durable than PLA, but neither of them are close to properties of dedicated flexible filament. Not all printers can handle TPU or similar materials, and I want to design my rovers to be printable even on cheap basic printers that only handle PLA.

The potential for breakage highlights another problem with tightly integrated designs: if the living hinge breaks, the entire component has to be reprinted. I’m still undecided about using 3D-printed living hinges, so there will be a few more rounds of experimentation to gather more data. But if I want to use living hinges printed from plastic filament not intended to be flexible, I should at least change the design to be a multi-piece part. If the living hinge itself is a smaller separate component, it can be reprinted quickly for replacement in case of breakage. Which I don’t think is a huge risk when rolling around, but unintentional sharp jolts happen a lot when I am trying to open up this rover’s equipment bay.

Most of the focus for Micro Sawppy Beta 2 (MSB2) is on building a body that resembles real Mars rovers, so its miniaturized rocker-bogie suspension is largely unchanged relative to MSB1, inheriting all the same flaws. The most notable suspension changes are in how its steering servos are installed.

Relative to MSB1, steering servo body is flipped around front-back so the bulk of the servo body faces away from the steering knuckle. The upside is that it allows that bracket to be narrower thus saving space. The downside is that wire routing becomes more convoluted as the wires jut out away from the body and have to double back. It “wastes” some wire length but that has minimal impact, as I’ve determined I had to use servo extension cables anyway.

The other change is that the top half of the bracket is no longer in line (when viewed from the top) with the bottom half housing wheel driving servo, it has been rotated 90 degrees to be in line with wheel travel direction. This approach has several benefits, starting from a cleaner look when the rover is traveling straight ahead. It also increases the steering angle range, giving the bracket more distance before it would make contact with a suspension arm. This change, combined with the fact it is now narrower to begin with, allows much more room for the robot arm.

But there are a few downsides. Since it was printed 45 degrees relative to the 3D print bed, this design doesn’t enjoy the strength offered by a design aligned with print layers. To compensate for this, I made the bracket thicker in high-stress areas, but it still suffered breaks along layer lines in ways that the previous design would not.

Another downside is that it further compromised ground clearance, increasing the chances this plastic might impact obstacles on the ground. I think it’s still acceptable in light of modifications other have made to Sawppy V1, but it is definitely a step backwards.

The lesson I learned from this experiment is: while an one-piece design would satisfy the goals of reducing parts count, it is hard to satisfy all objectives and still remain in one piece. The thicker yet still more breakable bracket made it more difficult to assemble, which also made it harder to diagnose problems and repair them. And neither of these single piece designs allowed manual steering trim adjustment, which is a feature on my wish list.

In light of these experimental results, for the next version of micro rover steering mechanism I will go to a multi-piece design. I also decided to go multi-piece on differential link but for slightly different reasons.

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.