Devlog #2 — ROVER “Target Fidelity”

Hey again! Been a while, and a LOT happened since devlog #1. Last time I had the math roughly done and was about to start modeling. Now the whole mobility system is basically designed in Fusion, joints and all. Let me catch you up.

Round tubes and printed joints

First big change: I switched the arms from square tube to round aluminum tube (Ø25 for the rocker, Ø20 for the bogie). Easier to find and easier to bond into joints. And instead of bending tubes or bolting plates, all the intersections are now 3D-printed PETG joints — the tubes slide straight in and get pinned with M4 bolts. The cool part is the joints set the angles (160° at the rocker apex, 150° at the bogie V), so every tube is just cut straight at 90°. Way simpler to build.

The differential rabbit hole

Okay this is where I spent most of my time. I had a “differential” that was just a rigid bar, and after reading the JPL docs more carefully I realized that’s not actually a differential — it doesn’t keep the chassis level, each side just does its own thing. The real JPL rover uses a rocking balancín with two link rods (bieletas) that connect the two rockers so when one climbs, the other drops and the body stays level.

So I rebuilt it properly: a balancín that pivots in the center, two bieletas going down to clamps on each rocker. Took me a bunch of tries to get right. Things I learned the hard way:

• The bieletas need ball joints on both ends, not rigid connections, or the whole thing locks up (the balancín and rocker rotate in different planes).
• For the leveling to be symmetric, the balancín arm and the clamp offset have to be the same distance — I went with 180 mm on both, giving a clean 1:1 ratio.
• I put the balancín right over the rocker joint and the bieletas kept hitting the chassis edge. Fixed it by mounting the whole differential on top of the lid, outside the sealed box, like JPL does.

Steering and leveling

The wheels don’t mount straight to the tubes anymore. The wheels sit in C-mounts with MG996R servos . All three mounts on a side share the same 90 mm vertical drop so the wheels end up level on flat ground. Had to angle the sockets (75° on the bogie, 80° on the rocker) to cancel out the tube tilt so the wheels hang vertical.

Picked the motors (and a reality check)

Bumped my mass estimate up to 30 kg to be safe, and that changed everything for the motors. Ran the torque numbers and a standard JGB37 wasn’t gonna cut it for climbing. Found the JGB37 50RPM at I+D that does 40 kg·cm — that’s the one. Pairing 6 of them with 2× BTS7960 drivers (one per side), a PCA9685 for the steering servos, and a 12V LiPo to handle the ~26A peaks when climbing. Still need to confirm the encoder version for navigation.

What’s next

• Finish the differential clamp + bieleta details and double-check the 1:1 in Motion Study
• Start the print-and-test phase
• Source the motors and get them spinning

That’s it for 2! Slowly turning a dream into an actual machine. Thanks for reading!

Devlog #1 — ROVER “Target Fidelity”

Hello! First devlog for my Stardance project, so let me quickly show what I’m building.

ROVER “Target Fidelity” is a low-cost autonomous rover that drives itself over rough terrain like dirt, gravel and grass. It grabs soil and plant samples on its own and even runs biology and geology tests onboard. Think tiny Mars rover, but built with a 3D printer, cheap motors and a Raspberry Pi instead of a NASA budget. It uses a rocker-bogie suspension (same thing Perseverance has) so it keeps all six wheels on the ground no matter how rough it gets. That’s the goal, starting with the movement and the arm.

This first log is all about the ideation process.

Research first

I didn’t wanna just wing the suspension so I dug into how rocker-bogie actually works. Two papers shaped the design: Verma et al. (2017) for the formulas to size the arms and wheels, and Singh & Jain (2020) for how the load spreads out and how it climbs stuff. I also read NASA JPL’s Open Source Rover docs for the differential and joints.

Main takeaways:

• Six wheels in a 3-3 layout keeps at least four touching the ground all the time.
• It’s fully passive, no springs no shocks, the geometry does all the work.
• The bogie pivot position matters a lot for grip. I went with Case 2 (centered pivot) cause it splits weight evenly between front and middle wheel, so less spin on loose dirt.

The calculations

Worked out the sizes for what I wanted: chassis 600 mm wide by 1200 mm long, ~150 mm/s, able to climb 1.5x the wheel size.

Started with the wheel. Went 200 mm diameter (cause why not), ran the speed formula and got ~14.3 rpm at the wheel for my target speed. Fine for most gearmotors, and if I need more rpm I’ll add gears or change the diameter.

Then the wheelbase. Subtract front and rear wheel radius from the chassis length, gave me 1000 mm between axles. Placed the three wheels per side (front at 0, middle at 430 mm, rear at 1000 mm) and stuck the bogie pivot right between the front and middle wheel.

For the arms I used the angle formulas but with chiller terrain angles instead of the mount-everest climbing one the paper uses. Got a rocker arm ~800 mm total (two 400 mm pieces) at 10° and bogie arms of 223 mm each at 15°. The bogie ends up a V with its two sockets meeting at 150°. Leaves ~158 mm of clearance under the chassis.

Also ran the loads. Rear wheel takes the most (40%), so that’s the part I gotta stress test hardest later. My 300 mm obstacle target is only 75% of the mechanism’s max, so comfy margin there, or at least I hope so.

What’s next

Now that the math is kinda ready, I’m into the make and re-make phase to see how accurate the model is. Plan is to model everything for 3D printing and build a first test version using plastic tubes instead of aluminum for the arms, so I can test the geometry and run the motors without dropping a ton of money. Rather find out something’s wrong on a cheap plastic prototype than after cutting expensive aluminum.

Next goals:

• Finish the printable joints
• Build a cheap plastic test frame
• Get the drive motors spinning

That’s it for devlog #1! Next time I should have actual parts to show. Thanks for reading! (sorry if my English is off, not my first language.)