Wheeled robot kits calling themselves rovers are plentiful, but rarely do they faithfully represent the JPL rovers sent to Mars. ServoCity’s Bogie Runt Rover at least is a six-wheel chassis with rocker-bogie suspension, but is missing the corner wheel steering system. Looking for commercially available products out there with six wheel drive rocker-bogie and four corner steering, I was almost ready to conclude none existed until I came across the 4tronix M.A.R.S Rover Robot.
The name is an acronym for Mobile Autonomous Robotic System and the autonomy comes from the brainpower of either a Raspberry Pi Zero or BBC micro:bit. Out of all the commercial products I’ve come across, M.A.R.S. has the most mechanically faithful model of Curiosity and Perseverance suspension geometry. It has roughly similar proportions, and uses a differential arm over the body. Sojourner, Spirit, and Opportunity use a differential inside the body in order to maximize top surface area for solar panels. The nuclear-powered Curiosity and Perseverance didn’t have such constraint. By moving the differential above, they gained body interior volume.
Perusing the assembly instructions I see the structural components are all printed circuit boards (PCB) which is not usually considered structural material but apparently can be made to work at this scale. Six wheel drive comes from N20 micro gear motors, and four wheel steering comes from MG90S metal gear micro servos. Both of these are fairly widely available for maker projects and good choices.
My quibble with the M.A.R.S. rover are with its rotational motion joints. First up are the rocker-bogie suspension components, each of the joints are a M3 screw inside a PCB hole. I’m queasy about using fasteners as axles, since their threads are not really designed to carry a load. There are no bearings on these joints and the screws have to be tightened precisely. Too tight, and the joints would bind. Too loose, and the rover will wobble. This seems awfully finicky and would loosen as the rover travels.
Second, each wheel’s weight is transferred directly into the N20 micro gearbox. Examining pictures of these gearboxes online, it does not appear the gearbox is designed to handle loads perpendicular to rotational axis. Perhaps the rover is lightweight enough the load would be fine, but all I see are metal shafts turning inside holes in brass plate.
And third, each of the corner steering wheel assemblies are attached directly to the steering micro servo’s horn. Despite the metal gears, MG90S type servos still has a great deal of flexibility and play in its gearbox in the face of loads perpendicular to rotational axis. We can see each corner steering assembly wobbling noticeably while in motion:
(I also smiled when I realized this video cuts out right when the rover starts tackling a very challenging vertical climb. We’ll never know if it managed the climb.)
The final quibble I have with this design is that the steering axis is not aligned with the wheel. In less formal terms, it means the wheel is sideways offset from its corresponding steering servo instead of sitting directly under the servo. As a result, when the steering servo rotates, its wheel isn’t pivoting about a point. Instead, it is dragged through an arc. This adds a great deal of mechanical stress to the steering mechanism. It also makes the desired wheel velocity harder to calculate through a turn, but it appears the M.A.R.S. software skips that detail. All left side wheels are commanded to turn at the same rate, and all right side wheels are the same with each other. As a result we can see a bit of wheel skid through a turn, reintroducing some of the problems of using a strictly skid steer system as the Bogie Runt Rover did.
In conclusion M.A.R.S. has the mechanical sophistication of corner steering in addition to a good representation of rocker-bogie suspension, but its current iteration does not yet take full advantage. Still, it looks like a nifty little kit and the price of one hundred pounds is significantly lower than the cost of parts for Sawppy.
This concludes my quick tour of commercially available rover kits. But before I move on to DIY rovers, there’s a rover category that bridges across both “commercial product” and “do it yourself”: LEGO rovers.