Polaris is a polar rover designed to prospect for ice at the poles of the Moon. I mentored this project as teaching assistant of the graduate-level course Mobile Robot Design.

Polaris is a polar rover designed to prospect for ice at the poles of the Moon. The video above shows a time-lapse of the assembly process, which lasted about one month and included many steps related to mechanical and electrical assembly, software integration, and operational testing.

Each of Polaris’ four wheels is driven by an actuation stack consisting of a large brushless motor, harmonic drive, and numerous custom components.

One of the machined wheel motor housing components

Various machined wheel motor housing components.

Photo-realistic CAD of Wheel motor assembly

Exploded view of wheel motor assembly

Completed parts of Polaris

The Polaris chassis is made of carbon fiber composite. To create the chassis, foam molds were cut and sanded and epoxy, sealant and release agent were applied to the carbon fiber. The part was baked in an oven to cure the epoxy. The chassis measures 1.8 m by 1.4 m yet only weighs about 10 kg.

Polaris Side Beams

Polaris chassis and bucket wheel in foreground

The chassis is an ‘H’ shaped carbon fiber structure to which aluminum radiator, baseplate and torque tube are mounted.

The rover’s computer, motion controller, batteries and power components are mounted to the baseplate of the chassis. Electronic components that generate higher heat… like motor amplifiers and DC-DC converters … are mounted to the radiator. Switches, connectors and displays are mounted externally for access by rover operators.

Four rocker arms provide a passive suspension via chains, sprockets and transverse torque tube. To test this setup, the torque tube, all four shoulder assemblies, and chains were fitted and tested for motion.

Each main shaft has a large sprocket which is used to link the rotation of the two shoulders on that side of the rover. The two larger shoulders have additional shafts and sprockets for chain tensioning and the differencing mechanism.

The two shafts located towards the bottom relate to the passive averaging suspension. The larger one, known as the differencing shaft, contains a sprocket and is connected to the torque tube. The other shaft is an idler that redirects the chain loop to increase tooth engagement on the differencing shaft sprocket. The differencing shafts on opposing shoulders are connected through a carbon fiber torque tube which completes the system. The differencing effect is achieved by routing the chain “over” one of the differencing shaft sprockets, and “under” the sprocket on the opposing shoulder. The result is that rocker arms on either side of the chassis rotate together and opposite to the other side. This is illustrated in the appended video where one chain loop is rotated in one direction, and the other side rotates the opposite way.

This assembly provides a simple and compact implementation of passive averaging suspension, which is stiff and exhibits low backlash.