Chassis

The development of the chassis for the rover BEAR from the Technische Universität Berlin started in summer 2016. The main projects goal is the participation in the European Rover Challenge.
The conditions set by the competition create several special requirements regarding the chassis construction.

A maximum weight of 50kgs is given by the competition rules. Compared to NASA Rover Curiositys weight of 900kgs, BEAR will be an absolute lightweight.
Apart from the chassis, the rover contains an onboard computer, a camera mast, a drilling device and a manipulator. They have to fulfill the mass restriction altogether. The chassis’ maximum mass was restricted to 25 kgs by the internal project management.

Especially the terrain conditions had a big influence in the choice of a chassis configuration. The competition site consists of sandy or gravelly soil, covered with several elevations of various inclines. There are no larger rocks in the area, which could be a major obstacle for the wheels.
Possible sandy sections make it necessary to provide grousers on the wheels, to guarantee propulsion in every situation.

The majority of currently operated space rovers are built with a so called rocker bogie suspension containing six or eight wheels. Its main feature is its good all-terrain mobility, provided by a differential gear in the middle of the chassis (see Fig. 01).

Technische Universitaet Berlin, TUB, Fakultaet IV Verkehrs- und Maschinenesysteme, Institut fuer Luft- und Raumfahrt, Modell des MarsRovers “SEARS”

 Figure 1: TU Rover SEAR with an eight wheeled rocker-bogie suspension

BEARs drilling device shall later be able to collect a soil sample from a depth of 15cm which is then analyzed in a miniature laboratory on the rover. To protect the rover from unwanted mechanical loads and to ensure a straight drilling path, the drill subsystem must be mounted exactly in the middle of the rover. This makes it impossible to use a differential gear between the two sides of the chassis which obliterates the main advantage of the rocker bogie system. Due to the missing rocks, the rover BEAR does not even need improved climbing abilities.
Another task is the exact positioning of the rover for more precise manipulator operations. To make this possible it is necessary to make every wheel steerable round its axis. Therefor needed steering devices on every wheel causes a significant increase of weight. Thus only four steerable wheels can be used within the mass limit. Due to the chosen drill position and the strict mass requirement a system with four separately mounted though steerable wheels has been chosen instead of the rocker bogie suspension (see Fig. 02).

Figure 2: CAD-model of a steerable chassis with compression struts

Maneuvering in rough terrain always causes shocks to the chassis and the mounted subsystems. A suspension strut on each wheel will reduce the mechanical loads and the caused disturbances.

After the final design was chosen, the basic dimensions of all components were calculated using common engineering methods of rough calculations. Based on those dimensions, the CAD model was created. It was rather obvious, that the roughly calculated components would largely exceed the mass limit.
A numeric FEA program was used in order to determine the mechanical loads even on more complex structures in more detail than ever possible by hand. The contacts between the single parts, the bolts and bearings were simulated as detailed as possible to reduce the mass of the chassis while still keeping a sufficient strength (see Fig. 03).
This way it was ensured, that even the more complex structures in the chassis of BEAR will resist all occurring loads and withstand all adversities of the European Rover Challenge.

Figure 3: Stress test of a screw connection using the FEA