Study of Passive Steering Mechanism for Mars Surface Exploration Rovers

Abstract

In planetary exploration, rovers are used to move across the surface to obtain high-resolution topographic elevation maps, as well as to conduct mining and rock sampling. However, because of the harsh planetary environment, surface exploration poses many technical challenges. In this study, we assume an unmanned exploration rover that is small enough to be carried on a Japanese launch vehicle, and its weight is limited by the payload capacity of the launch vehicle. It is also important that the rover is not prone to failure. The surface of Mars is covered with fine sand called regolith, and this sand can get between the mechanisms and cause them to fail. Therefore, a rover with lightweight and simple mechanisms is needed. In addition, Mars has a large slope, and there are not only rocky terrain but also uneven terrain such as depressions and craters. For smooth exploration, the rover must travel a safe and efficient path to avoid tipping over or getting stuck. However, wheel slippage on the regolith makes it difficult to follow the target path precisely and can result in getting stuck. If an active steering mechanism is omitted in order to configure a simple and lightweight traveling system, and if the path following is attempted to be performed by the difference in rotation speed between left and right traveling mechanisms such as wheels and crawlers when traveling on a curved path, the ground of the traveling system is forced to skid, which places a severe load on the traveling system. Therefore, this study proposes a lightweight, simple, and passive mechanism that reduces skidding and provides good path-following performance. The proposed mechanism uses a passive Ackermann mechanism without actuators, and the steering is performed by the difference in rotational speed between the left and right wheels. Since there is no actuator for steering, the mechanism is lightweight and simple. In order to suppress steering in unintended directions, a weak spring is added in the direction that returns the steering angle to a straight line. This paper describes the results of a prototype wheeled rover mechanism incorporating the proposed mechanism in the size assumed and confirming its ability to follow a target path by running experiments on soil simulating the surface of Mars. The experimental results show that the proposed mechanism has a higher ability to follow the target path than a rover without a steering mechanism. In addition, the rover's ability to follow the target path was improved by correcting the steering angle with respect to the rotational speeds of the left and right wheels.