High-Speed Traversal of Rough Terrain Using a Rocker-Bogie Mobility System

Abstract

The Rocker-Bogie Mobility system was designed to be used at slow speeds. It is capable of overcoming obstacles that are on the order of the size of a wheel. However, when surmounting a sizable obstacle, the vehicles motion effectively stops while the front wheel climbs the obstacle. When operating at low speed (greater than 10cm/second), dynamic shocks are minimized when this happens. For many future planetary missions, rovers will have to operate at human level speeds (~1m/second). Shocks resulting from the impact of the front wheel against an obstacle could damage the payload or the vehicle. This paper describes a method of driving a rocker-bogie vehicle so that it can effectively step over most obstacles rather than impacting and climbing over them. Most of the benefits of this method can be achieved without any mechanical modification to existing designs – only a change in control strategy. Some mechanical changes are suggested to gather the maximum benefit and to greatly increase the effective operational speed of future rovers. The Rocker-Bogie System: One of the major shortcomings of current planetary rovers is that they are slow. In order to be able to overcome significantly rough terrain (i.e., obstacles more than a few percent of wheel radius) without significant risk of flipping the vehicle or damaging the suspension, these robots move slowly and climb over the obstacles by having wheels lift each piece of the suspension over the obstacle one portion at a time. NASA’s currently favored design, the rocker-bogie [1,2], uses a two wheeled rocker arm on a passive pivot attached to a main bogie that is connected differentially to the main bogie on the other side (see Figure 1). The body of the rover is attached 1 School of Aerospace & Mechanical Engineering, University of Oklahoma, 865 Asp Ave., Rm. 212, Norman, OK, 73019 USA. dpmiller@ou.edu, tzeliang2@hotmail.com. www.amerobotics.ou.edu to the differential and so is suspended at an angle that is the average of the two sides. The ride is further smoothed by the rocker which only passes on a portion of a wheel’s displacement to the main bogie. Each wheel is independently driven, and cab independently steered. The maximum speed of the robots operated in this way is limited to eliminate as many dynamic effects as possible, and so that the motors can be geared down so that the wheels can individually lift a large portion of the entire vehicle’s mass. In order to go over an obstacle, the front wheels are forced against the obstacle by the rear wheels. The rotation of the front wheel then lifts the front of the vehicle up and over the obstacle. The middle wheel is the pressed against the obstacle by the rear wheel and pulled against the obstacle by the front, until it is lifted up and over. Finally, the rear wheel is pulled over the obstacle by the front two wheels. During each wheel’s traversal of the obstacle, forward progress of the vehicle is slowed or completely halted. This is not an issue for the operational speeds at which these vehicles have been operated to date. Figure 1: Side View of Rocker-Bogie Configuration