Vehicles with cooperative redundancy of multiple steering systems: A hybrid shaft/hydrostatic drive system

Cooperative redundancy of multiple steering systems are used to maximise traction, manoeuvrability and stability of a wheeled vehicle operating on difficult terrain. Cooperative redundancy is achieved if all the wheel angle steering effects and the drive wheel speed steering effects have a single theoretical instant centre. This means they all reinforce each other without conflict. This maximises energy efficiency and minimises ground damage and tyre wear. In previous work, close coupled wheel motors where used to drive the wheels. In this work, each wheel is connected to the output shaft of a reverse differential. The primary input to each differential is by means of a mechanical shaft drive which delivers power from the vehicle gearbox to each reverse differential. The secondary input to each reverse differential is provided by a hydrostatic motor which is used to correct the speed of each wheel in order to achieve cooperative redundancy. The hydrostatic motors will only be driven when the vehicle is turning. The hydrostatic motors will be stationary when the vehicle is proceeding in a straight line. Equations for the correct speed of the hydrostatic motors are derived. If the vehicle is to be capable of turning about any instant centre, the wheels must be capable of turning through a large angle range. The necessary range is 180° if the wheels can be driven in both forward and reverse directions. A mechanical drive to such wheels is only feasible if a vertical kingpin drive is involved. However, such a drive suffers from an inevitable but unwanted coupling between the turning of the wheel and the rotation of the wheel. Means of compensating for this unwanted effect are also described.