**3.2 Options for a new hybrid steering system**

There are some initial decisions to make in order to design a hybrid steering system for any kind of mobile equipment. First, the new system has to allow the human operator to drive himself the equipment anytime i.e. to have the highest priority. Second, the new control system has to be stable and to preserve the accuracy of the previous one under any operation condition. The electrical component strongly depends on the electric power supply, rising real problems from the security point of view. Some electrical power supply systems can reduce the risk of the electrical systems failure. The main problem is to use the same steering cylinders with two kinds of hydraulic metering valves. Both of them need to have close center, or to be isolated upon the request of the driver by an additive directional valve. The relative small fuel consumption introduced by an open center ORBITROL hydraulic steering units can be preserved by driving the input with an electric actuator like a stepping motor with 100-200 pulses/revolution. This option needs a special design of the stepping motor in order to insert it into a steering column. On the other hand, the use of an open center flow valve leads to a poor tracking ability that increases with the steering wheel speed [17]. A typical steady state diagram of such a hydro mechanical feedback steering unit shows a small force capability for high turning speeds. The modern product range has to cover applications of all types of mobile equipment.

This section presents the structure and the performance of a new electro hydraulic steering system for articulated mobile equipment remote controlled. The new concept was designed and tested in the Fluid Power Systems of the University "POLITEHNICA" of Bucharest in order to point out the accuracy and the stability of a control solution based on a dual input (digital and analog) IAC-R REXROTH valve
