**4. Hybrid steering systems for trucks and buses**

The high-speed trucks and buses offer an important field of applications to the hybrid servo systems. However, the linear mechanical feedback of the classical steering box is still used in modern front axles with independent suspension arms (**Figure 33**) [24].

A hybrid effect can be introduced by a brushless servomotor placed on the steering column, or in a mixing box SC, in series with the torsion bar, under the shape of a module with two inputs, which controls the rotary valve (**Figure 34**).

The rotary valve opening has a high frequency oscillations (**Figure 35a**), but the steering process remains continuous (**Figure 35b**).

The basic rotary steering box of the trucks and buses has the same structure as the linear one, the only difference coming from the feedback: the piston motion is fed back to the torsion bar with a ball screw (**Figure 36**). A brushless motor placed in series with the input shaft can control the motion of the rotary valve (**Figure 37**).

The brushless motor controls the torsion bar by a complex torque and angle sensor composed by Hall sensors and a rotary inductive wheel (**Figure 38**).

**Figure 33.** *Front axle of a modern truck with linear steering box.*

**Figure 34.** *Simcenter Amesim simulation of a hybrid linear steering box [25].*

## *Advanced Applications of Hydrogen and Engineering Systems in the Automotive Industry*

**Figure 35.**

*Valve opening and rod displacement for a hydraulic steering system [4]: (a) rotary valve angle opening [degree]; (b) rod displacement [m].*

**Figure 36.** *Steering box with ball screw feedback [22].*

**Figure 37.** *Hybrid steering box [26].*

*Hybrid Steering Systems for Automotive Applications DOI: http://dx.doi.org/10.5772/intechopen.94460*

**Figure 38.** *Brushless motor with complex control functions [26].*

**Figure 39.** *Components of the hybrid steering system placed in a Volvo truck [26].*

A general view of the hybrid system of a Volvo truck shows the minimum changes needed in the standard version of the car for introducing the hybrid feature (**Figure 39**).

The HYDRAPULSE Company from U.S.A. introduced a special kind of hybrid driving system. EHSU is a ruggedized electric hydraulic steering pump (electrohydraulic) unit with integrated motor, controller, and closed-loop feedback that is specifically designed for mobile steering assist and e-steering applications. The EHSU (**Figure 40**) is available in both high and low voltage models and a variety of pump displacements and power levels. There is no electrical feedback from the steering arm! [27].
