Hydraulic steering systems are making way for electromechanical steering systems for various reasons

Hydraulic steering mechanism seen on most automobiles may soon be a thing of past. The progress in electromechanical devices and systems is poised to replace hydraulic steering systems. A hydraulic steering system in an automobile employs a belt-driven pump to provide hydraulic pressure to the system. This hydraulic pressure is generated by a rotary vane pump, which is driven by the vehicle's engine via the belt. As the speed of the engine increases, the pressure in the hydraulic fluid also increases, hence a relief valve is incorporated into the system to allow excess pressure to be bled away. 

While travelling in a straight line the twin hydraulic lines in the hydraulic steering system provide equal pressure to both sides of the steering wheel gear. When torque is applied to the steering wheel, the hydraulic lines provide unequal pressures and hence assist in turning the wheels in the intended direction. Modern hydraulic steering systems offer variable assist or variable steering assistance. An electronic pressure valve reduces the hydraulic pressure of the power steering lines as the vehicle's speed increases. 

Speaking at the recent steering-tech 2008, organised by TÜV SÜD Automotive GmbH and the University of Munich in Germany, Dr. Wolfgang Runge, general manager (Research and Development vehicle steering) at ZF expressed that the steering systems of the future will get by without hydraulics. He added that luxury vehicles will also be increasingly equipped with electromechanical steering systems. Runge pointed out that achievable fuel economy will range around 0.8-litre per 100 kms. 

Steering systems have been under the scanner of various automakers and suppliers for various reasons including emissions. According to the sources in the steering industry, more stringent efficiency requirements aimed at reducing CO2 emissions, together with the ever more widespread implementation of sophisticated features such as advanced driver assistance systems, are today the impetus behind the technological changeover to electromechanical steering systems. These latest changes, combined with the demand for shorter development lead times and cost reduction targets present a real challenge for the automotive and steering system supply chain add sources. 

Interestingly, experts in the auto industry opine that hydraulic steering only offers cost benefits when comparing the basis system. As soon as a variable transmission ratio is added, one achieves the same costs with the electromechanical steering system, assuming appropriate quantities. ZF, which produced nearly 10 million electromechanical steering units, has supplied them to smaller cars, which currently seem to be the right candidates for such a system. To reach out to luxury cars, designers are engaged in the development of electromechanical steering systems that use a clever design and control of two electro motors. This is claimed to do away with the need to dial-in 42V on-board supply system. Electromechanical power steering is limited to smaller vehicles. This is because the 12volt electrical system is limited to 80amps of current which, in turn, limits the size of the motor to less than one-kilowatt (12.5 volts times 80 amps equals 1000 watts). Vehicles such as trucks and SUVs require a more powerful motor, which brings 42V supply into the picture. 

As more and more automakers seek to arrive at electromechanical systems that will suite various kinds of automobiles, the new direct steering system developed by an Ingolstadt-based automaker enables a variable steering ratio as well as different functions for active vehicle stabilisation. Seen on the 2009 Audi A4, at the heart of the system is a superposition gear that operates with no play and modifies the steering ratio in response to vehicle speed. Together with the ESP electronic stabilisation program, the dynamic steering system keeps the new Audi A4 stable by means of slight, extremely rapid steering movements. The superimposed transmission is located in the steering column and combined with an electric motor. It is of the 'Harmonic Drive' type that has already proved highly successful in robotics and space travel. 

The harmonic drive transmission has only three main operating components. An electric motor turns an elliptical inner rotor which, by way of a ball bearing, alters the shape of a thin sunwheel connected to the steering input shaft. At the vertical axes of the ellipse, the sunwheel meshes with the teeth on the inside of an annulus or ring gear, which acts on the steering output shaft. When the inner rotor is turned, the main axis of the ellipse changes and with it the extent of gear tooth meshing. Since the sunwheel has fewer teeth than the annulus, there is relative movement between them - the superimposed movement used to alter the effective steering ratio. The dynamic steering ratio can vary by almost 100 per cent, depending on the car's road speed and the chosen Audi drive select mode. Changes in the ratio take place continuously and imperceptibly. 

When parking, the dynamic steering system is extremely direct, with only two turns of the steering wheel from lock to lock, and very little effort needed thanks to a high degree of power assistance. At typical country-road speeds, the direct response and level of power assistance are reduced slightly, but even then the driver will not have to move his or her hands to another point on the steering wheel when negotiating a sharp corner. At high speeds on motorways or similar roads, a more indirect steering ratio and less power assistance ensure that the car has a high degree of straight-line stability; in theory, four turns of the wheel from one steering lock to the other would be necessary. 

 The BMW Active Front Steering (AFS), at the other end, is integrated into hydraulic steering systems at BMW. Masterminded electronically, this system varies the steering transmission ratio under normal driving conditions at low and medium road speeds, making the steering more direct, reducing the steering effort, and thus significantly enhancing the car's performance in city traffic, when parking and in bends. At high speeds, in turn, the steering becomes more indirect and thus provides superior directional stability and a very good, docile steering response. Networked to Dynamic Stability Control (DSC), active front steering is able to intervene as soon as the car starts to become unstable by monitoring the yaw rate and changing the steering angle accordingly. This reduces the number of DSC interventions at the lower end of the control range. 

 The Mercedes Direct-Steer System seen on the new SL takes all of the positive attributes of the speed-sensitive power steering and adds to them a variable steering ratio that changes depending on the steering angle. In and around the central position, the steering has an indirect ratio for excellent straight-line stability, meaning a high level of handling safety. The direct-steer system's ratio starts to increase very rapidly at a steering angle of just 5 degrees, and feels quite direct from a steering angle of around 100 degrees. The result is a reduction in the required steering angle, as it is known in technical circles. In other words, relatively small turns of the steering wheel are sufficient for changing course. And, indeed, the number of steering wheel turns from lock to lock is reduced by around 25 per cent with the new direct-steer system.