Electrohydraulic Power Steering
A large automotive manufacturer released a sports car, which had a rear engine / rear drive power train arrangement. It also incorporated the usual sports car equipment such as manual multispeed transmission, and excellent steering and braking. As the interest regarding the car grew within the marketplace, the manufacturer added more features such as more horsepower, even better styling, etc. The marketplace also began to expand to customers interested in the aspect of owning such a car, but with actual day to day driving requirements that did not actually fit the car's aggressive capabilities. In fact, these customers were more interested in looking sporty, while desiring the more typical amenities found in general family cars, those amenities including power steering.
My design team was selected to research and develop a power steering system that would give both the ease of power steering, while actually increasing the steering responsiveness (as is possible with power steering). This latter was initially limited in the early cars due to the fact that a driver can only supply so much input into a manual steering system before reaching normal driver physical capacity. With the addition of power assist, the required driver input level drops drastically, while the actual steering system output potential (due to the power assist) rises, which then allows much more latitude in the selection of the aggressiveness of the mechanical advantage within the overall system. This allows the car to steer even more quickly, which fit well with the car's increase in engine horsepower, with the bigger, more aggressive tires, etc.
The challenge was to add the power supply for the steering system to the vehicle without any tear up to the existing engine layout, to the chassis, etc. The selection process quickly settled on "typical" hydraulic boost (found still in most cars and trucks world wide), as it allowed an easy design of the steering gear (many exist). The design of the power supply (read "pump") was where the challenge proved to be. The engine could not accommodate a standard belt driven pump, nor could the chassis accommodate the plumbing from rear of car to front, so a front mounted electric driven pump was settled on. The team focused on the design challenges of such a new pump / motor / controls package.
The design requirements had the typical industry list of cost effectiveness, durability, and reliability, but in addition, the system had to deliver variable effort steering (light steering at static or near static, while heavier (less assist) steering at speed. Also, as the unit was to be front mounted, the device was in an area of the car that had no masking noises, meaning that the unit had to be as quiet as possible in order to avoid owner issues.
Eventually the design settled out and was released with a brushed electrical motor, a vane pump, an attached electronic controller, and a fluid reservoir, all mounted on a bracket which utilized laminated steel / polymer / steel construction for its noise deadening properties. Lastly, due to the typical vehicle requirements of cold temperature (-40 C) operation, a special mineral based fluid was developed with acceptable viscosity at both ends of the temperature spectrum.
To see the resume of the expert associated with this case study, see the link below.
