The invention relates to a sensor adapted to measure the torque between two rotatable shafts. Specifically, the present invention relates to a device that measures the torque between an input shaft connected to a steering wheel and a pinion shaft connected to a steering rack of an automobile.
A large number of automobiles are equipped with power steering. The most common power steering system presently is a hydraulic power steering system. The steering mechanism of a vehicle includes an input shaft which is connected to the steering wheel of the vehicle. Further, a pinion shaft is mechanically linked to the wheels of the vehicle such that rotation of the pinion shaft provides steering to the vehicle. The input shaft and the output shaft are coupled by a torsion bar. In a hydraulic power steering system, a pump provides pressurized hydraulic fluid to provide force to assist rotation of the pinion shaft. The power assist provided by the hydraulic fluid is controlled by rotational deflection of the torsion bar that interconnects the input shaft and the pinion shaft. As the amount of torque between the two shafts increases, the power steering system provides more force to assist in rotation of the pinion shaft. In this way, the amount of torque that the operator of the vehicle must input to the system is controlled.
Unfortunately, hydraulic power steering systems are inefficient, primarily due to the need to size components needed to respond to steering demands through a broad range of operating conditions. In response to the challenge of providing more energy efficient steering systems, electric power assisted steering systems have been developed. In an electric power assisted system, an electric motor provides force to a steering rack of the vehicle to assist in turning the wheels of the vehicle. Just as with the hydraulic system, the end result is that the amount of torque the operator must input is controlled. The amount of torque applied by the operator should not be zero. It is preferred that the operator should have to exert some input torque in order to receive tactile feedback from the steering system and to have a xe2x80x9cfeelxe2x80x9d for the vehicle. Just as with the hydraulic system, the electric power assisted steering system uses a torsion bar located between the input shaft and the pinion shaft, whereby torque deflection between the two shafts is localized along the torsion bar and can be measured to determine the magnitude of the applied torque from the operator.
Traditionally, the torsion bar would be equipped with strain gages to determine the amount of deflection. However, systems using strain gages mounted directly to the shaft are susceptible to damage and wear. The shaft of the steering mechanism of the vehicle rotate and must be able to withstand some amount of axial movement due to the movements of the vehicle. In another known system, a mechanical shuttle converts the relative angular displacement of the input and output shafts, occurring in the torsion bar, to linear movement of a surface of the shuttle. A potentiometer, or other sensor, is mechanically linked to detect the position of the shuttle surface. These systems suffer from the problems inherent in the use of the mechanical shuttle and the contact based sensing system, namely hysteresis, reliability and sensitivity limitations. Therefore, there is a need for a device that will measure the amount of torque being transferred between the input shaft and the pinion shaft of a steering mechanism that is insulated from the rotational and axial movements of the input shaft and pinion shaft within the vehicle.
A torque sensing device adapted to measure the torque between two shafts of a steering mechanism and to send a corresponding signal to an electrical power assisted steering system of an automotive vehicle includes a housing, an input shaft rotatably supported within said housing and adapted to connect to a steering wheel of the automobile, and a pinion shaft rotatably supported within the housing and adapted to engage a steering rack of the automobile.
A torsion bar is positioned between and interconnects the input shaft and the pinion shaft and is adapted to allow rotational movement of the input shaft and the pinion shaft relative to one another. The input shaft and the pinion shaft are supported within the housing to allow limited axial movement of the input shaft and the pinion shaft within the housing. At least one magnet is fixedly mounted to one of either the input shaft or the pinion shaft.
A stator assembly having a first stator ring and a second stator ring is fixedly mounted to one of either the input shaft or the pinion shaft and extends axially over the magnet. A pair of flux brushes is fixedly supported within the housing and is located in spaced relation to one of the first and second stator rings of the stator assembly. The flux brushes are spaced apart from one another by an axial distance, thereby defining a gap therebetween. A sensing device is positioned within the gap and is adapted to measure the magnetic flux passing between the flux brushes.