An electrical assist power steering system requires sensing of torque applied to the steering wheel. An electric power steering system generally comprises a torque sensor for outputting a signal corresponding to a value of the steering torque of the driver, an electric motor for driving an assist apparatus which assists the steering force, and an electronic control unit (ECU) for determining current flowing through the electric motor in accordance with the output signal of the torque sensor. Generally, in an electric power steering system, a torque sensor converts a torsional torque of a steering shaft, which is generated by steering of a steering wheel connected to the steering shaft, to an analog signal (voltage) and outputs it to an electronic control unit (ECU). In the ECU, an analogue to digital (AD) converter circuit converts the analogue signal to a corresponding digital value. Torque measurement is performed to detect the torque caused to occur by a driver's operation using a steering wheel in an electromotive power steering system, or to detect the torque in the rotational direction of tires when running. The torque indicates how much force the driver is exerting to move the wheel. The output signal from a torque sensor is fed into a control unit which controls the electrical motor of the assist unit. When the torque sensed is high, the assist applied to the steering linkage will be high. When the torque sensed is low, the assist applied to the steering linkage will be low. In such a power-assisted steering system the output signals from the first and second receiver means may be processed to determine the rate of rotation of the vehicle steering shaft and a digital position measurement of the angular displacement of the steering shaft. This information can be used in a vehicle control microprocessor to modify or control the suspension and/or braking characteristics of the vehicle. The motor for steering assist is so driven as to obtain power assistance in accordance with the steering torque detected by the torque sensor, thereby reducing the power to operate the steering member. An electric power steering system is structured so that a steering mechanism to which the steering wheel is coupled through a steering shaft is provided with a torque sensor that detects the steering torque applied to the steering wheel and a motor that assists the operation of the steering mechanism and the motor is driven in accordance with the steering torque detected by the torque sensor to thereby reduce the operation force on the steering wheel. Typically, the torque sensing apparatus is connected between an input shaft connected to a steering wheel and a pinion or output shaft. The input shaft is connected to the steering wheel and the output shaft is connected to a pinion of a rack and pinion steering gear. The torque sensor includes a torsion bar connecting the input shaft to the output shaft. The detected torque is used to control an assist electric motor, an electrically driven pump, or a solenoid valve in response to a driver's input torque applied to a vehicle steering wheel. With the development of electric power steering systems, the need for a torque sensing apparatus which can accurately detect a torque produced by a steering shaft has been highlighted.

Torque sensors performs torque detection based on a torque detecting voltage which is a voltage difference between a first voltage and a second voltage on the basis of respective inductance changes of a pair of coils having inductances changing in opposite directions corresponding to torque. A typical torque sensor is attached to a member for measuring torque applied to that member. The torque sensor provides an electrical output signal having a characteristic which is functionally related to the torque applied to the member. The output signal is typically used to control an associated device. Torque measurement was previously accomplished using contact-type sensors directly attached to the shaft. One such sensor is a "strain gauge" type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by strain is measured by a bridge circuit or other well known means. However, contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. Also, they are very expensive and are thus commercially impractical for competitive use on vehicle steering systems. As a non-contact method for detecting torque, a magnetostrictive torque sensor using a magnetostrictive material is known. A magnetostrictive torque sensor makes use of the magnetostrictive effect in which magnetic permeability changes when a mechanical strain is imparted to a ferromagnetic material, the phenomenon being one of the phenomena of magnetic strain. The magnetostrictive effect may be termed as the change of dimensions of a material when exposed to a magnetic field or its inverse effect, i.e. the change in magnetization of a material as a result of external stress. The magnetostrictive effect is associated with ferromagnetic materials. Magnetoelastic torque sensor technology operates on the specific manifestation of magnetoelastic termed the Inverse Wiedemann Effect in which a magnetic field can be distorted to arise in the space around a torsionally stressed member. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Application of torque on the shaft strains the magnetostrictive material and causes changes in the magnetic permeability of the sensor. The changes in the permeability alter flux and thus induce electromotive force in the detecting coil. The applied torque is detected based on the induced electromotive force.