Magnetoelastic Torque Sensor Utilizing a Thermal Sprayed Sense-Element for Automotive Transmission Applications (2003)

A Magnetoelastic based Non-Contacting, Non-Compliant Torque Sensor is being developed by Siemens VDO for automotive transmission applications. Such a sensor would benefit the automotive industry by providing the feedback needed for precise computer control of transmission gear shifting under a wide range of road conditions and would also facilitate cross-platform usage of a common transmission unit. Siemens VDO has prototyped transmission torque sensors operating on the principle of Inverse-magnetostriction, also referred to as the Inverse-Joule Effect and the Villari Effect. Magnetostriction, first documented in the mid 1800's, is a structural property of matter that defines a material's dimensional changes as a result of exposure to a magnetic field. Magnetostriction is caused when the atoms that constitute a material reorient in order to align their magnetic moments with an external magnetic field. This effect is quantified for a specific material by its saturation magnetostriction constant, which is a value that describes a material's maximum change in length per unit length. Inverse-magnetostriction, conversely, defines changes in a material's magnetic properties in response to applied mechanical forces. Material that is highly magnetostrictive and elastic in nature is referred to as being magnetoelastic. The premise of the Siemens VDO torque sensor design is that a magnetoelastic material can be bonded to a cylindrical shaft and magnetized in its mechanical quiescent state to create a sense-element. While under torque, principle tensile and compressive stress vectors in the form of counter-spiraling, mutually orthogonal helices develop in the shaft and are conveyed to the magnetoelastic sense-element giving rise to a measurable magnetic field change. The magnetic field deviation that arises from the magnetoelastic sense-element is directly proportional to the magnitude of the imposed torque. In effect, the magnetic field is modulated by torque. A sensitive magnetometer then translates the field strength into an analog voltage signal, thereby completing the torque-to-voltage transducer function. Critical to the success of the Siemens VDO torque sensor design is an intimate attachment of the sense-element to the torque-bearing member. Inconsistencies in the boundary between the sense-element and the torque-bearing member will result in aberrant coupling of stresses into the sense-element manifesting in performance degradation. Boundary inconsistencies can include such imperfections as voids, contaminates, lateral shearing, and localized zones of stress pre-load. Such inhomogeneities may be inherent to an attachment method itself or may subsequently be caused by systemically rendered malformations. Thermal spray, the process where metal particles are deposited onto a substrate to form a coating, was used to address the issue of securely affixing magnetic material to a torque-bearing member. In addition to achieving the prerequisite of an intimate and secure bond, the thermal spray process can be regulated such that the deposited magnetic material is pre-loaded with the internal stresses needed to invoke the inverse-magnetostriction effect. Summarizing, the passive nature of the magnetic sense-element provides an intrinsically simple kernel for the Siemens VDO torque sensor that makes for a highly reliable and stable design.