机械专业毕业设计（论文）外文翻译.docx

附录：magnetoelastic torque sensor utilizing a thermal sprayed sense-element for automotive transmission applicationsbrian d. kilmartinsiemens vdo automotive corporationabstracta 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 1800s, is a structural property of matter that defines a materials 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 materials maximum change in length per unit length.inverse-magnetostriction, conversely, defines changes in a materials 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. the thermal spray process adds robustness to the mechanical aspect by permitting torque excursions to an unprecedented 2000% of full scale (per prototype validation testing of certain constructs) without the need for ancillary torque limiting protection devices. furthermore, accuracy, repeatability, stability, low hysteresis, rotational position indifference, low cost and amenability to the high-volume manufacturing needs of the automotive marketplace are all attributes of this torque sensing technique. when coupled with a magnetometer that is grounded in well- established fluxgate technology, the resultant sensor is inherently dependable and can potentially establish a new standard for torque measuring sensors. introductionas is well known, automotive transmissions are designed to alter the power transfer ratio between the engine and the drive wheels effectively optimizing engine loading. the engine thereby runs in a narrow and efficient operating band even though the vehicle travels over a wide range of speeds. for automatic transmissions, shift valves select the gear ratio based generally on the throttle position, engine vacuum and the output shaft governor valve state. with the advent of electronic sensors and computerized engine controllers, transmission shift functions have been migrating towards closed-loop operation under software processing control. along with this progression came the realization that the transmission output torque would provide a valuable feedback parameter for shift and traction control algorithms. the measurement of output torque, however, proved elusive due to the extremely harsh operating conditions. one particular suv application under consideration required 1% accuracy in measurements of roughly 2700 nm with possible torque excursion of 4700 nm; all while exposed to temperature extremes -45 to +160 o c. one method for measuring torque is to examine the physical stresses that develop in a shaft when it is subjected to an end-to-end twisting force. the principle stresses are compressive and tensile in nature and develop along the two counter-spiraling, mutually orthogonal 45 o helices. they are defined by the equation : t = tr / jwhere t is the torque applied to the shaft, r is the shaft radius and j is the polar moment of inertia.setting p r4/ 2 = j for a solid cylindrical shaft and r = d/2 yields: t = 16t / p donce again, t is the torque applied to the shaft and d is the shaft diameter.furthermore, the degree of twist experienced by the shaft for a given torque is given by2: q = 32(lt) / (p d4g)where l is the length of the shaft, t is the applied toque, d is the diameter of the shaft and g is the modulus of rigidity of the shaft. the modulus of rigidity defines the level of elasticity of the shaft material, thus, a lower g value would manifest in a shaft with a higher degree of twist for any given applied torque. torque induced stresses that occur in the shaft material are transferred into an affixed magnetic coating and give rise to measurable changes in its surrounding magnetic field that are directly proportional to the magnitude of the applied torque; with the polarity of the magnetic field, i.e., north or south, governed by the direction of the applied torque. in essence, this is the premise of torque sensing by means of inverse magnetostriction. torque sensor embodimentto effectively invoke the inverse-magnetostriction effect, the magnetic material must be correctly pre-loaded with stress anisotropy in its quiescent state. in the case of a cylindrically shaped magnetic element, the anisotropic forces must be circumferential (i.e., tangential) in nature and can be either compressive or tensile depending on the polarity or sign of the materials saturation magnetostriction constant. achieving a homogenous pre-load throughout the magnetic material is crucial if the sensor is to accurately interpret torque regardless of its rotational position within a stationary magnetometer. positive magnetoelastic devicesearlier efforts to create such a torque sensing element relied on a sense element made of material with a positive saturation magnetostriction constant. this embodiment was realized with a ring-shaped magnetoelastic element made from 18% nickel-iron alloy that intrinsically requires tensile circumferential pre- loading 3 . such a pre-load was achieved by pressing the ring onto a tapered area of the base shaft effectively stretching it. the effect of tensile stress on the magnetic hysteresis behavior is shown in figure 1 where the remnant inductance, b r , nearly triples. the “easy-axes” of the magnetic domains align circumferentially due to the anisotropy defined by the principal tensile stress vector. when magnetically biased, the system in effect operates as a circumferentially shorted magnet with b approaching b r and h approaching zero. negative magnetoelastic devicesto advance the state of the art, siemens vdo automotive has opted for a magnetoelastic element witha negative saturation magnetostriction constant. in this case, the alloy is very high in nickel content exhibiting a saturation magnetostriction, l s , in the range of -3e-5 dl/l and requires the stress pre-load to be tangentially compressive in nature. to achieve this embodiment, the magnetoelastic material that constitutes the sense element is “deposited” onto the base shaft using a high- velocity-oxygen-fuel (hvof) thermal spray process. the coating thickness is only 0.5mm with an axial length of 25mm. the sense element material is endowed with compressive stress by means of precise control of the thermal spray process parameters. this proprietary procedure transforms a deposition process that normally confers isotropic material properties into one that renders the requisite stress anisotropy. prototype fabricationmagnetoelastic elementthe specification for the shaft requires the measurement of torque levels of 2700 nm with no deleterious effects following exposures of up to 4700 nm. operating temperature is -45 o c to 160 o c. by converting from the earlier torque sensor “pressed-on ring” concept to one based on a magnetoelastic material with a negative saturation magnetostriction constant, l s , the design is advanced in several respects. primarily, its resiliency against stress/corrosion cracking is enhanced by 1) the inherent insusceptibility of high nickel content alloys towards corrosives and 2) by the lower porosity of material in compression. this is in distinct contrast with the high iron content ring placed in tension which is vulnerable to fissuring, material creep and stress corrosion cracking which can, over time, relieve the necessary anisotropic forces causing performancedegradation.an important consequence of using the thermal spray technology is the intimate bond provided between the deposited magnetoelastic element and the base shaft. by using a thermal spray process, the boundary whereby torque induced stresses are transferred is free of such imperfections as voids, galled or furrowed material and localized stress gradients that are all characteristically associated with the pressed-on ring technique. these imperfections can induce aberrations in the magnetic field shape thereby imparting torque measurement errors relative to the rotational position of the shaft with respect to a stationary magnetometer. furthermore, the strong bond at the interface effectively eliminates the slippage commonly associated with the interference fit of a pressed-on ring during extreme torque exposures. any movement at this interface will manifest as a biasing of material stresses causing a zero-shift measurement error. this is not a concern when the magnetoelastic element is deposited using an hvof thermal spray gun. torque excursions to an unprecedented 2000% of full scale have been successfully applied directly to prototype sensors without ancillary torque limiting protection devices. in addition, depositing the magnetoelastic element onto a rotating shaft provides an inherently mechanically balanced assembly that imposes no angular velocity (rpm) or angular acceleration limits on the system. other thermal spray technology attributes are its amenability to high volume manufacturing environments, the robustness of the process insuring consistent reproducibility, and an overall reduction in fabrication steps such as the elimination of machining procedures to mass-produce rings, cutting operations for precisely matching tapers on the shaft and ring, and pressing operations to install rings onto shafts. magnetic field shapingcontributions from the mechanical mounting tolerances of system components (e.g., bearings and bushings) can manifest as a misalignment between the centroid centerlines of the magnetometer and the magnetoelastic element. once calibrated, any displacement in the positional relationship between these two components will alter the systems transfer function, possibly causing the overall error to exceed specification. the sharply focused nature of the magnetic field radially emanating from the magnetoelastic element during the application of torque (see figure 3) accentuates this effect. this error can be minimized by shaping the physical structure of the magnetoelastic element resulting in a contouring of the magnetic field to a more favorable shape. as shown in figure 4, the magnetic field is made to be less pronounced with an hourglass shaped magneto elastic element and sensitivity to misalignment is, thus, reduced. in this example, the magneto elastic element is contoured such that the air gap between the magneto elastic element and the magnetometer is reduced when axial displacement between their centroid centerlines occurs. the expected reduction in magnetic signal strength caused by this displacement is thus compensated by the air gap reduction.shafts can be fabricated with a variety of contoured surface adaptations and the thermal sprayed magnetoelastic elements shape will expectedly follow suit. as is evident, a pressed-on ring manifestation of the magnetoelastic element would be incompatible with this technique. various contours are being considered for further reducing the sensitivity to misalignment and for improving other performance parameters such as magnetic field strength and hysteresis. cylindrical shaft shown with superimposed associated magnetic field (i.e., radially directed flux density)contoured shaft (hourglass shape) shown with superimposed associated magnetic field (i.e., radially directed flux density)in figures 3 and 4, the spatial image of the shaft is mapped using a laser displacement system and the superimposed magnetic field is mapped in 3-space with a hall cell. magnetometerrounding out the torque sensor hardware complement is a non-contacting magnetometer that translates the magnetic signal emitted by the shafts sense element into an electrical signal that can be read by system-level devices. coupling the torque signal to some interim conditioning electronics magnetically is an attractive option due to its “non-contacting” attribute. a signal transference scheme capable of spanning an air gap is advantageous since it requires no slip rings, brushes or commutators that can be affected by wear, vibration, corrosion or contaminants. the fundamental magnetometer embodiment, shown in figure 5, is circular with the shaft passing through its center. the magnetometer encompasses the magnetoelastic element of the shaft and the shaft is allowed to freely rotate within the fixed magnetometer. power and the output signal pass through the magnetometers wiring harness. transmission torque sensor magnetometerthe magnetometer actually performs several functions beyond measuring a magnetic fields strength. these functions include magnetic signal conditioning, electrical signal conditioning, implementation of self-diagnostics, and the attenuation of magnetic and electromagnetic noise sources. the magnetic detection method chosen for the torque sensor is fluxgate magnetometry, also known as saturable-core magnetometry. this is a well-established technology that has been in use since the early 1900s. fluxgate magnetometers are capable of measuring small magnetic field of strengths down to about 10 -4 a/m (or 10 -6 oe) with a high level of stability. this performance is roughly three orders of magnitude better than that achieved by hall effect devices. although many fluxgate designs use separate drive and pickup coils, the torque sensor magnetometer was designed to use a single coil for both functions. magnetic signal conditioning is accomplished by use of flux guides integral to the magnetometer. these flux guides amplify the magnetic signal radiating from the shafts sense element prior to detection by the fluxgates thereby improving the signal-to-noise ratio. the flux guides provide additional signal conditioning by integrating inhomogeneities in the magnetic signal relative to the shaft rotational position that might otherwise be misinterpreted as torque variations. the flux guide configuration is shown in figure 6 and a magnetic simulation of the resulting field concentration is shown in figure 7. flux guides surrounding magnetoelastic elementaxial view of magnetic simulation with flux guide materials relative dc permeability set to 50,000 (e.g., hymu “80”) to further improve the magnetometers immunity to stray signals present in the ambient, common-mode rejection schemes are employed in the design of both the electronic and magnetic circuits. for example, wherever possible, differential circuitry was used in the electronic design in order to negate common-mode noise. this practice was carried over to the magnetic design through the use of symmetrically shaped flux guides and symmetrically placed fluxgates that cancel common- mode magnetic signals that originate outside the system. finally, to augment the electrical and magnetic common- mode rejection strategies, emi and magnetic shielding practices were incorporated into the design to further improve the signal-to-noise ratio. stray magnetic and electro-magnetic signals found in the ambient are prevented from reaching the fluxgates and the shafts m