This invention relates to sensors that measure torque applied to a shaft. It particularly relates to sensors having a sleeve torsionally engaged with the shaft and sensing means responsive to torsional strain in the sleeve.
Many known torque sensors operate by responding to magnetostrictive effects resulting from strain in a stressed member or transducer. Some of these are in commercial production. Efforts have been directed toward using magnetostrictive effects to measure the torque applied to the steering wheel by the driver of a motor vehicle. One known design torsionally engages a sleeve having desirable magnetostrictive properties to a portion of the steering wheel shaft. Another design uses the magnetostrictive properties of a current production steering wheel shaft to eliminate the cost of attaching a sleeve to the steering wheel shaft. In a third known design magnetostrictive material is beam or vapor deposited on a steering wheel shaft. The known designs have not proved entirely satisfactory. Known methods of attaching a sleeve require processes that are not easily adapted to large volume production. The same is true of beam or vapor deposition of magnetostrictive materials. Efforts to use the shaft itself have suffered from the difficulty of obtaining shafts consistently having desired magnetostrictive properties. For measuring steering torque in an automobile the ideal sensor would be inexpensive and compatible with existing steering wheel shafts.
The expression xe2x80x9ctorsionally engagedxe2x80x9d is used herein to describe engagement between a first element and a second element for transmitting torque therebetween. It includes engagement for transmitting torque by a rigid attachment such as a weld or adhesive joint or both elements being made of one piece of material. It also includes engagement by means that transmit only torque exemplified by a wrench socket engaging the head of a bolt. The expression xe2x80x9ctorsionally engagedxe2x80x9d is used to cover a broad range of torque transmitting engagement means that may or may not transmit forces in addition to torque.
A torque sensor incorporating a sleeve of magnetostrictive material is described in U.S. Pat. No. 5,351,555 issued Oct. 4, 1994 to Garshelis. Particular attention is focused on the Garshelis patent because it is believed to offer the lowest cost sensor responsive to torque applied to a magnetostrictive sleeve. However, the invention is applicable to any torque sensor having a sleevelike transducer that is torsionally stressed when torque is applied to a shaft.
The Garshelis design provides a sleeve (xe2x80x9ctransducerxe2x80x9d) permanently magnetized in its circumferential direction. Garshelis discusses attachment of the transducer to the torsionally stressed shaft and (column 15 beginning at line 7) describes requirements which must be met by the chosen method of attachment:
xe2x80x9cproper operation . . . requires that there be no slippage between any of the components at their interfaces. . . . Somewhat less obvious, but no less important, is the requirement that there be no inelastic strain in shaft 8 in any cross section which includes the transducer 4. Thus, all strains associated with the transmission of torque must be fully recoverable when the torque is relaxed.xe2x80x9d
and in column 16 beginning at line 5
xe2x80x9cAs already indicated, the transducer 4 and underlying shaft must act as a mechanical unit. Rigid attachment of the transducer 4 either directly or indirectly to shaft 8 is crucial to proper operationxe2x80x9d.
In fact, attachment by adhesive bonding (using known adhesives and known designs) or interference fit (Garshelis"" preferred method) do not satisfy the above quoted requirements. All known designs based on adhesive bonding or interference result in peak stresses exceeding the capabilities of the bond.
In column 16 beginning at line 5 and continuing through line 23 of column 17 Garshelis discusses three categories of torsional engagements between the transducer and the shaft. The categories are 1) salient point, i.e. splines, knurls, teeth etc. at the ends of the transducer mating with similar features on the shaft; 2) distributed, i.e. adhesive bonding or interference fit; 3) diffuse, i.e. welding or brazing the ends of the transducer to the shaft. The first xe2x80x9c1) salient pointxe2x80x9d and the last xe2x80x9c3) diffusexe2x80x9d work well but manufacturing methods for achieving these attachments are not easily adapted to automotive manufacturing procedures.
About friction or adhesive bonding Garshelis states (column 16 lines 37 through 41):
xe2x80x9cThis bonding limits the maximum measurable torque to a lower value than might otherwise be handled by the shaft 8 alone or transducer 4 alone, but is advantageous for other reasons as indicated previously.xe2x80x9d
Accordingly, Garshelis expresses a known need for an xe2x80x9cadvantageousxe2x80x9d process such as adhesive bonding that does not limit the maximum measurable torque to xe2x80x9ca lower value than might otherwise be handled by the shaft 8 alone or transducer 4 alonexe2x80x9d. Garshelis goes on to state (column 16 lines 41 through 47):
xe2x80x9cPress or shrink fits can be used to obtain the desired circular anisotropy, and can provide very substantial gripping forces which as a practical matter will not be broken by expected torques on shaft 8. With clean, degassed (and perhaps deoxidized) surfaces, the effective coefficient of friction can rise without limit and act somewhat like a weld.xe2x80x9d
Providing xe2x80x9cclean, degassed (and perhaps deoxidized) surfacesxe2x80x9d on the elements before they are joined by press or shrink fits is expensive and time consuming. It is difficult to assure such qualities in many millions of parts as required for automotive production. It is not stated in the Garshelis patent but it is believed that to achieve in a press fit an effective coefficient of friction that xe2x80x9ccan rise without limit and act somewhat like a weldxe2x80x9d as stated in Garshelis the xe2x80x9cclean, degassed (and perhaps deoxidized) surfacesxe2x80x9d must be joined and heat treated at high temperatures in a suitable atmosphere for many hours. To obtain a shrink fit heat treatment is believed to be required both to achieve an effective coefficient of friction that xe2x80x9ccan rise without limit and act somewhat like a weldxe2x80x9d and to cause the shrinkage required for a shrink fit.
Another method of achieving an interference fit between the transducer and the shaft is described by Garshelis with reference to FIGS. 14, 15 and 16. In this method the shaft is hollow and an expander is drawn through the shaft to expand it thereby providing the desired hoop stress. This process also is believed to be difficult and expensive to implement in mass production of steering wheel shafts.
The following numerical examples will clarify the issues related to attaching a sleeve by adhesive or interference fit (without heat treatment or other processes to achieve an effective coefficient of friction that xe2x80x9ccan rise without limit and act somewhat like a weldxe2x80x9d). In column 10 lines 3 through 5 Garshelis cites the example of a shaft diameter of 0.5 inch (1.27 centimeters) and a transducer wall thickness in the 0.030 to 0.050 inch (0.076 centimeters to 0.127 centimeters) range. The wall thickness is important to achieve sufficient magnetic flux (Garshelis column 10 lines 24 through 31). From the well known fact that torque transmitted by a shaft is distributed as the third power of the radius it follows for the case of the aforementioned 0.5 inch diameter shaft that if the transducer and shaft have similar shear moduli (which is likely to be the case) 36 percent of the total torque will be transferred to the transducer in the case of 0.030 inch transducer wall thickness and 52 percent of the total torque will be transferred to the transducer in the case of 0.050 inch transducer wall thickness. A possible diameter of a steering wheel shaft of an automobile is 2 cm and it might be subjected to a maximum torque of 600 newton-meters (450 ft-lbs). Such a torque might be applied by a large healthy male driver after the wheel reached the end of its travel. At the one centimeter radius of the outer surface of the steering wheel shaft 600 newton-meters torque creates a tangential force of 60,000 newtons (13500 lbf). If 36 percent of the torque is transmitted to the transducer 21,600 newtons (4860 lbf) must be transferred between the transducer and the shaft by the attachment means. The fraction of the force transferred between the transducer and the shaft would be 36 percent in the case of the 2 centimeter diameter shaft if the inside diameter of the transducer is also 2 centimeters and the thickness of its wall is 1.2 millimeters (0.047 inches). The fraction would be much larger if the inside diameter of the transducer is larger and the thickness remains 1.2 millimeters. Assuming an adhesive shear strength of 10 newtons per square millimeter (1419 psi) and assuming means exist for providing constant shear stress over the area of adhesive attachment, transferring 21,600 newtons requires 21.6 square centimeters or 3.5 centimeters of shaft length of bonded area at each end of the transducer.
The second example is an interference fit. If the transducer wall thickness is 1.2 millimeters and is stressed to a hoop stress of 700 mpa (100,000 lbf/in2) and the coefficient of friction is 0.3, 1575 newtons (353 lbf) of shear force can be transferred per millimeter of length. Transferring the aforementioned 21,600 newtons of shear force requires about 1.4 centimeters of shaft length of contact with the shaft at each end of the transducer.
In summary, in the case of a two centimeter diameter steering wheel shaft, both bonding by adhesive and attachment by press fit would require contact with the shaft for one to four axial centimeters beyond each end of the active area of the transducer to transmit the forces encountered in operation assuming uniform shear forces. To prevent higher stresses that would cause adherence to fail the shear force must be distributed uniformly over the area of attachment. In fact, known technology does not enable the hereinabove reproduced requirements (Garshefis column 16 lines 37 through 41 and column 16 lines 41 through 47) to be achieved with any amount of adhesive or conventional press fit adherence area because the shear stresses peak at the ends of the attachment regions and exceed the maximum shear capabilities of adhesives and/or press fits.
A substantial difference is now evident between xe2x80x9cdistributed attachmentxe2x80x9d (adhesive, friction) and xe2x80x9csalient point attachmentxe2x80x9d and xe2x80x9cdiffuse attachmentxe2x80x9d. In the latter two attachment is truly at the ends of the transducer and the transducer operates as a unit with the shaft. This is also true in the aforementioned case where the effective coefficient of friction rises without limit and acts somewhat like a weld which is believed to be properly categorized as a xe2x80x9cdiffuse attachmentxe2x80x9d. In the xe2x80x9cdistributed attachmentxe2x80x9d cases attachment forces are required to be distributed over lengths of shaft such as the aforementioned one to four centimeter attachment regions at each end of the transducer.
It will also be appreciated from the above numerical examples taken with the following that where the transducer has a constant thickness as illustrated in FIGS. 1, 3, 4 and 6 through 16 of Garshelis (all of the figures that illustrate transducers) the end portions of the transducer do not xe2x80x9cact as a mechanical unitxe2x80x9d with the steering wheel shaft unless the ends are effectively welded to the shaft. For the transducer to xe2x80x9cact as a mechanical unitxe2x80x9d with the steering wheel shaft it must twist as the steering wheel shaft twists over its entire length. However, if a constant thickness transducer is attached by adhesive or press fit, sufficient torque to twist the transducer and cause it to xe2x80x9cact as a mechanical unitxe2x80x9d with the steering wheel shaft is only achievable in a xe2x80x9ccentral regionxe2x80x9d between the aforementioned attachment regions. Outside the xe2x80x9ccentral regionxe2x80x9d the torque available to twist the transducer diminishes with distance from the xe2x80x9ccentral regionxe2x80x9d because the transmission of torque is xe2x80x9cdistributedxe2x80x9d and the twisting of the transducer diminishes as the torque diminishes with distance from the central regions causing the torsional strain of the transducer and the shaft to be different far from the central regions. In Garshelis"" words cited hereinabove: xe2x80x9cThis bonding limits the maximum measurable torque to a lower value than might otherwise be handled by the shaft 8 alone or transducer 4 alone.xe2x80x9d
An object of this invention is to provide a torque sensor transducer which can be attached by adhesive to a torque carrying shaft and which will then operate xe2x80x9cas onexe2x80x9d with the torque carrying shaft.
A general object of this invention is to provide a torque sensor which also overcomes certain disadvantages of the prior art.
The present invention provides a torque sensor for measuring the torque applied to a shaft. It comprises a magnetostrictive sleeve torsionally engaged with two shear levelers. The shear levelers are bonded by adhesive to the shaft. The shear levelers have flared ends and regions of varying torsional elasticity that operate to level the shear stress in the adhesive. The term xe2x80x9clevelxe2x80x9d is used herein with reference to shear stress in adhesives to describe causing the shear stress to be constant and without peaks over the area bonded by adhesive. It may include being constant at two or more different levels at two or more areas bonded by adhesive.
Further, in accordance with the invention, the torque sensor is attached to the shaft by adhesive which is stressed in shear without stress peaks thereby enabling designs wherein the adhesive can transfer torques approaching the yield limit of the shaft.
Further, in accordance with the invention, the shear levelers have varying torsional stiffnesses to provide a uniform shear stress in the adhesive.
Further, in accordance with the invention, the shear levelers have flared ends and varying thickness adhesive at the flared ends further distributes stress in the adhesive and enables designs wherein the adhesive transmits torques that approach the yield torque of the shaft.
Further, in accordance with a first embodiment of the invention, low magnetic permeability isolation rings magnetically isolate the shear levelers from the magnetostrictive central segment.
Further, in accordance with the aforementioned first embodiment of the invention, the isolation rings are welded to the shear levelers and the magnetostrictive central segment.
Further, in accordance with the aforementioned first embodiment of the invention, the magnetostrictive central segment is pressed onto a stack of washers with crowned outer circumferences that maintains the transducer in its cylindrical shape and minimizes the torque that must be accumulated by the shear levelers. Great hoop stress in the magnetostrictive central segment is achieved by heat treatment after the magnetostrictive central segment is pressed onto the stack of washers.
Further, in accordance with the aforementioned first embodiment of the invention, each washer of the aforementioned stack of washers with crowned outer circumferences is coated with a thin layer of material that evaporates during heat treatment thereby leaving each washer separated from adjacent washers and therefore free to rotate without friction when the transducer is torsionally strained.
Further, in accordance with a second embodiment of the invention, the shear levelers are unitary with a low magnetic permeability middle segment upon which the magnetostrictive central segment is pressed and welded and possibly shrunk whereby great hoop stress in the magnetostrictive central segment is achieved which advantageously provides desirable magnetic properties.
Further, in accordance with the aforementioned second embodiment of the invention, the shear levelers are unitary with a low magnetic permeability middle segment upon which the magnetostrictive central segment is placed and welded and great hoop stress in the magnetostrictive central segment is achieved by expanding the middle segment and the magnetostrictive central segment together which advantageously provides desirable magnetic properties.
Further, in accordance with a third embodiment of the invention, the shear levelers are unitary with the magnetostrictive central segment and annular grooves are provided between the shear levelers and the magnetostrictive central segment. The grooves enhance magnetic anisotropy and provide surfaces against which force may be applied to facilitate installation of the transducer on the shaft.
Further, in accordance with the invention, a torque sensor comprises a circularly symmetric magnetic element centered on the rotation axis of a magnetostrictive element for providing a lower reluctance magnetic field path and less sensitivity to a bent shaft or other asymmetry.
A complete understanding of this invention may be obtained from the description that follows taken with the accompanying drawings.