Abstract:
A torque sensor comprising a shaft of magnetostrictive material; a pair of magnets having oppositely sensed poles positioned circumferentially around the shaft so as to induce a localized magnetic field in the shaft between the poles. A torque applied to the shaft is sensed by a flux detector positioned circumferentially between the magnet poles so as to detect a component of the localized magnetic field which escapes from the shaft as a result of the torque. With this design, there is no need to permanently magnetize the shaft or a collar attached to the shaft, as in the prior art. The prior art manufacturing step of permanently magnetizing the collar or its shaft is also thus eliminated, greatly simplifying the manufacturing.

Description:
BACKGROUND ART  
         [0001]    This invention relates to torque sensors, more specifically to magnetostrictive torque sensors.  
           [0002]    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. This inverse magnetostrictive effect is sometimes referred to as the magnetoelastic effect, but the term magnetostrictive is used exclusively in the present document. Generally, the magnetostrictive effect is associated with ferromagnetic materials.  
           [0003]    U.S. Pat. No. 5,351,555 discloses a non-contact torque sensor of the magnetostrictive type which can be used with rotating shafts. As illustrated in FIG. 12 of the accompanying drawings, the torque sensor comprises a collar  120  fitted tightly onto a shaft  116 . The collar  120  is magnetized circumferentially around the shaft as indicated by arrows in the figure. When the shaft  116  is torqued, the torque is transmitted to the collar  120  and induces a helical magnetic field therein. A component of the helical field is sensed by an externally positioned magnetic flux detector  118  from which the magnitude of the torque can be inferred.  
           [0004]    Although these designs work well, they have been criticized for several reasons. One problem is that under high torque conditions it is possible that slippage of the collar on the shaft may occur. Another issue is the manufacturing costs associated with making and fitting the collar to the shaft, which have been said to be too high.  
           [0005]    WO 99/21150, WO 99/21151 and WO 99/56099 disclose various designs of torque sensor which address the shortcomings of the collar-based designs. In these more recent designs, a portion of the shaft itself is magnetized, thereby allowing a separate magnetized collar to be dispensed with. FIG. 13 of the accompanying drawings illustrates an example of these collarless designs. A shaft  116  has integral portions thereof  122  magnetically polarized in the circumferential direction, i.e. around the shaft, as in the collar-based designs. Multiple polarized regions are preferred, with adjacent domains being oppositely magnetically polarized, as illustrated in the figure in which two such domains are shown. Torquing of the shaft causes a change in the magnetic field external to the shaft, which is measured by a suitable magnetic flux detector  118 , similar to the collar-based designs.  
           [0006]    One problem common to all these designs is their dependence on the permanent polarization of the collar or shaft. The magnetic polarization is induced during manufacturing, but manufacturing variations cause variations in the polarization strength, which in turn cause variations in sensitivity between different sensors. Although measures are proposed in WO 99/56099 to control this variation, the measures are quite complex. More seriously, all of the above-mentioned designs depend on, and assume, long-term stability of the magnetically polarized part of the sensor. If the magnetic polarization decays, then a given external torque applied to the shaft will result in a lower output from the magnetic flux detector. Periodic recalibration of the sensor will therefore be required if absolute sensitivity is needed. If the decay is more serious, then remagnetization of the magnetized part of the torque sensor or more likely replacement of the whole sensor will be necessary.  
           [0007]    It is therefore an aim of the invention to provide a torque sensor that reduces the prior art dependence on magnetic polarization strength.  
         SUMMARY OF THE INVENTION  
         [0008]    According to a first aspect of the present invention there is provided a torque sensor comprising: a shaft comprising magnetostrictive material; a pair of opposite magnet poles positioned circumferentially around the shaft so as to induce a localized magnetic field in the magnetostrictive material between the opposite magnet poles; and a torque-sensing flux detector positioned to detect a component of the localized magnetic field which escapes from the magnetostrictive material when the shaft is torqued.  
           [0009]    In this way, there is no need for a permanently magnetically polarized part of the torqued component, i.e. the shaft or its collar, which is essential to all the abovereference prior art designs. The magnetic field in the torqued component is instead induced with an external magnetic field. As a result, the magnetic field strength is easily quantified, for example by additional flux detectors arranged close to the external magnet poles. Another consequence of the proposed design is that the prior art manufacturing step of permanently magnetizing the collar or shaft is completely eliminated, thus greatly simplifying the manufacture of the torque sensor.  
           [0010]    The magnetostrictive shaft may be made from a wide range of ferromagnetic materials including many types of stainless steel, tool steel and Ni—Fe alloys. This gives the advantage that the magnetostrictive torque sensor can be retrofitted to a component including a shaft that is torqued, since the shaft does not need to be magnetized or have extra coatings or collars attached to it.  
           [0011]    In one embodiment, the magnetostrictive torque sensor comprises a magnetostrictive shaft that has a main body of non-ferromagnetic material surrounded by a layer of magnetostrictive material. A sensor of this kind is especially advantageous, since it allows design freedom in the choice of the shaft. For example, the shaft can be made of a non-ferromagnetic metal or composites.  
           [0012]    The magnets may be permanent magnets or electromagnets.  
           [0013]    Further flux detectors positioned adjacent to the magnets can be provided to give an independent measurement of the strength of the magnets. The output of the further flux detectors can be combined with the output from the flux detectors positioned to measure torque so that variations in the strength of the magnets can be compensated for. This arrangement has the advantage over the prior art where the permanent circumferential magnetic field in either the shaft or its collar cannot be quantified during measurement. The prior art sensors require a known torque to be applied to them in order to check the calibration, which is impractical or difficult to achieve in some situations, e.g. for a steering column in an automobile.  
           [0014]    The invention may find a wide range of applications. For example, a torque sensor embodying the invention may be incorporated in an automobile steering column or an automotive gearbox. In the case of a gearbox, the torque sensor may be arranged to measure torquing of the gearbox main shaft and arranged inside the main gearbox casing, for example.  
           [0015]    According to a second aspect of the invention there is provided a method of sensing torque comprising:  
           [0016]    (a) providing a shaft comprising magnetostrictive material;  
           [0017]    (b) applying an external magnetic field to the shaft using a pair of opposite magnet poles positioned circumferentially around the shaft so as to induce a localized magnetic field in the magnetostrictive material between the opposite magnet poles;  
           [0018]    (c) torquing the shaft so that a component of the internal magnetic field escapes from the magnetostrictive material; and  
           [0019]    (d) detecting the escaped component of the internal magnetic field and providing a torque signal responsive thereto.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:  
         [0021]    [0021]FIG. 1 is a schematic view of a magnetostrictive torque sensor according to a first embodiment of the invention;  
         [0022]    [0022]FIG. 2 is a cross-sectional view showing the magnetostrictive torque sensor of the first embodiment;  
         [0023]    [0023]FIG. 3 is a cross-sectional view of a magnetostrictive torque sensor according to a second embodiment of the invention;  
         [0024]    [0024]FIG. 4 shows the interconnections between the two flux detectors of FIG. 3  
         [0025]    [0025]FIG. 5 is schematic drawing of a magnetostrictive torque sensor according to a third embodiment of the invention;  
         [0026]    [0026]FIG. 6 is schematic drawing of a magnetostrictive torque sensor according to a fourth embodiment of the invention;  
         [0027]    [0027]FIG. 7 is a schematic drawing in cross-section of a magnetostrictive torque sensor according to a fifth embodiment of the invention;  
         [0028]    [0028]FIG. 8 shows the interconnections between four flux detectors of FIG. 7;  
         [0029]    [0029]FIG. 9 is a schematic cross-section of a magnetostrictive torque sensor according to a sixth embodiment of the invention;  
         [0030]    [0030]FIG. 10 is a schematic diagram of a feedback system employed for controlling the magnetic field in the sixth embodiment;  
         [0031]    [0031]FIG. 11 is a section of an automotive gearbox with a torque sensor embodying the invention;  
         [0032]    [0032]FIG. 12 is a schematic view of a prior art magnetostrictive torque sensor according to U.S. Pat. No. 5,351,555; and  
         [0033]    [0033]FIG. 13 is a schematic view of a prior art magnetostrictive torque sensor according to WO 99/21 150.  
     
    
     DETAILED DESCRIPTION  
       [0034]    [0034]FIG. 1 is a schematic diagram of a magnetostrictive sensor  8  according to a first embodiment of the invention. The magnetostrictive sensor  8  comprises a shaft  16  that can be subjected to a torque  38  to be sensed, as illustrated by an arrow in the figure. The shaft  16  is made of a magnetostrictive material, but is not itself permanently magnetized (in contrast to the prior art designs referred to in the introduction). Instead of permanently magnetizing the shaft, the shaft  16  is magnetized in situ by an external magnetic field generated by a pair of permanent magnets  10  and  12 .  
         [0035]    The shaft  16  is made of the magnetostrictive material “Terfenol-D” which has a very high magnetostrictive coefficient λ=ΔL/L of the order of 2000×10 −6 . Generally, materials with magnetostrictive coefficients of at least 20×10 −6  are preferred for the shaft, although a shaft made of material with a lower magnetostrictive coefficient will still function.  
         [0036]    Some examples of other suitable materials for the shaft are:  
         [0037]    1. maraging Ni—Fe alloys;  
         [0038]    2. 17-4 PH, 17-7 PH and 15-5 PH Stainless steels;  
         [0039]    3. tool steel;  
         [0040]    4. Ni—Fe with a Ni content between 14-28%;  
         [0041]    5. Ni—Fe with a Ni content between 42-65%;  
         [0042]    6. Ni—Fe Alloy 718 (AMS 5663E);  
         [0043]    7. Alloy Supermet 625;  
         [0044]    8. Permenorm 5000 H2;  
         [0045]    9. Dimag 1, 2 and Dimag X;  
         [0046]    10. Fe/B/Si alloys.  
         [0047]    The permanent magnets  10  and  12  are arranged so that a pair of opposite magnet poles  11  and  13  (i.e. North and South) are positioned circumferentially around the shaft and facing the shaft so that a localized magnetic field is induced in the magnetostrictive material of the shaft between the opposite magnet poles  11  and  13 . In the figure, the permanent magnets  10  and  12  are arranged diametrically opposite to each other about the magnetostrictive shaft  16  with one of their end faces  11  and  13  lying tangential to the surface of the magnetostrictive shaft  16 .  
         [0048]    In an alternative construction (not shown), a single permanent magnet could be used in place of a pair of magnets with the individual poles of the single permanent magnet being arranged in the same positions as the poles  11  and  13  of the two magnets of the first embodiment. For example, a horseshoe-like shape of the magnet would allow for such a construction.  
         [0049]    A magnetic flux detector  18  is positioned circumferentially between the permanent magnets  10  and  12  so as to detect a component of the localized magnetic field which escapes from the magnetostrictive material when the shaft is subjected to the torque  38 . The preferred flux detector  18  is a saturated coil detector. However, a variety of other flux detectors could be used, e.g. Hall detector, magnetoresistance, magnetotransistor or MAGFET (Magnetic Field Effect Transistor) sensors.  
         [0050]    [0050]FIG. 2 is a cross-sectional view through the magnetostrictive sensor  8  of the first embodiment. The magnetic flux paths within the shaft  16  are indicated with lines  22 . The flux paths lead from the North pole  11  of the magnet  10  across an air gap to the surface of the shaft  16  and then extend within the shaft  16  generally in the North-to-South direction before crossing a further air gap between the surface of the shaft  16  and the South pole  13  of the magnet  12 . The magnetic field inside the shaft  16  brings about an alignment of the magnetostrictive domains in the local direction of the magnetic field.  
         [0051]    The permanent magnets  10  and  12  are connected by a magnetic loop member  20  having a half circle shape in section. The loop member  20  is joined to the distal ends of the permanent magnets  10  and  12  so that there is a high degree of closure to the magnetic field  22  thereby increasing the magnetic field permeating the magnetostrictive shaft  16  and reducing stray fields.  
         [0052]    In the present embodiment, the permanent magnets  10  and  12  are made of sintered NiFeBr alloy and produce a magnetic flux density of approximately 350 mT. However, lower (or higher) strength magnets could also be used. An appropriate value of the magnetic flux density produced by the permanent magnets will depend on the composition and dimensions of the magnetostrictive shaft and the range of torques  38  that will be applied to the magnetostrictive shaft.  
         [0053]    In use, the magnetostrictive shaft  16  produces a magnetic field in response to a torque  38  applied to the magnetostrictive shaft  16  that forces realignment of the magnetostrictive domains in the shaft. The magnetic field generated by the torque  38  perturbs the magnetic field produced by the permanent magnets  10  and  12 . The flux detector  18  detects this perturbation. The output signal from the flux detector  18  is generally indicative of the magnitude of the torque  38 . To the extent that the response is not perfectly linear, this can be compensated for by calibration in combination with appropriate processing of the output signal. The shaft  16  may be a steering column of an automobile or other vehicle, for example, and the outputs from the flux detector  18  used for feedback into an electronic power assisted steering control system.  
         [0054]    [0054]FIG. 3 shows a torque sensor according to a second embodiment of the invention. As in the first embodiment, the shaft  16  is made of magnetostrictive material and is magnetized in situ by an external magnetic field generated by a pair of permanent magnets  10  and  12  which penetrate into the shaft  16 . The permanent magnets  10  and  12  are arranged so that a pair of opposite magnet poles  11  and  13  (i.e. North and South) are positioned circumferentially around the shaft facing the shaft. The opposite magnet poles induce a localized magnetic field in the magnetostrictive material of the shaft that passes between the opposite magnet poles  11  and  13 . A pair of magnetic flux detector  18  is positioned radially outside the shaft circumferentially between the permanent magnets  10  and  12  so as to detect a component of the localized magnetic field which escapes from the magnetostrictive material when the shaft is subjected to a torque  38 . It is noted that, with a hollow shaft construction, it would also be possible to arrange the flux detector(s)  18  inside the hollow part of the shaft. A ring  30  of magnetic material is also provided to connect the distal ends of the magnets  10  and  12 , and to hold both the magnets  10  and  12 , and the flux detectors  18 .  
         [0055]    The torque sensor  8  of the second embodiment is principally distinguished from that of the first embodiment in that the shaft  16  is hollow, rather than solid. The internal space within the hollow shaft serves to exclude magnetic flux, thereby concentrating the induced magnetic field radially outward of the torque axis, where the strain induced by the torque  38  will be higher and closer to the flux detectors  18 . A hollow shaft is also more uniformly stressed by torsion than a solid shaft.  
         [0056]    [0056]FIG. 4 shows the electrical interconnections of the two flux detectors  18  of the second embodiment. As illustrated, the flux detectors  18  are wired in series. Moreover, the connections are such that the current induced by the magnetic flux in the two flux detectors sums together. This is achieved when terminals of like polarity are arranged circumferentially adjacent to each other as viewed around the outer surface of the shaft  16 . This connection scheme is geometrically opposite to what would be done with the prior art, in which terminals of opposite polarity in circumferentially adjacent detectors would be connected together. This is because, in the prior art, the magnetic flux flows unidirectionally around the circumference of the shaft, for example clockwise, as a result of the unidirectional permanent magnetic poling. By contrast, in the present case, there are two circumferential field components one flowing clockwise and the other anti-clockwise around the shaft, as a result of the magnetic field being externally induced by the magnet poles.  
         [0057]    [0057]FIG. 5 shows a torque sensor according to a third embodiment of the invention. The arrangement is generally similar to that of the first embodiment in respect of the permanent magnets  10  and  12 , the flux detector  18  and the shaft  16 . However, in the third embodiment, the design of the shaft  16  is different. Instead of a simple shaft made of magnetostrictive material, the main body  40  of the shaft  16  is made of a material that is not magnetostrictive. The main body  40  is illustrated as being solid, but may be hollow or of any other internal construction. The main body  40  of the shaft has arranged on its outer surface adjacent to the permanent magnets  10  and  12  a layer of magnetostrictive material  36 . This construction produces a similar concentration of the induced magnetic field as in the hollow shaft arrangement described above. Namely, the induced magnetic field is concentrated in the outer part of the shaft where the torque  38  is highest and proximal to the flux detectors.  
         [0058]    In one example, the magnetostrictive layer  36  has a thickness of 1.5 mm and is deposited by a conventional thermal spraying process. Wire flame spraying is suitable. This process involves sputtering a pure metal or alloy wire with oxygen and fuel. Layers of up to 5 mm in thickness can be deposited without difficulty using thermal spraying. If thinner layers are preferred, conventional electroplating may be used. Layers of up to about 0.1 mm in thickness can be conveniently provided by electroplating.  
         [0059]    [0059]FIG. 6 shows a torque sensor according to a fourth embodiment of the invention. The arrangement is generally similar to that of the first embodiment in respect of the permanent magnets  10  and  12 , the flux detector  18  and the shaft  16 . However, in the fourth embodiment, a further alternative shaft design is employed. The shaft  16  comprises a main body  32  of magnetostrictive material, such as maraging NiFe alloy. The outer surface of the main body  32 , in the active region of the shaft, has arranged thereon a layer  34  of low permeability material. In the present example, the low permeability layer is made of aluminum of typical thickness 0.5 to 1 mm. The low permeability layer  34  may be thermally sprayed or electroplated. The low permeability layer  34  has arranged thereon a layer  36  of magnetostrictive material. In the present example, the magnetostrictive layer  36  is made of Ni—Fe alloy, typically of thickness between 0.5 to 5 mm. The magnetostrictive layer  36  may be deposited by thermal spraying or electroplating. The purpose of the low permeability layer  34  is to resist penetration of the externally induced magnetic field into the magnetostrictive main body  32  of the shaft. In other words, the layer  34  isolates the active outer magnetostrictive layer  36  from the main body of the shaft, which would otherwise strongly influence the induced magnetic field pattern, owing to its magnetostrictive properties. It is therefore possible to select a material for the main body of the shaft which is magnetostrictive, without significantly altering the performance of the sensor. It will be appreciated that the main body of the shaft need not be solid, but may be hollow or of any other internal structure.  
         [0060]    [0060]FIG. 7 shows in cross-section a torque sensor according to a fifth embodiment of the invention. The torque sensor  8  comprises four permanent magnets  10 ,  12 ,  10 ′ and  12 ′ arranged radially within and held in place by a ring  30  which is made of a magnetic material to give closure to the magnetic field  22  generated by the permanent magnets. The magnets are arranged so that circumferentially adjacent magnets have opposite poles  11 ,  13 ,  11 ′,  13 ′ facing the shaft  16  in a North-South-North-South sequence. In addition, the magnets are arranged at equal angular intervals of 90°, although this angular spacing is not critical and could be varied. With this arrangement, the magnetic flux penetrates the shaft  16  predominantly in its radially outer regions, thus achieving similar advantages to the above-described embodiments where flux is excluded from the central region either by use of a hollow shaft or provision of a flux excluding non-magnetic layer. The present embodiment has the additional advantage that the desired concentration of magnetic flux in the radially outer parts of the shaft is achievable with a simple solid shaft.  
         [0061]    The torque sensor is provided with four flux detectors  18  arranged circumferentially between the permanent magnets  10 ,  12 ,  10 ′ and  12 ′ for measuring the magnetic flux component attributable to torque in the shaft  16 . The flux detectors  18  are conveniently held in place by the ring  30 . By increasing the number of flux detectors  18 , variations in the signal caused by changes in the distance between the outer surface of the shaft and the flux detectors, e.g. owing to eccentricity in the shaft or its mounting, can be compensated for for smaller rotations of the shaft. This is advantageous for applications in which the shaft is not spinning rapidly, but is only being torqued as a result of small angular rotations of less than one or more full rotations. One example of such an application is the steering column of an automobile where the full lock-to-lock rotation may only be about one full rotation.  
         [0062]    A further feature of the fifth embodiment is the provision of an additional flux detector  19  arranged adjacent to one of the permanent magnets  12 ′ in the air gap between the magnet pole  13 ′ facing the shaft and the shaft. To avoid confusion with the flux detectors  18 , the flux detector  19  is referred to as the magnet-monitoring flux detector in the following, and the flux detectors  18  as torque-sensing flux detectors. The magnet-monitoring flux detector  19  serves to provide an independent measure of the excitation energy that is supplied to the shaft  16  by the permanent magnet  12 ′. A perturbation in the strength of the magnetic field  22  supplied by the permanent magnets, caused for example by temperature fluctuations or aging of the permanent magnets, can thus be factored out of the output signal of the torque-sensing flux detectors  18 . It will be appreciated that the magnet-monitoring flux detector  19  could be positioned adjacent to any of the permanent magnets. Further, it will be appreciated that multiple magnet-monitoring flux detectors may be provided, e.g. one for each active magnet pole.  
         [0063]    [0063]FIG. 8 shows the electrical interconnections of the four flux detectors  18  of the fifth embodiment. As illustrated, the flux detectors  18  are wired in series. Moreover, the connections are such that the current induced by the magnetic flux in the four flux detectors sums together. This is achieved when terminals of like polarity are arranged circumferentially adjacent to each other as viewed around the outer surface of the shaft  16 . This connection scheme is geometrically opposite to what would be done with the prior art, as described further above in relation to the second embodiment.  
         [0064]    [0064]FIG. 9 shows in cross-section a torque sensor according to a sixth embodiment of the invention. This embodiment is analogous to the first embodiment, but uses electromagnets  60  and  62  in place of permanent magnets for inducing the magnetic field in the shaft  16 . A field-monitoring flux detector  19  is provided in addition to a torque-sensing flux detector  18 . The field-monitoring flux detector  19  can be used to control the magnetic field generated by the electromagnets, as illustrated in FIG. 1  0 , as well as or instead of for passively compensating the signals generated by the torque-sensing flux detector  18 .  
         [0065]    [0065]FIG. 10 is a block schematic drawing of the torque sensor of the sixth embodiment with associated electromagnet control system. Variation in the strength of the magnetic field  22  produced by the electromagnet  60  is detected by the magnet-monitoring flux detector  19 . The magnet-monitoring flux detectors  19  feeds an output signal to a control circuit  42 , the output signal being indicative of the magnitude of the instantaneous field of the electromagnet  60 . The control circuit  42  then controls the power supplied to the coils of the electromagnets  60  and  62  via control of the electromagnets&#39; power supply  44 . Accordingly, the power supplied to the electromagnets can be stabilized in a feedback loop.  
         [0066]    It will be understood that in any of the above embodiments, the number of flux detectors  18  may be varied as described in relation to the other embodiments to provide further variants.  
         [0067]    It will also be understood that in any of the above embodiments, one or more magnet-monitoring flux detectors may be provided, e.g. one only, or one for each active magnet pole.  
         [0068]    [0068]FIG. 11 is a section of an automotive gearbox with a torque sensor  8  embodying the invention arranged therein. The torque sensor  8  is arranged around the main shaft  80 . The clutch  82  is also evident.  
         [0069]    It will be appreciated that although particular embodiments of the invention have been described, many modifications/additions and/or substitutions may be made within the spirit and scope of the present invention.