Eddy current sensor assembly for shaft torque measurement

Technical Field

0001In the past it has been known that the magnetic permeability of ferromagnetic materials changes when stress is applied to the material. Such a property of a material is referred to as magnetostriction. In addition, induced stress on a conductive material alters its electrical conductivity. Taking advantage of these two properties, many devices have been built to sense changes in the electrical conductivity and magnetic permeability of a ferromagnetic shaft in order to determine the torque that the shaft is under. However, many of these prior inventions either require a device that completely surrounds the shaft rather than a device that is placed adjacent to the shaft at a single point, or that a device is placed directly over the ferromagnetic shaft at a single point. The difficulty of the prior art sensors that completely surround the shaft is that they are bulky and difficult to install and remove. The difficulty of the prior art sensors that are placed over the shaft at a single point is that they work only on ferromagnetic shafts and that they lack sensitivity and reproducibility as applied to different shafts. The present invention overcomes these problems by first providing a press-fitted magnetoelastic ring over the shaft whose torque is measured and then providing a sensor over the magnetoelastic ring at a single point along its circumference that detects changes of the ring's electrical conductivity and magnetic permeability due to the stresses induced by the shaft.

Background Art

0002<patcit id="pcit0001" dnum="DE3437379"><text>DE 3437379</text></patcit> discloses a sensing device for determining the change in permeability of a highly magnetostrictive surface on a shaft. The device utilises probes which operate in diametrically opposed pairs.

0003According to the present invention there is provided a device for sensing a torque applied through a shaft, the device comprising: <ul id="ul0001" list-style="none" compact="compact"><li>a magnetoelastic ring press-fitted on the shaft whose torque is measured;</li><li>a sensor placed near the magnetoelastic ring at a single point on its circumference, the sensor including</li><li>an excitation coil driven by an oscillator for inducing eddy currents inside the material of the magnetoelastic ring; and</li><li>a sensing coil for sensing the magnetic field generated by the induced eddy currents inside the magnetoelastic ring;</li><li>an electrical circuit configured to receive signals from said sensing coil to detect changes in the magnetoelastic ring's electrical conductivity and magnetic permeability responsive to applied torque forces in the shaft.</li></ul>

0004Preferred features of the invention are set out in the attached subclaims.

Brief Description of the Drawings

0005<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a schematic showing the generation of eddy currents inside a conductive material;</li><li><figref idref="f0002">Figure 2</figref> is an electrical schematic of an absolute eddy current sensor which can be used in one embodiment of the present invention;</li><li><figref idref="f0003">Figure 3</figref> is an electrical schematic of a differential eddy current sensor which can be used in another embodiment of the present invention;</li><li><figref idref="f0002">Figure 4</figref> is a side view of a shaft under torque indicating lines of compression and tension within the shaft;</li><li><figref idref="f0004">Figure 5</figref> is a side view of a shaft with eddy current sensors which can be used in one embodiment of the present invention;</li><li><figref idref="f0003">Figure 6</figref> is a side view of a shaft with eddy current sensors which can be used in another embodiment of the present invention;</li><li><figref idref="f0003">Figure 7</figref> is a side view of a shaft with eddy current sensors which can be used in another embodiment of the present invention;</li><li><figref idref="f0004">Figure 8</figref> is a side view of a shaft with an eddy current sensor which can be used in another embodiment of the present invention;</li><li><figref idref="f0004">Figure 9</figref> is a side view of a shaft with an eddy current sensor which can be used in another embodiment of the present invention;</li><li><figref idref="f0005">Figure 10</figref> is a side view of a shaft with eddy current sensors which can be used in another embodiment of the present invention;</li><li><figref idref="f0005">Figure 11</figref> is an end view of the shaft with eddy current sensors as shown in <figref idref="f0005">Fig. 10</figref>;</li><li><figref idref="f0005">Figure 12</figref> is a side view of a shaft with a differential eddy current sensor which can be used in another embodiment of the present invention;</li><li><figref idref="f0005">Figure 13</figref> is a side view of a shaft with a differential eddy current sensor which can be used in another embodiment of the present invention;</li><li><figref idref="f0006">Figure 14</figref> is a side view of a shaft with eddy current sensors which can be used in another embodiment of the present invention;</li><li><figref idref="f0006">Figure 15</figref> is an end view of the shaft with eddy current sensors as shown in <figref idref="f0006">Fig. 14</figref>;</li><li><figref idref="f0006">Figure 16</figref> is a side view of a shaft with eddy current sensors which can be used in another embodiment of the present invention;</li><li><figref idref="f0007">Figure 17</figref> is an end view of a shaft with an eddy current sensor according to the embodiment of <figref idref="f0007">Fig. 18</figref>;</li><li><figref idref="f0007">Figure 18</figref> is a side sectional view of a shaft with an eddy current sensor which can be used in another embodiment of the present invention;</li><li><figref idref="f0007">Figure 19</figref> is a side view of a device according to the present invention;</li><li><figref idref="f0008">Figure 20</figref> is a side view of a shaft with an eddy current sensor which can be used in another embodiment of the present invention; and</li><li><figref idref="f0008">Figure 21</figref> is an end view of the shaft with an eddy current sensor as shown in <figref idref="f0008">Fig. 20</figref>.</li></ul>

Best Mode for Carrying out the Invention

0006While the invention is susceptible of embodiment in many different forms, there is shown in the drawings and described in detail preferred embodiments of the invention. It is to be understood that the present disclosure is to be considered only as an example of the principles of the invention. This disclosure is not intended to limit the broad aspect of the invention to the illustrated embodiments. The scope of protection should only be limited by the accompanying claims.

0007Referring to <figref idref="f0001">Fig. 1</figref>, the electrical conductivity (σ) of a given material provides a measure of how easily electrical currents flow through the material. Its magnetic permeability (µ) provides a measure of how easily magnetic flux lines permeate through the material. These two parameters determine the way a given material interacts with electric and magnetic fields. When a piece of material 100 is exposed to an excitation coil 102 powered by an oscillator 104 magnetic flux (represented by magnetic flux lines 1 06) is created a round the coil 102 and a magnetic field 108 is applied to the material 100. The magnetic field 108 induces eddy currents 110 in the material which create an opposing magnetic field 112. Eddy currents (EC) are circulating currents in conductive materials that are produced as a result of time-varying magnetic flux passing through the material. Therefore, ECs are induced inside a conductive material that is near an electrical coil powered by an AC current. The EC generation is due to the interaction of the material's conductive electrons and the AC magnetic field that is generated around the coil by the current through it and depends on the material's electrical conductivity and magnetic permeability. Referring to <figref idref="f0003">Fig. 3</figref>, when a ferromagnetic material, such as a steel shaft, is under torque, the material's electrical conductivity and magnetic permeability are changed in relation to the amount of torque across the material. By sensing the change in electrical conductivity and magnetic permeability, one can determine the amount of torque being applied through the shaft.

0008Referring to <figref idref="f0002">Fig. 2</figref>, an absolute EC sensor 1 consists of an excitation coil 2 driven by an oscillator 4 powered by a power source 6 that, when energized, induces eddy currents inside any conductive material at close proximity to it. A sensing coil 8 senses the magnetic field generated by the induced electrical currents inside the conductive material. The current generated in the sensing coil 8 by the sensed magnetic field is supplied to a bridge balancing circuit 10, an amplifier 12, a demodulator 14 and a filter 16 to create a sensor output 17. The generated magnetic field is determined by the path and strength of the eddy current flow, which in turn depends on the material geometry and its σ and µ local values. Thus, absolute EC sensors 1 with a single excitation/sensing element pair have been successfully used for characterization of conductive materials.

0009Referring to <figref idref="f0003">Fig. 3</figref>, a differential EC sensor has two excitation/sensing element pairs and is used primarily for the detection of small-localized metallurgical or geometrical anomalies of conductive components. <figref idref="f0003">Fig. 3</figref> shows such a sensor that is similar to that of <figref idref="f0002">Fig. 2</figref> but adding a second excitation element 2' and a second sensing element 8'. The excitation elements 2 and 2' of a differential EC sensor generate magnetic fields that have approximately the same amplitude and phase. However, its sensing elements 8 and 8' generate signals of approximately equal amplitude but have opposite phase.

0010Referring to <figref idref="f0002">Fig. 4</figref>, torque transmitted through a shaft 18 generates a shear stress throughout the shaft 18 that varies from zero at its center to a maximum value at its surface. Lines of tension 20 and lines of compression 22 are formed along the shaft 18 as a result of the applied torque. These torque-induced tension and compression lines 20, 22 are normal to each other and are formed at ±45° from the axis of symmetry. A magnetoelastic ring 24 that is press-fitted onto the shaft 18 would also experience the generated shear stress, and lines of tension 20 and lines of compression 22 are formed in the ring 24 as well.

0011The electrical conductivity (σ) and the magnetic permeability (µ) values of the magnetoelastic ring 24 are altered by the generated shear stress with the values being different along the lines of tension from those along the lines of compression (i.e. σ_tension # σ_compression and µ_tension ≠ µ_compression). Thus, a differential EC sensor placed over the magnetoelastic ring 24 so that one of its excitation/sensing element pairs focuses along the lines of tension and the other excitation/sensing element pair focuses along the lines of compression (<figref idref="f0004">Fig. 5</figref>), would sense the local value difference of σ and µ along the magnetoelastic ring and would provide a measure of the applied shaft torque.

0012Alternatively, depending on the application constraints, two magnetoelastic rings 24 may be press-fitted onto the shaft 18. In this situation, two differentially hooked-up EC sensors, one focusing along the lines of tension of the first ring and the other along the lines of compression of the second ring, would provide similar test results (<figref idref="f0003">Fig. 6</figref>).

0013Referring to <figref idref="f0003">Fig. 7</figref>, in certain applications where high torque-sensitivity is required, knurled grooves 28 can be placed on the press-fitted magnetoelastic rings over the outside diameter surface in order to increase the EC sensor response. Under this configuration, the sensor excitation/sensing element pair that focuses on the lines of tension has knurl grooves that are placed parallel to the direction of lines of tension. In the same way, the magnetoelastic ring where the sensor excitation/sensing element pair focuses on the lines of compression has knurl grooves that are placed parallel to the direction of lines of compression.

0014The EC sensors of the present invention may be implemented in one of several ways. Referring to <figref idref="f0004">Fig. 8</figref>, the simplest embodiment involves attaching a single excitation/sensing element-pair EC sensor 30 along with its associated electronics to a fixed position adjacent the ring 24 that has been pressed onto the rotating shaft 18. In this configuration, loads or moments acting upon the torque-carrying shaft could affect the EC sensor output and thus, this type of assembly is recommended for shaft applications that carry only pure torque. Here, the shaft could have any type of shape, size, and alloy composition and have been made by any given manufacturing process. Referring to <figref idref="f0004">Fig. 9</figref>, in certain applications, hysteresis effects can be minimized with the use of a hardened metal coupling sleeve 31 press-fitted between the s haft OD surface and the magnetoelestic ring ID surface. While not described explicitly, any of the embodiments later described in the present application may also use a coupling sleeve 31 in order to minimize hysteresis effects. Additionally, below, each separate configuration is described with respect to single excitation/sensing element-pair sensors. However, one of ordinary skill in the art would recognize the single excitation/sensing element-pair sensors or sensors where the electronics and sensing coils are separate could be used interchangeably.

0015Referring to <figref idref="f0003">Fig. 6</figref>, a single element-pair sensor 30 is attached in a fixed position over each of two magnetoelastic rings 24, respectively, that are pressed onto shaft 18 such that one sensor 30 is focused upon the lines of compression and the other sensor 30 is focused upon the lines of tension. The two sensors are then differentially connected together in order to cancel out load and moment effects on the sensor assembly output. Referring to <figref idref="f0004">Fig. 5</figref>, the configuration of <figref idref="f0003">Fig. 6</figref> is modified such that the two rings 24 are replaced with a single ring 24 of greater width.

0016Referring to <figref idref="f0005">Figs. 10 and 11</figref>, single element-pair sensors 30 are placed in a fixed position about a rotating shaft 18. The sensors 30 are placed about the same circumferential line and angularly displaced about that line. One sensor 30 is focused upon the lines of compression and one sensor 30 is focused upon the lines of tension. The sensors 30 are attached in a differential sensor pair in order to cancel out load and moment effects on the sensor assembly output. Summation of the sensor pair signals constitutes the sensor assembly output.

0017Referring to <figref idref="f0005">Fig. 12</figref>, dual element-pair differential sensors 32 are used. A dual element-pair differential sensor 32 is a complete sensor that incorporates two sensing elements 8, 8', as described above, and their corresponding electronics. The dual element-pair sensor 32 is placed over two magnetoelastic rings 24 press-fitted onto a torque-carrying shaft 18 so that each sensing element 8 and 8' is axially aligned along the shaft 18 with the other and adjacent one of the rings 24. Alternately, the dual element-pair sensor 32 could be axially placed over a single magnetoelastic ring 24 press-fitted onto a torque-carrying shaft 18, as in <figref idref="f0005">Fig. 13</figref>. Here, for each dual element-pair sensor 32, one sensing element 8 is focused along the lines of tension and the other sensing element 8' along the lines of compression. In this configuration, the effect of loads or moments acting upon the torque-carrying shaft 18 have on the sensor output are cancelled out by the differential nature of each sensor.

0018Referring to <figref idref="f0006">Figs. 14 and 15</figref>, as in <figref idref="f0005">Figs. 12 and 13</figref>, two dual element-pair differential sensors 32 are placed over and near a magnetoelastic ring 24 press-fitted onto a torque-carrying shaft 18 so that the sensors 32 and their corresponding sensing elements 8, 8' are circumferentially located and angularly displaced about the shaft. Here, for each dual element-pair sensor 32, one sensing element 8 is focused along the lines of tension and the other sensing element 8' along the lines of compression. In this configuration, the effect that loads or moments acting upon the torque-carrying shaft have on the sensor output are cancelled out by the differential nature of each sensor. Summation of each sensor signal constitutes the sensor assembly output.

0019Referring to <figref idref="f0006">Fig. 16</figref>, multiple single element-pair EC sensors 30 and their corresponding electronics are placed over and near a magnetoelastic ring 24 press-fitted onto a torque-carrying shaft 18 so that some of the sensors 30 are axially located along the shaft 18 and some circumferentially located around the shaft. In both the axial and circumferential sensor sets, adjacent sensors 30 are focused alternatingly along lines of tension and lines of compression. Differential sensor pairs are formed among both the axially and circumferentially placed sensors 30 by selecting one sensor focusing on lines of tension and another on lines of compression in order to cancel out load and moment effects on the sensor assembly output. Summation of all sensor signals constitutes the sensor assembly output. It can be understood by one of ordinary skill in the art that a pair of single element-pair sensors 30 may be replaced a single dual element-pair sensor 32.

0020Referring to <figref idref="f0003 f0004 f0005 f0006">Figs. 5-16</figref>, each configuration contemplates the sensor being held to a fixed point near the rotating shaft. While in many situations this is acceptable, this configuration allows the distance between the shaft and the sensor to vary due to an out-of-round condition of the shaft or misalignment or wear of the b earing t hat support the s haft. A s a result, the sensor excitation field strength at the magnetoelastic ring surface will tend to fluctuate with the varying distance, which would cause the sensor output to fluctuate also. In order to hold the sensor at a fixed distance with respect to the magnetoelastic ring, in <figref idref="f0007">Figs. 17-19</figref>, there is show an arrangement where the sensor 30 is held to a sensor support block 38. The sensor support block 38 is attached to an assembly ring 39 and each is kept from rotating by a flange 44 (<figref idref="f0007">Fig. 19</figref>) held to a reference point (not shown) and mounted to the rotating shaft by two bearings 40, 42. As such the distance between the sensor 30 and the rotating magnetoelastic ring 24 is kept substantially constant. As can be understood by one of ordinary skill in the art any of the above sensor configurations of <figref idref="f0004 f0003 f0005 f0006">Figs. 5-16</figref> (incorporating single and dual element sensors) can be implemented by mounting them in the arrangement of <figref idref="f0007">Figs. 17-19</figref>.

0021Referring to <figref idref="f0008">Figs. 20 and 21</figref>, the arrangement of <figref idref="f0007">Figs. 17-19</figref> is modified such that the sensor 30 mounted upon the bearings 40, 42 may be installed as a single unit. This is accomplished by providing a sleeve 46 onto which magnetoelastic sleeve 24 and bearings 40, 42 have been pressed. The unit is then pressed onto the shaft 18.