Abstract:
An apparatus is provided to reduce strain transferred from a body, such as a shaft, to a strain sensor, such as a SAW device. The strain sensor is capable of measuring a maximum amount of strain. A strain absorber has the strain sensor mounted thereon and is arranged to mount the strain sensor to the body. The strain absorber is arranged to transfer a reduced amount of strain from the body to the strain sensor so that the strain on the strain sensor is no greater that the maximum amount of strain.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates to reducing the strain level in a system such as in a torque sensing system. 
       BACKGROUND 
       [0002]    Torque is measured for a variety of applications. For example, it is known to control a vehicle engine in response to the torque measured on the transmission shaft of the vehicle. In at least some instances, the torque on a shaft or other mechanism is measured by measuring the strain on the surface of the shaft and then converting this measured strain to torque. Frequently, strain on a shaft is measured by a surface acoustic wave (SAW) device that is mounted on the surface of the shaft. The maximum measurable strain that can be measured by a typical SAW device is 500 microstrain. However, in many cases, the strain on a shaft is much higher than 500 microstrain. For example, the strain on a shaft can be as high as 2500 microstrain. The typical SAW device is unsuitable for measuring strain in these higher strain applications. 
         [0003]    The present invention is directed to an arrangement that solves this or one or more other problems. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with one aspect of the present invention, an apparatus for reducing strain transferred from a body to a strain sensor comprises a strain sensor and a strain absorber. The strain sensor is capable of measuring a maximum amount of strain. The strain absorber has the strain sensor mounted thereon and is arranged to mount the strain sensor to the body. The strain absorber is arranged to transfer a reduced amount of strain from the body to the strain sensor so that the strain on the strain sensor is no greater that the maximum amount of strain. 
         [0005]    In accordance with another aspect of the present invention, an apparatus for reducing strain between a shaft and a SAW device comprises a SAW device, a shaft, and a strain absorber. The SAW device is capable of measuring a maximum amount of strain. The shaft is capable of exhibiting strain in excess of the maximum amount. The strain absorber is mounted to the shaft and has the SAW device mounted thereon. The strain absorber is arranged to transfer a reduced amount of strain from the shaft to the SAW device so that the strain on the SAW device is no greater that the maximum amount of strain. 
         [0006]    In accordance with yet another aspect of the present invention, a torque measuring system comprises a SAW device, a strain absorber, and a strain-to-torque converter. The SAW device is capable of measuring a maximum amount of strain, and the SAW device provides a strain measuring signal. The strain absorber is arranged to mount the SAW device to a body whose strain is to be measured, the strain absorber has the SAW device mounted thereon, and the strain absorber is arranged to transfer a reduced amount of strain to the SAW device so that the strain on the SAW device is no greater that the maximum amount of strain. The strain-to-torque converter is arranged to convert the strain measuring signal to a torque measuring signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other features and advantages will become more apparent from the detailed description when taken in conjunction with the drawings in which: 
           [0008]      FIG. 1  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to one embodiment of the present invention; 
           [0009]      FIG. 2  is a top view of the embodiment shown in  FIG. 1 ; 
           [0010]      FIG. 3  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to another embodiment of the present invention; 
           [0011]      FIG. 4  is a top view of the embodiment shown in  FIG. 3 ; 
           [0012]      FIG. 5  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to yet embodiment of the present invention; 
           [0013]      FIG. 6  is a side view of the embodiment shown in  FIG. 5 ; 
           [0014]      FIG. 7  a top view of the embodiment shown in  FIG. 5 ; 
           [0015]      FIG. 8  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to still embodiment of the present invention; 
           [0016]      FIG. 9  is a top view of the embodiment shown in  FIG. 8 ; 
           [0017]      FIG. 10  is a side view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to a further embodiment of the present invention; 
           [0018]      FIG. 11  a top view of the embodiment shown in  FIG. 10 ; 
           [0019]      FIG. 12  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to a yet further embodiment of the present invention; 
           [0020]      FIG. 13  a top view of the embodiment shown in  FIG. 12 ; 
           [0021]      FIG. 14  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to a still further embodiment of the present invention; 
           [0022]      FIG. 15  a top view of the embodiment shown in  FIG. 14 ; 
           [0023]      FIG. 16  is a front view of an arrangement including a shaft having a strain sensor mounted thereto by a strain absorber according to another embodiment of the present invention; 
           [0024]      FIG. 17  a top view of the embodiment shown in  FIG. 16 ; 
           [0025]      FIG. 18  illustrates a shaft having sensors mounted to a shaft and diametrically opposite to one another; and, 
           [0026]      FIG. 19  illustrates a first embodiment of a torque sensing system; and, 
           [0027]      FIG. 20  illustrates a second embodiment of a torque sensing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    As shown in  FIGS. 1 and 2 , a strain sensing system  10  includes a shaft  12 , a strain sensor  14 , and a strain absorber  16  that mounts the strain sensor  14  to the shaft  12 . The shaft  12  may be any mechanism that exhibits strain measurable by the strain sensor  14 . 
         [0029]    The strain sensor  14  includes a housing  18 , a SAW device  20 , and an terminal  22 . However, the strain sensor  14  may include other components. For example, the strain sensor  14  may include components that communicate the measurement of the SAW device  20  from the rotating platform formed by the shaft  12 , the strain sensor  14 , and the strain absorber  16  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  14 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  20  by way of an antenna connected to the terminal  22 . The SAW device  20  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  18  and the strain absorber  16  may be integrally formed. 
         [0030]    The strain absorber  16  is a circular hollow structure having a cylindrical side wall  24  that forms a cavity therein and that supports an integrally formed circular cover  26  at one end thereof. The strain sensor  14  is attached to the upper surface of the circular cover  26  by a bonding agent or other suitable attachment apparatus. The opposing end of the cylindrical side wall  24  conforms to the shaft  12  and is attached to the shaft  12  by welds, a bonding agent, or other suitable attachment apparatus. 
         [0031]    As shown in  FIGS. 3 and 4 , a strain sensing system  30  includes a shaft  32 , a strain sensor  34 , and a strain absorber  36  that mounts the strain sensor  34  to the shaft  32 . The shaft  32  may be any mechanism that exhibits strain measurable by the strain sensor  34 . 
         [0032]    The strain sensor  34  includes a housing  38 , a SAW device  40 , and an terminal  42 . The strain sensor  34  may include other components. For example, the strain sensor  34  may include components that communicate the measurement of the SAW device  40  from the rotating platform formed by the shaft  32 , the strain sensor  34 , and the strain absorber  36  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  34 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  40  by way of an antenna connected to the terminal  42 . The SAW device  40  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  38  and the strain absorber  36  may be integrally formed. 
         [0033]    The strain absorber  36  is a quadrilateral hollow structure having a side wall  44  of quadrilateral cross-section that forms a cavity therein and that supports an integrally formed quadrilateral cover  46  at one end thereof. The strain sensor  34  is attached to the upper surface of the quadrilateral cover  46  by a bonding agent or other suitable attachment apparatus. The opposing end of the side wall  44  conforms to the shaft  32  and is attached to the shaft  32  by welds, a bonding agent, or other suitable attachment apparatus. 
         [0034]    As shown in  FIGS. 5 ,  6 , and  7 , a strain sensing system  50  includes a shaft  52 , a strain sensor  54 , and a strain absorber  56  that mounts the strain sensor  54  to the shaft  52 . The shaft  52  may be any mechanism that exhibits strain measurable by the strain sensor  34 . 
         [0035]    The strain sensor  54  includes a housing  58 , a SAW device  60 , and an terminal  62 . The strain sensor  54  may include other components. For example, the strain sensor  54  may include components that communicate the measurement of the SAW device  60  from the rotating platform formed by the shaft  52 , the strain sensor  54 , and the strain absorber  56  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  54 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  60  by way an antenna connected to of the terminal  62 . The SAW device  60  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  58  and the strain absorber  56  may be integrally formed. 
         [0036]    The strain absorber  56  is an elongated solid structure having rounded ends  64  and  66 , a top surface  68 , and a bottom surface  70  that conforms to the shaft  52 . The strain sensor  54  is attached to the top surface  68  of the strain absorber  56  by a bonding agent or other suitable attachment apparatus. The bottom surface  70  of the strain absorber  56  is attached to the shaft  52  such as by welds  72  and  74  at the ends  64  and  66 , or by a bonding agent or other suitable attachment apparatus. As shown in  FIG. 6 , a recess  76  is formed in the bottom surface  70  of the strain absorber  56 . 
         [0037]    As shown in  FIGS. 8 and 9 , a strain sensing system  80  includes a shaft  82 , a strain sensor  84 , and a strain absorber  86  that mounts the strain sensor  84  to the shaft  82 . The shaft  82  may be any mechanism that exhibits strain measurable by the strain sensor  84 . 
         [0038]    The strain sensor  84  includes a housing  88 , a SAW device  90 , and an terminal  92 . The strain sensor  84  may include other components. For example, the strain sensor  84  may include components that communicate the measurement of the SAW device  90  from the rotating platform formed by the shaft  82 , the strain sensor  84 , and the strain absorber  86  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  84 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  90  by way of an antenna connected to the terminal  92 . The SAW device  90  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  88  and the strain absorber  86  may be integrally formed. 
         [0039]    The strain absorber  86  is a solid cylindrical button having a top surface  94  and a bottom surface  96 . The bottom surface  96  conforms to the shaft  82 . The strain sensor  84  is attached to the top surface  94  of the strain absorber  86  by a bonding agent or other suitable attachment apparatus. The bottom surface  96  of the strain absorber  86  is attached to the shaft  82  such as by welds, by a bonding agent, or by other suitable attachment apparatus. 
         [0040]    As shown in  FIGS. 10 and 11 , a strain sensing system  100  includes a shaft  102 , a strain sensor  104 , and a strain absorber  106  that mounts the strain sensor  104  to the shaft  102 . The shaft  102  may be any mechanism that exhibits strain measurable by the strain sensor  104 . 
         [0041]    The strain sensor  104  includes a housing  108 , a SAW device  110 , and an terminal  112 . The strain sensor  104  may include other components. For example, the strain sensor  104  may include components that communicate the measurement of the SAW device  110  from the rotating platform formed by the shaft  102 , the strain sensor  104 , and the strain absorber  106  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  104 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  110  by way of an antenna connected to the terminal  112 . The SAW device  110  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  108  and the strain absorber  106  may be integrally formed. 
         [0042]    The strain absorber  106  is a solid member having a top surface  114  and a bottom surface  116  that stands off from and conforms to the shaft  102 . The strain sensor  104  is attached to the top surface  114  of the strain absorber  106  by a bonding agent or other suitable attachment apparatus. 
         [0043]    The strain absorber  104  has a first hole  118  at one end and a second hole  120  at the other end. The first hole  118  receives a stud  122  having a threaded portion  124  and a flared portion  126 . The second hole  120  receives a stud  128  having a threaded portion  130  and a flared portion  132 . A first nut  134  is threaded onto the threaded portion  124  of the stud  122  and a second nut  136  is threaded onto the threaded portion  130  of the stud  128  to fasten the strain absorber  104  to the shaft  102 . The flared portions  126  and  132  are flared by a selected amount to stand off the strain absorber  104  from the shaft  102  by a desired amount. The studs  122  and  128  conduct strain from the shaft  102  through the strain absorber  106  to the strain sensor  104 . 
         [0044]    As shown in  FIGS. 12 and 13 , a strain sensing system  140  includes a shaft  142 , a strain sensor  144 , and a strain absorber  146  that mounts the strain sensor  144  to the shaft  142 . The shaft  142  may be any mechanism that exhibits strain measurable by the strain sensor  144 . 
         [0045]    The strain sensor  144  includes a housing  148 , a SAW device  150 , and an terminal  152 . The strain sensor  144  may include other components. For example, the strain sensor  144  may include components that communicate the measurement of the SAW device  150  from the rotating platform formed by the shaft  142 , the strain sensor  144 , and the strain absorber  146  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  144 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  150  by way of an antenna connected to the terminal  152 . The SAW device  150  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  148  and the strain absorber  146  may be integrally formed. 
         [0046]    The strain absorber  146  is in the form of either a solid or hollow ring that encircles and conforms to the shaft  142 . The strain sensor  144  is attached to an outer surface  154  of the strain absorber  146  by a bonding agent or other suitable attachment apparatus. For example, the strain sensor  144  may rest on a flattened area of the outer surface  154  of the strain absorber  146 . An inner surface  156  of the strain absorber  146  is attached to the shaft  142  such as by welds, by a bonding agent, or by other suitable attachment apparatus. 
         [0047]    As shown in  FIGS. 14 and 15 , a strain sensing system  160  includes a shaft  162 , a strain sensor  164 , and a strain absorber  166  that mounts the strain sensor  164  to the shaft  162 . The shaft  162  may be any mechanism that exhibits strain measurable by the strain sensor  164 . 
         [0048]    The strain sensor  164  includes a housing  168 , a SAW device  170 , and an terminal  172 . The strain sensor  164  may include other components. For example, the strain sensor  164  may include components that communicate the measurement of the SAW device  170  from the rotating platform formed by the shaft  162 , the strain sensor  164 , and the strain absorber  166  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  164 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  170  by way an antenna connected to of the terminal  172 . The SAW device  170  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  168  and the strain absorber  166  may be integrally formed. 
         [0049]    The strain absorber  166  is in the form of either a solid or hollow ring that encircles and conforms to the shaft  162 . The strain sensor  164  is attached to an outer surface  174  of the strain absorber  166  by a bonding agent or other suitable attachment apparatus. For example, the strain sensor  164  may rest on a flattened area of the outer surface  174  of the strain absorber  166 . An inner surface  176  of the strain absorber  166  is attached to the shaft  162  such as by welds, by a bonding agent, or by other suitable attachment apparatus. 
         [0050]    In addition, the inner surface  176  of the strain absorber  166  may have opposing flats  178  and  180  that cooperated with opposing flats  182  and  184  of the shaft  162 . The flats  178 ,  180 ,  182 , and  184  provide for polarized mating between the strain absorber  166  and the shaft  162 . 
         [0051]    As shown in  FIGS. 16 and 17 , a strain sensing system  200  includes a shaft  202 , a strain sensor  204 , and a strain absorber  206  that mounts the strain sensor  204  to the shaft  202 . The shaft  202  may be any mechanism that exhibits strain measurable by the strain sensor  204 . 
         [0052]    The strain sensor  204  includes a housing  208 , a SAW device  210 , and an terminal  212 . The strain sensor  204  may include other components. For example, the strain sensor  204  may include components that communicate the measurement of the SAW device  210  from the rotating platform formed by the shaft  202 , the strain sensor  204 , and the strain absorber  206  to a stationary platform that may include, for example, a controller or other signal processing circuitry that receives and suitably processes the signal from the strain sensor  204 . These other components may include, for example, an RF transmitter that transits an RF signal based on the strain measured by the SAW device  210  by way of an antenna connected to the terminal  212 . The SAW device  210  and these other components may be integrated in the same IC or as separate ICs or may be arranged as separate circuits as desired. As further desired, the housing  208  and the strain absorber  206  may be integrally formed. 
         [0053]    The strain absorber  206  is in the form of either a solid or hollow ring that encircles and conforms to the shaft  202 . The strain sensor  204  is attached to an outer surface  214  of the strain absorber  206  by a bonding agent or other suitable attachment apparatus. For example, the strain sensor  204  may rest on a flattened area of the outer surface  214  of the strain absorber  206 . An inner surface  216  of the strain absorber  206  is attached to the shaft  202  such as by snap fitting into grooves in the shaft  202 , or by other suitable attachment apparatus. 
         [0054]    In addition, the inner surface  216  of the strain absorber  206  may have opposing projections  218  and  220  that cooperate with opposing indents  222  and  224  of the shaft  202 . The projections  218  and  220  and the indents  222  and  224  provide for polarized mating between the strain absorber  206  and the shaft  202 . 
         [0055]    As shown in  FIG. 18 , a strain sensing system  230  includes a shaft  232 , first and second strain sensors  234  and  236 , and first and second strain absorbers  238  and  240 . The first strain absorber  238  mounts the first strain sensor  234  to the shaft  232 , and the second strain absorber  240  mounts the second strain sensor  236  to the shaft  232  so that the second strain sensor  236  is substantially diametrically opposed to the first strain sensor  234 . The first and second strain absorbers  238  and  240  may have any of the configurations shown above. 
         [0056]    The strain that is sensed by a strain sensor can be typically on the order of 2500 microstrain. By properly dimensioning the strain absorbers with respect to the shafts on which the strain absorbers mount the strain sensors, the strain absorbers described above are able to reduce that strain on the strain sensors to at or below the 500 microstrain that is typically the maximum strain that a strain sensor such as SAW device can measure. 
         [0057]    The strain absorber in each of the embodiments described above may be formed from any suitable material such as metal, plastic, an elastomeric material, etc. The material of the strain absorber, for example, may be the same as the material that is used for the shaft that supports the strain absorber. The dimensions of the strain absorber are dependent on the strain produce in the shaft. The strain produced in turn depends on the shaft diameter, the applied torque, and the shaft material. Also, the dimensions of the strain absorber may be optimized to provide the desired amount of strain reduction between the strain on the surface of its corresponding shaft and the strain on the strain sensor supported by the strain absorber. Further, the material of the strain absorber may be selected to affect the amount of strain reduction that is desired. 
         [0058]      FIG. 19  illustrates a torque sensing system  250  having a strain sensor  252  and a strain-to-torque converter  254 . The strain sensor  252  may be any type of strain sensor, such as any of the strain sensors described above in connection with  FIGS. 1-18 . The strain-to-torque converter  254  may be any type of processor, such as an ASIC, a field programmable gate array, a microcomputer, etc., capable of converting the strain measurement provided by the strain sensor  252  to a torque output. For example, since the strain vs. torque relationship is known, the strain-to-torque converter  254  may implement an equation, a look up table, etc. in order to make the strain to torque conversion. 
         [0059]      FIG. 20  illustrates a torque sensing system  260  having a first strain sensor  262 , a second strain sensor  264 , a comparator  266 , and a strain-to-torque converter  268 . The first strain sensor  262  may be any type of strain sensor, such as any of the strain sensors described above in connection with  FIGS. 1-18 . Similarly, the second strain sensor  264  may be any type of strain sensor, such as any of the strain sensors described above in connection with  FIGS. 1-18 . The comparator  266 , for example, may be a summer having an input to receive the output of the first strain sensor  262  and an input to receive the output of the second strain sensor  264 . The output of the summer  266  is the average between the outputs of the first and second strain sensors  262  and  264 , which eliminates bending load errors. The strain-to-torque converter  268  may be any type of processor, such as an ASIC, a field programmable gate array, a microcomputer, etc., capable of converting the strain measurement provided by the summer  266  to a torque output. 
         [0060]    Each of the strain sensors  234  and  236  could consist of two resonators mounted 45° apart about the shaft axis. The purpose of using two resonators (F 1 , F 2 ) in each of the strain sensors  234  and  236  would be to eliminate common mode errors such as thermal related errors, centrifugal force errors acting on the sensor during rotation, etc. The common mode errors are eliminated by taking the difference of the two resonator outputs (F 1 −F 2 ). 
         [0061]    Certain modifications of the present invention have been discussed above. Other modifications of the present invention will occur to those practicing in the art of the present invention. For example, SAW devices have been disclosed above as the elements of the strain sensors that measure strain on a shaft. However, devices other than SAW devices can be used in connection with the present invention to measure strain on a shaft. 
         [0062]    Also, strain sensors are described above as including RF devices that use antennas to transmit their strain or torque measurements to a stationary receiver. However, the strain or torque measurements may be communicated to a stationary device by use of mechanisms other than antennas and RF transmissions. 
         [0063]    In addition, the strain sensors as described above measure strain on shafts. Instead, the strain sensors can be used to measure strain on mechanisms other than shafts. 
         [0064]    Moreover, the strain measurement may be converted to a torque measurement by either the rotating platform or by the stationary platform. Thus, the strain-to-torque converter  254  may be located with the strain sensor  252  on the rotating platform or instead may be located on the stationary platform while the strain sensor  252  is located on the rotating platform. Similarly, the summer  266  and the strain-to-torque converter  268  may be located with the first and second strain sensors  262  and  264  on the rotating platform or instead may be located on the stationary platform while the first and second strain sensors  262  and  264  are located on the rotating platform. As a further alternative, the summer  266  may be located with the first and second strain sensors  262  and  264  on the rotating platform and the strain-to-torque converter  268  may be located on the stationary platform. Accordingly, either the strain measurement or the torque measurement or an intermediate signal may be communicated from the rotating platform to the stationary platform. 
         [0065]    Furthermore, the drawings show strain sensors being mounted to the exterior surfaces of the strain absorbers. In certain embodiments, the strain sensors could instead be mounted to interior surfaces of the strain absorbers. 
         [0066]    Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.