Abstract:
A turbocharger includes a cylindrical wall and a non-ferromagnetic compressor wheel within the cylindrical wall. The non-ferromagnetic compressor wheel has fins. A magnetoresistive sensor housing is threaded through the cylindrical wall and houses a permanent magnet and at least one magnetoresistor. The permanent magnet is positioned so as to induce eddy currents on the fins. The permanent magnet magnetically biases the magnetoresistor, and the magnetoresistor senses rotation of the non-ferromagnetic compressor wheel.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates to the use of a magnetoresistive sensor to sense the speed of a rotating non-ferromagnetic device such as the wheel of a turbocharger compressor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Turbocharging is the force feeding of an engine with air under pressure in order to improve the fuel economy, emissions, and performance of the engine. In a typical turbocharger, an engine&#39;s exhaust is used to drive a turbine wheel which in turn drives a compressor wheel through a shaft that interconnects the turbine and compressor wheels. The compressor wheel of the turbocharger draws air into the turbocharger and moves the air by centrifugal force to the outlet of the turbocharger for supply to the engine.  
         [0003]     Frequently, a turbocharger is controlled in an open loop manner, meaning that the speed of the compressor wheel is not used to provide feedback in order to control the speed of the compressor wheel. Therefore, in order to avoid an over speed condition that can damage or destroy the turbocharger, the turbocharger is run far below its maximum speed. However, running a turbocharger too far below its maximum speed results in less than optimal performance of the engine supplied by the open loop controlled turbocharger.  
         [0004]     Therefore, it is desirable to sense the speed of a turbocharger&#39;s compressor wheel so that the turbocharger can be controlled nearer to its maximum speed. Sensing the speed of the compressor wheel of the turbocharger also has other advantages. For example, sensing the speed of the compressor wheel allows the turbocharger to be controlled so that it runs very near it&#39;s maximum speed limit, where its performance is best. Moreover, many turbocharger warranty claims are caused by over speed conditions, and many of these over speed warranty claims are due to the inability of current turbocharger control systems to accurately sense and control the speed of the turbocharger&#39;s compressor wheel.  
         [0005]     Compressor wheels are typically made from aluminum, which is a non-ferromagnetic material. Therefore, it is problematic to sense the speed of such compressor wheels magnetically. For example, magnetoresistive sensors are currently used to sense ferrous metal targets but not non-ferrous metal targets.  
         [0006]     Typically, a magnetoresistive sensor is biased by a stationary magnet. When the ferrous metal target being sensed by the magnetoresistive sensor has teeth and slots, the bias of the magnetoresistive sensor is influenced by the pole piece effect from the target teeth and slots as they pass in front of the magnetoresistive sensor and magnet. Such magnetoresistive sensors have not been used to sense the speed of non-ferromagnetic turbocharger compressor wheels.  
         [0007]     The present invention is directed to a magnetoresistive sensor that is arranged to sense the speed of a non-ferromagnetic turbocharger compressor wheel.  
       SUMMARY OF THE INVENTION  
       [0008]     According to one aspect of the present invention, an apparatus comprises a non-ferromagnetic compressor wheel of a turbocharger, a permanent magnet, and at least one magnetoresistor. The non-ferromagnetic compressor wheel has fins. The permanent magnet is positioned so as to induce eddy currents on the fins. The magnetoresistor is positioned with respect to the non-ferromagnetic compressor wheel and the permanent magnet so as to be magnetically biased by the permanent magnet and so as to sense rotation of the non-ferromagnetic compressor wheel.  
         [0009]     According to another aspect of the present invention, an apparatus comprises a non-ferromagnetic compressor wheel of a turbocharger, a magnetic field sensor housing, a permanent magnet, and an active magnetic field sensor. The non-ferromagnetic compressor wheel has fins. The magnetic field sensor housing is attached to a structure in proximity to the non-ferromagnetic compressor wheel. The permanent magnet is disposed within the magnetic field sensor housing and is positioned so as to induce eddy currents on the fins. The active magnetic field sensor is disposed within the magnetic field sensor housing and is positioned with respect to the non-ferromagnetic compressor wheel and the permanent magnet so as to be magnetically biased by the permanent magnet and so as to sense a magnetic field induced by the eddy currents to thereby detect rotation of the non-ferromagnetic compressor wheel.  
         [0010]     According to still another aspect of the present invention, a method of sensing rotation of a non-ferromagnetic compressor wheel of a turbocharger comprises the following: inducing eddy currents in fins of the non-ferromagnetic compressor wheel; sensing a magnetic field induced by the eddy currents by use of an active magnetic field sensor so as to produce pulses having a pulse rate dependent upon a speed at which the non-ferromagnetic compressor wheel rotates; and, reducing the pulse rate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     These and other features and advantages of the present invention will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which:  
         [0012]      FIG. 1  illustrates a compressor section of a turbocharger where the compressor section includes a compressor wheel and a magnetoresistive sensor for sensing the speed of the compressor wheel according to the present invention;  
         [0013]      FIG. 2  illustrates the compressor wheel of  FIG. 1  in additional detail;  
         [0014]      FIG. 3  illustrates the relationship between the compressor wheel and the magnetoresistive sensor of  FIG. 1 ;  
         [0015]      FIGS. 4 and 5  are respective isometric and side views of the magnetoresistive sensor of  FIG. 1 ;  
         [0016]      FIG. 6  illustrates the magnetoresistive sensor of  FIG. 1  in additional detail;  
         [0017]      FIG. 7  illustrates the permanent magnet of the magnetoresistive sensor of  FIG. 1 ;  
         [0018]      FIG. 8  illustrates a magnetoresistive bridge of the magnetoresistive sensor of  FIG. 1 ;  
         [0019]      FIG. 9  illustrates a processing circuit that can be advantageously used with the magnetoresistive sensor of  FIG. 1 ; and,  
         [0020]      FIG. 10  shows an alternate mounting arrangement for the magnetoresistive sensor. 
     
    
     DETAILED DESCRIPTION  
       [0021]     A compressor section  10  of a turbocharger is shown in  FIG. 1  and includes a turbocharger compressor wheel  12  that rotates within a cylindrical chamber  14  formed by a cylindrical wall  16 . The turbocharger compressor wheel  12  is typically rotated by a turbine wheel (not shown) and the turbine wheel may be suitably controlled to rotate the turbocharger compressor wheel  12  at a desired speed. Accordingly, the turbocharger compressor wheel  12  draws air into the cylindrical chamber  14  (from above as shown in  FIG. 1 ) and supplies the air under pressure through an outlet  18  to an engine such as a diesel or gasoline engine.  
         [0022]     A magnetoresistive sensor  20  is received in an aperture of the cylindrical wall  16  in order to sense the speed at which the turbocharger compressor wheel  12  rotates within the cylindrical chamber  14 .  
         [0023]     As shown in  FIG. 2 , the turbocharger compressor wheel  12  has a shaft  22  and a plurality of fins  24  radiating out from the shaft  22 . The turbocharger compressor wheel  12  is driven by the turbine wheel that is suitably coupled to the shaft  22 . The fins  24  are optimally shaped to compress air and to impel the compressed air through the outlet  18  as the shaft  22  rotates the fins  24 .  FIG. 3  shows an exemplary geometric relationship between the fins  24  of the turbocharger compressor wheel  12  and the magnetoresistive sensor  20 .  
         [0024]     As shown in  FIGS. 4 and 5 , the magnetoresistive sensor  20  includes a housing  26 . The housing  26  has a first portion  28  that is externally threaded so that the magnetoresistive sensor  20  can be threaded through the cylindrical wall  16  and into position where it can sense the rotation of the turbocharger compressor wheel  12 . The housing  26  has a second portion  30  that is faceted to receive a wrench or other tool to facilitate the turning of the magnetoresistive sensor  20  in order to thread the magnetoresistive sensor  20  through the cylindrical wall  16  of the compressor section  10 . The magnetoresistive sensor has a third portion  32  through which electrical leads  34  may be run in order to couple the magnetoresistive sensing elements located within the housing  26  to a controller or other apparatus that is located externally of the housing  26 . The housing  26 , for example, my be a stainless steel housing made from 300 series stainless steel.  
         [0025]      FIG. 6  shows a magnetoresistive subassembly  36  that is housing within the housing  26  of the magnetoresistive sensor  20 . The magnetoresistive subassembly  36  includes a chip carrier  38 , a magnetoresistive chip  40  supported by the chip carrier  38  on one side thereof, and a permanent magnet  42  supported by the chip carrier  38  on another side thereof. Accordingly, the chip carrier  38  is sandwiched between the magnetoresistive chip  40  and the permanent magnet  42 . Alternatively, the permanent magnet  42  may be supported by the magnetoresistive chip  40 . In this case, the magnetoresistive chip  40  is supported on the chip carrier  38 , and the permanent magnet  42  is supported on the magnetoresistive chip  40 . Other orientations of the chip carrier  38 , the magnetoresistive chip  40 , and the permanent magnet  42  relative to one another are also possible. The permanent magnet  42  is shown in  FIG. 7  and includes a flat surface that abuts the magnetoresistive chip  40  as shown in  FIG. 6 .  
         [0026]     A North-South axis of the permanent magnet  42  extends between the North and South poles of the permanent magnet  42 . This North-South axis is parallel to the longitudinal axis of the magnetoresistive sensor  20 . For example, the magnetoresistive sensor  20  may be positioned with respect to the turbocharger compressor wheel  12  so that the North-South axis of the permanent magnet  42  intersects the shaft  22  of the turbocharger compressor wheel  12 . The permanent magnet  42  magnetically biases the magnetoresistors of the magnetoresistive sensor  20 .  
         [0027]     As shown in  FIG. 8 , the magnetoresistive chip  40  comprises four magnetoresistors  44 ,  46 ,  48 , and  50  formed in a semiconductor substrate as a Wheatstone bridge. The junction of the magnetoresistors  44  and  46  is coupled to a source that is shared with a comparator  52  which may be an operational amplifier. The junction of the magnetoresistors  48  and  50  is coupled to a reference potential such as ground. The junction of the magnetoresistors  46  and  50  is coupled to the positive input of the comparator  52 , and the junction of the magnetoresistors  44  and  48  is coupled to the negative input of the comparator  52 . An amplifier may be placed upstream of the comparator  52  as necessary.  
         [0028]     Alternatively, instead of integrating the magnetoresistors  44 ,  46 ,  48 , and  50  on a semiconductive substrate to form a chip, the magnetoresistors  44 ,  46 ,  48 , and  50  may be formed as discrete elements mounted, for example, on a printed circuit board. Also, in the case where the magnetoresistors  44 ,  46 ,  48 , and  50  are integrated on a semiconductive substrate, the comparator  52  may be likewise integrated on the same substrate, in which case the output of the comparator  52  is brought out of the magnetoresistive sensor  20  by way of the leads  34 . Alternatively, the comparator  52  may be external of the housing  26  in which case the leads  34  are used to couple the output of the magnetoresistors  44 ,  46 ,  48 , and  50  to the comparator  52 . As a still further alternative, fewer or more than four magnetoresistors may be used in the magnetoresistive sensor  20 .  
         [0029]     With the arrangement as described above, eddy currents are induced in the fins  24  of the turbocharger compressor wheel as the fins  24  are rotated by the permanent magnet  42 . These eddy currents flowing in the aluminum fins  24  of the turbocharger compressor wheel  12  at high RPM cause a magnetic field that opposes the magnetic field created by the permanent magnet  42 . The magnetoresistors  44 ,  46 ,  48 , and  50  of the magnetoresistive sensor  20  detect this magnetic field created by these eddy currents. The magnetoresistive sensor  20  is placed in a region to detect the magnetic field induced by the eddy currents in order to produce a signal that can be used to measure the travel of each of the fins  24  past the magnetoresistive sensor  20 . The measurement of the number of the fins  24  per given duration of time can be used to determine the speed of the turbocharger compressor wheel  12 .  
         [0030]     The sensed speed of the turbocharger compressor wheel  12  can be used for a variety of purposes. For example, the sensed speed can simply be recorded. During warranty negotiations, this record provides evidence of whether or not the speed specification of the turbocharger had been exceeded by the customer. Instead of recoding all speed readings for this purpose, only the maximum compressor speed need be stored. Accordingly, as each new compressor speed reading is made, it is compared to the stored maximum compressor speed reading and, if the new compressor speed reading is greater than the stored maximum compressor speed reading, the new compressor speed reading becomes the stored maximum compressor speed reading. The stored maximum compressor speed reading can be used for a variety of purposes. For example, if the stored maximum speed of the compressor exceeds design specifications, warranty claims can be refuted. Additionally or alternatively, the sensed speed can be used by a controller to eliminate most or all over speed conditions altogether.  
         [0031]     Moreover, it may be necessary to divide down the number of pulses per revolution produced by the magnetoresistive sensor  20  in response to rotation of the turbocharger compressor wheel  12  due to limitations of control processors that keep track of the sensor output at high RPM. Compressor wheels also have different numbers of fins from one turbocharger to another.  
         [0032]     Accordingly, a circuit  60  as a shown in  FIG. 9  may be used to regulate the number of output pulses per revolution of the turbocharger compressor wheel  12 . The output of the magnetoresistive sensor  20  is coupled to a counter  62  whose outputs are selectively coupled as inputs to a NAND gate  64 . The outputs of the counter  62  that are coupled to the NAND gate  64  may be selected to produce a desired divide-by number N. Thus, the counter  62  and the NAND gate  64  together divide the pulse rate at which the magnetoresistive sensor  20  emits pulses by N. A J-K flip-flop  66  further reduces this pulse rate by two. The output of the J-K flip-flop  66  is coupled to the base of an NPN transistor  68  whose output forms the output of the circuit  60 . In this manner, the circuit  60  can be used to divide down the number of pulses per revolution produced by the magnetoresistive sensor  20  in response to rotation of the turbocharger compressor wheel  12  so as to meet limitations of the control processors that keep track of the sensor output. The circuit  60  can also be used to regulate the number of pulses per revolution produced by the magnetoresistive sensor  20  in response to rotation of the turbocharger compressor wheel  12  to a consistent number regardless of the number of fins of a compressor wheel. The duty cycle of the pulses at the output of the NPN transistor  68  is 50%.  
         [0033]     As in the case of the arrangement shown in  FIG. 8 , the circuit  60  may be integrated on the same substrate as the magnetoresistors of the magnetoresistive sensor  20 , in which case the output of the circuit  60  is brought out of the magnetoresistive sensor  20  by way of the leads  34 . Alternatively, the circuit  60  may be external of the housing  26  in which case the leads  34  are used to coupled the output of the magnetoresistors  44 ,  46 ,  48 , and  50  to the circuit  60 .  
         [0034]     Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present invention. For example, magnetoresistive elements as disclosed above are active magnetic field sensors (requiring a voltage stimulus) that are used to sense the magnetic fields induced by the eddy currents flowing on the surfaces of the fins of the compressor wheel. These magnetoresistive elements can be magnetoresistors, giant magnetoresistors (GMR), anisotropic magnetoresistors (AMR), etc. Alternatively, other active magnetic field sensors such as Hall effect sensors can be used to sense the magnetic fields induced by the eddy currents flowing on the surfaces of the fins of the compressor wheel.  
         [0035]     Also, the first portion  28  of the housing  26  is described above as being externally threaded so that the magnetoresistive sensor  20  can be threaded through the cylindrical wall  16 . Instead, the housing  26  may be unthreaded and instead may have a flange for screw mounting to the cylindrical wall. Such a mounting arrangement is shown in  FIG. 10  where the magnetoresistive sensor  20  has a housing  70  with a flange  72  that is arranged to receive one or more screws for fastening the magnetoresistive sensor  20  to the cylindrical wall  16 .  
         [0036]     Moreover, the magnetoresistive sensor  20  is described above as being mounted into the cylindrical wall  16  in order to sense the speed at which the turbocharger compressor wheel  12  rotates within the cylindrical chamber  14 . Alternatively, the magnetoresistive sensor  20  could instead sense the compressor wheel through the turbo housing. Instead of boring a hole all the way through the housing, a blind hole that has a thin face could receive magnetoresistive sensor  20  with the magnetoresistive sensor  20  detect rotation through the thin face.  
         [0037]     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.