Abstract:
A variable reluctance sensor is shown for determining the speed or position of a moveable target object. The sensor has a magnet which is coupled to a pole piece. A wire coil is located annularly around the pole piece. The magnet generates flux in the coil which is changed by the movement of the target object. A second bucking magnet is annularly located around the coil and generates a flux field to reinforce the flux in the coil and to prevent flux from leaking from the wire coil and pole piece. In this manner, the voltage output from the coil is increased due to the decrease in flux leakage. A processor unit is coupled to the wire coil. The processor unit measures the reluctance generated in the coil from the movement of the target object.

Description:
FIELD OF INVENTION 
     This invention relates to a variable reluctance sensor. More specifically, this invention relates to a reluctance sensor which enhances low output signals by reducing magnetic flux leakage. 
     BACKGROUND 
     It is desirable in many applications to determine the precise position or speed of various objects which linearly traverse a defined path or a rotational path. For example, the position or speed of numerous items in the automotive field such as transmission input and output shafts, crankshaft or tone wheels for anti-lock braking systems. A common type of sensor used in automotive components is a variable reluctance sensor which functions by sensing the change in reluctance from a ferrous target as the air gap between the sensor and the target changes. This is usually accomplished by cutting slots in the target and passing it by the sensor. 
     A variable reluctance sensor is used because it is rugged and is of relatively low cost. The variable reluctance sensor typically has a pole piece with a coil of wire. A magnet is located in the sensor and generates a magnetic field in the coil. The target object is a ferrous material such as steel or iron whose movement effects the flux within the coil generated by the magnet. For example, a target object may be a gear whose teeth are in proximity to the variable reluctance sensor. The reluctance is measured from the pole piece or magnet and is proportional to the distance from the target object. The magnet may either be in the front of the pole or the rear of the pole piece. Another configuration is having a stacked magnetic material forming a rectangular pole piece having alternating ferrous and magnetic material. The reluctance change on one end of the stack also changes the flux paths in the other end which creates the voltage change in the coil. 
     The output of the variable reluctance sensor depends on the rate of change of magnetic flux in the coil. The voltage generated by the coil is proportional to the number of turns in the coil. Given the air gap between the pole piece and the target object as well as magnetic paths through the pole piece and other sensor assembly, some of the magnetic flux leaks and does not affect the coil. Generally, it is desirable to have a higher voltage output for easier reading of the sensor output. Thus, the more leakage of magnetic field flux, the lower the voltage output. At low speeds of the target object, lower rate of change of flux is generated in the coil, which combined with inherent leaks, results in a lower voltage output from the sensor. 
     The present reluctance sensors may be enhanced in order to increase low voltage output. For example, added ferrous paths may be created by providing additional pole pieces near the target object. The additional pole pieces allow the capture of additional flux thus increasing overall voltage output. However, these modifications add cost and complexity to the system, nullifying the advantages of the variable reluctance sensor. Such corrections also require extra processing for the additional pole pieces which increase the complexity and cost of the device. 
     Thus, there exists a need for an increased output reluctance sensor. There is also a need for a reluctance sensor which does not require excessive processing components. There is a further need for a reluctance sensor which provides high output from target objects with low flux levels. 
     SUMMARY OF THE INVENTION 
     The present invention may be embodied in a variable reluctance sensor for determining the position of a moveable target object. The sensor has a magnet and a pole piece coupled to the magnet. A wire coil is located annularly around the pole piece. A second bucking magnet is annularly located around the coil which generates a flux field. A control unit is coupled to the wire coil which measures the sinusoidal signal generated in the coil from the movement of the target object. 
     The present invention may also be embodied in a method of increasing the voltage output from a variable reluctance sensor. The sensor has a pole piece, a coil located annularly around the pole piece, and a magnet. A bucking magnet is added around the coil to produce flux to cancel leaking flux from the sensor to increase coil voltage output. 
     It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of a variable reluctance sensor according to one embodiment of the present invention. 
     FIG. 2 is an exploded perspective view of the variable reluctance sensor of FIG. 1 
     FIG. 3 is cross sectional flux diagram of the magnetic fields surrounding the variable reluctance sensor in FIG. 1 without the addition of bucking magnets. 
     FIG. 4 is cross sectional flux diagram of the magnetic fields surrounding the variable reluctance sensor in FIG. 1 with the addition of bucking magnets. 
     FIG. 5 is a circuit diagram model of the variable reluctance sensor according to one embodiment of the present invention. 
     FIG. 6 is a graph of the output of the variable reluctance sensor with the placement of the bucking magnets according to the present invention in comparison with a standard variable reluctance sensor. 
     FIG. 7 is a circuit schematic of a control circuit used by the variable reluctance sensor according to one embodiment of the present invention. 
     FIG. 8 is a cross sectional flux diagram of a second variable reluctance sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the present invention is capable of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. 
     Referring now to the drawings and more particularly to FIGS. 1-4 which show a perspective, exploded perspective and cross sectional views of a variable reluctance sensor generally indicated at  10 , embodying the general principles of the present invention. 
     In general, the variable reluctance sensor  10  senses the rotation of a target object  12 . The target object must be made of a ferrous material such as steel and may move in a linear or angular motion relative to the variable reluctance sensor  10 . The target object  12  in this example is a gear and the sensor  10  detects the rotation of the gear. Of course, it is to be understood that any rotational or linear movement device may be used with the sensor  10 . The target object  12  which is sensed by the sensor  10  is typically rotated around an axis (not shown) to insure strict angular movement. 
     The outputs of the sensor  10  are coupled to a control system  14 . The sensor  10  has a disk shaped magnet  16  which is coupled to a pole piece  18 . The pole piece has a proximal end which is close to the target object  12  and a distal end. There is an air gap between the proximal end of the pole piece  18  and the target object  12 . The pole piece  18  has a platter  20  which is located on the distal end of the pole piece  18  and supports the magnet  16 . The pole piece  18  is a ferrous material and has a wire coil  22  which is coupled to the control system  14 . It is to be understood that the magnet  16  may be located in any appropriate location to generate a magnetic field within the coil  22 . For example, the magnet  16  may be located on a platter on the proximal end of the pole piece  18 . 
     A circular covering  24  is located over the coil  22  to protect the coil  22 . The circular covering  24  is typically an insulator material such as plastic. Two pairs of bucking magnets  26  and  28  are coupled around the coil  22 . Alternatively, a single ring magnet with one pole on the inside of the ring and another pole on the outside could be used for the magnets  26  and  28 . The target object  12  moves rotationally and has a series of gear teeth  30 . Since the target object  12  is metal, its movement changes the flux in the wire coil  22 . 
     The flux generated by the magnet  16  and shaped by the target object  12  is shown in FIG. 3 which is a cross section of the sensor  10 . The flux generates a voltage output from the coil  22  which is coupled to the control system  14 . FIG. 3 models the magnetic flux lines without the bucking magnets  26  and  28 . In this case a series of flux lines  32  are leaking from the pole piece  18  and the coil  22 . The leaking flux reduces the electrical output measurable from the coil  22 . The leaking flux thus significantly affects the output at low speeds of the target object  12  because the generated flux change is proportional to the speed of the target object  12  and thus is low to begin with. 
     The operation of the sensor  10  will now be explained with reference to FIG. 4 which is a flux diagram of the sensor  10  in FIG. 1 with the bucking magnets  26  and  28 . The movement of the target object  12  shapes a magnetic field in the coil  22  with a series of flux lines. The bucking magnets  26  and  28  force the flux from the magnetic field to travel through the pole piece  18  and the coil  22 . The bucking magnets  26  and  28  thus prevent leaking of the flux from the pole piece  18  and therefore increase the flux change and ultimate voltage from the coil  22 . 
     FIG. 5 shows an electrical circuit representation  58  of the interaction between the coil  22  and the target object  12  in FIGS. 1-4. The target object  12  is represented by a target resistor  60 . The gap between the target object  12  and the sensor  10  is modeled by an air gap resistor  62 . The flux leaking from the end of the pole  18  opposite the platter is represented by a resistor  64  while the flux leaking from the pole  18  to the coil  22  is represented by a resistor  66 . Two resistors  68  and  70  are in parallel and represent the flux leaks from the platter  20  and the magnet  16 . 
     The pole piece  18  is represented by two resistors  72  and  74  which create a voltage drop representing the flux generated in the coil  22  ultimately detected by the control circuit  14 . The platter  20  is represented by a resistor  76 . The magnet  16  is represented by a voltage source  78 . The current in the circuit diagram  58  represents the flux. Thus, it is beneficial to maximize the voltage measured across the output represented by the resistors  72  and  74 . The bucking magnets  26  and  28  create flux which replaces the leakage flux and enhances the flux generated in the coil. The bucking magnets  26  and  28  are modeled as voltage sources  80  and  82  which are in series with the resistors  72  and  74 . Using the electrical model, additional voltage sources result in greater outputs. The increase in current as a result of the voltage sources  80  and  82  on the resistors  64  and  66  results in greater voltage/flux across the resistors  72  and  74 . The elimination of the leakage flux and the addition of the flux from the two bucking magnets  26  and  28  thus increases the reluctance output. The bucking magnets  26  and  28  may be increased in length to optimize the cancellation of flux leakage. Additionally, the length of the bucking magnets  26  and  28  may be altered to produce the largest voltage output for a selected target and air gap configuration. 
     The output of the sensor  10  may be shown in FIG. 6 which is a voltage graph  100  representing the voltage outputs from the coil  22  in FIG. 1. A bottom trace  102  represents the voltage output of the sensor  10  without the bucking magnets  26  and  28 . A top trace  104  represents the voltage output of the coil  22  with bucking magnets  26  and  28 . As may be seen, the flux leakage is reduced and the corresponding voltage is higher resulting in a more usable sensor. 
     FIG. 7 is a schematic of the control circuit  14  which receives an output from the coil  22  of the sensor  10 . The motion of the target object  12  generates a sinusoidal signal from the coil  22  as the magnetic flux changes due to the velocity of the target object  12 . The outputs of the sensor  10  are coupled to a zero detection circuit  110  which detects the time when the sinusoidal signal crosses zero in order to determine the frequency of the signal which is proportional to the speed of the target. It is to be understood that any other appropriate circuit may be used to detect changes in the flux. For example, a peak detection circuit may be used instead of the zero crossing detection circuit  110 . 
     The zero crossing detection circuit  110  has an input  112  which is coupled to the output of the sensor  10 . A diode  114  clips the negative part of the voltage signal. The input  112  is coupled to the negative input of an operational amplifier  116 . The positive input of the operational amplifier  116  is coupled to a reference resistor  118 . Another resistor  120  is coupled to a voltage source  122 . The resistor  120  and resistor  118  provide a reference level voltage to compare the negative input of the operational resistor  118 . The output of the operational amplifier  116  therefore goes high when the negative input is higher than the reference voltage indicating a zero crossing point. 
     The output of the operational amplifier  116  is coupled to a micro-controller  124 . The micro-controller  124  may be any specific, dedicated controller or a programmable microprocessor, application specific integrated circuit (ASIC) or any other comparable circuit. The micro-controller  124  processes the output of the operational amplifier  116  by reading the number of high pulses in a certain time period indicating the frequency of the teeth  30  detected by the sensor  10  and therefore the position or speed of the target object  12 . The micro-controller  124  uses this data to control devices. In this example, the micro-controller  124  is coupled to a transmission shift assembly  126 . The micro-controller  124  will activate the transmission shift assembly when the target object  12  which determines the transmission shaft speed reaches a certain speed. 
     FIG. 8 is a perspective view of a variable reluctance sensor  200  according to the present invention which senses the absolute position of a linearly moving target object  202 . In this case the target object is a rod  204  which is attached to the traveling portion of a suspension system  206 . The suspension system  206  holds a wheel  208 . By determining the position of the rod  204 , the position of the wheel  208  may be determined for adjustments by the suspension system. 
     The outputs of the sensor  200  are coupled to a control system  210  which is identical to the control system  14  described with reference to FIG. 7 above. The sensor  200  is similar to the sensor  10  described with reference to FIGS. 1-3 above. The sensor  200  has a disk shaped magnet  212  which is coupled to a pole piece  214 . The pole piece has a proximal end which is close to the target object  202  and a distal end. There is an air gap between the proximal end of the pole piece  214  and the target object  202 . The pole piece  214  has a platter  216  which is located on the distal end of the pole piece  214  and supports the magnet  212 . The pole piece  214  is a ferrous material and has a wire coil  218  which is coupled to the control system  210 . It is to be understood that the magnet  212  may be located in any appropriate location to generate a magnetic field within the coil  218 . 
     A covering  220  is located over the coil  218  to protect the coil  218 . The circular covering  218  is typically an insulator material such as plastic. Two pairs of magnets  222  and  224  are coupled around the coil  218 . 
     The rod  204  is made of a ferrous material such as steel and has a series of teeth  226 . Since the rod  204  is metal, its movement changes the flux in the wire coil  218  by the movement of the teeth  226  relative to the sensor  200 . The flux generated by the movement of the rod  204  generates a voltage output from the coil  218  which is coupled to the control system  210 . The magnets  222  and  224  force the flux from the magnetic field to travel through the pole piece  214  and the coil  218 . The magnets  222  and  224  thus prevent leaking of the flux from the pole piece  214  and therefore increase the flux change and voltage output from the coil  218 . 
     Of course, the present invention may be employed in any application which requires determination of linear position or rotational position. Examples in the automotive field include suspension travel, crankshaft or crankshaft rotation and positioning, wheel speed and transmission shaft speed. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the present invention without departing from the spirit or scope of the invention. Thus, the present invention is not limited by the foregoing descriptions but is intended to cover all modifications and variations that come within the scope of the spirit of the invention and the claims that follow.