Magnetic position sensor system composed of two reference magnetoresistors and a linear displacement sensing magnetoresistor

[0002] The present invention relates to a method of sensing position using magnetoresistors.

[0003] It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.

[0004] Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position formation during cranking. Various combinations of magnetoresistors and single dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 570,016, 5,731,702, and 5,754,042).

[0005] The shortcoming of MR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of MR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive MR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.

[0006] To compensate for the MR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the MR device, it is also desirable to make the current of the current source automatically increase with the MR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the MR device. It is also known that air gap variations between the MR device and ferromagnetic materials or objects will affect the resistance of MR devices with larger air gaps producing less resistance and decreased output signals.

[0007] What is needed is a position sensor capable of self compensation over wide ranges of temperature and air gaps, including tilts.

[0008] The present invention is a position sensor system capable of self compensation over wide temperature ranges and air gaps, including tilts. It employs three matched MRs (also commonly referred to as MR elements) with either one common bias magnet or separate bias magnets. The MRs are aligned in the direction of movement of a magnetic target. The middle MR is the actual position sensor. The two outer MRs serve as reference sensors which sense the magnetic field at the limits of the position sensing range. The cooperating magnetic target assures that one of the two outer MR elements is always exposed to some maximum magnetic field, B_MAX, corresponding to a position X_MAX, and the other MR element is always exposed to some minimum magnetic field, B_MIN, corresponding to a position X_MIN, such that B_MIN≦B_X≦B_MAX, corresponding to X_MIN≦X≦X_MAX, where B_Xis the magnetic field measured by the middle MR and varies with the position, X, of the target.

[0009] The actual position, X, is computed assuming a linear relation between MR resistance in the magnetic field range from B_MINto B_MAXand the position, X, of the target. This is accomplished by making the magnetic field a linear function of position in the range X_MINto X_MAXand using MRs whose resistance is a linear function of magnetic field in the range B_MINto B_MAX.

[0010] Alternatively, the shape of the magnetic field profile can be tailored to the MR characteristics in the operating magnetic field range B_MINto B_MAXto yield the desired linear relation between MR resistance and position X. That is, the sensor always operates on the linear part of the MR resistance versus magnetic field characteristic curve in the magnetic field range B_MINto B_MAX.

[0011] Accordingly, it is an object of the present invention to provide a magnetic position sensor system capable of self compensation over wide ranges of temperatures, air gaps and tilts.

[0012] FIG. 1 depicts an example of the preferred environment of use of the present invention.

[0013] FIG. 2 depicts a plot of MR resistance versus magnetic field and position according to the present invention.

[0014] FIG. 3 shows an example of an analog circuit implementing the present invention.

[0015] FIG. 4 shows an example of a circuit employing a digital processor implementing the present invention.

[0016] FIG. 5 is a flow diagram for the digital processor of FIG. 4.

[0017] FIG. 1 depicts an example of the preferred environment of use of the present invention. The single MR sensor 10, preferably stationary, employs three magnetoresistors, MR1, MR2, and MR3, which are biased by a permanent magnet 12, wherein the magnetic flux 14, 16, and 18 emanating therefrom are represented by the dashed arrows. The magnetic flux 14, 16, and 18 pass from the permanent magnet 12 through the magnetoresistors MR1, MR2, and MR3 and through the air gaps 20, 22, and 24 to the target 30.

[0018] The matching of MR1, MR2, and MR3 ensures that the sensing elements are equally affected by temperature and air gap and that the resistance of each MR is the same for the same magnetic field exposure. If the magnetic field is a linear function of position, then the resistance of MR1, MR2, and MR3 will be linear functions of magnetic field and position also and, by properly designing an electric circuit, the MR output voltages will also be linear functions of resistance and position.

[0019] The target 30 is made of a magnetic material (for example a ferromagnetic material), having in this example a tooth 32, a selectively shaped tooth edge profile 26, and a space 34, and is designed to have the magnetic field profile 36 which is conveyed with the target as the target moves. The target 30 may have other configurations besides that shown in FIG. 1 and may be appropriately shaped to provide any desirable magnetic field profile similar to the magnetic field profile 36. The target 30 moves in the X direction 38 and is constrained to move in a known range having a maximum value X_MAXand a minimum value X_MIN. The magnetic profile 36 in conjunction with the constricted range of movement of the target between X_MINand X_MAXensures that MR1 is always exposed to B_MAXand MR3 is always exposed to B_MINwhereas MR2 is exposed to B_MINwhen it is at the position X_MINand to B_MAXwhen it is at the position X_MAXwherein MR2 is exposed to a magnetic field B_X, where B_MIN≦B_X≦B_MAX, at the position X, where X_MIN≦X≦X_MAX, having a magnetic profile which is a linear function of position between X_MINand X_MAX.

[0020] FIG. 2 depicts a plot of MR resistance versus magnetic field and MR resistance versus position according to the present invention. The target 30 is designed and appropriately shaped such that the magnetic field range between B_MINand B_MAXfalls on the linear part 40 at low temperatures, or the linear part 42 at high temperatures, and not around the respective parabolic regions 44 or 46 of the magnetic field plot of FIG. 2 by which the magnetic field is a linear function of position and by which the resistance of MR1, MR2, and MR3 are linear functions of magnetic field and position also. It can be seen from FIG. 2 that the magnetic field range remains in the linear region as the temperature varies. That is, the linearity of the magnetic field is stable as the temperature varies. It can also be seen in FIG. 2 that the MR resistances vary widely at different temperatures such as R′_1MINat point 41′ at a low temperature and R″_1MINat point 41″ at a high temperature, wherein R′_1MINis the resistance of MR1 at X_MINat a low temperature and R″_1MINis the resistance of MR1 at X_MINat a high temperature. Similarly, R′_3MAXat point 43′ is the resistance of MR3 at X_MAXat a low temperature and R″_3MAXat point 43″ is the resistance of MR3 at X_MAXand R₂′ at point 45′ is the resistance of MR2 at X_Xat a low temperature and R₂″ at point 45″ is the resistance of MR2 at X_X. As shown in FIG. 2, the position measurement of X_Xat point 48 is not affected even with widely varying resistances at different temperatures. A similar plot can be ascribed to MR resistance versus air gap. A change in air gap will not affect the position measurement of X_Xas long as the region between B_MINand B_MAXis on a linear part of the MR resistance curve such as 40 and 42 in FIG. 2. A smaller air gap would simply shift the MR resistance values toward higher magnetic field values while a larger air gap would simply shift the MR resistance values toward lower magnetic field values.

[0021] From FIG. 2, it can be deduced that:

[0022] (X_X−X_MIN)/(X_MAX−X_MIN)=(B_X−B_MIN)/(B_MAX−B_MIN)=(R₂−R₃)/(R₁−R₃)  (1)

[0023] where R₁, R₂, and R₃are the resistances of MR1, MR2, and MR3, respectively, at an arbitrary temperature. By properly designing an electric circuit, the MR output voltages will also be linear functions of resistance and position and equation (1) above may be equated to (V_MR2−V_MR3)/(V_MR1−V_MR3) where V_MR1, V_MR2, and V_MR3are the voltages across MR1, MR2, and MR3, respectively. Therefore, from the above relationships:

[0024] X_X=X_MIN+(X_MAX−X_MIN)(R₂−R₃)/(R₁−R₃)  (2)

[0025] and

[0026] X_X=X_MIN+(X_MAX−X_MIN)(V′_MR2−V′_M3)/(V′_MR1−V′_MR3).  (2′)

[0027] By proper design, X_MINmay be set to zero resulting in simplified equations (3) and (3′):

[0028] X_X=(X_MAX−X_MIN)(R₂−R₃)/(R₁−R₃)  (3)

[0029] and

[0030] X_X=(X_MAX−X_MIN)(V′_MR2−V′_MR3)/(V′_MR1−V′_MR3).  (3′)

[0031] FIG. 3 shows an example of an analog circuit 50 implementing the present invention. In FIG. 3, R₄, R₅, R₆, and R₇have, preferably, the same value, say R, such that R>>R₁>R₂>R₃where R₁, R₂, and R₃are the resistances of MR1, MR2, and MR3, respectively. Under these conditions:

[0032] V₁=(R₂+R₃)/(R₁+R₂+R₃)*V_S  (4)

[0033] V₂=R₂/(R₁+R₂+R₃)*V_S  (5)

[0034] V₁₁=(V_S+V₂)/2=(1+R₂/(R₁+R₂+R₃))*(V_S/2)  (6)

[0035] and

[0036] V₂₂=V₁/2=(R₂+R₃)/(R₁+R₂+R₃)*(V_S/2)  (7)

[0037] where V_Sis the power supply voltage and V₁, V₂, V₁₁, and V₂₂are the voltages at the nodes shown in FIG. 3.

[0038] The output V_Dof differential amplifier 52 (i.e. an OP AMP) is:

[0039] V_D=V₁₁−V₁=(V_S/2)*((R₁−R₃)/(R₁+R₂+R₃))  (8)

[0040] and the output V_Nof differential amplifier 54 is:

[0041] V_N=V₂−V₂₂=(V_S/2)*((R₂−R₃)/(R₁+R₂+R₃))   (9)

[0042] whereby the output V_OUTof analog divider 56 is:

[0043] V_OUT=K*(V_N/V_D)=K*(R₂−R₃)/(R₁−R₃)  (10)

[0044] wherein K is the gain of analog divider 56 and is adjusted for maximum sensitivity or K is adjusted to satisfy other system requirements. For example, K may be adjusted such that V_OUThas a value of zero when position X_MINis at MR2 and a value of 5 volts when position X_MAXis at MR2. Hence,

[0045] (R₂−R₃)/(R₁−R₃)=V_OUT/K  (11)

[0046] and equations (2) and (3) may be expressed as:

[0047] X_X=X_MIN+(X_MAX−X_MIN)V_OUT/K  (2″)

[0048] and

[0049] X_X=(X_MAX−X_MIN)V_OUT/K  (3″)

[0050] Thus, since the gain K and the range X_MAX−X_MINare known, the position X_Xcan be determined from the voltage V_OUT. In the example of FIG. 3, the order in which MR1, MR2, and MR3 are connected (i.e. MR1 to MR3 to MR2) is important. Other possibilities will be appreciated by those skilled in the art and may require the use of additional differential amplifiers or OP AMPS.

[0051] FIG. 4 shows an example of a circuit 60 employing a digital processor 62 (i.e. digital signal processor, microcontroller, microprocessor, etc.) for implementing the present invention. In FIG. 4, R₁′, R₂′, and R₃′ are the resistances of MR1, MR2, and MR3, respectively. V_S′ is the value of the supply voltage and is implicitly known to the digital processor 62, for example, as an input or stored in the digital processor's memory. The position range, X_MINand X_MAXare, preferably, stored in memory also. The values of V₁′ and V₂′ are input to the digital processor 62 and can be expressed as:

[0052] V₁′=V_S′*(R₂′+R₃′)/(R₁′+R₂′+R₃′)  (12)

[0053] and

[0054] V₂′=V_S′*R₃′/(R₁′+R₂′+R₃′)  (13)

[0055] wherein V′_MR1, V′_MR2, and V′_MR3are the values of the voltages across MR1, MR2, and MR3, respectively, and can be expressed as:

[0056] V′_MR1=V_S′V₁′  (14)

[0057] V′_MR2=V₁′V₂′  (15)

[0058] and

[0059] V_MR3V₂′.  (16)

[0060] The value of the output voltage V′_OUTis computed by the digital processor 62 and can be expressed as:

[0061] V′_OUT=K′*(V′_MR2−V′_MR3)/(V′_MR1−V′_MR3)  (17)

[0062] wherein K′ is the gain and is adjusted for maximum sensitivity or K′ is adjusted to satisfy other system requirements. For example, K′ may be adjusted such that V′_OUThas a value of zero when MR2 is at the position X_MINand a value of 5 volts when MR2 is at the position X_MAX. Hence,

[0063] (V′_MR2−V′_MR3)/(V′_MR1−V′_MR3)=V′_OUT/K′  (18)

[0064] and equations (2′) and (3′) may be expressed as:

[0065] X_X=X_MIN+(X_MAX−X_MIN)V′_OUT/K′  (2′″)

[0066] and

[0067] X_X=(X_MAX−X_MIN)V′_OUT/K′.  (3′″)

[0068] Thus, since the gain K′ and the range X_MAX−X_MINare known, the position X_Xcan be determined from the voltage V′_OUT.

[0069] FIG. 5 is a flow diagram for the digital processor 62 of FIG. 4. The procedure starts at block 70 where initialization of the digital processor 64 is accomplished. At block 72 the values of V₁′ and V₂′ are entered into the digital processor 62 and the values of V′_MR1, V′_MR2, and V′_MR3are computed at block 74 according to equations (14), (15), and (16). The gain K′ is selected at block 76 and the output voltage V′_OUTis computed and output at block 78. V′_OUTis computed according to equation (17). If at decision block 80 the procedure is not done, then control passes to block 72. Otherwise, the procedure ends at block 82. If desired, the value of X_Xmay be computed according to equation (2′″) or (3′″) and output as well. The method of accomplishing this would involve another computation block being implemented in FIG. 5, and is well known to those skilled in the art.

[0070] To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.