Method and apparatus for calculating offset of wheatstone bridge type sensor

Abstract

A method and an apparatus for calculating an offset of a Wheatstone bridge type sensor are described. The offset calculation method includes measuring resistances between nodes of a Wheatstone bridge type sensor, calculating an offset of the sensor using the measured resistances and providing information on the calculated offset. Accordingly, the offset of the Wheatstone bridge type sensor can be rapidly and easily calculated independently from the size of a bias current, and ultimately. Furthermore, time required to measure can be reduced and thus a sensor fabrication cost can be reduced, and also, mass production can be enhanced.

Claims

1. An apparatus for a planar hall resistance (PHR) sensor comprising a first node, a second node, a third node and a fourth node arranged in order, the apparatus comprising: a first measurement unit configured to measure a first resistance between the first node and the second node when the PHR sensor is mounted in the apparatus; a second measurement unit configured to measure a second resistance between the second node and the third node when the PHR sensor is mounted in the apparatus; a third measurement unit configured to measure a third resistance between the third node and the fourth node when the PHR sensor is mounted in the apparatus; a fourth measurement unit configured to measure a fourth resistance between the fourth node and the first node when the PHR sensor is mounted in the apparatus; a reference offset calculator connected to the first, second, third and fourth measurement units and configured to apply the first, second, third and fourth resistances to a predetermined formula to calculate a reference offset of the PHR sensor, the reference offset being a value independent from a bias current of the PHP sensor; a display connected to the reference offset calculator and configured to display the reference offset; and an input device connected to the reference offset calculator configured to receive a user input of a first bias current, wherein, in response to the user input, the reference offset calculator is further configured to compute a first offset for the PHR sensor based on the reference offset and the first bias current and wherein the display is further configured to display the first offset.

2. The apparatus of claim 1, wherein the reference offset calculator is further configured to multiply the reference offset and the first bias current to compute the first offset.

3. The apparatus of claim 1, wherein the reference offset calculator is configured to compute the reference offset based on the following relationship: (RARB+RCRD)/2, where RA is the first resistance, RB is the second resistance, RC is the third resistance and RD is the fourth resistance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:

(2) FIG. 1 is a view showing an operation method of a PHR sensor;

(3) FIG. 2 is a view showing four types of real PHR sensors;

(4) FIG. 3 is a view showing results of measurement of offset voltages in real PHR sensors shown in FIG. 2;

(5) FIG. 4 is a view showing a model of the PHR sensor shown in FIG. 2;

(6) FIG. 5 is a view showing an equivalent circuit of the model shown in FIG. 4;

(7) FIG. 6 is a view provided to explain resistance values between nodes;

(8) FIG. 7 is a block diagram showing an offset calculation apparatus according to an exemplary embodiment of the present disclosure;

(9) FIG. 8 is a flowchart provided to explain an offset calculation method according to an exemplary embodiment of the present disclosure;

(10) FIG. 9 is view showing results of real measurement of RA, RB, RC, and RD and errors;

(11) FIG. 10 is a view showing results of calculation of R1, R2, R3, and R4 based on the results of real measurement of FIG. 9, and errors; and

(12) FIG. 11 is a view provided to illustrate verified performance of the results of calculation of offset voltages according to an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

(13) Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings.

(14) FIGS. 4 and 5 illustrate a planar hall resistance (PHR) sensor model, and an equivalent circuit, respectively. The PHR sensor is a kind of sensor for measuring magnetism and is of a Wheatstone bridge type.

(15) The PHR sensor is formed in a ring type and has resistance values of resistances R1, R2, R4, and R4 changed according to a correlation between a direction of a current and a direction of an external magnetic field when a bias current is applied. In this case, variations in resistance values of diagonally opposite resistances are the same, whereas variations in resistance values of adjacent resistances are contrary to each other.

(16) That is, as resistance values of resistances R1 and R3 increase, resistance values of resistances R2 and R4 decrease, and, as resistance values of resistances R2 and R4 increase, resistance values of resistances R1 and R3 decrease.

(17) As described above, the PHR sensor increases resistance values of one pair of diagonally opposite resistances of the four resistances and decrease resistance values of the other pair of diagonally opposite resistances according to an external magnetic field (an input magnetic field), and outputs an intensity of a magnetic field as a voltage based on a difference between the resistance values (see FIG. 1).

(18) When a bias current I flows, an offset voltage (Voffset) of the PHR sensor may be calculated based on the following equation:
Voffset={(R2*R4R1*R3)/(R1+R2+R3+R4)}*IEquation (1)

(19) According to the above-described equation, when R2*R4 equals R1*R3, the offset voltage is 0. That is, there is no offset when R2*R4=R1*R3 although R1, R2, R3, and R4 are all not the same.

(20) In addition, the sign (+/) of the offset voltage may be determined according to a magnitude relation between R2*R4 and R1*R3. Specifically, when R2*R4>R1*R3, the offset voltage is a positive (+) voltage, and, when R2*R4<R1*R3, the offset voltage is a negative () voltage.

(21) As shown in FIG. 6, a resistance value RA (R12) measured between node 1 and node 2 is a parallel resistance of R1 and R2+R3+R4 rather than R1. Likewise, a resistance value RB (R23) measured between node 2 and node 3 is a parallel resistance of R2 and R1+R3+R4, a resistance value RC (R34) measured between node 3 and node 4 is a parallel resistance of R3 and R1+R2+R4, and a resistance value RD (R41) measured between node 4 and node 1 is a parallel resistance of R4 and R1+R2+R3.

(22) RA, RB, RC, and RD may be expressed by the following equations:
RA=R1(R2+R3+R4)Equation (2)
RB=R2(R1+R3+R4)Equation (3)
RC=R3(R1+R2+R4)Equation (4)
RD=R4(R1+R2+R3)Equation (5)

(23) Since R1, R2, R3, and R4 are not really measured, RA, RB, RC, and RD, which are resistances between nodes, should be measured and then four simple simultaneous equations in four variables should be solved in order to obtain R1, R2, R3, and R4 values.

(24) A difference between resistances of adjacent nodes may be calculated based on the above-described equations as follows:
RARB=(R1*R3+R1*R4R2*R3R2*R4)/(R1+R2+R3+R4)Equation (6)
RBRC=(R1*R2+R2*R4R1*R3R3*R4)/(R1+R2+R3+R4)Equation (7)
RCRD=(R1*R3+R2*R3R1*R4R2*R4)/(R1+R2+R3+R4)Equation (8)
RDRA=(R1*R4+R3*R4R1*R2R1*R3)/(R1+R2+R3+R4)Equation (9)

(25) In addition, when both sides of equations (6) and (8) are added and both sides of equations (7) and (9) are added, the following equations are obtained:
(6)+(8)=RARB+RCRD=2(R2*R4R1*R3)/(R1+R2+R3+R4) Equation (10)
(7)+(9)=RCRD+RDRA=2(R1*R3R2*R4)/(R1+R2+R3+R4)Equation (11)

(26) From equation (10) or (11), following equation (12) may be derived:
(R2*R4R1*R3)/(R1+R2+R3+R4)=(RARB+RCRD)/2Equation (12)

(27) The left-hand side of equation (12) corresponds to a reference offset in equation (1) indicating an offset voltage of a PHR sensor. Accordingly, when the right-hand side of equation (12) is substituted for equation (1), the offset voltage of the PHR sensor in which a reference offset is expressed by RA, RB, RC, and RD rather than R1, R2, R3, and R4 may be obtained, which is expressed as follows:
Voffset={(RARB+RCRD)/2}*IEquation (13)

(28) Much calculation and much time are required to calculate an offset voltage by obtaining R1, R2, R3, and R4 from RA. RB, RC, and RD.

(29) However, using equation (13), the offset voltage of the PHR sensor may be calculated rapidly and easily without complicated calculation. As shown in equation (13), the size of the offset may be determined simply by calculating the reference offset (RARB+RCRD)/2 with resistances between nodes.

(30) Furthermore, since equation (13) provides the reference offset voltage which is independent from the bias current, that is, which has nothing to do with a change in the bias current, equation (13) has high availability.

(31) An apparatus for calculating an offset voltage of a PHR sensor using equation (13) will be described in detail with reference to FIG. 7. FIG. 7 is a block diagram showing an offset calculation apparatus according to an exemplary embodiment of the present disclosure.

(32) As shown in FIG. 7, the offset calculation apparatus according to an exemplary embodiment of the present disclosure includes an RA measurement unit 111, an RB measurement unit 112, an RC measurement unit 113, an RD measurement unit 114, an offset calculator 120, an offset information provider 130, and an input unit 140.

(33) The RA measurement unit 111, the RB measurement unit 112, the RC measurement unit 113, and the RD measurement unit 114 are resistance measurement devices which measure RA, RB, RC, and RD values of the PHR sensor, respectively. The RA, RB, RC, and RD measured by the measurement units 111-114 are transmitted to the offset calculator 120.

(34) The offset calculator 120 calculates a reference offset (RARB+RCRD)/2 according to above-described equation (13). When a user inputs information (I) on a bias current value through the input unit 140, the offset calculator 120 may calculate an offset voltage [Voffset={(RARB+RCRD)/2}*I] of the PHR sensor.

(35) The offset information provider 130 is a display which visually outputs the reference offset and the offset voltage calculated by the offset calculator 120, and may be implemented by using a liquid crystal display (LCD), a 7-segment, or the like.

(36) Furthermore, the offset information provider 130 and the input unit 140 may be integrated into a touch screen.

(37) An offset calculation process performed by the offset calculation apparatus shown in FIG. 7 will be described in detail with reference to FIG. 8. FIG. 8 is a flowchart showing an offset calculation method according to an exemplary embodiment of the present disclosure.

(38) As shown in FIG. 8, when the PHR sensor is mounted in the offset calculation apparatus, the RA measurement unit 111 measures the RA value of the PHR sensor (S210), the RB measurement unit 112 measures the RB value of the PHR sensor (S220), the RC measurement unit 113 may measure the RC value of the PHR sensor (S230), and the RD measurement unit 112 measures the RD value of the PHR sensor (S240).

(39) Then, the offset calculator 120 calculates a reference offset and an offset voltage using the results of measurement in steps S210 to S240 (S250).

(40) In addition, the offset information provider 130 displays the offset information calculated in step S250 and provides the same to the user (S260).

(41) FIG. 9 illustrates a minimum value of real measurements RA (R12), RB (R23), RC (R34), and RD (R41) regarding sensor types #2, #3, and #4 shown in FIG. 2, and an error rate which is calculated with reference to the minimum value, and FIG. 10 illustrates results of calculation of R1, R2, R3, and R4 based on the results of real measurement of FIG. 9, and errors.

(42) As described above, in deriving the results of calculation shown in FIG. 10, much calculation and much time are required.

(43) In table of FIG. 11, 1) measured values indicate results of real measurement of offset voltages after a bias current is applied according to a related-art method, 2) calculated values after resistance values are measured indicate results of calculation of offset voltages using R1, R2, R3, and R4 suggested in FIG. 10, and 3) algorithm applied indicates results of calculation of offset voltages using RA (R12), RB (R23), RC (R34), and RD (R41) suggested in FIG. 9 according to an exemplary embodiment of the present disclosure.

(44) As shown in FIG. 11, it can be seen that the results of calculation of offset voltages according to an exemplary embodiment of the present disclosure are not greatly different from the results of real measurement or calculation of offset voltages using other methods.

(45) The method and the apparatus for calculating the offset voltage of the PHR sensor have been described by referring to preferred embodiments.

(46) The PHR sensor mentioned in the above-described embodiments is a kind of a magnetic sensor or a current sensor having a Wheatstone bridge type, and is merely an example. The technical idea of the present disclosure can be applied to other types of sensors having the Wheatstone bridge type.

(47) Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.