JPH0452415B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for calibrating a magnetometer using a magnetic field detection element such as a Hall element, and in particular to a method for calibrating a magnetometer using a magnetic field detection element such as a Hall element. The magnetic field detection value can be automatically calibrated according to changes in sensitivity. Prior Art In a conventional magnetometer using this type of magnetic field detection element, for example, a Hall element, the Hall element driving section is temperature-compensated, or the magnetic field detection probe is inserted into a magnetic field shielded casing. The magnetic field detection value was calibrated by measuring the offset value of the magnetic field detection. In addition, from the magnetic field measurement value and the environmental temperature at the time of measurement, a calibration curve of the magnetic field measurement value against temperature change was used to determine the accurate magnetic flux value of the measured magnetic field, but the temperature of the measurement environment and the magnetic field detection For example, it is impossible to eliminate measurement errors due to differences in the temperature of the probe itself.
The conventional method of calibrating this type of magnetometer has the disadvantage that it cannot be calibrated with sufficient accuracy. Furthermore, in a magnetometer using a Hall element, the magnetic field detection offset value generally fluctuates based on temperature changes of about 0.5 Gauss/°C, so in particular when detecting weak magnetic fields of 1 Gauss or less, Temperature changes in degrees Celsius greatly affect magnetic field measurements. Therefore, the conventional magnetometer using a Hall element is
It has problems such as being unsuitable for measuring weak magnetic field strength, and furthermore, it cannot automatically correct the offset of magnetic field detection, which limits its use as a magnetic field sensor for automatic control. There were also drawbacks. Summary of the Invention It is an object of the present invention to solve the above-mentioned conventional problems and eliminate their drawbacks, and to automatically correct fluctuations in magnetic field detection sensitivity caused by temperature changes of magnetic field detection elements using Hall elements etc. Therefore, it is an object of the present invention to provide a method for calibrating a magnetometer using a magnetic field detection element, which makes it possible to obtain accurate magnetic field measurement values. That is, in the calibration method of the present invention, in a magnetometer in which three magnetic field detecting elements each detecting magnetic fields in three directions orthogonal to each other are arranged in combination, each magnetic field detection output X, Y,
Z, each magnetic field detection sensitivity R _x , R _y , R _z and each magnetic field detection offset a, b, c, and the true value M of the magnetic field strength in the detected magnetic field (X-a/R _x ) ² + (Y-b/R _y ) ² + (Z-c/R _z )
² = M ² , each of the magnetic fields when the magnetic flux meter is directed in at least six directions and the three magnetic field detection elements detect each magnetic field for which the true value M of the strength of the magnetic field is known. From at least six sets of simultaneous equations obtained by substituting the values of the detection outputs X, Y, and Z and the true value M of the known magnetic field strength, each of the magnetic field detection sensitivities R _x , R _y , R _z The magnetometer is calibrated by calculating the values of each of the magnetic field detection offsets a, b, and c, and the magnetic field to be detected in any direction is detected by the three magnetic field detection elements, and each of the magnetic fields is When the values of the detection outputs X, Y, and Z are obtained, the true value M of the strength of the magnetic field in the detected magnetic field in any direction can be calculated from the relational expression. It is. EXAMPLES The present invention will be explained in detail below using examples with reference to the drawings. First, FIG. 1 shows an example of the configuration of an apparatus for calibrating a magnetometer according to the method of the present invention. In the illustrated configuration, a magnetic field measuring probe 4 shown surrounded by a dotted line includes three magnetic field detecting elements, for example, Hall elements 5, 6, and 7, and these three magnetic field detecting elements 5, 6, 7 are arranged in combination so as to be able to detect magnetic fields in three directions that are not on the same plane, for example, three mutually orthogonal directions x, y, and z. Moreover, those magnetic field detection elements 5, 6, 7 are 3
They are driven and operated by respective drive circuits 1, 2, and 3. For example, when the magnetic field detection element is a Hall element, a constant current or a constant voltage is supplied to each control terminal of these elements. Each magnetic field detection output voltage is supplied via three DC amplifier circuits 8, 9, and 10 to an arithmetic processing unit 11, which is indicated by a dotted line and constituted by, for example, a microcomputer. In the arithmetic processing unit 11, the magnetic field detection output voltages in each direction x, y, and z supplied to input terminals #1, #2, and #3 are sequentially switched by a multiplexer 12 and guided to an analog-digital converter 13. , and supplies it to the arithmetic unit 14 in the form of digital data. In the arithmetic unit 14, according to the program of the calibration method of the present invention stored in advance in the read-only memory 16, and
The random access memory 17 is used in the calculation process to perform calculations as will be described later on the input data, remove the offset included in the magnetic field detection output, and then generate an output that represents the direction, strength, etc. of the magnetic field to be measured. Data is retrieved via interface 15. Note that when the illustrated device is used as a magnetometer, output data is supplied to and displayed on an appropriately configured display section (not shown). However, in order to directly obtain temperature information of the magnetic field detection element, which is one of the causes of offset included in measurement data when the above-mentioned device is used as a magnetometer, the configuration shown in Figure 2 is required. change. That is, as described above, the drive circuit 18
The magnetic field detection output of the magnetic field detection element 19 driven by is supplied to the arithmetic processing unit 21 via the amplifier circuit 20, and the internal resistance value appearing between the input terminals of the magnetic field detection element 19 is also supplied to the arithmetic processing unit 21,
The temperature of the magnetic field detecting element to be used is determined based on an internal resistance-temperature characteristic curve that has been previously measured and stored, and the offset value of the measurement data generated at that temperature is calculated.
For example, when a Hall element is used as the magnetic field detection element 19 and driven at a constant voltage by the drive circuit 18, the constant voltage value and the current value appearing between the input terminals of the Hall element 19 are supplied to the arithmetic processing unit 21, and the driving When constant current is driven by the circuit 19, the constant current value and the voltage value appearing between the input terminals of the Hall element 19 are supplied to the arithmetic processing section 21, and the element internal resistance value determined from the input voltage and current value is calculated based on the element temperature. The element temperature is determined by comparing it with a calibration value that has been measured and stored in advance. Next, when the circuit device shown in Figure 1 is used as a magnetometer, the detection output value is automatically calibrated by determining offsets and changes in detection sensitivity caused by changes in the measurement environment such as temperature. The magnetometer calibration method of the present invention will be explained. Now, a magnetic field detection element, for example, a Hall element 5,
6 and 7 are arranged so that their respective magnetic field detection directions are orthogonal to each other, the magnetic field detection elements 5,
Due to variations in the magnetic field detection sensitivity and input/output resistance of 6 and 7, and differences in the amplification degree and offset of the amplifier circuits 8, 9, and 10, the magnetic field detection sensitivity and offset value vary depending on the x, y, and z directions of the magnetic field. Each one is different. Now, if the true strength of the magnetic field in each of the x, y, and z directions is M _x , M _y , and M _z , then
The amplified output values X, Y, and Z mentioned above are each expressed by the following equations. X=R _x・M _x +a Y=R _y・M _y +b Z=R _z・M _z +c (1) Here, R _x , R _y , and R _z are magnetic field detection in each direction of x, y, and z, respectively. It is the detection sensitivity of the system, and a, b, and c are the offset values of each of these systems, all of which are functions of temperature. Note that, physically, the magnetic field detection sensitivities R _x , R _y , and R _z correspond to Hall constants, and the amplified outputs X, Y, and Z correspond to Hall electromotive force. Therefore, first, a reference uniform magnetic field whose true value of magnetic field strength is known, such as earth's magnetism, is detected using a magnetometer having such a configuration. If the strength of the uniform magnetic field is M, the x, y, and z directions are perpendicular to each other, so the relational expression (2) holds: M _x ² + M _y ² + M _z ² = M ² (2) Two. Then, by substituting equation (1) into equation (2), the following equation (3) is obtained. (X-a/R _x ) ² + (Y-b/R _y ) ² + (Z-c/R _z )
² = M ² (3) Therefore, since the magnetic field strength M of the reference uniform magnetic field is constant, the right-hand side M ² of equation (3) is constant, and the three systems are arranged in combination with the magnetic field detection directions orthogonal to each other. The magnetic field detection values X, Y, and Z when the magnetic field detection elements 5, 6, and 7 are oriented in various directions are given by the offset values a, b, and c, as shown in It is on a spherical surface centered on point P (a, b, c), and always satisfies the relationship of equation (3). This (3)
Since the formula includes six variables a, b, c, R _x , R _y , R _z , each magnetic field when the magnetic field detection directions of the magnetic field detection elements 5, 6, and 7 are changed in six ways. By obtaining the detected values X, Y, and Z, respectively,
Six simultaneous equations are obtained, and each offset value a,
b, c and each magnetic field detection sensitivity R _x , R _y , R _z can be calculated accurately. Note that the direction of the magnetic field measurement probe, and therefore each magnetic field detection element 5, 6,
If the magnetic field detection values X, Y, Z when repeatedly measured with the directions of the magnetic field detection elements 5, 6, and 7 fixed in six different ways differ each time due to changes in temperature, etc., the directions of the magnetic field detection elements 5, 6, and 7 are changed in seven or more ways to obtain the respective detected magnetic field values, and six variables a, b, c, R _x , R _y , and R _z are calculated using the least squares method or the like. Next, a method of correcting the detected magnetic field values measured in six directions as described above will be explained. Now, define the matrix Γ as follows. Γ＝X ² ₁ Y ^{2 1} _Z ² ₁ X ₁ Y ₁ Z _{1 1} X ² ₂ Y ² ₂ Z ² ₂ X ₂ Y ₂ Z ₂ 1 … … X ² ₆ ^{Y 2 6} _{Z 2 6 6} 1 (4) Here, X _k , Y _k , Z _k (k=1, 2,..., 6) are measured values in six directions. Now, regarding the determinant (4) of Γ consisting of 6 rows and 7 columns, for example, the determinant of the remaining 6 rows and 6 columns after removing the first column (X ² ₁ , X ² ₂ , ..., X ² ₆ ) is Expressed as ΓX ² , the second
The remaining determinant after removing the columns (Y ² ₁ , Y ² ₂ , ..., Y ² ₆ ) is expressed as ΓY ² , and in the same manner, the 6th column (Z ₁ ,
When the residual determinant after removing Z ₂ ,..., Z ₆ ) is expressed as ΓZ, each variable a, b, c,
R _x , R _y , and R _z are expressed by the following equation (5).

[Formula] A calculation program for calculating each variable based on this formula (5) is stored in the read-on memory 16 in the calculation processing unit 11, and the calibration button switch is pressed to send the calculation program to the calculation processing unit 11. Execute calculations according to the program. That is, the measurement data obtained by sequentially pointing the magnetic field measurement probe 4, which is a combination of magnetic field detection elements 5, 6, and 7 in the six directions mentioned above, is digitized by the analog-to-digital converter 13 in the arithmetic processing section 11. Then, it is sequentially stored in the random access memory 17, read out when the number of measurement points required for calculation processing is reached, and the calculation unit 14 automatically performs the calibration calculation according to the program read out from the read-only memory 16. The obtained variables a, b, c, R _x , R _y , R _z
The value of is stored in the random access memory 17 again. During normal magnetic field measurement, the true value of the magnetic field measurement, which has been subjected to the arithmetic processing of equation (5), is taken out via the interface 15. Therefore, each variable obtained as described above,
That is, in order to determine the relationship between the offset values a, b, c and detection sensitivities R _x , R _y , R _z of each system and the measured environmental temperature, the environmental temperature is also measured at the same time as the magnetic field measurement. This temperature can be measured by (1) directly measuring the temperature of a magnetic field detection element such as a Hall element, or (2) as explained in Figure 2, by applying a voltage between the input terminals of the Hall element and input voltage. Detect the value of the current flowing between the terminals or the value of the current flowing between the input terminals and the voltage appearing between the input terminals, and calibrate in advance the change in internal resistance value determined from these currents and voltages due to temperature. This can be done by leaving it in place. Next, the method of the above-mentioned item (2), that is, the indirect determination of the magnetic field detection element temperature will be explained in detail. First, FIG. 4 shows an input equivalent circuit of a magnetic field detection element, such as a Hall element. An offset in magnetic field detection is caused by a change in internal resistance value determined from the current voltage between the input terminals of the Hall element due to temperature. Further, changes in magnetic field detection sensitivity also occur due to changes in the element constant of the Hall element. That is, a constant voltage or constant current is supplied to the drive terminal of the Hall element, that is, the input terminal that supplies the element drive voltage or current, to drive the element.
When the input resistance changes, the current flowing between the input terminals or the voltage appearing between the input terminals changes, so the state of the element can be known by measuring the input voltage and current values. Detecting the state of the element in this manner is equivalent to using the Hall element itself as a temperature sensor, and the state of the element can be detected more accurately than a normal temperature sensor that measures only temperature. In addition, to obtain the temperature characteristic curve of offset voltage value and detection sensitivity, increase the temperature in steps of, for example, 5°C within the desired temperature range, and calculate the offset voltage value and detection sensitivity at each temperature as described above. It is preferable to calculate the temperature, plot the temperature, and linearly interpolate the intermediate temperature points. By detecting the state of the Hall element, such as its temperature, from the Hall element itself as described above,
It becomes possible to automatically determine the offset value versus temperature characteristic and the detection sensitivity versus temperature characteristic of the Hall element. Effects As is clear from the above explanation, according to the present invention, the offset value and detection sensitivity, which change depending on the temperature of a magnetic field detection element, such as a Hall element, are automatically calibrated by calculation to detect weak signals. Even in the case of a strong magnetic field, a special effect can be obtained in that the true value of the strength of the magnetic field can be easily and accurately determined.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of the configuration of a magnetic field detection device used in the calibration method of the present invention, FIG. 2 is a block diagram showing another example of the configuration of the magnetic field detection device, and FIG. A diagram showing the principle of calibration, and FIG. 4 is a circuit diagram showing an equivalent circuit of a Hall element. 1, 2, 3, 18... Drive circuit, 4... Magnetic field measurement probe, 5, 6, 7, 19... Magnetic field detection element (Hall element), 8, 9, 10, 20... Amplification circuit, 11, 21... Arithmetic processing unit, 12... Multiplexer, 13... Analog-digital converter, 14... Arithmetic unit, 15... Interface,
16...Read-only memory, 17...Random access memory.

Claims (1)

[Scope of Claims] 1. In a magnetometer configured by combining and arranging three magnetic field detection elements that respectively detect magnetic fields in three directions perpendicular to each other, each magnetic field detection output X, Y of the three magnetic field detection elements , Z, each magnetic field detection sensitivity R _x , R _y ,
(X-a/R _x ) ² + (Y-b/R _y ) is established between R _z and each magnetic field detection offset a, b, c and the true value M of the magnetic field strength in the detected magnetic field. ² + (Z-c/ _Rz )
² = M ² , each of the magnetic fields when the magnetic flux meter is directed in at least six directions and the three magnetic field detection elements detect each magnetic field for which the true value M of the strength of the magnetic field is known. From at least six sets of simultaneous equations obtained by substituting the values of the detection outputs X, Y, Z and the true value M of the known magnetic field strength, each of the magnetic field detection sensitivities R _x , Ry, R _z and The magnetic flux meter is calibrated by calculating the values of the magnetic field detection offsets a, b, and c, and the magnetic field to be detected in any direction is detected by the three magnetic field detection elements to detect each magnetic field. Magnetic field detection characterized in that when the values of the outputs X, Y, and Z are determined, the true value M of the strength of the magnetic field in the detected magnetic field in any direction can be calculated from the relational expression. Calibration method of magnetometer using element.