Fluxgate Magnetic Sensor 900
First, the principle of detection by a conventional fluxgate magnetic sensor 900 will be briefly described. As illustrated in FIG. 1, a toroidal winding is provided, as an excitation coil 2, to a ring-shaped magnetic core 1 (illustrated by a dashed circular line in FIG. 1) which is made of a high magnetic permeability material such as permalloy, and a solenoidal winding is further provided to the outer side of the ring-shaped magnetic core 1 as a detection coil 4, all together constituting a sensor unit 10.
An excitation current IH with a frequency f0, which has, for example, a rectangular waveform symmetrically alternating between positive and negative, is supplied to the excitation coil 2 from an excitation circuit 20. The excitation current IH causes a magnetomotive force in the excitation coil 2, and the resulting magnetic flux φ is linked with the detection coil 4. Where the value of the excitation current IH is set so as to have a magnitude sufficient to magnetically saturate the ring-shaped magnetic core 1, the ring-shaped magnetic core 1 is periodically magnetically saturated.
When the ring-shaped magnetic core 1 is excited at the frequency f0, a pulse waveform with a frequency 2f0 is generated in the detection coil 4 as a sensor output signal SS. Where an external magnetic field H such as the earth's magnetism exists, the pulse width and amplitude of the pulse waveform vary depending on the magnitude of the external magnetic field H. Accordingly, the pulse waveform with the frequency 2f0 is detected by means of a sensor output signal detection unit 30 as a magnetic sensor output SM, enabling measurement of the external magnetic field H. The details of the operating principle are described in, e.g., Japanese Patent Application Laid-Open No. 2009-92381 (Hereinafter “Patent literature 1”).
However, in this case, the amplitude of the sensor output signal SS has a linear relationship with the magnitude of the external magnetic field only when the external magnetic field is small. Accordingly, the fluxgate magnetic sensor 900 in FIG. 1 is problematic because of its poor linearity and thus narrow measurement range.
Fluxgate Magnetic Sensor 910
As art for avoiding the aforementioned problem, there is a method using a feedback magnetic field H′. A conventional one-axis closed-loop fluxgate magnetic sensor 910 will be described with reference to FIG. 2. The fluxgate magnetic sensor 910 creates a feedback magnetic field H′ that is equal in absolute value but opposite in polarity to an external magnetic field H, and performs feedback control so that a magnetic field in a sensor unit 10 is consistently a zero magnetic field.
A signal detection/feedback unit 130 converts an output of the sensor unit 10 into a current, and gives feedback to a detection coil 4 using the current. As a result of the feedback current flowing in the detection coil 4, a feedback magnetic field H′ having polarity opposite to that of the external magnetic field H is generated, and the feedback current increases until the absolute value of the feedback magnetic field H′ becomes equal to that of the external magnetic field H. The feedback current reaches equilibrium when the magnetic field present in the detection coil becomes zero. Since this current is proportional to the external magnetic field H, the external magnetic field H can be detected by detecting the current. In such a manner as described above, a fluxgate magnetic sensor device with good linearity and thus, a wide measurement range can be provided.
Multi-Axis Fluxgate Magnetic Sensor 920
A conventional multi-axis fluxgate magnetic sensor 920 will be described with reference to FIGS. 3A and 3B. Here, a description will be given taking a three-axis one.
As with the fluxgate magnetic sensor 900, a toroidal winding is provided to a ring-shaped magnetic core (not illustrated) as an excitation coil 2-1, and a solenoidal winding is provided to the outer side of the ring-shaped magnetic core as a second detection coil 4. A solenoidal winding is further provided to the outer side of the ring-shaped magnetic core as a first detection coil 3 in such a manner that the axis is orthogonal to the second detection coil 4. The ring-shaped magnetic core, the excitation coil 2-1, the first detection coil 3 and the second detection coil 4 are included in a first/second sensor unit 110.
Furthermore, a toroidal winding is provided to a ring-shaped magnetic core (not illustrated) as an excitation coil 2-2, and a solenoidal winding is provided to the outer side of the ring-shaped magnetic core as a third detection coil 5. The ring-shaped magnetic core, the excitation coil 2-2 and the third detection coil 5 are included in a third sensor unit 210. FIGS. 3A and 3B each illustrate coordinate axes x, y and z for indicating the relationship in axis directions between the first detection coil 3, the second detection coil 4 and the third detection coil 5. The third sensor unit 210 is disposed in such a manner that an input axis (for example, the z-axis) of the third detection coil 5 is orthogonal to each of input axes (for example, the x-axis and the y-axis) of the first detection coil 3 and the second detection coil 4. In FIG. 3A, a front view of the third sensor unit 210 is illustrated, and in FIG. 3B, a plan view of the third sensor unit 210 is illustrated. The first detection coil 3 is connected to a signal detection/feedback unit 130-1, the second detection coil 4 is connected to a signal detection/feedback unit 130-2, and the third detection coil 5 is connected to a signal detection/feedback unit 130-3. The signal detection/feedback units 130-1, 130-2 and 130-3, upon receipt of sensor output signals SS1, SS2 and SS3 from the first detection coil 3, the second detection coil 4 and the third detection coil 5, each create a feedback current as with the signal detection/feedback unit 130 in FIG. 2, and also create first, second and third magnetic sensor outputs SM1, SM2 and SM3, respectively. The respective detection coils 3 to 5 create first to third feedback magnetic fields H′x, H′y and H′z from the respective feedback currents, and perform sensing by means of a method similar to that of the fluxgate magnetic sensor 910 in FIG. 2. Thus, a three-axis fluxgate magnetic sensor with input axes orthogonal to one another is provided.
Conventionally, when a multi-axis fluxgate magnetic sensor is used to, e.g., control the orientation of an oil field drilling tool, there is a problem in that the magnetic characteristics, etc., of the ring-shaped magnetic core vary according to the temperature because of the frictional heat accompanying the drilling or, e.g., geothermal heat, causing changes in the output. Such changes in the magnetic characteristics are often non-linear with respect to the temperature, and thus, the temperature characteristic of a sensor output is inevitably non-linear with respect to the temperature. For art in which a temperature detection mechanism using a thermistor is provided in a signal processing circuit to compensate for a gain decrease in the circuit, which is caused by a temperature increase, Japanese Patent Application Laid-Open No. 2002-71773 (hereinafter “Patent literature 2”) is known.
Patent literature 2 is effective for use in the case where it is only necessary to simply keep a gain of no less than a certain value, but is problematic in that it is insufficient for use in the case where the linearity of the temperature characteristic curve is required.
Furthermore, while when multiple axes are employed in a ring-shaped magnetic core-equipped fluxgate magnetic sensor, detection coils must be arranged so that the axes of the coils are orthogonal to each other, it is known that the two orthogonal axes cause interference, resulting from the heterogeneity of the material of the ring-shaped magnetic cores and in addition, the interference amount has temperature dependency (Reference literature 1: P. Brauer, J. M. G. Merayo, O. V. Nielsen, F. Primdahl and J. R. Petersen, “Transverse field effect in fluxgate sensors”, Sensors and Actuators A: Physical, 1997, vol. 59, Volume 59, p.p. 70-74).
Accordingly, the conventional multi-axis fluxgate magnetic sensor 920 has a problem in that interference IFAX occurs between orthogonal feedback magnetic fields (for example, between the first feedback magnetic field H′x and the second feedback magnetic field H′y), disabling accurate sensing.
Although measuring the interference amount in advance and performing compensation can be considered, the interference amount has temperature dependency, and the temperature dependency is non-linear with respect to the changes in the temperature. Thus, the interference amount cannot easily be compensated for. If the change in the interference amount with respect to the temperature is substantially linear, the interference amount can easily be compensated for; however, if the change in the interference amount with respect to the temperature is non-linear and compensation is performed using a polynomial approximation, although it is desirable that the order of the compensation formula be high, the order of the compensation formula is ordinarily restricted because of the limits of the compensation system. If an order that is high enough to respond to non-linear change in data cannot be attained, the compensation residual increases, disabling provision of a sensor sufficient for practical use.
Here, a brief description is provided below with regard to an interference amount having temperature dependency and the temperature dependency being non-linear with respect to the changes in the temperature (see Reference literature 1). First, magnetic flux Φd in each detection coil can be expressed by the following expression:
      Φ    d    =                              L                      d            ⁢                                                  ⁢            0                          ⁡                  (          t          )                    ×                        i          d                ⁡                  (          t          )                      +                  μ        0            ⁢              A        core            ×                        ∫          0                      2            ⁢            π                          ⁢                              ∑                          n              =              1                                      N              d                                ⁢                                                    h                t                1                            ⁡                              (                                  n                  ,                  θ                                )                                      ×                          M              ⁡                              (                                  t                  ,                  θ                                )                                      ×                          ⅆ                              ×                d                ⁢                                                                  ⁢                θ                                                        
Here, Φd denotes the entire magnetic flux in the coil, Ld0(t) denotes a self-inductance of the detection coil (where no magnetic core is provided), id(t) denotes the detection current, μ0 denotes a magnetic permeability of a ring-shaped magnetic core, Acore denotes a cross-sectional area of the ring-shaped magnetic core, d is a diameter of the ring-shaped magnetic core, θ denotes an angle expressed as a rotational position of the ring-shaped magnetic core, M(t, θ) denotes magnetization in the tangential direction of the ring-shaped magnetic core, ht1(n, θ) denotes a component in tangential direction of a magnetic field in the ring-shaped magnetic core (which is generated by a n-th detection winding) and Nd denotes the number of turns of the detection coil.
Here, the magnetization M(t, θ) in the tangential direction of the ring-shaped magnetic core is proportional to a magnetic susceptibility χ(θ) and inversely proportional to the square of saturated magnetization Ms (see Reference literature 1). The magnetic susceptibility χ(θ) is an amount depending on a component in the rotational direction of the ring-shaped magnetic core, which causes the interference between the orthogonal axes. Also, it is known that saturated magnetization Ms generally decreases with a temperature increase, and it has a non-linear characteristic. Accordingly, the magnetization M(t,θ) is a function between the magnetic susceptibility χ(θ) and the temperature, and the amount of the interference between the orthogonal axes non-linearly varies according to the temperature. Because of the non-linearity of the interference amount, it is difficult to compensate for the interference amount, disabling accurate sensing.
For example, the temperature characteristic of the sensor sensitivity of the conventional multi-axis fluxgate magnetic sensor 920 exhibits a temperature coefficient that rapidly increases upon reaching a certain temperature, as indicated by a solid line in FIG. 4. It is presumed that this is attributable to the material, etc., of the ring-shaped magnetic core having a non-linear temperature coefficient, which affects the amount of interference between the orthogonal axes. The details of the measurement method in FIG. 4 will be described later.