ELECTROMAGNETIC FLOWMETER SENSOR CAPABLE OF DETECTING MAGNETIC FIELD AND MAGNETIC PERMEABILITY Field of The Invention The invention belongs to the field of flow measurement technologies, relates to an electromagnetic flowmeter sensor, and particularly relates to an electromagnetic flowmeter sensor capable of detecting a magnetic field and a magnetic permeability. Background of The Invention The evolution of the electromagnetic flowmeter is also regarded as the evolution of the excitation mode from the perspective of the magnetic field. The excitation mode mainly experiences the following four evolution stages from the perspective of magnetic field stabilization and compensation: 1. Constant voltage excitation mode, in which square waves and sine waves both exist. Since the copper resistance changes with the temperature, the excitation current changes accordingly, which results in change of the magnetic field and affects the measurement precision. Therefore, this mode is generally abandoned. 2. Constant current excitation mode. After the electromagnetic flowmeter adopting the constant current excitation mode is used for a period of time, it needs to detect whether the current changes, and if yes, the current needs to be adjusted to the original value, or an actually measured value is input into the electromagnetic flowmeter for software compensation. Most of the existing electromagnetic flowmeters adopt this mode. However, although this mode is applicable to many scenarios, the measurement precision is affected and the long-term stability is also undesirable when the magnetic permeability changes, for example, in the presence of iron ore slurry and magnetic circuit corrosion. 10002 1 2002. 8 IIliI I 3. Current feedback mode. The automatic compensation of the actually measured value to the change of the excitation current saves the trouble of periodic regulation or compensation of the excitation current, but still cannot be applied when the magnetic permeability changes, so that this mode has the same defect as the constant current excitation mode. Moreover, when the excitation current changes direction, the current applies an impact force on the excitation coil. After the electromagnetic flowmeter is used for a long time, its insulating layer is aging, and frequent impact of the electromagnetic force may result in interturn short-circuit. It does not matter that the magnetic field is stabilized through the constant current or the change of the magnetic field is compensated through current feedback, once the interturn short-circuit occurs, the magnetic field is weakened, and the measured flow is below the destination flow. 4. Reference coil mode was once regarded as a magnetic field feedback mode by mistake. It has been discovered from careful study that since the reference coil is used to be wound with the excitation coil, most of the signals are induced voltage signals caused by the change of the leakage flux component, and since the signal component caused by the main measured flux change is small, the reflected change of the magnetic field is limited. Therefore, this mode is substantially an excitation voltage feedback mode. In view of the above, when the magnetic permeability changes, the existing electromagnetic flowmeter sensor obtains imprecise measured values and has unstable performance. The discussion of the background to the invention included herein including reference to documents, acts, materials, devices, articles and the like is included to explain the context of the present invention. This is not to be taken as an admission or a suggestion that any of the material referred to was published, known or part of the common general knowledge in Australia or in any other country as at the priority date of any of the claims. 10002 2 2002. 8111 III lii I I Summary of the Invention Accordingly, the invention is directed to providing an electromagnetic flowmeter sensor capable of detecting a magnetic field and a magnetic permeability, and the electromagnetic flowmeter sensor is capable of measuring a fluid with a variable magnetic permeability. In accordance with one aspect of the present invention, there is provided an electromagnetic flowmeter sensor capable of detecting a magnetic field and a magnetic permeability, the sensor including flanges and a measuring tube. The flanges are installed on two ends of the measuring tube. The sensor also includes a reference coil and an excitation coil fixedly provided on the measuring tube, wherein the reference coil and the excitation coil are separately wound. The reference coil is kept a certain distance away from the excitation coil to substantially avoid an application range of leakage flux. In some embodiments, the excitation coil is fixed on an external side wall of the measuring tube, and the reference coil is wound into an elongated shape, like a center shaft of the excitation coil which is located in the middle of the excitation coil. In some embodiments, the reference coil is close to an external side wall of the measuring tube, and the reference coil is larger than the excitation coil by full size and encloses the excitation coil. In some embodiments, a magnetic circuit board is disposed on an external side wall of the measuring tube, two ends of the magnetic circuit board are fixed on the measuring tube through pole shoes, and the reference coil is wound on the magnetic circuit board. In some embodiments, a magnetic circuit board is disposed on an external side wall of the measuring tube, two ends of the magnetic circuit board are fixed on the measuring tube through pole shoes, the excitation coil is fixed on one of the pole shoes, and the reference coil is fixed on the other one of the pole shoes. In some embodiments, the reference coil is a single coil or formed by several serially-connected coils. 10002 3 2002. 8111 III lii I I The invention has the following beneficial effect. The electromagnetic flowmeter sensor of the invention is capable of measuring a fluid with a variable magnetic permeability, and the proportion of signals reflecting the main flux change among the reference coil signals is raised by more than four times and is generally not affected by eddy current in the alternating current excitation mode. As a result, the zero flow is stable and the measurement precision is improved. Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof. Brief Description of the Drawings FIG. 1 is a front view of an electromagnetic flowmeter adopting a center-shaft winding manner according to Embodiment 1; FIG. 2 is a top view of the electromagnetic flowmeter adopting a center-shaft winding 10002 3a 2002. 8111 III lii I I manner according to Embodiment 1; FIG. 3 is a front view of an electromagnetic flowmeter adopting an enclosed-type winding manner according to Embodiment 2; FIG. 4 is a top view of the electromagnetic flowmeter adopting an enclosed-type winding manner according to Embodiment 2; FIG. 5 is a front view of an electromagnetic flowmeter adopting a bead-type winding manner according to Embodiment 3; and FIG. 6 is a top view of the electromagnetic flowmeter adopting a bead-type winding manner according to Embodiment 3. List of Reference Numerals: 11. flange; 12. measuring tube; 13. excitation coil; 14. reference coil; 15. electrode; 21. pole shoe; 22. excitation coil; 23. flange; 24. measuring tube; 25. reference coil; 26. electrode; 31. pole shoe; 32. excitation coil; 33. flange; 34. measuring tube; 35. reference coil; 36. magnetic circuit board. Detailed Description of the Preferred Embodiments The invention is directed to realize magnetic field feedback, which is the development trend of the electromagnetic flowmeter technology as can be seen from the evolution of the electromagnetic flowmeter. As long as the magnetic field is stabilized or the change of the magnetic field can be compensated in time, the limits of the electromagnetic flowmeter can be eliminated. The 10002 4 2002. 8 IIliI I invention is applicable to scenarios that the magnetic permeability changes, for example, in the presence of iron ore slurry, which ensures the measurement precision of the electromagnetic flowmeter in actual applications and improves the long-term stability of the electromagnetic flowmeter. The specific implementation of the invention is illustrated in detail below with reference to the accompanying drawings. Embodiment 1 Referring to FIG. 1 and FIG. 2, this embodiment provides an electromagnetic flowmeter sensor capable of detecting a magnetic field and a magnetic permeability, which includes flanges 11 and a measuring tube 12. The flanges 11 are installed on two ends of the measuring tube 12. A reference coil 14 and an excitation coil 13 are fixedly provided on an external side wall of the measuring tube 12, and the reference coil 14 and the excitation coil 13 are separately wound at an interval. The reference coil 14 is not wound together with the excitation coil 13, and is located away from the excitation coil by a distance as far as possible according to the assembly space. The reference coil 14 is not only located away from the flanges 11 but also located away from an electrode 15. In the case of a large-aperture electromagnetic flowmeter, since the excitation coil is close to the outer side of the measuring tube, the reference coil may be formed into an elongated shape, like a center shaft of the excitation coil which is located in the middle of the excitation coil. This installation mode is called a center-shaft installation mode. The center-shaft installation mode is applicable to a large-aperture electromagnetic flowmeter. The reference coil may be one or two or more serially-connected coils. The reference coil may be formed by a wound enamel wire, which has a low cost, or formed by one or more flexible circuit boards, which achieves good consistence. The number of turns of the reference coil is determined by experiments according to a matching voltage of a converter circuit. Generally, in rated excitation, the output voltage of reference signal is about 100 mV. The principle is that the reference coil keeps a certain distance away from the excitation coil and tries to avoid the application range of the leakage flux. It can be seen from the experiment that, compared with the conventional reference coil, in the invention, the proportion of signals reflecting the main flux change among the reference coil signals is raised by more than four times and is generally not affected by eddy current during alternating current excitation, which is a 10002 5 2002. 8 IIliI I substantial difference. When a conventional reference coil is adopted, with the increase of the sensor aperture, due to the impact of the eddy current, the phase of the reference signal may deviate from the measured phase of the magnetic field, and the phase difference may reach 150, resulting in that a part of quadrature interference signal will convert to in-phase interference signal. When the reference coil of the invention is adopted, the phase difference between the the reference signal and the actually magnetic field may be controlled within 10, so that the zero point of the sensor is stabilized and the measurement precision is improved. Embodiment 2 Referring to FIG. 3 and FIG. 4, the difference between this embodiment and Embodiment 1 is that, flanges 23 are installed on two ends of a measuring tube 24. A reference coil 25 and an excitation coil 22 are fixedly provided on an external side wall of the measuring tube 24. The reference coil 25 is close to the casing of the measuring tube 24, and is larger than the excitation coil 22 by full size and encloses the excitation coil 22, which is called an enclosed-type installation mode and is applicable to a medium-aperture electromagnetic flowmeter. The excitation coil 22 is fixedly wound around a magnetic circuit board. The magnetic circuit board is fixed on the external side wall of the measuring tube 24 through pole shoes 21. An electrode is disposed on the external side wall of the measuring tube 24 and at a position away from the reference coil 25. Embodiment 3 Referring to FIG. 5 and FIG. 6, the difference between this embodiment and Embodiment 1 is that, flanges 33 are installed on two ends of a measuring tube 34. A reference coil 35 and an excitation coil 32 are fixedly provided on an external side wall of the measuring tube 34. The excitation coil 32 is fixedly wound around a magnetic circuit board 36. The magnetic circuit board 36 is fixed on the external side wall of the measuring tube 34 through pole shoes 31. An electrode is disposed on the external side wall of the measuring tube 34 and at a position away from the excitation coil 32. The reference coil 35 is wound on the magnetic circuit board 36, which is called a bead-type installation mode and is applicable to a small-aperture electromagnetic flowmeter. Since the small-aperture electromagnetic flowmeter has a limited space, it is impossible for the reference coil to keep a long distance away from the excitation coil in the installation modes in Embodiment 1 and Embodiment 3, so that the reference coil may be wound on the magnetic circuit board and at a 10002 6 2002. 8 position close to the electrode while away from the pole shoes. The reference coil may be a single wound coil or formed by several serially-connected coils. Embodiment 4 The difference between this embodiment and Embodiment 3 is that, the excitation coil is disposed on one of the pole shoes, and the reference coil is disposed on the other one of the pole shoes. This installation mode is applicable to an ultimately small-aperture electromagnetic flowmeter. The reference coil is generally fabricated in two methods. One is winding with an enamel wire, and the other is fabricating with a flexible circuit board. When a flexible circuit board fixing method is used in fabrication of a small-aperture electromagnetic flowmeter sensor, the cost is not increased and the consistence is good. For a large-aperture electromagnetic flowmeter sensor, for example, having a diameter of 2 m to 3 m, the size of the reference coil is large. Since there is no such big flexible circuit board, when this method is adopted, several flexible circuit boards may be serially connected for the fabrication of the reference coil. In this case, the cost is apparently lower if an enamel wire winding method is adopted. The enamel wire is a main category of the winding wire, and consists of a conductor and an insulating layer. An enamel wire is formed by first annealing and softening a bare wire, then painting the bare wire several times, and finally baking the bare wire. The reference coil may be fixed by various methods. No matter which method is adopted, the reference coil must be securely fixed without any change in shape or position so as not to affect the measurement precision. A simple method is to adhere the reference coil to a conduit with a glass fiber tape. Before the adhesion, the surface of the conduit needs to be cleaned. Moreover, to secure the adhesion, silica gel can be applied for a better effect. The invention has the following advantages: 1. Since the component for reflecting the measured magnetic field is substantially raised among the reference coil signals of the invention, the electromagnetic flowmeter truly realizes magnetic field feedback. It can be seen from the experiment that, the difference between the theoretical value and the actually measured value of an equivalent magnetic field of the 10002 7 2002. 8 IIliI I electromagnetic flowmeter is small. Since the alternating current excitation is generally used for industrial control, the condition of the fluid is rather complex, and it is inconvenient to calibrate the sensor in all the frequencies. The meter coefficients in all frequencies are similar due to the magnetic field feedback, and a desirable frequency can be selected according to the actual condition. 2. A fluid with a variable magnetic permeability, such as iron ore slurry, can be measured, which eliminates the limits of the electromagnetic flowmeter. Currently, radioactive elements are used to measure the concentration of iron ore concentrate slurry, which has low precision, is risky in maintenance and management due to nuclear radiation, and has a high cost. If the effective concentration of the iron ore concentrate is measured through the magnetic permeability, the measurement is safe and convenient, and the mass flow of the iron ore concentrate can be directly obtained based on its volume flow and concentration. Therefore, the sensor achieves the functions of measuring the electromagnetic flow, wet grade of the iron ore concentrate slurry, and mass flow of the iron ore concentrate, which is a breakthrough in this field and can solve problems unsettled for a long time. 3. A big defect of on-site check is avoided. In scenarios that the magnetic permeability of the fluid does not change, for example, in the presence of water, the state of the sensor may be detected by calculating the change of the magnetic permeability, so as to determine whether interturn short-circuit or magnetic circuit corrosion occurs, and the change of the magnetic field is compensated to reduce the measurement error when the sensor is faulty. If the interturn short-circuit occurs, the measure value of the magnetic permeability is reduced, and the fault can be found easily. The magnetic permeability may be detected online or offline, in which a constant sine-wave excitation current is input, a reference coil wave form is detected, and the magnetic permeability is calculated. If the magnetic permeability does not change, it is indicated that the sensor has no fault. The offline detection method is particularly applicable to a constant current square-wave excitation electromagnetic flowmeter. Herein, the description and application of the invention are illustrative, and the scope of the invention is not intended to be limited to the above embodiments. Variations and changes to the embodiments disclosed herein are possible. Replacement made to the embodiments and equivalent parts are known to persons of ordinary skill in the art. It should be apparent to persons skilled in the 10002 8 2002. 8 IIliI I art that, the invention can be implemented in other forms, structures, arrangements, ratios and through other components, materials, and parts without departing from the spirit or essential features of the invention. 10002 9 2002. 8 liiI I