Method for Detecting Common Mode and Other Interfering Magnetic Fields

Abstract

A method detects a proportion of a common mode magnetic field transmitted together with a signal magnetic field each emitted by one of at least two magnetic field sensors (S1; S2), wherein the magnetic field sensors (S1; S2) are connected in at least one electric circuit, and at least two differential drive clocks (A; B) reverse the current flowing in the electric circuit.

Claims

1. A method comprising: detecting an interfering portion of a common mode magnetic field transmitted together with a signal magnetic field each emitted by one of at least two magnetic field sensors (S1; S2), connecting the magnetic field sensors (S1; S2) in at least one electric circuit, and with at least two differential drive clocks (A; B) reversing the current flowing in each of the electric circuits.

2. The method according to claim 1, further comprising providing a centre tap (CT) is arranged between the magnetic field sensors (S1; S2).

3. The method according to claim 1, further comprising the following steps: allocating an electrical component, implementing an electrical resistance (R1; R2) to each of the magnetic field sensors (S1; S2) in the electric circuit, taking measurements of (A+) and (B−) during an interval of the drive clocks (A; B) switching at a frequency of 10's of KHz, where the drive clock (A) is at a high value and the drive clock (B) is at a low value, calculating a positive current (A+), flowing through the sensor (S1) by adding up the value of the current (iS1), flowing from the magnetic field sensor (S1) to the electrical component (R1) allocated to the magnetic field sensor (S1) and the current of the common mode field icm1, interfering with the magnetic field sensor (S1), calculating a negative current (B−), where the drive clock (B) is low, by summing the value of the current (iS2), flowing to the magnetic field sensor (S2) from the electrical component (R2), allocated to the magnetic field sensor (S2) and the current of the common mode field (icm2), interfering with the magnetic field sensor (S2), taking measurements of (B+) and (A−) during the interval of the drive clock (A; B) switching at a frequency, where the drive clock (B) is at a high value and the drive clock (A) is at a low value, calculating a positive current (B+), where the drive clock (B) is high, by adding up the value of the current (iS2), flowing from the magnetic field sensor (S2) to the electrical component (R2), allocated to the magnetic field sensor (S2) and the current of the common mode field (icm2), interfering with the magnetic field sensor (S2), calculating a negative current (A−), flowing through the sensor (S1), by summing up the value of the current (iS1), flowing to the magnetic field sensor (S1) from the electrical component (R1), allocated to the magnetic field sensor (S1) and the current of the common mode field icm1, interfering with the magnetic field sensor (S1), summing up both the sum of the positive current (A+) and the negative current (B−) and the sum of the positive current (B+) and the negatives current (A−), calculating the residual interfering field detection (IFD).

4. A device for magnetic field detection, the device comprising at least one electric circuit having differential drive voltages, at least two magnetic field sensors (S1; S2) being connected in the electric circuit, and emitting at least one signal magnetic field, wherein at least one electrical component, implementing an electrical resistance (R1; R2) is allocated to each of the magnetic field sensors (S1; S2) in the electric circuit.

5. The device according to claim 4, wherein a centre tap (CT) is arranged between the at least two magnetic field sensors (S1; S2).

6. The device according to claim 4, further comprising at least two drive clocks (A; B), the drive clocks being connected into the electric circuit such that the drive clocks generate the differential drive voltages to reverse the current flowing the electric circuit.

7. The device according to claim 4, wherein the electric circuit is a bi-directional drive.

8. The device according to claim 4, wherein the electrical component implementing an electrical resistance (R1; R2) is a resistor (R1; R2).

9. The device according to claim 4, wherein the device is a bipolar magnetometer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] FIG. 1 shows a drive clock A and an opposite drive clock B with varying polarities.

[0073] FIG. 2 shows a drive clock A and an opposite drive clock B with polarities varying relative to FIG. 1.

[0074] FIG. 3 shows a drive clock A and an opposite drive clock B with common mode interfering magnetic fields.

[0075] FIG. 4 shows the electric circuit comprising a centre tap (CT).

[0076] FIG. 5 shows the electric circuit comprising a centre tap (CT), with the electric current flowing in the opposite direction relative to FIG. 4.

[0077] FIG. 6 shows the electric circuit comprising a resistor.

[0078] FIG. 7 shows the electric circuit comprising a resistor, with the electric current flowing in the opposite direction relative to FIG. 6.

[0079] Reference numerals in the written specification and in the figures indicate corresponding items.

DETAILED DESCRIPTION

[0080] According to the FIG. 1 and FIG. 2, magnetic field sensors S1 and S2 are arranged in a current circuit. Any type of magnetic field sensors can be used for the sensors S1 and S2.

[0081] Said sensors S1 and S2 are measuring fields in opposite directions. To operate a flux gate sensor, according to the invention, a frequency has to be set which can also be addressed to as a drive clock frequency with a preset value usually in the 10's of kilohertz (KHz) range depending on the number and characteristics of the flux gates.

[0082] In a given phase 1 a drive clock A is high and an opposite drive clock B is low.

[0083] Whereas in a given phase 2, drive polarities are switched, so that a drive clock B is high and the drive clock A is low.

[0084] For this reason, the currents between said sensor S1 and the sensor S2 move backwards and forwards.

[0085] In a given phase 1, the current moves from drive clock A to drive clock B, whereas in a given phase 2, the current moves from B to A.

[0086] The term clock is referred to as a 5 V (Volt) digital clock. Thus, the voltage can be changed from 0 V to 5 V and the other way round. It goes without saying that different voltage values can be applied as well. This principle can also work with a single ended drive clock.

[0087] Thus, the currents move backward and forward through the sensing network, formed by the sensors S1 and S2. In other words, the currents moving between the sensors S1 and S2 are biphasic currents.

[0088] In the present application, the biphasic current refers to two phases or pulses of two different intensities, alternating with each other during a treatment. Thus, the currents are shifted in either directions between the sensors 1 and 2.

[0089] In other words, it is the function of the magnetic sensors S1 and S2 to sense the changes in the current.

[0090] The sensing direction of the sensor S1 in the example is from drive clock A to drive clock B, whereas the sensing direction of a sensor S2 is in the opposite way, from the drive clock B to the drive clock A.

[0091] Now, by way of an example, a common mode field shows a direction from drive clock A to drive clock B. As direction does not matter, said common mode field could also go from said drive clock B to said drive clock A.

[0092] As shown in the FIG. 3, by way of example, when the drive clock A is high and the drive clock B is low, with the current i1 moves from the drive clock A to the drive clock B the common mode field could strengthen the field that is measured with the sensor 1.

[0093] In other words, the field is measured, using the sensor S1 is directed in the same direction as the common mode field. According to the FIGS. 1 and 2, both the current and the common mode which is interfering the magnetic fields are in the same direction.

[0094] In this case, the sensor S1 is not only measuring the signal field but is also measuring the common mode field.

[0095] In other words, the current signal i1 is a function of both the signal field and the interfering field.

[0096] In the same example, the current i2 is a function of the signal field of which the interfering field is deducted.

[0097] This is due to the opposite directions of both the sensing field of the sensor S1 and the sensor S2.

[0098] In other words, when in a phase 2 the currents are reversed, the magnitude and the direction of a correction current is remain the same magnitude and direction, when the sensing field is present and there is no interfering common mode magnetic field.

[0099] So, the currents i1 and i2 do not show the signal of the sensors only. The currents and i2 rather show the signal of the sensor and the common mode interfering field added to this signal.

[0100] Therefore, a measurement field is corrects the difference between the two sensing field directions i1 and i2.

[0101] The signal of the sensor S1 represents the signal of the sensor S1 and the common mode field. Thus, the signal of the sensor S1 represents the current due to the signal field of the sensor S1 and added to it the current due to the common mode field cm1.

[0102] Thus, the current i1 comprises the component of the signal of the sensor S1 added to it the component of the common mode interfering field.

[0103] Also, with the current of the sensor S2 is the signal of the sensor S2, with the common mode field deducted from the signal of the sensor S2. Which is due to oppositely directed current of the sensor S2 and the common mode field.

[0104] According to the invention, one of the sensors S1, S2 is adapted to be a common mode sensor and the respective other sensor S2, S1 is adapted to be a differential mode sensor.

[0105] It goes without saying that when the currents i1 and i2 are reversed, the system is oriented the other way round.

[0106] So, in the differential mode, is1 represents the measurements which is taken to detect the stress applied to the material.

[0107] In other words, the signal is1 is the information which is wanted. However, the signal is1 is interfered by the external common mode field.

[0108] As shown in the FIGS. 4 and 5, the factor which is to be detected by the measurement is the signal is1, which however is interfered by the common mode field cm1.

[0109] Thus, the signal i1 in a phase 1 is made up of two components, being the signal is1 and the icm1.

[0110] Signals is1 and the icm1 are summed up, what one is left with is the measurement current ic.

[0111] It is the measurement current ic, which is used for measuring, with the common mode field disappearing.

[0112] Even though the common mode field is present, the common mode field is not visible in the signal ic, when there is a perfect system.

[0113] The same applies to the phase 2. Here, the way the currents distribute, the signal ic equals is1 plus icm1, whereas in phase 2 currents distribute as (−is2) plus icm2.

[0114] When signals of the phase I and phase II are added together the correction current also remains is =is1 plus is2.

[0115] FIG. 6 and FIG. 7 show that even though the currents are bidirectional and even though there is a common mode field present, when the common mode fields are identical, the common mode fields disappear from the measurements.

[0116] This is due to the fact that the measurement current IC is made up of the signal (−i1) and the signal (−i2).

[0117] The signal of the sensor S2 represents the signal of the sensor S2 and the common mode field.

[0118] When the drive clock A is set from 0 V to 5 V level, consequently the drive clock B is set to a 0 V level. This state is called phase 1. In the example according to the FIG. 5, the drive clock A passes from 0 V to 5 V. Consequently, the drive clock B transitions from 5 V to 0 V.

[0119] In said phase 1, the currents flow from drive clock A to drive clock B.

[0120] As shown in the FIGS. 6 and 7 the system can also be set in an opposite manner. In phase 2 a drive clock A transitions from 5 V to 0 V, whereas the drive clock B is shifted from 0 V to 5 V.

[0121] The transition from 5 V to 0 V of the drive clocks A or B depends on a fixed kilohertz-frequency to which the actual drive clock is adjusted.

[0122] By way of example, the drive clock A and B is switching from 5 V to 0 V and back to 5 V ad infinitum with a 50% duty cycle and a period of 20 μS, respectively.

[0123] As drive clocks A and B are inverted, when the drive clock A is low, then the drive clock B is high and the other way around. Thus, when the drive clock B is low, then the drive clock A is high.

[0124] Therefore, the current flows from A to B or B to A for said half period, being 10 μS, respectively.

[0125] The invention uses at least one traditional sensor.

[0126] Contrary to the sensor of the state-of-the-art, the invention changes the way, the coils of the sensor are connected.

[0127] Further, the invention measures the centre tap position arranged between two sensors. In doing this, the presence of the stray fields is detected.

[0128] The detection can be done, implementing a single channel, without driving up extra costs.

[0129] It is one object of the invention to extract hidden information, as to the magnitude of the external magnetic field, disturbing the signal, issued by the sensors.

[0130] It is one purpose of the invention to set up the following four equations.

[0131] In the FIG. 7 (phase 1) the drive clock A is set to a value higher than the drive clock B.

[0132] Thus there is a current flowing from the drive clock A towards the centre tap CT of i1. Current i2 flows from the centre tap (CT) to the drive clock B.

[0133] Therefore, the following formulas apply:

TABLE-US-00001 A.sub.+ = −i.sub.s1 − i.sub.cm1 Positive current during A high B.sub.− = i.sub.s2 − i.sub.cm2 Negative current during B low B.sub.+ = −i.sub.s2 + i.sub.cm2 Positive current during B high A.sub.− = i.sub.s1 + i.sub.cm1 Negative current during A low

[0134] The values of the signals A+ and B− are added up and combined with added values of the signals B+ and A−.

[0135] Then, the difference between the sums of the values of the signals A+ and B− and the sums of the values of B+ and A− are calculated.

[0136] What remains is interfering field detection:

IFD=2icm1+2icm2.0