GB2156530A

GB2156530A - Method and apparatus for measuring a magnetic field

Info

Publication number: GB2156530A
Application number: GB08508069A
Authority: GB
Inventors: Radivoje Popovic; Peter Gruber
Original assignee: Landis and Gyr AG; LGZ Landis and Gyr Zug AG
Current assignee: Siemens Building Technologies AG; Landis and Gyr AG
Priority date: 1984-03-28
Filing date: 1985-03-28
Publication date: 1985-10-09
Also published as: GB8508069D0; GB2156530B; FR2562261A3; DE3502871A1; FR2562261B3; DE3502871C2; CH667535A5

Abstract

Apparatus for measuring a magnetic field (HM), which apparatus may form part of the measuring means of an electricity meter, comprises an electromagnetic first control circuit (2, 8b, 3b, 4, 5) and an electrical second control circuit (2, 8b, 3c, 9) which are operated alternately. A sensor (2) measures a magnetic field difference (HM - HK) between the magnetic field (HM) to be measured and a compensation magnetic field (HK). A switch (3) feeds the measurement result either to the first or to the second control circuit. The first control circuit (2, 8b, 3b, 4, 5), by way of a first controller (4), controls a compensation current (IK) which flows in a coil (5) and in that way produces the compensation magnetic field (HK), so that the voltage (UO) at the output of the sensor (2) becomes zero. The second control circuit (2, 8b, 3c, 9) detects and adjusts the working point of the sensor (2) in such a way that the magnetic field (HM) to be measured is equal to the sum comprising the compensation magnetic field (HK) and an apparently-present magnetic field. The apparently-present magnetic field is equal to a multiple of a predetermined reference value (HR) of the compensation magnetic field (HK), corrected by the effective value of all 'offset' voltages in the electrical second control circuit (2, 8b, 3c, 9). <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring a magnetic field This invention relates to a method and an apparatus for measuring magnetic field.

Magnetic field measurements can be used for example in electricity meters for determining the electrical current which is the governing factor of the power consumption, by measuring the proportional magnetic field produced by the electrical current.

Swiss patent specification No. 591 699 discloses a current measurement operation using what is referred to as the compensation measuring process wherein a magnetic field sensor measures the difference between two magnetic fields. In that arrangement, the first magnetic field is the proportional magnetic field which is produced by the current to be measured, by means of a first coil winding, while the second magnetic field is a compensation magnetic field which is produced by the output signal of the magnetic field sensor after suitable amplification by means of a second coil winding. The two coil windings are arranged in space in such a way that the two magnetic fields have parallel but opposite directions. The measuring circuit operates in such a way that the difference between the two magnetic fields is regulated to zero.In that case, the two magnetic fields and thus also the numbers of ampere turns of the two coil windings are identical. If the current to be measured is of a very high value and if the number of turns of the two coil windings are approximately equal, that means that the current in the second coil winding must also be very high. However, it is extremely difficult to generate such high currents, for example of the order of magnitude of some amperes, at the output of the magnetic field sensor, when using exclusiviely semiconductor components.

From a first aspect, the present invention provides a method of measuring a magnetic field, wherein a compensation magnetic field is produced as an actual value by means of a first control circuit, the compensation magnetic field being of such a strength that the output voltage of a sensor included in the control circuit becomes approximately zero, and wherein the working point of the sensor is detected and adjusted by means of a second control circuit also including the sensor, in such a way that the magnetic field to be measured is equal to the sum comprising the compensation magnetic field and an apparently-present magnetic field which is equal to a multiple of a predetermined reference value of the compensation magnetic field corrected by the effective value of all 'offset' voltages in the second control circuit, and the two control circuits are set in operation alternately in respect of time, the absolute value of the compensation magnetic field being held below the predetermined reference value of the compensation magnetic field.

From a second aspect, the present invention provides apparatus for measuring a magnetic field comprising a first control circuit for producing a compensation magnetic field of such a strength that the output voltage of a sensor included in the first control circuit is approximately zero, a second control circuit, which also includes the sensor provided for detecting and adjusting the working point of the sensor, wherein the second control circuit includes an electrical input of the sensor, and a changeover switching means for setting the two control circuits in operation alternately in respect of time.

Using a method and an apparatus of this type makes it possible to measure the high magnetic fields and thus also high electrical currents by means of the compensation measuring process, without an excessively high electrical current flowing in the second coil winding (which has only a small number of turns approximately equal to that of the first coil) winding, so that the current in the second coil winding can be easily produced by means of a semiconductor component at the output of the magnetic field sensor, with simultaneous compensation of all "offset" voltages which are active in the semiconductor component and in the magnetic field sensor, and which at the same time make it possible to convert analog measurement signals into digital signals.

Embodiments of the invention will now be described in greater detail by way of example and with reference to the accompanying drawings, in which: Figure 1 shows a block circuit diagram of one embodiment of a compensated magnetic field sensor; Figure 2 shows a block circuit diagram of a voltage detector; Figure 3 shows a time diagram of a magnetic field HM to be measured, which is assumed to be a sinusoidal alternating field; Figure 4 shows an idealized time diagram of a compensation current IK; Figure 5 shows a non-idealized first period, illustrated in detail form, of the time diagram shown in Fig. 4; Figure 6 shows a block circuit diagram of a push-pull circuit of two compensated magnetic field sensors; Figure 7 is a time diagram of the output voltage U, of the first compensated magnetic field sensor in Fig. 6;; Figure 8 is a time diagram of the output voltage U2 of the second compensated magnetic field sensor in Fig. 6; Figure 9 is a first alternative embodiment of a coil assembly; Figure 10 is a second alternative embodiment of a coil assembly; Figure ii is a third alternative embodiment of a coil assembly; and Figure 12 shows a block circuit diagram of a compensated magnetic field sensor with microcomputer.

The same reference numerals denote the same components in all the figures of the drawings.

A compensated magnetic field sensor 1 shown in Fig. 1 comprises a sensor 2, a switch 3, a first controller 4, a coil 5, a resistor 6, a voltage detector 7, a first difference means 8a, a second difference means 8b, an analog-digital converter 8c and a second controller 9.

The sensor 2 is for example a known semiconductor magnetic field sensor which has at least a first electrical input. If the input stage of the sensor 2 is a differential stage, its second electrical input is for example to be connected to earth.

In Fig. 1 the switch 3 is shown in the form of an electromagnetic relay purely for reasons of representation in the drawing. In Fig. 1 it is shown in the current-less released condition and has a relay coil 3a, a first change-over switch 3b, an opening contact 3c and a second change-over switch 3d. In practice however, instead of the electromagnetic relay, any known and controllable semiconductor switching device with suitable switching contact equipment is used.

The first controller 4 includes a voltage storage means and for example a known trans conductance amplifier, that is to say, an amplifier whose output current is proportional to its input voltage. The second controller 9 also includes a voltage storage means and is formed for example by means of an operational amplifier which is wired up in the form of a known non-inverting amplifier.

The coil 5 which generally has only a small number of turns is an element which is arranged in space as close as possible to the sensor 2 so that a compensation current 1K which flow through the coil 5 generates a compensation magnetic field HK which is of parallel and opposite direction to the magnetic field HM which is to be measured and which is produced for example in a manner which is not shown in Fig. 1 by a current 1M to be measured, by means of a further coil.The difference (HM - HK) between the two magnetic fields is measured by the sensor 2 whose output is taken to the negative input of the second difference means 8b; in turn, when the relay coil 3a is in a current-less condition, the output of the difference means 8b is taken on the one hand by way of the closing contact of the first change-over switch 3b to the input of the first controller 4 and on the other hand by way of the opening contact 3c to the input of the second controller 9. The positive input of the second difference means 8b and a first terminal of the resistor 6 are connected to earth. The output of the second controller 9 is permanently taken to the first electrical input of the sensor 2. The opening contact of the first change-over switch 3b connects the output of the first difference means 8a to the input of the second controller 4.The output of the first controller 4 is connected on the one hand by way of the coil 5 to the second terminal of the resistor 6, to a measuring input 10 of the voltage detector 7, to an analog output 11 of the compensated magnetic field sensor 1 and, when the relay coil 3a is in a current-less condition, by way of the closing contact of the change-over switch 3d to the input of the analog-digital converter 8c, and, on the other hand, directly to the negative input of the first difference means 8a.The voltage detector 7 has a first output 1 2 which is connected to a terminal of the relay coil 3a, a secod output 1 3 which is at the same time a control output 14 of the compensated magnetic field sensor 1, a bus output 1 5 which at the same time is a part of a digital output 1 6 of the compensated magnetic field sensor 1, a first start input 1 7 and a second start input 1 8. The last two are at the same time the first and second start inputs 1 9 and 20 of the compensated magnetic field sensor 1. The other terminal of the relay coil 3a is connected to a positive feed voltage Vcc.

The input of the digital-analog converter 8c is also connected to earth by way of the opening contact of the second change-over switch 3d.

The bus output of the digital-analog converter 8c, together with the bus output 1 5 of the voltage detector 7, forms the digital output 1 6 of the compensated magnetic field sensor 1.

The respective positive inputs of the two difference means 8a and 8b are reference or desired value inputs while their negative inputs respectively represent actual value inputs.

The sensor 2, the second difference means 8b, the first change-over switch 3b, the first controller 4 and the coil 5 together form an electromagnetic first control circuit M. The sensor 2, the second difference means 8b, the opening contact 3c and the second controller 9 represent an electrical second control circuit E. The resistor 6 is a current-voltage transducer and together with the voltage detector 7 forms a current comparator 6; 7 which serves to compare the measured compensation current 1K to a reference value 1R The current comparator 6, 7 and the switch 3 together form a change-over switching means 3, 6, 7.

The voltage detector 7 which is used in Fig.

1 and which is shown in Fig. 2 includes a first comparator 21, a second comparator 22, a first OR-gate 23, an up-down counter 24, a second OR-gate 25, a delay member 26, a third OR-gate 27, a D-type flip flop 28 and an NPN-transistor 29. The measuring input 10 of the voltage detector 7 is connected to the non-inverting input of the first comparator 21 and to the inverting input of the second comparator 22. The inverting input of the first comparator 21 receives a first reference voltage UR = R.IR and the non-inverting input of the second comparator 22 receives a second reference voltage - UR = - R.lR, wherein R is the value of the resistor 6 and 1R is the reference value of the compensation current K For example, 1R = 1 mA.The output of the first comparator 21 is taken to the up-down input U/D of the up-down counter 24 and to a first input of the first OR-gate 23 while its second input is in turn connected to the output of the second comparator 22. The output of the first gate 23 is connected to the clock input CL of the up-down counter 24, to a first input of the second gate 25, to a first input of the third gate 27 and to the second output 1 3 of the voltage detector 7. The first start input 1 7 of the voltage detector 7 is connected to a second input of the second gate 25 and to the reset input R of the D-type flip flop 28. The output of the second gate 25 is connected by way of the delay member 26 to a second input of the third gate 27 whose third input is connected to the second start input 1 8 of the voltage detector 7.The output of the third gate 27 is taken to the clock input of the D-type flip flop 28 whose Q-output is fed back to its D-input, while its Q-output is connected to the base of the NPN-transistor 29. The collector of the NPN-transistor 29 forms the first output 1 2 of the voltage detector 7 and the emitter of the NPN-transistor 29 is connected to earth. The bus output of the up-down counter 24 forms the bus output 1 5 of the voltage detector 7.

In Fig. 3, the magnetic field HM to be measured is assumed to be sinusoidal alternating field, while in Fig. 4 the illustrated compensation current IK' in terms of absolute value, is never greater than the reference value R The rearward edge of each current pulse was assumed in Fig. 4 as being ideally perpendicular, while in Fig. 5 it is shown in non-ideal form in respect of the first period, and comprises a portion of the duration At, which falls for example linearly with time. The time curve in the diagrams in Figs. 3 to 5 is described in greater detail in the functional description set out hereinafter.

The arrangement shown in Fig. 6 includes a first compensated magnetic field sensor 1 a, a second compensated magnetic field sensor 1 b and a further switch 30, for example a trigger relay which has a relay coil 30a and a change-over switch 30b. The two compensated magnetic field sensors 1 a and 1 b are identical and are of the same construction as the compensated magnetic field sensor 1 shown in Fig. 1. In this case also the trigger relay may be replaced by a suitable semiconductor circuit, for example a further D-type flip flop with transistors connected on the output side thereof. If a trigger relay is used, the relay coil 30a has a first component coil 31 and a second component coil 32.

The analog output 11 a of the first compensated magnetic field sensor 1 a is connected to the output of the push-pull circuit by way of the opening contact of the change-over switch 30b while the analog outut 11 b of the second compensated magnetic field sensor 1 b is connected to the push-pull circuit output by way of the closing contact of the change-over switch 30b.The control output 1 4a of the first compensated magnetic field sensor 1 a is taken to the second start input 20b of the second compensated magnetic field sensor 1 b and to a first terminal of the second component coil 32 while the control output 1 4b of the second compensated magnetic field sensor 1 b is conversely connected to the second start input 20a of the first compensated magnetic field sensor 1 a and to a first terminal of the first component coil 31. The second terminals of the component coils 31 and 32 are connected together and to the positive feed voltage Vcc while the first start input 1 9b of the second compensated magnetic field sensor 1 b is connected to earth.The digital outputs 1 6a and 1 6b of the two compensated magnetic field sensors la and 1 b are shown for reasons of completeness.

The alternate mode of operation of the two compensated magnetic field sensors 1 a and 1 b of the push-pull circuit will be seen from the time diagrams in Figs. 7 and 8 and will be described in greater detail in the functional description set out hereinafter.

The coil of the first alternative embodiment of a coil assembly as shown in Fig. 9 comprises a metal spiral flat coil whose turns are for example square or rectangular and which is applied to the surface of a semiconductor crystal 33. The uncompensated sensor 2 is diffused in the semiconductor crystal 33 at the surface thereof. Disposed directly above the magnetic field sensor 2 is a narrow air gap between two ferromagnetic thin films 34 and 35 which are both fixed on the surface of the crystal 33 between said surface and one half of the coil 5, in such a way that one thin film is disposed in a relationship of extending the other.Fig. 9 shows only a half of the coil 5, more specifically that half in which the compensation current 1K in the metal conductors of the coil 5 flows into the plane of the paper, normal thereto, which is shown in symbolic terms by a respective small cross in regard to each of the conductors of the coil 5.

The air gap between the two thin films 34 and 35 lies approximately underneath the middle of the coil half illustrated. The assembly 33, 34, 35 comprising the semiconductor crystal 33 and the thin films 34 and 35 is completely covered over upwardly by an insulating layer 36, comprising for example SiO2, which completely electrically insulates the assembly 33, 34 and 35 from the metal conductors of the coil 5. The arrangement in space is such that the magnetic field difference (HM - HK) between the magnetic field HM to be measured and the compensation magnetic field HK is effective in the longitudinal direction of the thin films 34 and 35 in parallel relationship to the surface of the magnetic field sensor 2.

The coil 5 of the second alternative embodiment of a coil assembly as shown in Fig. 10 comprises a minicoil which is wound in conventional fashion on a coil former 37 which is for example of square or rectangular crosssection. The assembly 33, 34 and 35 referred to with reference to the description of Fig. 9 is disposed in the iron core space of the coil former 37, in such a way as to fill that space as completely as possible.

The two thin films 34 and 35 comprise for example Ni Fe in Figs. 9 and 10, and are for example approximately 1y in thickness.

The coil 5 of the third alternative embodiment of a coil assembly as shown in Fig. 11 is wound on a flat annular ferro-magnetic core 38 which has a small air gap in which the sensor 2 is arranged. The coil 5 with its core 38 and the sensor 2 are mounted on a small insulating plate 39, for example a ceramic plate, which in the vicinity of the centre of the annular ferro-magnetic core 38 has an opening through a current conductor 40 is passed approximately normal to the plane of the ceramic plate 39. Flowing in the current conductor 40 for example is a current 1M which produces the magnetic field HM to be measured.

The compensated magnetic field sensor with microcomputer, as shown in Fig. 12, comprises the actual compensated magnetic field sensor 1 which is of the construction shown in Fig. 1, a differentiator 41, a sample/hold circuit 42, an analog-digital converter 43, a microcomputer 44 amd a digitalanalog converter 45. The positive input of the first difference means 8a is taken in the compensated magnetic field sensor (see Fig.

1) to an input terminal 46 of the compensated magnetic field sensor 1. The input terminal 46 which is normally connected directly to earth is connected to the output of the digital analog converter 45 in the arrangement shown in Fig. 12. In Fig. 12, the analog output 11 of the compensated magnetic field sensor 1 is taken in the specified sequence by way of the differentiator 51, the sample/hold circuit 42 and the analog-digital converter 43 to a data input-output of the microcomputer 44. The data input-output is also additionally connected to the input of the digital-analog converter 45. The second start input 20 of the sensor 1 is earthed and is thus inoperative. It will be appreciated that the digital components of the circuit shown in Fig. 1 2 may also be replaced by corresponding analog components.

Functional description As already mentioned, the compensated magnetic field sensor 1 shown in Fig. 1 includes two control or regulating circuits which are operative in alternate succession in respect of time. In that respect, the electrical second control circuit E is operative in each case only for a brief time, for a short period At. The sensor 2 which is included in both control circuits M and E measures the magnetic field difference (HM - HK) between a magnetic field HM to be measured and a compensation magnetic field HK and converts it into an electrical voltage UO The switch 3 carries intera alia the measuring result of the sensor 2 either to the first control circuit or to the second control circuit.

If, as a result of a control operation: HM = HK (1), that gives: NM.lM= NKIK (2) and lK=(NM/NK).lM (3) wherein NK denotes the number of turns of the coil 5. NM represents the number of turns of a coil (not shown) by means of which for example a current 1M to be measured produces the magnetic field HM. If the value of the current 1M to be measured is high, a low value is generally selected for NM. If on the other hand the sensor 2 is a semiconductor component of correspondingly small dimensions, then the coil 5 may also have only a small number of turns, for reasons of space.In that case, NMNK and in accordance with equation (3), 1K1M' whereby the compensation current 1K would also be of a high value and could never be directly produced by the first control ler 4 which is also a semiconductor compo nent. In order nonetheless to accomplish that, the electrical second control circuit E is required, which becomes operative as soon as the compensation current 1K which is measured by means of the resistor 6, in terms of absolute value, reaches the reference value I,, or the voltage drop across the resistor 6, in terms of absolute value, reaches the value of the reference voltage UR. The reference value HR of the compensation magnetic field HK corresponds to the reference value 1R of the compensation current 1K Thus, the absolute value of the compensation current 1K is always held beneath a predetermined reference value IR The same applies in regard to the absolute value of the compensation magnetic field HK which is always kept below its predetermined reference value HR.

The measurement is started at the time t = 0, when for example the sinusoidal mag netic field HM to be measured, which is shown in Fig. 3. is of the value zero, by at that time a start pulse being applied to the first start input 1 9 of the compensated magnetic field sensor 1 shown in Fig. 1, the second start input 20 of which is only required in the push-pull circuit shown in Fig. 6, and thus can be disregarded here.

By way of the first start input 1 7 of the voltage detector 7 (see Fig. 2) and by way of the second OR-gate 25, the start pulse reaches the input of the delay member 26 which has a delay time At. During that delay time, the switch 3 is in the position shown in Fig. 1, that is to say, the opening contact of the first change-over switch 3b, the opening contact 3c and the opening contact of the second change-over switch 3d are closed, so that the two controllers 4 and 9 each have a respective reference value of zero. In the vicinity of the start time, the compensation current 1K is equal to zero and thus the compensation magnetic field HK is also equal to zero so that the magnetic input signal HM - HK of the sensor 2 is also approximately zero.By means of the electrical second control circuit E and the input signal UE at the electrical input of the sensor 2, the second controller 9 controls its output voltage UO to zero, the magnetic input signal HM - KKO of the sensor 2 being operative as an interference parameter or value.The electrical input signal UE thus compensates the entire effect of the "offset" voltages of the electrical second control circuit E, in particular the effect of the "offset" voltages of the sensor 2, and the effect of the interference parameter HM - HKO. The electrical voltage required in the second controller 9, for producing the electrical input signal UE or for nullifying the output voltage UO of the sensor 2 is stored within the second controller 9 in a voltage storage means, for example a capacitor (not shown) so that, when the opening contact 3c is opened in the subsequent operating phase, the electrical input signal UE is available at the input of the sensor 2 and thus remains operative during that subsequent operating phase.

The input signal UE which is produced and detected by the electrical second control circuit E sets the working point of the sensor 2.

After the expiry of the short period At which is selected to be of sufficient length that the electrical second control circuit E on the one hand has sufficient time to settle in and the voltage storage means on the other hand has sufficient time to become charged, a logic value ''1" appears at the output of the delay member 26 (see Fig. 2) and, by way of the third gate 27, switches over the D-type flip flop 28 which was previously set to the zero position shown in Fig. 2, at the start time t = 0, by means of its reset input R. Thus, a logic value "1" appears at the Q-output of the D-type flip flop 28, and switches over the switch 3 by way of the NPN-transistor 29. In Fig. 1, the opening contact 3c opens so that both the charging process in respect of the voltage storage means and also the control operation of the electrical second control circuit E are interrupted.However, the effect of the above-mentioned "offset" voltages is still compensated by the voltage stored in the voltage storage means, by way of the electrical input of the sensor 2. At the same time, the switching over of the switch 3 causes closure of the closing contact of the changeover switch 3b which thus renders the electromagnetic first control circuit M operative. The electromagnetic first control circuit M also has a reference value of zero and as its actual value has the output voltage UO of the sensor 2. The electromagnetic first control circuit M thus produces a compensation magnetic field HK of such strength that the output voltage UO of the sensor 2 contained in the electromagnetic first control circuit M becomes approximately zero.On the assumption that the magnetic field HM to be measured is on the rise since the start time t = 0, the compensation current 1K also rises, beginning from zero, in proportion to the magnetic field HM to be measured. As soon as the compensation current 1K which is positive in this case reaches the reference value 1R or its voltage drop caused across the resistor 6 reaches the value of the reference voltage UR, the first comparator (see Fig. 2) switches the switch 3 back again into its original position shown in Fig.

1.

That occurs in the following fashion: The voltage detector 7 which continuously monitors the voltage drop across the resistor 6 switches over the change-over switch 3, either by means of the first comparator 21 (see Fig.

2) when a positive voltage drop reaches the value of the reference voltage UR, or by means of the second comparator 22 (see Fig.

2) when a negative voltage drop reaches the value of the reference voltage - UR. If one of those two situations occurs, then a logic value "1" appears in the voltage detector 7 (see Fig. 2) at the output of the first gate 23 and thus also at the output of the third gate 27, that logic value actuating the D-type flip flop 28 and thus also the switch 3 in Fig. 1, by way of the NPN-transistor 29 and the first output 12 of the voltage detector 7.

The switch 3 is not switched back at infinite speed, as shown in Fig. 4, with a perpendicular rearward edge of the compensation current pulses, but in the manner shown in Fig. 5, with a rearward edge which falls for example linearly with time, of a duration At. It is assumed that the contacts 3b and 3c are switched over approximately at the same time.

In other words, during the period At, on the one hand the compensation current 1K is regulated to zero by means of the first controller 4 while on the other hand the second controller 9 is brought into operation to control the output voltage Uo of the sensor 2 to zero.

During that phase of operation, the magnetic field difference (HM - HK) again acts as an interference parameter on the electrical control circuit E. At time tl (see Fig. 5), that is approximately zero as HMHK, and at time t2 it reaches the value HM as HK-O. Thus, after the expiry of the short period of time At, there is a fresh value of the output signal UE of the sensor 2, which, besides the effect of the above-mentioned "offset" voltages, also takes account of the "offset" effect of the magnetic field HM at the time t2 which corresponds to the compensation magnetic field HK = HR at the time t1 plus a small residual field.At time t2, a first period in the variation in respect of time of the compensation current 1K is concluded and the delay member 26 (see Fig. 2) can initiate a second period. At the moment at which the compensation current 1K reached the reference value 1R during the first period, the logic value "1" which is produced by the first comparator 21 (see Fig. 2) was also passed by way of the second gate 25 to the input of the delay member 26 so that, after expiry of the period At, a logic value "1" again appears at the output thereof and thus also at the output of the third gate 27 which again actuates the switch 3 by way of the Dtype flip flop 28 and the NPN-transistor 29 so that the electrical second control circuit E is again taken out of operation and the electromagnetic first control circuit M is put back into operation. The second period of the compensation current 1K is initiated in that way and the compensation current 1K again rises, beginning at the zero value, to the reference value 1R Further periods of a positive compensation current follow (see Fig. 4), more particularly up to the time at which the magnetic field HM to be measured begins to sink.Although briefly before that time the compensation current 1K first still rises positively, it generally no longer reaches the reference value IRT but begins previously to sink and becomes negative. This time, the second comparator 22 (see Fig. 2) switches over the switch 3 when the negative compensation current 1K falls below the reference value - 1R' or, which comes to the same thing, when the absolute value of the compensation current 1K exceeds the reference value 1R The subsequent compensation current periods are negative until the magnetic field HM to be measured again changes in direction and assumes a rising configuration. From that time, the compensation current periods are again positive (see Fig. 4), etc.

In the process as just described above, both at the start time and also whenever the measured compensation current 1K reaches the reference value IRT in absolute terms, that is detected, the reference value of the compensation current 1K is set to zero and the arrangement is briefly switched over from the electromagnetic first control circuit M to the electrical second control circuit E.During each of the short periods At, on the one hand 1K and HK are set to zero, that is to say, at the time t2, the sensor 2 is compensated in purely electrical mode, and on the other hand, by means of the electrical second control circuit E, the input signal UE and thus the working point of the sensor 2 are so detected and adjusted that the magnetic field HM to be measured becomes equal to the sum comprising the compensation magnetic field HK and a magnetic field which is apparently present and which is equal to a multiple of the predetermined reference value HR of the compensation magnetic field HK, corrected by the effective value of all residual "offset" voltages in the electrical second control circuit E.In that connection, the multiple is equal to the number of periods of the positive compensation current 1K minus the number of periods of the negative compensation current IKB which elapsed from the start time to the generation of the fresh value of UE, in which respect obviously only the periods which are fully expired at the time of reaching the reference value 1R and - 1R respectively are counted.

After the expiry of the period At the new working point of the sensor 2 is set by means of the electrical second control circuit E. Then, the electrical control circuit E is opened and at the same time generation of the compensation current 1K is rendered operative again, by means of the electromagnetic first control circuit M.

It was assumed in Fig. 4 that the reference value 1R is very small. If that is the case, the front edges of the compensation current periods can be assumed to be linear and the periods themselves can be assumed to be triangular, in a first approximation.

The up-down counter 24 shown in Fig. 2 counts the number of times that the changeover switch 3 is switched over; in that respect, the up-down counter 24 counts forwards in binary mode when the positive compensation current 1K reaches the reference value 1R and it counts down in binary mode when the negative compensation current reaches the reference value - I,. As each count value of the up-down counter 24 has a calibration value IR, that means that the digital value is continuously stored in the up-down counter 24, that value corresponding to the instantaneous value of the magnetic field to be measured, with each bit having the valence 1R In other words: the magnetic field HM to be measured is digitised and its digital instantaneous value is stored in the up-down counter 24 whose parallel outputs form a bus output which is connected by way of a bus connection to the bus output 1 5 of the voltage detector 7. The bus output 1 5 of the latter is in turn a part of the digital output 1 6 of the compensated magnetic field sensor 1.

The periods of the compensation current 1K, which have not been fully concluded at the time of reaching the reference value 1R and - 1R respectively and which are thus also not counted represent digitisation errors which decrease in magnitude in proportion to a decreasing magnitude of the reference value 1R In order to increase the degree of accuracy in the digitisation operation, the digitised value of the magnetic field HM which is a multiple of the reference value HR, during the operating phase of the first control circuit M, can be detected with a degree of accuracy which is higher by the compensation current 1K which is converted by means of the analogdigital converter 8c.That is effected by addition, by the voltage drop which is produced by the compensation current 1K in the resistor 6 being passed by way of the closing contact of the second change-over switch 3d to the analog-digital converter 8c and then as a digital value to the digital output 1 6 of the compensated magnetic field sensor 1 where that digital value represents the lowermost bits of the magnetic field HM to be measured.

As already mentioned, the magnetic field HM to be measured is generally not constant during the period of time At, but is variable, so that the compensation current 1K at the end of the period At, at the time t2 (see Fig. 5), should not be of the value zero but should already have an initial value 1KA2 That initial value correction can be effected by means of one of the three methods set out below.

A first method involves using the push-pull circuit shown in Fig. 6. In that circuit, two identical compensated magnetic field sensors 1a and 1b are operated in a push-pull mode so that, when in one thereof the electromagnetic first control circuit M is in operation, in the other the electrical second control circuit E is operative. By means of its change-over switch 30b, the further switch 30 conducts to the output of the push-pull circuit the analog output signal of that compensated magnetic field sensor 1 a or 1 b whose electromagnetic first control circuit M is in fact in operation.

The configuration in respect of time of the analog output voltages U, and U2 respectively of the two compensated magnetic field sensors 1 a and 1 b is shown in Fig. 7 and Fig. 8 respectively, which show that the period of time At of one of the two compensated magnetic field sensors 1 a or 1 b is respectively bridged in respect of time by the rise time tR of the other compensated magnetic field sensor 1 b and 1 a respectively. In that connection, the rise time tR begins in each case at the beginning of the period At of the other compensated magnetic field sensor, that is to say, at a time at which the magnetic field to be measured still had no time to leave its instantaneous value. Thus, no initial value correction is also required within the individual compensated magnetic field sensors 1 a and 1 b.That is automatically achieved by the interconnection of the two compensated magnetic field sensors la and ib.

In a second method, the value 1R and the steepness or slope (dlK/dt) of the compensation current 1K is assumed to be known, at the time tl, at the beginning of the respective period At, as it can be measured. On the assumption that the period At is very short, it can be assumed for example in a first approximation that the magnetic field HM to be measured, from the time t1, varies further with the same degree of steepness (dlK/dt)t, as that which it possessed at the time t1.It is therefore possible to assume the presence of a linear relationship between the initial value 1K.t2 at the time t2 at the beginning of the next period of the compensation current 1K and the final value 1K.tl = 1R at the time tl at the beginning of the duration At of the current period of the compensation current 1K That then gives: lK.,2 = (dlK/dt)t, . At (4).

The values 1R and At are known and are stored for example in a random access memory (RAM) of the microcomputer 44. The steepness dlK/dt of the compensation current 1K is continuously determined by means of the differentiator 41 of the circuit shown in Fig.

12, and is respectively sampled at the time t1, when the measured compensation current 1K reaches the reference value IR, by means of the sample/hold circuit 42, and the sampled analog value (dlK/dt), of the steepness is respectively stored in the random access memory of the microcomputer 44 after conversion thereof into a digital value, which is effected by means of the analog-digital converter 43. Then, by means of the formula (4), the microcomputer 44 computes the digital value of the initial value Kt2 which in turn is also stored under a suitable address in the RAM of the microcomputer 44.Until the expiry of the period At, the initial value 1K,t2 which is calculated in that way is applied instead of the zero value as a reference value for the compensation current IKW to the positive input of the first difference means 8a, by way of the input terminal 46 of the compensated magnetic field sensor 1 (see Fig. 1).

Thus, after expiry of the period At, the calculated initial value 1K.I2 of the compensation current 1K is set instead of 1K = O. In the following operating phase in which the electromagnetic first control circuit M produces the compensation effect, the positive input of the first difference means 8a is again to be connected to earth in a manner which is not shown in the drawing, for example by means of a relay contact.

In third method, the second method can be modified insofar as, although the values in respect of steepness (dlK/dt)t1, which are converted into digital values by means of the analog-digital converter 43, are stored in the random access memory of the microcomputer 44, in that case however, by means of the microcomputer 44, a respective correction value which applies at the end of a respective operating period At of the electrical second control circuit E, is computed, in respect of the magnetic field HM which is digitised at time tl, in dependence on the sum value 1K.tl+ (dlK/dt)" .At,wherein 1K.lI=1Risthe value of the compensation current 1K at the time tl at the beginning of the operating period At of the electrical second control circuit E.

It will be appreciated that, in regard to method two or method three, it is possible to operate with analog values instead of with digital values, provided that the digital components are replaced by corresponding analog components.

Noise which occurs can have an adverse effect on the level of accuracy of the measurement, or can even drive the two controllers 4 and 9 into saturation. Although an increase in the gain factors of the controllers would increase the useful signal, it would also amplify the effect of the noise so that in the ultimate analysis there would be no advance. The use of a magnetic field concentrator as shown in one of Figs. 9 to 11 reduces the adverse influence of noise. In Figs. 9 and 10, the magnetic field concentrator comprises the two thin films 34 and 35 while in Fig. 11 it comprises the ferromagnetic core 38. The field lines of the compensation magnetic field HK generated by the coil 5 are concentrated by the iron-bearing material of the thin films 34 and 35 in the respective air gap or of the ferromagnetic core 38, that is to say, they are amplified by a constant factor ,u. The amplified compensation magnetic field !L.HK is then measured in the respective air gap by the sensor 2. In that way, the useful signal is amplified without also simultaneously amplifying the noise so that the percentage proportion thereof in relation to the measured compensation current 1K falls and therefore has a less severe influence on the measuring operation.

Claims

1. A method of measuring a magnetic field, wherein a compensation magnetic field is produced as an actual value by means of a first control circuit, the compensation magnetic field being of such a strength that the output voltage of a sensor included in the control circuit becomes approximately zero, and wherein the working point of the sensor is detected and adjusted by means of a second control circuit also including the sensor, in such a way that the magnetic field to be measured is equal to the sum comprising the compensation magnetic field and an apparently-present magnetic field which is equal to a multiple of a predetermined reference value of the compensation magnetic field corrected by the effective value of all 'offset' voltages in the second control circuit, and the two control circuits are set in operation alternately in respect of time, the absolute value of the compensation magnetic field being held below the predetermined reference value of the compensation magnetic field.

2. A method according to claim 1, wherein the compensation magnetic field is produced by means of a compensation current and both at the start time and also whenever the compensation current reaches a reference value in terms of absolute value, that is detected, the reference value of the compensation current is set to zero and the arrangement is briefly switched over from the first control circuit to the second control circuit.

3. A method according to claim 2, wherein the rate of change of the compensation current is continuously detected by means of a differentiator, whenever the compensation current reaches the reference value at a time t1, the analog values of the rate of change are sampled by means of a sample/ hold circuit, the sampled analog values of the rate of change are then respectively converted into digital values by means of an analogdigital converter, said digital values are then respectively stored in a random access memory of a microcomputer, an initial value of the compensation current is computed by means of the microcomputer using the formula (dlK/dt)t, .At, wherein t is the respective operating duration of the second control circuit and (dlK/dt)" is the rate of change sampled at time t1, and the initial value of the compensation current is used as a reference value for the compensation current until the expiry of the period At.

4. A method according to claim 2, wherein the rate of change of the compensation current is continuously detected by means of a differentiator, whenever the compensation current reaches the reference value at a time t1, the analog values of the rate of change are sampled by means of a sample/ hold circuit, the sampled analog values of the rate of change are then respectively converted into digital values by means of an analogdigital converter, said digital values are then respectively stored in a random access memory of a microcomputer, and a respective correction value, which is applicable at the end of a respective operating period t of the second control circuit, for the magnetic field which is digitised at the time t1, is computed by means of the microcomputer in dependence on the sum value 1K,tl = (dlK/dt)t, At wherein IKt, is the value of the compensation current at the time tl at the beginning of the operating period At of the second control circuit, and (dlK/dt)t, is the rate of change sampled at time t1.

5. Apparatus for measuring a magnetic field comprising a first control circuit for producing a compensation magnetic field of such a strength that the output voltage of a sensor included in the first control circuit is approximately zero, a second control circuit, which also includes the sensor provided for detecting and adjusting the working point of the sensor, wherein the second control circuit includes an electrical input of the sensor, and a changeover switching means for setting the two control circuits in operation alternately in respect of time.

6. Apparatus according to claim 5, wherein besides the sensor, the first control circuit also comprises a difference means, a change-over switch, a first controller and an element for producing the compensation magnetic field.

7. Apparatus according to claim 6, wherein the first controller includes a trans conductance amplifier.

8. Apparatus according to claim 6 or claim 7, wherein the element for producing the compensation magnetic field is a coil through which flows a compensation current.

9. Apparatus according to claim 8, wherein the coil is a metal spiral flat coil which is applied to the surface of a semiconductor crystal, the sensor is diffused into the semiconductor crystal at the surface thereof, a narrow air gap is provided between two ferromagnetic thin films above the sensor and approximately below the middle of one half of the coil, and the two thin films are secured in a condition of being completely electrically insulated on the surface of the semiconductor crystal between said surface and the half of the coil, in such a way that one thin film is disposed in line with the other.

1 0. Apparatus according to claim 8, wherein the coil is a minicoil which is wound on a coil former, in the iron core space of which is disposed a semiconductor crystal filling said space, the sensor is diffused into the semiconductor crystal at the surface, a narrow air gap is provided between two ferromagnetic thin films above the sensor, and the two thin films are secured in a condition of being electrically insulated on the surface of the semiconductor crystal, in such a way that one thin film is disposed in line with the other.

11. Apparatus according to claim 8, wherein the coil is wound on a flat annular ferromagnetic core which has a small air gap in which a sensor is disposed, wherein the sensor and the coil with its core are mounted on an insulating plate which in the vicinity of the centre of the annular ferromagneic core has an opening thorugh which a current conductor, in which flows a current for producing the magnetic field to be measured, passes approximately normal to the plane of the plate.

1 2. Apparatus according to any one of claims 5 to 11, wherein besides the sensor, the second control circuit also comprises difference means, an opening contact and a second controller.

1 3. According to any one of claims 5 to 12, wherein a capacitor is provided in the second controller as a voltage storage means.

14. Apparatus according to any one of claims 5 to 13, wherein the change-over switching means comprises a resistor, a voltage detector and a switch.

1 5. Apparatus according to claim 14, wherein the voltage detector includes a first comparator and a second comparator whose outputs are connected to respective inputs of an OR-gate.

1 6. Apparatus according to any one of claims 5 to 15, wherein forming a part of the measuring means of an electricity meter.

1 7. An arrangement for measuring a magnetic field comprising two apparatuses each according to claim 5, which each have a respective analog output connected to the output of the arrangement by way of a common further change-over switch, and which are operated in push-pull mode.

18. An arrangement according to claim 17, when forming a part of the measuring means of an electicity meter.

1 9. Apparatus or an arrangement for measuring a magnetic field, substantially as herein described with reference to and as illustrated in the accompanying drawings.

20. A method of measuring a magnetic field, substantially as herein described with reference to and as illustrated in the accompanying drawings.