GB2427700A

GB2427700A - Magnetic field measurement with continuous calibration

Info

Publication number: GB2427700A
Application number: GB0518433A
Authority: GB
Inventors: Inessa Bolshakova; Roman Holyaka; Sergei Gerasimov
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-06-29
Filing date: 2005-09-09
Publication date: 2007-01-03
Anticipated expiration: 2025-09-09
Also published as: UA82496C2; FR2887991A1; GB2427700B; FR2887991B1; GB0518433D0

Abstract

A method for measuring a quasi-stationary magnetic field B comprises detecting an output voltage from a galvano-magnetic transducer 1, independently detecting a change in the magnetic field over a period of time and, if this change exceeds a threshold, calibrating the galvano-magnetic transducer by comparing the change in its output voltage with the change in magnetic field. The independent detection of the change in magnetic field over time may be achieved using an inductive coil 2. The galvano-magnetic sensor may be a Hall Effect transducer and may be placed in the centre of the inductive coil. The invention may have particular application in a high radiation environment in which the output from a Hall sensor is liable to drift.

Description

METHOD FOR MEASURING QUASI-STATIONARY MAGNETIC FIELD

The invention relates to measurement instrumentation, namely to the methods for measuring the magnetic field based on inductive and galvanomagnetic measuring transducers, and

can be used for measuring the magnetic fields in

thermonuclear fusion reactors.

A known method for measuring the variable magnetic field is based on the measurement of the voltage integral at the measuring coil and further calculation of magnetic field induction change [Green M.I. Search coils. 1/ CAS.

Measurement and alignment of accelerator and detector magnets. Geneva. 1998. p. 163, Fig. 19.]. Change of magnetic flux through the coil, which is an informative parameter of change of magnetic field to be measured, causes a voltage (an electromotive force) at the terminals of the measuring coil. This voltage is integrated within certain time interval by the integrator, the signal of which results from the measurement of magnetic field change during integration time.

The disadvantage of this measurement method is impossibility of long term measurement of magnetic fields, and, in particular, of quasi-stationary magnetic fields or pulse magnetic fields with considerable pulse duration etc. The reason for this is, firstly, impossibility to determine initial value of magnetic field, which existed in the beginning of integration, secondly, low accuracy of integrator operating during long time integration process and minute voltage changes on measurement coil.

A magnetic field measurement method based on the

measuring of the output voltage of the galvanomagnetic transducer, in particular, of the semiconductor Hall transducer, and, further calculation of magnetic field induction using previously given value of transducer sensitivity is known, [Popovic R.S. Hall effect devices: magnetic sensor and characterization of semiconductors. TOP Publishing Ltd. 1991. P. 188. Fig. 4.22.]. By applying a voltage (current) supply to galvanomagnetic transducer in a certain way, charge carrier flow is being formed. Under the action of Lorentz force on moving charge carriers a signal is being generated in the galvanomagnetic transducer, for example, voltage difference on Hall transducer output leads.

This voltage is an informative signal of magnetic field measurement process. A coefficient of conversion from measured voltage to magnetic field induction is known in advance and constant.

Unlike magnetic field measurement method based on coils [Green M.I. Search coils. II CAS. Measurement and alignment of accelerator and detector magnets. Geneva. 1998. P. 163, Fig. 19.], the above mentioned method [Popovic R.S. Hall effect devices: magnetic sensor and characterization of semiconductors. lOP Publishing Ltd. 1991. P. 188. Fig. 4.22.] based on the galvanomagnetic transducer provides measuring of actual magnetic field value, and not its changes in time. So, the latter has no limitations for long term measurements of constant or quasi- stationary magnetic fields. However, its disadvantage is low accuracy of

magnetic field measurement under extreme operation

conditions, particularly, under high penetrating radiation conditions. The reason for this is a change of galvanomagnetic transducers' electrical and physical parameters under penetrating radiation; in particular, a change of galvanomagnetic transducers sensitivity under long term charged particles or neutron action.

A magnetic field measurement method based on

galvanomagnetic transducer output voltage measurement and calculation of induction of magnetic field to be measured using the voltage value and the sensitivity of above mentioned transducer, the transducer sensitivity being determined in the process of its periodical calibration with

the use of test magnetic field, is known. Such the

calibration is performed in-situ, i.e. directly inside of the object, where measuring transducer is placed. Test magnetic field is provided with the calibration coil, within that the galvanomagnetic transducer is placed. The said coil and the galvanomagnetic transducer appropriately arranged in it are forming a unified construction, which is a

functionally integrated probe. Test magnetic field

magnitude, which is defined by coil current supply and is considered to be known, and measured galvanomagnetic transducer output voltage value, which is determined by the test magnetic field, are the informative quantities in the process of galvanomagnetic transducer sensitivity calculation. [Boishakova I., Holyaka R., Leroy C. Novel approaches towards the development of Hall sensor-based magnetometric devices for charged particle accelerators /1 IEEE Transactions on Applied Superconductivity. 2002. - Vol.12, N'l. - P. 1655-1658.].

The advantage of the above mentioned measurement method [Bolshakova I., Holyaka R., Leroy C. Novel approaches towards the development of Hall sensor-based magnetometric devices for charged particle accelerators II IEEE Transactions on Applied Superconductivity. - 2002. - Vol.12, Ns1. P. 1655-1658.1 is a possibility of periodical calibration, and, thereby, it provides a compensation of galvanomagnetic transducer sensitivity drift under long term penetrating radiation conditions. It is fundamentally important that in the process of periodical calibration there is no need to take the probe out of the object, where magnetic field is to be measured. This advantage is of fundamental importance in magnetic field measurements under radiation operation conditions, in particular, in reactors or charged particle accelerators. A drift of galvanomagnetic transducer parameters occurs under high radiation conditions, so it is necessary to perform their periodical calibration. Such the calibration should be performed in- situ.

The disadvantage of the above mentioned method [Bolshakova I., Holyaka R., Leroy C. Novel approaches towards the development of Hall sensor-based magnetometric devices for charged particle accelerators II IEEE Transactions on Applied Superconductivity. - 2002. - Vol.12, N'l. - P. 1655-1658.] is low accuracy of periodical in- situ calibration. It is caused by low stability of coil parameters, which produces test magnetic field. In order to produce the necessary magnitude of the test magnetic field (several milliTesla) the coil should have as small diameter as possible (several millimeters) and as many loops as possible (several hundreds) . Besides, a considerably large current should be applied to the coil (dozens-hundreds of milliamperes), which results in the coil's heating. So, such the multiturn small-sized coils at considerably large current values, and, first of all, interturn insulation, are not reliable enough. Especially such unreliability is observed in coils' operation at high temperatures in vacuum, where, firstly, a heat sink is not sufficient, and, secondly, problems with gas emission from compound or interturn insulation varnish have a place.

It is an object of the present invention to improve the known quasistationary magnetic field measurement method in order to improve a measurement accuracy and a measurement device operating reliability under long-term harsh operation conditions of high penetrating radiation, insufficient heat sink and restricted access by means of the periodical calibration of the galvanomagnetic transducer directly in the process of the measurement using for such the calibration the recurring induction changes of quasi-

stationary magnetic field to be measured.

The object stated is achieved as follows: for galvanomagnetic transducer calibration in the quasi- stationary magnetic field measurement method, comprising galvanomagnetic transducer output voltage measurement and further calculation of induction of magnetic field to be measured using the measured output voltage value and galvanornagnetic transducer sensitivity known in advance, which is determined by transducer periodical calibration through determination of ratio between magnetic field change and corresponding galvanomagnetic transducer output voltage, the change of magnetic field to be measured is used. This change is measured by the field inductive transducer, the calibration being performed only when the change of magnetic field to be measured exceeds a previously given value during some predetermined time interval.

The proposed method for measuring the quasi-stationary magnetic field will now be described in details, being taken in conjunctions with the drawings, in which:

Fig. 1 shows time dependences of magnetic field

induction and inductive field transducer output voltage.

Fig. 2 shows the linear approximation of the galvanomagnetic transducer transduction.

Fig. 3 shows a block diagram of the device that carries out the proposed method for measuring the quasi-stationary

magnetic field.

Fig. 4 shows the arrangement of galvanomagnetic

transducer 1 in the coil 2 of the inductive field

transducer.

Fig. 5 shows time discriminator layout.

The measuring of quasi-stationary magnetic field

accordingly to the present invention is performed by the measurement device based on galvanomagnetic transducer 1.

The example of time dependence of induction B of quasi- stationary magnetic field to be measured is shown in Fig.l upper epure, which shows the induction changes AB0 of quasi- stationary magnetic field to be measured being correspondent to the time intervals t=t2-t1, at increase and decrease of magnetic field induction B. According to the invention these induction changes AB0 of quasi- stationary magnetic field to be measured are used for the calibration of the measurement device based on galvanomagnetic transducer directly in the process of magnetic field measuring.

The calibration includes coefficient KB determination, which connects VH output voltage of the measurement device based on galvanomagnetic trasducer with magnetic field induction B by formula: VH=KB*B. (1) Galvanomagnetic transducer transduction function at its linear approximation is shown in Fig.2. Coefficient KB is given by the formula as follows: KBVH0, (2) where L\VHOVH(t2)-VH(tl)is a difference between the output voltages VH(tl) and VH(t2) of the measurement device based on galvanomagnetic transducer on boundaries of z1t=t-t1 time interval. By using KB coefficient defined in this way and using the equation (1), a magnetic field BHinduction is calculated for any t point of time (see Fig. 2) in accordance with the measured output voltage /VHx.

According to the invention the induction change zB0 of the quasistationary magnetic field to be measured are measured by magnetic field induction sensor, which is usually comprised of coil 2 placed in the magnetic field to be measured.

The voltage VCQIL at coil 2 output is proportional to the effective area A of its loops and to the rate of change perpendicular to A area of magnetic field B induction vector component: (3) di' where K is coefficient of proportionality.

According to the invention the change of induction of magnetic field to be measured is determined by integration of measured inductive field transducer output voltage by the formula V07=K.M, (4) where t is an integration characteristic time.

As far as for calibration of the galvanomagnetic transducer 1 it is necessary to determine the value of changes of magnetic field induction iB to be measured exactly at the site where galvanomagnetic transducer 1 is located, this transducer is placed within the coil 2 of the

inductive field transducer.

The proposed method for measuring the quasi-stationary magnetic field can be embodied in the device for measuring the quasi-stationary magnetic field, block diagram of which is shown in Fig.3. This device is comprised of galvanomagnetic transducer 1 in the form of Hall transducer, magnetic field inductive transducer coil 2, driver 3, which is connected to galvanomagnetic transducer 1 and which provides its operating, time discriminator 4, which is connected to coil 2 and determines time intervals, in which magnetic field change reached the value known in advance, on-line data storage 5, which is controlled by time discriminator 4 and stores the results of driver 3 output voltage measurement, and corrector 6, which is controlled by time discriminator 4 and provides device periodical calibration in accordance with the proposed method. The galvanomagnetic transducer 1 is located inside of the coil 2 of magnetic field inductive transducer (Fig.4), the galvanomagnetic transducer 1 sensitivity axis (in particular, normal N to Hall transducer area) coinciding with coil 2 axis. Consequently, the measurement of magnetic field induction changes is provided by inductive transducer in the same section of the field in which the magnetic field induction being measured by means of Hall transducer.

Only galvanomagnetic transducer 1 and coil 2 of magnetic field inductive transducer are located in the area of

magnetic field measurement under extreme operation

conditions. The other elements of magnetic field measurement device are located out of the extreme operation conditions area that prevents the influence of destabilization factors existing in the area of extreme operation conditions.

Magnetic filed measurement is performed by galvanomagnetic transducer 1 (Hall transducer) . Driver 3 provides galvanomagnetic transducer signal formation and is comprised of galvanomagnetic transducer current supply stabilizer, amplifier and analog-digital converter of galvanomagnetic transducer signal.

In order to simplify the disclosure of the invention a linear approximation of transduction function of magnetic

field measurement device with Hall transducer as

galvanomagnetic transducer will be considered at first. Then driver's output voltage VH can be described as follows: VH=KH*KA*IHB, (5) where KH is a magnetic sensitivity of Hall transducer; KA is a coefficient of signal transformation by driver; H is Hall transducer supply current; B is magnetic field induction vector component, which is perpendicular to Hall transducer area.

In the process of long term exploitation of magnetic field measurement device under extreme operation conditions, the galvanomagnetic transducer magnetic sensitivity KH is being changed which is followed by measurement inaccuracy.

In general case, destabilization sources could be the transformation coefficient KA and supply current H. That is why galvanomagnetic transducer calibration provides for the determination of single coefficient KB, which connects output voltage VH of the measurement device with magnetic

field induction in accordance with formula (1)

Time discriminator 4 determines time interval, on the boundaries of which the magnetic field change, measured by means of the coil 2, reaches the given value AB0. In accordance with the invention, voltage VCOIL from magnetic field inductive transducer coil 2 is applied to the input of the time discriminator 4, and the output of the time discriminator 4 controls the operation of on-line data storage 5 and corrector 6 of this device.

An exemplary embodiment of time discriminator 4 layout and its links with other elements of the device are shown in Fig. 5. Such the time discriminator is comprised of integrator operational amplifier OAl, resistor R, capacitor o and switch SW), two comparators operational amplifiers 0A2, 0A3), logical element LE and timer TM.

The voltage VOUT at the integrator output is given by the formula (4), where r = RC is integrator time constant.

So the magnetic field inductive transducer coil 2 and

integrator provide magnetic field induction change

measurement, the result of which is almost independent of destabilization factors, in particular, of high penetrating radiation level and temperature. It must be emphasized that possible change of electrical and physical parameters of coil 2 wire has no effect on the signal in any way. In accordance with formula (3), the signal, which is picked up from coil 2, is determined not by electrical and physical parameters of wire, but only by its loops' effective area A, which is not affected by the radiation.

The time discriminator operation is demonstrated by time epures (Fig.l), which show a time change of magnetic field induction B(t), integrator output voltage VOUT(t) and operating pulses Si, S2, S3. Timer TM (Fig.5) forms a sequence of synchronizing pulses S1(t), duration and sequence period of which are fixed P1 = P2 = P3 = const.

Synchronizing pulses S1(t) periodically turn on the switch SW, which nulls the voltage on integrator output, VOUT(ti) = 0. Just after each synchronizing pulse termination (t1) a periodical integration process starts (magnetic field measurement) and rise-up edge of the pulse S2(t) is formed for controlling the on-line data storage. This pulse acts as a function of command, which selects and stores the output signal VH(tl) of driver 3 of the galvanomagnetic transducer 1 in the on-line data storage. In most cases, this signal has already been formed digitally by the analog-digital converter of this driver. Thus, voltage VH(t) is recorded in the on-line data storage 5 and is informative value of magnetic field induction B(t1), which is measured by means of the galvanomagnetic transducer 1.

Further operation of time discriminator depends on magnetic field B(t) change rate. In a case of sufficiently fast change of magnetic field, the voltage VOUT at the integrator output will exceed one of reference values +Vo (for example, the first period P in Fig.l) or -V0 (the third period P3 in Fig.l) before the start-up of the next synchronizing pulse. These reference values of voltage determine the threshold values of magnetic field B0 change.

At the time point t2, when integrator output voltage is equal to one of the reference values VOUT = V0, droop of pulse S2(t) and rise-up edge of the pulse S3(t) of corrector 6 control are formed (Fig.1) . During this time output signal VH(t2) of the galvanomagnetic transducer driver is measured and recorded in the on- line data storage.

Otherwise, when magnetic field change does not exceed the threshold value A80 before period termination (for example, second period F2), synchronizing pulse S1(t) of the next period will reset the integrator. As the result, pulse S3(t) will not be formed. Thus, at minute magnetic field change, when integrator operating accuracy is low, the correction of transduction function of magnetic field measurement device is not performed according to the invention.

The corrector 6 (Fig.3) calculates coefficient KB, which connects a time interval At = t2 - t2, determined by time discriminator in which magnetic field AB0 change took place, with output voltages VH(tl), VH(t2) of galvanomagnetic transducer driver on the boundaries of this time interval, by the formula K VH(t2)-VH(tJ) 6 B () Taking into consideration the equation (4) and that during time t the magnetic field change is iB0, coefficient KB is given as KB=K..VH(t2)-VH(tI) (7)

C

where V0 is a value of the reference voltage of time discriminator layout.

Then, by measuring the galvanomagnetic transducer driver voltage value VHM at any time and by using coefficient KB, the calculation of magnetic field induction BM is performed.

In particular, at linear approximation of transduction function (Fig.2), accordingly to the invention the measurement of magnetic field induction will result in the determination of the induction value: BM=VHM. (8)

KB

At piecewise-linear approximation of transduction function of magnetic field measurement device, coefficient KBj is determined for each part j of transduction function.

At approximation of transduction function by polynomials, a set of equations, in which, in particular, coefficient KB(B) dependence is represented as first derivative dVH/dB, is used for calculation.

The proposed method allows improving-a quasi-stationary

magnetic field measurement accuracy (taking into

consideration in-situ calibration the measurement error does not exceed 0. 1%) and reliability of long term (several years) operation under harsh conditions, in particular, under high penetrating radiation and temperature conditions.

It will be understood that the invention is not restricted to the described and illustrated exemplifying embodiments thereof, and that these embodiments can be modified within the scope of the inventive concept illustrated in the accompanying Claims. iS

Claims

Claims 1. A method for measuring quasi-stationary magnetic field,

comprising a measuring of galvanomagnetic transducer output voltage and further calculation of the induction of magnetic field to be measured with use of said output voltage value and value of galvanomagnetic transducer sensitivity known in advance, being determined by galvanomagnetic transducer periodical calibration through definition of the ratio between magnetic field change and corresponding galvanomagnetic transducer output voltage change, characterised in that said calibration is performed directly in the process of magnetic field measuring, and magnetic field change, being used for calibration of the galvanomagnetic transducer, is the change of the magnetic field to be measured; said calibration is being performed only if a change of magnetic field to be measured exceeds a previously given value during some predetermined time interval.
2. The method according to claims 1 wherein the change of magnetic field to be measured in the process of calibration is measured with field inductive transducer.
3. The method according to claims I or 2 wherein the value of change of the magnetic field to be measured is determined through integration of the measured

output voltage of the field inductive transducer.
4. The method according to claims 1, 2 or 3 wherein the field inductive transducer contains a coil.
5. The method according to claims 1, 2, 3 or 4 wherein the field galvanomagnetic transducer is placed within the coil of the field inductive transducer.
6. A method for measuring quasi-stationary magnetic field substantially as herein described with reference to the accompanying figures.
7. Apparatus for calibrating a galvanomagnetic transducer during measurement of a quasi-stationary magnetic field comprising: a galvanomagnetic transducer providing an output voltage proportional to

magnetic field;

means for detecting a known change of the quasi-stationary magnetic

field, the change occurring over a time period;

means for measuring the change in output voltage of the galvanomagnetic transducer due to the change in magnetic field over the time period; and a means for calibrating the galvanomagnetic transducer using the ratio of magnetic field change and galvanomagnetic transducer output voltage change, wherein the calibration is performed only if the detected change in magnetic field exceeds a given value during some predetermined common interval.
8. An apparatus according to claim 7 wherein the change in magnetic field to be measured in the process of calibration is measured with a field inductive transducer.
9. An apparatus according to claims 7 or 8 wherein the value of change of the magnetic field to be measured is determined through integration of the measured output voltage of the field inductive transducer.
10. An apparatus according to claims 7, 8 or 9 wherein the field inductive transducer contains a coil.
11. An apparatus according to claims 7, 8, 9 or 10 wherein the field of galvanomagnetic transducer is placed within the coil of the field inductive transducer.
12. An apparatus for calibrating a galvanomagnetic transducer during measurement of a quasi-stationary magnetic field substantially as herein described with reference to the accompanying drawings.