GB2103395A

GB2103395A - An arrangement for compensating magnetic fields of movable bodies

Info

Publication number: GB2103395A
Application number: GB08216415A
Authority: GB
Inventors: Gerhard Trenkler; Wolfgang Bornhofft
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1981-06-06
Filing date: 1982-06-04
Publication date: 1983-02-16
Also published as: SE455448B; SE8203373L; FR2510805A1; FR2510805B1; DE3122686A1

Abstract

An arrangement for compensating magnetic fields of movable bodies 1 made from a magnetisable and/or electrically conductive material comprises current carrying windings 2 arranged inside or outside the bodies, their magnetic moments being controllable by means of magnetic sensors 3. The sensor signals are passed to a mathematical model in an electrical circuit (9, Fig. 2 not shown) which takes into account the mechanism whereby the magnetic fields are formed and the compensation fields and supplies output signals which control the winding currents in order to minimise the external magnetic field of the body. <IMAGE>

Description

SPECIFICATION An arrangement for compensating magnetic fields of movable bodies The present invention relates to an arrangement for compensating magnetic fields of a movable body.

It is known for current carrying windings to be capable of compensating for the external self-consistent magnetic fields of magnetisable bodies (except for a small residual field) if the size and direction of the current is adjusted accordingly in a measurement arrangement. If, however, a body is moved then the size and direction of the field changes in the case of bodies which are not fully symmetrical, since the body changes its position with respect to the earth's magnetic field. This field which depends on the position of the body can be partly compensated by other windings, if their winding currents follow the measurement signals which are produced by magnetic sensors connected to the body. The size and direction of the linear relationship between the sensor signals and the winding currents is adjusted in a measurement arrangement.If the body comprises conductive materials then, with a rotary movement therein, eddy currents are generated and their magnetic fields are added to the fields mentioned above. In the past other windings have been provided for compensating these additional fields and the size and direction of their currents are a linear function of the differential quotient of the output signals of the magnetic sensors. The size and direction are adjusted to fixed values again in a measurement arrangement.

These known methods have important disadvantages, some of which are mentioned below.

a) It is not possible for known changes in the magnetic properties of the body over a period of time to be taken into account.

b) Changes caused by external effects in magnetic properties of the body cannot be compensated.

c) Three windings are necessary for each spatial direction.

d) Fairly large changes in the location of the body are detrimental to compensation.

e) The eddy current fields are not adquately inhibited.

f) Failures in the magnetic sensors and/or windings are detrimental to compensation.

The present invention seeks to avoid these disadvantages and to ensure that the magnetic fields are compensated independently of location, time and attitude.

According to the present invention there is provided an arrangement for compensating magnetic fields of a movable body made from a magnetisable and/or electrically conductive material comprising current windings and magnetic sensors, the windings being controllable by the magnetic sensors wherein the output signals of the magnetic sensors are fed as input signals to an electrical device containing a mathematical model, the mathematical mode taking into account the mechanisms by which the magnetic fields inherent in the bodies are formed and the compensation fields caused by the current carrying windings and delivery output signals which control the currents in the windings so as to minimise the external magnetic field of the body.

The compensation process is considerably enhanced by the arrangement in accordance with the invention and the cost of equipment and current windings, which have to be laid, is reduced considerably.

The mathematical model is preferably constructed so that three orthogonal windings serve to compensate the magnetic field of the body.

The mathematical model may balance the effect of failure of one or more compensation windings by changing the current in the other compensation windings by adapting the parameters of the model.

Alternatively or in addition it may compensate for the effect of failure of one of more magnetic sensors by adapting the parameters of the model.

Significant parameters of the model may be determined either completely or partly in a measurement arrangement and are passed via a data line or data connection to a switching circuit of the electrical device.

The magnetic fields of the movable body may be ascertained in a measurement arrangement and the related field values are passed to the electrical device by a data line or data connection. The significant parameters of the model and/or other system parameters may be set by means of a keyboard, preferably an alphanumeric keyboard.

A screen may be used to display the significant parameters of the mathematical model and the state of compensation of the body.

The current windings may be arrangd outside, on, or inside the body.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which Figure 1 shows a cylindrical, magnetisable and conductive body in an homogenous field; and Figure 2 shows a circuit arrangement According to Figure 1 a hollow cylindrical member 1 comprising a magnetisable material is located in a homogeneous field with a permanent magnetisation Mp in the direction of the cylindrical axis. A compensation winding 2 is arranged around the body 1 and a current i(t) passes through it. Furthermore, a magnetic sensor 3 is shown in Figure 1.

The magnetic effect of the cylinder 1 can be described approximately as follows by means of a magnetic dipole having the moment mp in direction z: mp = Mp V, In which V is the volume of the cylinder. If the cylinder is located in a magnetic field Ha and if the coefficient of permeability is Blr + 1, then an approximate description of the magnetic effect is possible by a further dipole having the moment mj:

Wherein N is the demagnetisation factor.

When there is rotary movement of the body in the direction of the angle 0 as shown diagrammatically above the cylinder, a current I is induced in accordance with the induction rule, said current flowing in the walls of the cylinder in a circular direction with respect to its axis. The magnetic effect of the current can be approximated by a dipole with the magnetic moment mw: mw = i # Az where Az is the cross section of the cylinder. For the current i the following represents a first approximation where L is the inductance of the cylinder and R is the resistance in a circular direction:

In contrast to the dipole moments mp and mop the dipole moment mw is a time function which-depends on the rate of change of the angle.

The mechanism by which the self-constistent magnetic fields He of the body are formed can be. described as a mathematical model by the following formulae:

If the compensation winding is fed by a controllable constant current generator with the current is then the following is true if the magnetic effect is approximated by a dipole with the moment me:

where m = is As with As as the winding area. The compensation winding suppresses the self-consistent field of the body completely if: Her - Hsr = 0 He# - Hs# = 0 The above described arrangement can be represented in a block diagram as shown in Figure 2 which has three portions, i.e. a measuring element 4, an electrical device 5 and an adjusting element 6. The measuring element 4 comprises magnetic sensors 3 which detect the external field Ha and deliver an input signal x to the device 5. This device has an input unit 7, an output unit 8 and a switch arrangement 9 connected therebetween. Signals y pass out from the electrical device 5 into a current control unit 10 said signals producing the current i for the windings 11 of the compensation arrangement. The current carrying windings 11 produce a magnetic field which reduces the self-consistent field of the body to a minimum.

A mathematical model as follows:

is to be incoporated in the switching arrangement as an algorithm to be solved, assuming a linear relationship between the external field Ha and the input signals x of the electrical device 5 as well as between the current i of the control unit 10 and the output quantity y of the electrical deviceS. The parameters of this model function can be fixed as constants which depend on the material or the geometry and can follow known changes in these magnitudes. The model can be extended in its axes in order to compensate for the self-consistent fields of the body over all space and should be adapted accordingly in its parameters. This can be achieved either continuously or at certain intervals of time.

The mathematical model in the switching arrangement 9 can be effected in the form of an electronic computer. Several similar electrical devices 5 can be provided in parallel in order to make the compensation arrangement more reliable.

Claims

1. An arrangement for compensating magnetic fields of a movable body made from a magnetisable and/or electrically conductive material comprising current windings and magnetic sensors, the windings and magnetic sensors, the windings being controllable by the magnetic sensors, wherein the output signals of the magnetic sensors are fed as input signals to an electrical device containing a mathematical model, the mathematical model taking into account the mechanisms by which the magnetic fields inherent in the bodies are formed and the compensation fields caused by the current carrying windings and delivering output signals which control the currents in the windings so as to minimise the external magnetic field of the body.

2. An arrangement according to claim 1 wherein the parameters of the mathematical model continuously follow the changes in the magnetic properties of the body.

3. An arrangement according to claim 1 wherein the parameters of the mathematical model are matched to the changes in the magnetic properties of the body at predetermined intervals of time.

4. An arrangement according to claim 1 wherein the mathematical model in the electrical device is an electronic computer.

5. An arrangement according to any preceding claim wherein the mathematical model is constructed so that three orthogonal windings serve to compensate the magnetic field of the body.

6. An arrangement according to any preceding claim wherein a plurality of said electrical devices are connected in parallel in order to increase the reliability.

7. An arrangement according to any preceding claim wherein the mathematical model balances the effect of failure of one or more compensation windings by changing the currents in the other compensation windings by adapting the parameters of the model.

8. An arrangement according to any preceding claim wherein the mathematical model compensates for the effect of failure of one or more magnetic sensors by adapting the parameters of the model.

9. An arrangement according to any preceding claim wherein significant parameters of the mathematical model are determined either completely or partly in a measurement arrangement and are passed via a data line or data connection to a switching circuit of the electrical device.

10. An arrangement according to any preceding claim wherein the magnetic fields of the movable body are ascertained in a measurement arrangement and the related field values are passed to the electrical device by a data line or data connection.

11. An arrangement according to any preceding claim wherein significant parameters of the mathemati cal model or system parameters are set by means of an alphanumeric keyboard.

12. An arrangement according to any preceding claim wherein a screen is used to display the significant parameters of the mathematical model and the state of compensation of the body.

13. An arrangement according to any preceding claim wherein the currentwindings are arranged outside the body.

14. An arrangement according to any of claims 1 to 12 wherein the current windings are arranged on or in the body.

15. An arrangement for compensating magnetic fields substantially as herein described with reference to the accompanying drawings.