GB2168820A - A circuit arrangement with a hall generator supplied by a control current - Google Patents

Abstract

The invention provides a circuit arrangement with a Hall generator (10) which can be supplied by a control current, the Hall electrodes (12) (14) of which are each connected to an input of a differential amplifier (16). The control electrodes (20, 18) of the Hall generator (10) are each connected to a current source (42, 36) without being connected to a stable potential. One current source (42) supplies a control electrode (20) with a current (i1). The other current source (36) removes a current (i2) of equal magnitude from the other control electrode (18). The current sources (42, 36) may be supplied with control pulses from amplifier (54). <IMAGE>

Description

SPECIFICATION A circuit arrangement with a Hall generator supplied by a control current This invention relates to a circuit arrangement with a Hall generator supplied by a control current, the Hall electrodes of which are each connected to an input of a differential amplifier.

Hall generators are used, among other things, for measuring magnetic fields. If the Hall generator is supplied with a continuous control current, a Hall voltage is produced which is proportional to the intensity of the respective magnetic field in the Hall generator. The maximum permissible temperature of the Hall generator is influenced by the magnitude of the control current, by the mode of operation, that is by the cooling conditions, and by the ambient temperature. Thus the respective permissible control current must be adapted to the cooling conditions and the ambient temperature. So as to produce a sufficiently large Hall voltage, even with low magnetic field intensities, the control current must be increased. If the control currents are supplied to the Hall generators in the form of impulses, it is possible to use higher current amplitudes.The sensitivity can be increased hereby. The amplitudes of the control current impulses can be many times greater than the permissible direct control currents.

So as to provide a respective Hall generator with control current impulses, a control electrode can be connected to a current or voltage source, which periodically produces current or voltage impulses, while the second control electrode is connected to earth or another reference potential. In such a circuit, voltage jumps occur on the Hall electrodes in relation to the reference potential during supply of the respective Hall generator with a control current impulse even without a magnetic field in the Hall generator which jumps are of about equal size and have the same polarity. The height of the voltage jump corresponds to about half the voltage on the control electrodes. Owing to the voltage jumps, the differential amplifier connected to the Hall electrodes is controlled in common mode.It has been shown that the ratio of the semi-control voltage to the differential voltage produced by small magnetic fields can be greater than 80 dB when measuring stray flux on objects to be tested for structural faults. This means that the high common mode voltages particularly occurring with short control current impulse durations of about 5 wsec can hardly be suppressed by the differential amplifier.

The object of the present invention is to further develop a circuit arrangement of the type described in the outset, to the effect that when supplying the Hall generator with high-frequency control currents, common mode supply of the differential amplifier can be substantially avoided.

The object is achieved according to the invention in that the control electrodes of the Hall generator are each connected to a current source without being connected to a stable potential, such that one current source feeds a current into one control electrode and the other current source absorbs a current of equal magnitude from the other control electrode.

In this arrangement no voltage jump occurs on the Hall electrodes in relation to the respective reference potential of the differential amplifier when supplying with a control current impulse. The differential amplifier inputs are thus not supplied with high common mode signals. Low field intensities can thus be established with the Hall generator. It is possible to carry out numerous measuring cycles in a short space of time.

One useful embodiment is characterised in that the inputs of the differential amplifier connected to the Hall electrodes are each connected to a resistor and that the second contacts of the resistors are connected to the same reference potential as the differential amplifier. Acontact between the Hall electrodes of the Hall generator and the reference potential of the differential amplifier is achieved via the two resistors, preferably of equal size, between the Hall electrodes and the reference potential.

A preferred embodiment is described in claim 3. The circuit arrangement given in class 3 enables the adjustment to equal absolute values of the voltages of varying polarity occurring in relation to the reference potential on the Hall electrodes, when supplying the Hall generator with control currents in the absence of a field to be measured. The circuit arrangement can thus be adapted in simple manner to the respective conditions ofthe Hall generator.

In a favourable embodiment, the two current sources which can be supplied in each case by control impulses to produce a control current, are arranged between the positive and negative pole of a voltage source in series with the Hall generator. The circuitry expense is low with this arrangement.

For the simultaneous supply of the control input of the two current sources with control impulses, it is useful to connect the control input of the current source connected to the positive pole of the current source to the inverted output, and the control input of the current source connected to the negative pole to the non-inverted output of an amplifier supplied by common mode impulses.

The impuse duration of the control currents is preferably about 5 microseconds. This short impulse duration permits the supply of the Hall generators with high control currents and a small impulse-pause ratio. Furthermore, it is possible to repeat the measurement at short time intervals.

It is favourable to use circuit arrangements, as described in the claims, as Hall probes for measuring stray flux in the non-destructive testing of ferromagnetic objects for structural faults. The Hall probes can be arranged in one or more rows at a small spacing from the surface of the respective object.

Further details, advantages and characteristics of the invention not only emerge from the claims but also from an embodiment shown in a drawing.

Figure 1 shows a circuit diagram of an arrangement with a Hall generator which can be supplied by a control current, one Hall electrode of which is connected to a reference potential and Figure 2 shows a circuit diagram of an arrangement with a Hall generator which can be supplied by a control current, the Hall electrodes of which are not connected to a stable potential.

A Hall generator 10 is respectively connected with its two Hall electrodes 12, 14 to an input of a differential amplifier 16. One control electrode 18 is connected to earth potential, while the other control electrode 20 of the Hall generator 10 is connected to a current source or voltage source 22, which periodically supplies the Hall generator 10 with current or voltage impulses. The current or voltage 22 is connected to a pole 24 of a voltage source, the other pole of which, not shown, is connected to earth potential.

If the Hall generator 10on the control electrode 20 is supplied with a current or voltage impulse, then voltage jumps of equal polarity occur on the Hall electrodes 12 and 14 in relation to the reference potential, independently from a magnetic field to be measured. The voltage jumps are of about equal height. The equation is as follows: Uei Ue2 1/2 u, wherein u,l refers to the voltage on the Hall electrode 12, u,2 the voltage-on the Hall electrode 14 and u,, the voltage on the control electrodes 18, 20.

A common mode control of the differential amplifier 16 is caused by the voltage jumps uei and Ue2. If the Hall generator 10 is used as a Hall probe for measuring stray flux, which rises from structural disturbances in test samples in the non-destructive material testing with magnetic fields; Hall voltages are produced which are substantially lower than the common mode voltages. It has been shown that the ratio between the common mode voltages and the differential Hall voltage can be 2 80 dB in the determination of the previously mentioned structural faults.If the respective differential Hall voltage is referred to by A(ue1 then the following equation applies; 20 log .0.5 u, 2 80 [dB] (u,, - U,Z) A common mode voltage of 1/2 u5t (for example 5 V), which is established in the impulse supply of the Hall generator 10 within a very short time can thus hardly be suppressed by the differential amplifier 16 in relation to the differential voltage produced by the stray flux.

This disadvantage is avoided by the circuit arrangement shown in Figure 2. The same elements are provided with the same reference numerals in the arrangements shown in Figure 1 and 2.

The control electrode 18 in the circuit arrangement according to Figure 2 is connected to a contact of a current source 26, the other contact of which is connected to the negative pole 38 of a voltage source 40. The control electrode 20 is connected to the contact of a second current source 42, the other contact of which is connected to the positive pole 44 of the voltage souce 40. The current source 42 supplies the control electrode 20 with a current ii, while the current source 36 of the control electrode 18 drains a current i2. The two currents ii and i2 are of equal magnitude. The current sources 36 and 42 are pulsed current sources with very high internal resistances.

Resistors 46,48 of equal size are in each case connected to the inputs of the differential amplifier 16, the second contacts of which resistors are connected to a reference potential, for example earth potential. The current sources 36 and 42 each contain a control input 50, 52. The control input 50 of the current source 36 is connected to the non-inverted output of an amplifier 54, which has a further inverted output connected to the control input 52 of the current source 42. The input of the amplifier 54 is supplied with a periodical pulse sequence, which has a constant frequency and a constant, small impulse-pause ratio. The control electrodes 18, 20 are bridged by a preferably high-ohmic potentiometer 56, the tap 58 of which is connected to the Hall electrode 14.

The Hall generator 10 is arranged without a definite connection to a potential, that is floating, between the two current sources 36, 42. The Hall electrodes 12, 14 are connected via the resistors 46 and 48 to the reference potential.

During feeding of the current ii into the control electrode 20 and the removal of the current i2 of equal magnitude from the control electrode 18, a positive voltage u20 occurs on the control electrode 20 and a negative voltage u18 on the control electrode 18. With the large internal resistances of the current sources 36, 42, currents i1 and 2 of equal magnitude as well as a symmetrical construction of the Hall generator 10, the voltages u20 and us occurring on the control electrodes 20 and 18 are of equal magnitude and of opposite polarity. Differences owing to the structural conditions of the Hall generator 10 can lead to differences in the two voltages u20 and u18. By an alignment with the potentiometer 56, the two control voltages u20 and u18 can be brought into agreement inspite of the differences in the structural conditions of the respective Hall probe regarding size.

On the Hall electrodes 12 and 14, no voltage of equal polarity in relation to the reference potential of the differential amplifier 16 occurs inspite of supplying the Hall generator 10 with a control current in the case of measuring faults in a magnetic field. The voltage reduction u5t between the control electrodes 18 and 20 is produced by a resistor RH of the Hall generator 10 between the electrodes 18 and 20 according to the equation: = 11= 1 RH = i2 RH Since the Hall electrodes 18 and 20 conduct no voltages of equal polarity in relation to the reference potential of the differential amplifier 16, the common mode supply of the differential amplifier 16 is dispensed with.Thus, in the case of faults in the outer magnetic field to be measured, only voltages still occur on the differential amplifier 16 which result from the non-symmetry of the Hall probe and the ohmic or inductive zero component. The measurement of magnetic fields is thus no longer impaired by common mode signals on the differential amplifier inputs. It is possible to supply the Hall generator 10 with control currents of 5 Wsec duration, whereby the amplitudes amount to several times the permissible direct control current. Thus, low magnetic field intensities can also be measured accurately.

The circuit arrangement shown in Figure 1 can particularly be used in the non-destructive testing of ferromagnetic bodies for structural faults by magnetizing the respective body. The Hall generator 10 thereby serves as a Hall probe which is arranged on or close to the surface of the test sample, so as to measure the magnetic stray flux caused by structural disturbances. The Hall probes can usefully be arranged in the manner described in DE-PS 31 32808 at a small spacing from the surface of the body to be tested. The mark-to-space ratio of the control currents for the respective Hall generator 10 is then dependent on the number of Hall generators operated in succession. If, for example, 40 Hall probes are to be tested, then the mark-to-space ratio is 1/40. With a control current of 5 lisec duration, each hall generator is supplied every 2001use with a control current for 5 psec. The control frequency of the respective Hall sound is thereby 5 KHz.

Claims (9)

1. A circuit arrangement with a Hall generator which can be supplied by a control current, the Hall electrodes of which are each connected to an input of a differential amplifier, characterised in that, the control electrodes (18,20) of the Hall generator (10) are each connected to a current souce (42,36) without being connected to a stable potential, such that one current source (42) supplies a control electrode (20) with a current (i1) and the other current source (36) absorbs a current (i2) of equal magnitude from the other control electrode (18).

2. A circuit arrangement according to claim 1, characterised in that, the inputs of the differential amplifier (16) connected to the Hall electrodes (12, 14) are each connected to an alignable resistor (46,48) and that the two contacts of the resistors (46,48) are connected to the same reference potential as the differential amplifier (16).

3. A circuit arrangement according to claim 1 or 2, characterised in that, the control electrodes (18, 20) are bridged by a potentiometer (56), the tap (58) of which is connected to a Hall electrode (14).

4. A circuit arrangement according to one of the preceding claims, characterised in that, the two current sources (46,36) in each case supplied by control impulses to produce a control current are arranged between the positive and the negative pole (44,38) of a voltage source (40) in series with the Hall generator (10).

5. A circuit arrangement according to one of the preceding claims, characterised in that, the impulse duration of the control currents (i1, i2) iS about 5 microseconds.

6. A circuit arrangement according to one of the preceding claims, characterised in that, the current sources (46, 36) are pulsed current sources.

7. A current arrangement according to one of the preceding claims, characterised in that, the current source (42) has a control input (52) connected to an inverted output of an amplifier (54) and the other current source (36) has a control input (50) connected to the non-inverted output of the amplifier (54).

8. A circuit arrangement according to one of the preceding claims, characterised by the use of the Hall generator (10) as Hall probe for measuring stray flux in the non-destructive testing of ferromagnetic objects for structural faults.

9. A circuit arrangement substantially as herein described and as illustrated in the accompanying drawings.