GB2025630A - Electrical bridge circuit with compensation for long-term drift - Google Patents

Abstract

An electric sensing circuit, typically a metal detector, includes a bridge including inductors (L1, L2). The out-of- balance condition of the bridge is stored on a capacitor C1 and a compensating signal is applied to the bridge output at A1 in dependence upon the stored value of the charge. The stored value of the charge is only updated in response to relatively slow changes in the out-of-balance condition of the bridge, whereupon switch S1 is closed as soon as the imbalance voltage, delayed by network R6, C2, exceeds positive or negative threshold Vref1 or Vref2. Relatively rapid changes in the out-of-balance condition, whereupon the undelayed imbalance voltage exceeds a threshold Vref3, are used to provide an output signal for alarm or measuring purposes, and the arrangement is such that switch S2 is also operated to inhibit circuit A4, A5, S1 from automatically balancing the output signal giving the alarm or measurement. <IMAGE>

Description

SPECIFICATION Electrical bridge balancing circuit.

This invention relates to an electrical circuit which includes a bridge circuit and means for setting and holding the bridge circuit in balance. This invention has particular, but not sole, application to metal detection systems.

In many electrical circuits which include bridge circuits, for example many measuring instruments, it is necessary to maintain the bridge circuit in balance.

This can be effected by periodically re-baiancing the bridge, either manually or automatically. One of the problems with automatic balancing is that during the period that the bridge circuit is responding to effect a measurement the automatic balancing will start to correct for the error signal which is produced as representing the measurement.

According to the present invention there is provided an electrical sensing circuit including a bridge, sensing means arranged to produce an output indicative of the in or out of balance condition of the bridge, means arranged to store the value of said output in response to the value thereof charging at only less than a predetermined rate, and means responsive to said stored value for causing said sensing means to provide an output indicative of bridge balance.

Embodiments of this invention will now be described, by way of examples only, with reference to the accompanying drawings, in which: Figure 1 is an electrical circuit diagram of a first embodiment of metal detection system; Figure2 is an electrical circuit diagram of a second embodiment of metal detection system, having additional balancing facility, an audible alarm system and an arrangement for discriminating between ferrous and non-ferrous metals; Figure 3 is a vector diagram for explaining operation of the means for discriminating between ferrous and non-ferrous metals; and Figures 4 and 5 illustrate different probes for use with the system of Figure 2.

Referring firstly to Figure 1, there is shown a circuit diagram of a metal detection system which detects a piece of metal by responding to a change on inductance of one or other of two coils L1, L2 mounted in a probe (not shown), upon inductive coupling of that coil with the piece of metal.

The circuit comprises an oscillator OSC with centre tapped transformer T1. The two secondary winding sections and the coils L1, L2 are connected as a bridge the output of which is connected to an amplifier Al. A phase detector PD is connected to the output of amplifier Al and is supplied with reference voltage over a line X from the junction between coil L1 and its respective transformer secondary section.

An amplifier A2 is connected to the output of the phase detector PD and has its own output connected to a junction point Y.

Balancing for the bridge circuit is provided by a DC controlled bridge circuit comprising F.E.T.'s VR1 and VR2 (acting as variable resistors) and resistors R2 and R3. The junctions of the two F.E.T.'s with the respective resistors R2 and R3 are connected across the secondary of transformer T1 through resistors R4, R5 so that a proportion of the oscillator output is applied to VR1 and VR2. A control voltage which is stored on a capacitor C1 (as will be explained) is applied directly to the gate of F.E.T. VR2 and to the inverting input of an amplifier A6, to the noninverting input of which is applied an offset voltage 2VM. AmplifierA6 is an inverting DC amplifier with a gain of unity, so that when the value of the control voltage VC equals VM, the output of amplifier A6 is also VM.This amplifier output is connected to the gate of F.E.T. VR1, as shown. The junction of resistors R2 and R3 is connected to the input of amplifier Al.

Junction point Y is connected through a normally open electronic switch S1 to the storage capacitor C1. Point Y is also connected to an input of an amplifier trigger circuit A3 which is controlled by a reference voltage Vref3. The ouput of amplifier trigger circuit A3 drives an indicating meter IM, an alarm and a normally open electronic switch S2.

Junction pointY is also connected through a delay circuit R6, C2 to an input of both of two amplifier trigger circuits A4, A5. These two amplifier trigger circuits are controlled by reference voltages Vrefl and Vref2, respectively, which are negative and positive, respectively, but of equal magnitude less than that of Vref3. The outputs of amplifier trigger circuits A4, A5 are connected together to drive the normally open electronic switch So. when switch S1 is closed, it connects the junction point Y to the storage capacitor C1.

In operation, the oscillator OSC operates at, say 2 kilocycles and if the coils L1 and L2 are equal in resistance and inductance, the amplifier Al will provide zero output signal, assuming zero input to amplifier Al from the F.E.T. balancing bridge. In the latter connection, it will be noted, that if a voltage VC = VM is stored on capacitor C1, and therefore applied to the gate of F.E.T. VR2, an equal voltage VM will be provided by amplifier A6 and applied to the gate of F.E.T. VR1. Under this condition, the two F.E.T.'s have equal resistance and, because they are supplied with equal antiphase voltages from transformer Tithe net input to amplifier Al via resistors R2 and R3 is zero.

If an AC error signal should be provided by amplifier Al, representing some inbalance between coils L1 and L2, this error signal is converted to a DC voltage by the phase detector PD, which DC voltage is positive or negative according to the phase of the AC error relative to the phase of the reference signal provided over line X. This DC voltage is amplified by amplifier A2 and is applied to capacitor Cl, when electronic switch S1 is closed (as will be described).

Accordingly, the voltage VC stored on capacitor C1 depends upon the AC error signal produced by amplifier Al.

If the voltage VC stored on capacitor C1 differs from VM, then the gate voltages of the two F.E.T.'s will be different and an output signal will appear at the junction of resistors R2 and R3. This signal will correspond in phase to one or other section of the transformer secondary, depending whether the stored voltage VC is greater than or less than VM.

The arrangement is such that the signal thus applied to the input of amplifier Al from the balancing bridge is of appropriate phase to substantially cancel the original error.

The balancing bridge will continue to correct increasing errors until the DC control voltage on F.E.T. VR1 or VR2 causes cut off and then no greater AC correction signal is available.

The opening and closing of electronic switch S1 is controlled by the amplifier trigger circuits A4, A5 and their input delay circuit R6, C2. Should a DC error signal build up slowly at junction Y, either positively or negatively, then the respective one of amplifier trigger circuits A4, A5 will provide an output when the magnitude of the DC error signal exceeds Vrefl or Vref2. Switch S1 is thereby closed to re-establish the balance of the bridge circuit. Once the DC error signal at Y returns to zero, the output of whichever of amplifiers A4, A5 that has been operated disappears and the switch S1 is opened, however leaving capacitor C1 with the voltage required to maintain balance.

A rapid change in error signal at junction Y indicates that a piece of metal is being detected by one or other of coils L1, L2, because thermal drift or the like, will produce only relatively slow changes. If a rapid change occurs of magnitude Vref3 or more, then trigger amplifier A3 will produce an output to energize the alarm and drive the indicating meter.

The magnitude of Vref3 is greater than that of Vrefl and Vref2, and the delay circuit R6, C2 prevents amplifier A4 or A5 operating before amplifier A3.

The output of amplifier A3, when it is operated by the rapid change in error signal, closes switch S2 to prevent capacitor C2 charging and therefore preventing amplifier A4 or A5 from operating.

Accordingly, re-balancing can only take place in the normal condition, wherein no piece of metal is being detected and therefore switch S2 is kept open.

Slow changes of error signal, caused by thermal drift or the like, initiate the re-balancing before the error signal reaches a magnitude sufficient to operate amplifier A3. Once the error signal exceeds Vref3, amplifier A3 provides an output linearly related to the deviation of the bridge circuit from its balanced condition.

The system of Figure 1 is designed to balance out only the inductive components of the impedances of the bridge. The system can however be readily modified to balance out both the resistive and the inductive components of the bridge impedances and such a modified system is shown in Figure 2. The system of Figure 2 also includes an audible alarm system connected to the output of the bridge, the alarm system providing by means of a loudspeaker LS, a continuous tone which increases frequency as the coils L1, L2 are moved close to an object of ferrous metal, and a similar tone which is cyclically interrupted if the metal is non-ferrous. The system also includes a discriminatorto indicate that the probe (that includes the coils L1, L2) is correctly oriented (as will be explained hereinafter).

Referring now to Figure 2 in detail, in order to additionally balance out the resistive components of the bridge, the amplifier A1 feeds its outputs to another phase sensitive detector PDa supplied with a reference signal from line X through a 90" phase shift circuit PSi. Thus the detector PDa receives a reference signal appropriately phased to enable phase detection of the resistive component of the bridge.

An amplifier A2a amplifies the output of the phase sensitive detector PDa, and the outputs of the amplifiers A2, A2a are combined by a mixing amplifier A7. The output of the mixing amplifier A7 is fed to the threshold amplifier A3 for comparison with the reference signal Vref3 such that the ampli fierA3 provides an alarm output when the out of balance condition of the bridge exceeds a level set by Vref3. The output of the mixing amplifier A7 is also fed to the amplifier trigger circuits A4, A5 through the time constant circuit C2, R6. The switch S2 is operated when an output is produced by the amplifier A3, in order to short out the effect of the capacitor C2.Thus, one of the amplifiers A4, A5 will provide an output only when the combined output from mixing amplifier A7 changes at less than a predetermined rate defined by the time constant circuits C2, R6 to a level exceeding either Vrefl or 2.

Separate storage capacitors are provided to store a balancing signal for the resistive and inductive components of the bridge; capacitor C1 is connected to the amplifier A2 through switch S1 as in Figure 1, and an additional capacitor Cia is connected to the amplifier A2a through a switch Sla. Both the switches are operated in response to an output from the amplifiers A4, A5.

The D.C. voltage stored on the capacitor Cia controls a second balancing bridge which comprises FETVRla and VR2a with an associated amplifier A6A, all corresponding to the first balancing bridge and associated amplifier A6.

This second balancing bridge is supplied with AC from the transformer Ti's secondary through 90" phase shift circuits PS 2,3 such that the second balancing bridge has an output of an appropriate phase for balancing the resistive component of the coils L1, L2. The outputs of the two balancing bridges are combined together as an input to the amplifier Al.

Thus, in use, the two balancing bridges supply to the amplifier balancing signals which are effective to cause the amplifier Al to provide an output indicative that the bridge which includes coils L1, L2 is balanced in respect of both its inductive and resistive components, the value of the balancing signals being determined by the value of the charges stored on the capacitors C1, Cla, the stored values being updated when the combined outputs of the amplifier A7 drifts slowlyto exceed one of the thresholds set by the amplifiers A4, AS. The switches S1, Sla are then both closed to modify the values of charge or the capacitors C1, Cla. Thus, balance of the circuit is automatically effected in response to thermal drift or other long term changes in the operating condition of the circuit. In response to more rapid changes in the output of amplifier A7, such as will occur when the coils L1, L2, are moved over a metal object, the output will exceed Vref3, amplifier A3 will provide an output and the switch S2 will operate to prevent the amplifiers A4, A5 operat ing the switches S1, la hence preventing re balancing of the bridge. The output of the amplifier A3 causes the loudspeaker LS to be switched on as will now be described in detail.

The output of the amplifier A3 is fed through an operational amplifier A8, which operates as an OR gate, to operate an electrical switch S2 which gates a continuous audio signal produced by a variable frequency oscillator VFO to the loudspeaker LS. The oscil lator VFO operates at a frequency determined by the output voltage level of the mixing amplifier A7 and as a result, as the probe which includes the coils L1, L2 is moved closed to a metallic object the frequency of sound produced by the loudspeaker and the oscillator VFO, increases.

The system is arranged to discriminate between ferrous and non-ferrous objects. To this end, the circuit includes a detector D which detects the polarity of the output signal of the amplifierA2, which operates a low frequency oscillator LFO. The low frequency oscillator LFO operates at much lower frequency than the variable frequency oscillator VFO, and is used to operate the switch S2 cyclically.

The principle of operation of the detector D can be seen from Figure 3, which is a vector diagram of the phase of outputs of the amplifier Al (relative to the reference phase X) produced in response to the coils L1, L2, being passed over ferrous and non-ferrous metal objects respectively. When the coils L1, L2, are passed over a ferrous object, the amplifier Al provides an output V1 with a phase angle 01.

Experiments have shown that generally, for ferrous objects -IT/2 < 01 < II/2. Similarly, for non-ferrous ob ject, the amplifier Al provides an output V2 having a phase angle 2 where generally, II/2 < 2 < 3II/2.

Thus, ferrous objects generally provide a positive phase component in the inductive component of the amplifier Al, and non-ferrous objects generally provide a negative phase component in the induc tive component of the amplifier Al. This relationship manifests itself in the apparatus of Figure 2, in terms of the polarity of the output of the phase detector PD (which detects the phase of the inductive component fron Al). Accordingly, if the polarity of the output of the detector PD is negative, the metal being detected is in general of a non-ferrous variety. If the output of PD is negative, the polarity detector D provides an output which switches on the oscillator LFO causing the switch S2 to operate cyclically such that the ; loudspeaker produces a continuous tone for ferrous objects and a cyclically interrupted tone for non ferrous objects.

A practical form of a probe including the coils L1, L2, is shown in Figure 4. The probe consists of a rod having the coils L1, L2 wound along its length, the connections between the coils and the rest of the apparatus being effected by an electrical cable, not shown. For certain applications, in particular medical applications, it is desirable to form the probe as a flat spatula, in which case the coils are wound in the configuration shown in Figure 5, such that the spatula has flat opposed surfaces 10, 11.

A problem with the spatula type probe of Figure 5 is that a phase shift in output of the amplifier Al is produced depending on which side 10,11 of the probe is closest to a metallic object being detected.

This phase shift will cause the polarity detector D to provide incorrect results for the discrimination between ferrous and non ferrous objects. Accordingly, the apparatus will work properly only when for example the side 10 of the spatula is closest to the metallic object, and will give incorrect results when the side 11 is closest to the object. In certain situations, it is not possible to determine visually which side of the spatula is closest to the metallic object, for example when the spatula is inserted into an incision. It is however a feature of the apparatus of Figure 2 that is provides a characteristic low frequency tone from the loudspeaker in the event that the wrong side of the spatula is closest to the object being detected.

The phase shift produced by the wrong side of the spatula being closest to the object consists of 1 180 shift in the phase of the resistive component of the output of amplifier Al and accordingly the phase shift produces an inversion in the polarity of the output of the phase detector PDa which detects the phase of the resistive component). Such a polarity inversion is detected by a circuit SP which provides an output through the OR gate A8 to operate the switch S2 such that the oscillator VFO is connected to the loudspeaker. The combination of the inverted polarity signal from the phase detector PDa and the signal from the detector PD, will result in an unusually low magnitude signal being produced by the mixing amplifier A7, which will accordingly result in the oscillatorVFO operating at a low frequency. In proper operation of the apparatus, the amplifier A3 does not provide an output in response to such low magnitude signals from the amplifier A7, and hence the low frequency output produced in response to operation of the circuit SP, is not heard in proper operation of the system. Accordingly, this low tone is a characteristic indication that the probe of Figure 5 needs to be turned over such that the circuit D can operate properly.

Claims (18)

1. An electrical sensing circuit including a bridge, sensing means arranged to produce output, indicative of the in or out of balance condition of the bridge, means arranged to store the value of said output in response to the value thereof charging at only less than a predetermined rate, and means responsive to said stored value for causing said sensing means to provide an output indicative of bridge balance.

2. An electrical sensing circuit according to claim 1 wherein said output is only stored when the value thereof changes at less than a predetermined rate to exceed a predetermined value.

3. An electrical sensing circuit according to claim 2 including means arranged to provide an alarm signal in response to said output exceeding a predetermined value at a rate greater than said predetermined rate.

4. An electrical sensing circuit according to any preceding claim wherein said bridge is formed of inductors, an oscillator is arranged to supply an oscillatory electric current to the bridge, and said sensing means includes a phase sensitive detector arranged to compare the phase of a signal taken from the bridge with a reference phase taken from said oscillator.

5. An electrical sensing circuit according to claim 4 including a bridge balancing circuit comprising means for combining a further signal taken from the bridge with said signal applied to the phase sensitive detector, the value of said further signal being determined by said stored value of the output signal.

6. An electrical sensing circuit according to claim 5 wherein the reference phase and the phase of said further signal is appropriate for rendering the output from the phase sensitive detector indicative of the in or out of balance condition of the inductive components of the impedances of the bridge.

7. An electrical sensing circuit according to any preceding claim including means responsive to the phase of the signal supplied from the bridge to the sensing means, said means being adapted to indi catewhetherthe inductances of the bridge are being placed out of balance by the presence adjacent thereto of ferrous or non-ferrous metal.

8. An electrical sensing circuit including inductors connected to a bridge, an oscillator arranged to feed an oscillatory electric current to the bridge, first and second sensing means arranged to produce outputs respectively indicative of the in or out of balance condition of the inductive and resistive components of the impedances presented in the bridge, means for combining the outputs of said sensing means, means responsive to the combination of said outputs and arranged to store the value of each thereof only in response to the combination thereof changing its value at less than a predetermined rate to exceed a predetermined value, and first and second means responsive to said stored value respectively for causing said first and second sensing means to provide respective outputs indicative of balance of the said resistive and inductive components of the bridge.

9. An electrical sensing circuit according to claim 8wherein each said sensing means comprises a phase sensitive detector, and including means arranged to supply to the phase sensitive detectors a reference signal derived from said oscillator, and a phase shifter arranged to introduce a 90 phase shift between the reference signals applied to the phase sensitive detectors.

10. An electrical sensing circuit according to claim 8 or 9 including capacitors for storing the values of the outputs of said sensing means respectively, switching means arranged to connect the capacitors to said respective outputs selectively and a circuit responsive to the combination of said outputs and defining said predetermined rate and said predetermined level, said circuit being adapted to operate said switching means.

11. An electrical sensing circuit according to claim 10 wherein said capacitors are connected to respective bridge balancing circuits each of which is arranged to receive a signal from the bridge and supply to the sensing means a proportion of the signal in dependence upon the value of the charge stored by the respective capacitor, the signals supplied thereby to the sensing means having a 90 phase difference.

12. An electrical sensing circuit according to claim 9 including a polarity detector responsive to the output of the phase sensitive detector which indicates the in or out of balance condition of the inductive components of the bridge.

13. An electrical sensing circuit according to; claim 12 including a variable frequency oscillator arranged to produce an audible frequency oscillatory electrical signal the frequency of which is determined by the magnitude of said combination of said first and second outputs, a loudspeaker connected to the oscillator through a switching means, and means adapted to operate said switching means to apply the output of the oscillator to the loudspeaker in response to said combination of said first and second outputs exceeding a predetermined reference voltage.

14. An electrical sensing circuit according to claim 13 including an oscillator responsive to the output of said polarity detector and arranged to operate said switching means cyclically.

15. An electrical sensing circuit according to claim 13 or 14, including a further polarity detector responsive to the polarity of the output of the phase sensitive detector that indicates the in or out of balance condition of the resistive components of the bridge, differing from a predetermined polarity for indicating an incorrect physical arrangement of the inductances in the bridge.

16. An electrical sensing circuit according to claim 16 wherein said further polarity detector is arranged to operate said switching means.

17. A metal detector including an electrical sensing circuit as claimed in any preceding claim.

18. A metal detector substantially as herein described with reference to Figure 1 or 2 to 5 of the accompanying drawings.