Magnetic field detector

Abstract: A magnetic field detector, particularly for implantable medical apparatuses, comprises a coil (2) mounted at the free end of a resilient member (18) fixed at its opposite end portion, and a detecting device (20; 40, 42, 44, 46; 48, 50, 52; 54, 56) for detecting, as a measure of the magnetic field, the movement of the coil when fed with an electric current from a controllable current source (8). The resilient member is bent to form at least two shanks interconnected at one end. The coil is mounted at the free end of one of the shanks and the resilient member is fixed at the free end portion of the other shank.

Description: The present invention relates to a magnetic field detector, particularly for implantable medical apparatuses, comprising a coil mounted at the free end of a resilient member fixed at its opposite end portion, and a detecting device for detecting, as a measure of the magnetic field, the movement of the coil when fed with an electric current from a controllable current source. US-A-4,186,324 and US-A-4,887,032 show devices with a piezoelectric flexible bar member or a flexible bar member provided with a piezoelectric element which bar at its free end supports a coil and is fixed at the opposite end. If the coil is located in a magnetic field it will move if a current is passed through the coil and this movement of the coil will cause the piezoelectric bar to flex and a corresponding voltage will be produced by the piezoelectric arm or the associated piezoelectric element. These devices are used in servo accelerometer amplifier systems to produce error signals in order to restore a movable mass to a balanced condition (US-A-4,186,324) and for measuring the distance from an object to a reference point and the rate at which an object passes a reference point (US-A-4,887, 032). The document US-A-4,931,732 shows another device for detecting magnetic fields comprised in an apparatus for determining superconductive properties of material samples, which device discloses a magnetic field detector comprising a coil mounted on a resilient member said member being bent to form shanks interconnected at one end, said coil being mounted at the free end of one of the shanks. In our European patent application No. 91114252.9 which is a document according to Art. 54(3) EPC, a magnetic field detector, particularly for implantable medical apparatuses, is disclosed which comprises a movement sensitive sensor in the form of a piezoelectric plate to which a coil is attached, which coil is moving and thus affecting the sensor to deliver a signal when the coil is located in a magnetic field and a current is passed through the coil. The purpose of the present invention is to provide an improved magnetic field detector for determining magnetic field strength with higher sensitivity. This purpose is obtained with a magnetic field detector of the kind defined in the introduction of the description and with the characterizing features of claim 1. As the resilient member supporting the coil according to the invention is bent to form at least two shanks inter-connected at one end a mechanically more flexible construction is obtained compared to previously known constructions with a straight arm supporting the coil. The improved flexibility of the construction according to the invention results in a lower mechanical resonance frequency for the same dimensions of the device, which is an advantage, and a larger bending of the supporting member is obtained when the coil is tilted. If e.g. a piezoelectric element is applied to the resilient member over the bend to give an electric signal representative of the bending of the member this construction gives a higher sensitivity. According to the invention the resilient member supporting the coil is bent into a U-shape with the coil mounted inside the U-shaped member. Thus, the resilient member comprises two bending knees which still further improves the sensitivity of the detector. According to an advantageous embodiment of the detector according to the invention the detecting device comprises a light source directing a light beam towards a reflector fixed to the coil and means for recording the movement of the reflected light beam. If the recording means is situated at a long distance from the reflector of the coil, the coil movement will be translated into large readings at the recording means. Thus a detecting device with a high sensitivity is realized. According to further embodiments of the detector according to the invention the detecting device can comprise means for recording the interference pattern produced by incoming and reflected light to determine the movement of the coil from this pattern or the detecting device can comprise a variable capacitance device of the kind used in e.g. capacitive microphones. According to one more advantageous further development of the detector according to the invention the current pulses supplied to the coil are square pulses supplied through a capacitor. Oscillations of the mechanical system carrying the coil is then generated at the positive and the negative flanks of the pulse and these oscillations are similar but of opposite phase. According to still another advantageous embodiment of the detector according to the invention the capacitor is selected such that the electrical resonance frequency obtained with the inductance of the coil agrees with the mechanical resonance frequency of the system carrying the coil. An efficient transfer of energy will then be possible, that is, a maximum efficiency is obtained. This is of particular importance when using the detector according to the invention in for instance a pacemaker where no circuit is allowed to have a current consumption exceeding 1 µA. According to another advantageous further development of the detector according to the invention the current source is supplying an adjustable DC current to the coil in addition to the current pulses. This DC current results in a new working point and in this way the electrical resonance frequency can be adjusted. To get a coil with a high inductance the coil is, according to a further development of the invention, provided with a ferrite core. As examples chosen embodiments of the detector according to the invention will now be described more in detail with reference to the enclosed drawings, on which Fig. 1 shows an elevation view and Fig. 2 a side view of a magnetic field detector based on the principle of a moving coil, Fig. 3 shows equivalent electrical circuits of the detector together with electric signals used in its operation, Figs. 4 and 5 show the coil assembly in one embodiment of the detector according to the invention and Figs. 6 - 8 show schematically three different types of detecting devices for determining the movement of the coil of the detector according to the invention. Figs. 1 - 3 illustrate the construction and functioning of a magnetic field detector of the kind to which the detector according to the invention relates. The detector is provided with a coil 2 mounted at the free end of a piezoelectric bar 4 which is attached to a base 6 at its opposite end. The electronic circuitry of the detector is contained in the box 8 also mounted on the base 6. The electronic circuitry 8 comprises a current source which through the pins 1 and 2 of the electronic circuitry 8 is connected to the coil 2. This current source can supply an adjustable DC current to the coil to adjust the working point of the electrical equipment of the detector. This current source can also be operated as a pulse generator delivering electric pulses directly to the coil 2. Square pulses are preferably delivered from the electronic circuitry 8 to the coil 2 through a capacitor 10, connected between the pins 2 and 3 of the electronic circuitry 8. The capacitor 10 is connected in series with the coil 2 and the value of the capacitor 10 is selected such that the electrical resonance frequency of the electrical circuit will be close to the resonance frequency of the oscillating mechanical system. In this way a maximum coupling is obtained with an efficient energy transfer as a result. Pulses delivered by the electronic circuitry 8 are differentiated in the capacitor 10 and the pulse flanks will pass the capacitor and reach the coil 2. If the coil is located in a magnetic field the current passing the coil will excite an oscillation in the piezoelectric bar 4. This oscillation is observed as an electric signal across the piezoelectric bar 4 and supplied to the pins 5 and 6 of the electronic circuitry 8. Because of the above mentioned resonance the square pulse will generate a strongly damped electric pulse in the coil 2 with the same polarity as the flanks of the square pulse. Thus the oscillations generated by the positive and negative flanks of the square pulse are similar but of opposite directions. To get a sufficient high inductance the coil 2 is preferably provided with a ferrite core. Typical figures of the components are inductance L = 350 mH, , resistance R = 1 kohm, capacitance C = 150 nF and square pulses are delivered with a frequency of the order of 1 - 10 Hz. Fig. 3 shows the electrical equivalent to the components connected to the electronic circuitry in Figs. 1 and 2 as well as the signals appearing in the circuit. Square 12 illustrates pulses generated by a pulse generator in the electronic circuitry 8 and fed to the coil 2 through the capacitor 10. The pulse train is also fed to an oscilloscope 16 which on channel 1 (Ch 1) shows a pulse with an expanded time scale as compared to the pulses shown in square 12. The pulses supplied to the coil 2 produce an oscillation of the piezoelectric bar 4 in the presence of a magnetic field. The oscillations of the piezoelectric element give rise to a corresponding electric signal delivered to the pins 5 and 6 of the electronic circuitry 8. In the electronic circuitry 8 the signal from the piezoelectric bar 4 is high pass filtered, square 14, and supplied to channel 2 (Ch 2) of the oscilloscope, square 16. As appears from the oscillogram a damped oscillation of the piezoelectric bar is produced by the two flanks of the pulse, the oscillations at the positive and negative pulse flanks being in opposite phase. Figs. 4 and 5 show an advantageous construction of the mounting of the coil 2 on the resilient member. In this embodiment the resilient member is formed of a U-shaped sheet 18, the coil 2 being mounted inside the U-shape at the free end portion of the upper shank in the figures of the U-shaped member. The resilient member is attached to a base or carrier (not shown in Figs. 4 and 5) at the end portion of the lower shank in Figs. 4 and 5. The sheet 18 is formed of a resilient metallic material, like stainless steel, bronze, titanium, or a plastic material. On the outer side of the U-shaped member a piezoelectric sheet 20 is fixed over the bends 22, 24 and the web of the U-member. The embodiment shown in Figs. 4 and 5 is more flexible than the construction with a straight bar shown in Figs. 1 and 2 and results in a lower resonance frequency for essentially the same dimensions of the mechanical system. Further a larger bending, essentially in the bends 22, 24, is obtained in this embodiment when the coil is tilting. The piezoelectric element 20 delivers a corresponding electric signal to the electronic circuitry 8 and in this way a more sensitive detector is obtained. To the left in Figs. 4 and 5 a permanent magnet 26 is shown with its south pole directed towards the coil. The coil 2 is centered on the symmetry axis illustrated with the arrow 28. In Fig. 4 the coil 2 is fed with current such that it has a north pole at its upper end and its south pole at the lower end and in Fig. 5 the current and consequently the poles of the coil 2 are reversed. The arrows 30 and 32 above and below the coil 2 respectively illustrate forces which are acting on the coil because of the magnet 26. As appears the coil 2 is affected by oppositely directed forces in Figs. 4 and 5 which will tilt the coil 2 in different directions. Correspondingly different electric signals are then delivered by the piezoelectric element 20. Thus with the detector according to the invention the polarity of the magnet can be determined. Fig. 6 illustrates alternative means for detecting the movement of the coil 2. The outer surface 40 of the shank of the flexible member 18 on which the coil 2 is mounted is provided with a reflector or formed as a reflecting surface. A light source 42, preferably a laser, is directing a light beam towards the reflecting surface 40 and the reflected beam is recorded by suitable recording means or recording medium 46. Thus the reflected beam will create a luminous point 44 on the recording medium 46 which point will describe the oscillation of the coil 2 and the shank on which it is mounted. Fig. 7 illustrates an embodiment in which the oscillation of the coil and the flexible member 18 is detected by interference technique. Also in this case the outer side of the shank on which the coil 2 is mounted is provided with a reflector or a reflecting surface 40. A monochromatic light source 48 is directing a light beam towards the reflecting surface 40 and the interference pattern 50 produced by the direct incoming light and the reflected light will reproduce the oscillation of the coil 2 and the flexible member 18. The interference pattern 50 is recorded on a suitable recording medium 52. The appearance of the interference pattern 50 will be determined by the tilting of the coil 2 and consequently be used as a measure of the tilting and thus the magnetic field strength. Fig. 8 illustrates a further alternative means for detecting the movement of the coil 2 and the flexible member 18 comprising a variable capacitance device. The capacitance device comprises one essentially stationary condenser plate 54 (the fixation of the plate 54 not shown in the figure) and one condenser plate 56 mounted on the shank of the U-shaped member 18 which is carrying the coil 2. Thus when the coil 2 is moving the condenser plate 56 will move together with the coil 2 and the resulting variation in the distance between the two condenser plates 54 and 56 will result in a capacitance which is varying in response to the movement of the coil 2. This capacitance will consequently be a measure of the magnetic field measured by the detector. Instead of using a separate fix condenser plate 54 the lower shank of the U-shaped member 18 can form the fixed plate 58 of the variable capacitance device, cf. figure 8.

Claims: A magnetic field detector, particularly for implantable medical apparatuses, comprising a coil (2) mounted on a resilient member (18), and a detecting device (20; 40, 42, 44, 46; 48, 50, 52; 54, 56) for detecting, as a measure of a magnetic field, a movement of the coil when fed with an electric current from a controllable current source (8), said resilient member (18) being bent to form at least two shanks interconnected at one end by a bend (22,24), said coil (2) being mounted at the free end of one of the shanks and said resilient member being fixed at the free end portion of the other shank, characterized in that said resilient member (18) is bent into a U-shape and in that said coil (2) is mounted inside the U-shaped member at the free end of one of the shanks of the U-shape, said U-shaped member being fixed at the free end portion of its other shank. The detector according to claim 1, characterized in that a piezoelectric element (20) is applied to the resilient member (18) over the bend (22, 24) to give an electric signal representative of the bending of the member and thus of the movement of the coil (2). The detector according to claim 2, characterized in that said piezoelectric element is formed as a sheet (20) of piezoelectric material applied to the outer side of the U-shaped member (18) over its bends (22, 24) and its web. The detector according ta claim 1, characterized in that said detecting device comprises a light source (42) directing a light beam tcwards a reflector (40) fixed to the coil (2) and means (46) for recording the movement of the reflected light beam (Fig. 6). The detector according to claim 1, characterized in that said detecting device comprises a monochromatic light source (48) directing a light beam towards a reflector (40) fixed to the coil (2) and means (52) for recording the interference pattern (50) produced by incoming and reflected light to determine the movement of the coil from the interference pattern (Fig. 7). The detector according to claim 1, characterized in that said detecting device comprises a variable capacitance device having one stationary condenser plate (54; 58) and one condenser plate (56) coupled to the coil (2) for moving together with it (Fig. 8). The detector according to any one of the claims 1 - 6, characterized in that the current source (8) is supplying current pulses to the coil. The detector according to claim 7, characterized in that the current pulses are square pulses supplied by the current source to the coil (2) through a capacitor (10). The detector according to claim 8, characterized in that the capacitance value of the capacitor (10) is selected such that the electrical resonance frequency obtained together with the inductance of the coil (2) agrees with the mechanical resonance frequency for the mounted coil. The detector according to any one of the claims 8 - 9, characterized in that the current source (8) is supplying an adjustable DC current to the coil (2) in addition to the. current pulses. The detector according to any one of the claims 1 - 10, characterized in that the coil (2) has a ferrite core.

Inventors: EKWALL CHRISTER, EKWALL, CHRISTER
Applicants: PACESETTER AB
Application year: 1992
Main classification: G01R33
IPC codes: G01R33, A61B5, A61N1