Induction-type motion sensor

Abstract: The sensitivity of an induction-type motion sensor (1), comprising a magnetic circuit with a permanent magnet (12), to small movements increases when a stator (2) is devised with a core containing a permanent magnet (12) and an air gap (9), an electrical coil (3) is devised around the stator (2) and a rotor (1) is devised with a ferromagnetic moving element (4, 5) which, when the motion sensor is subjected to movement stress, passes in and out of the air gap (9), alternating the magnetic field in the stator (2) and inducing a current in the electrical coil (3).

Description: This invention relates to an induction-type motion sensor comprising a magnetic circuit with a permanent magnet, said magnetic circuit comprising a stator, a rotor and an electrical coil devised around the stator, said rotor comprising a pendulum arranged to oscillate when the motion sensor is subjected to movement stress, and a moving element is provided at the stator, at least partially in the stator's air gap, said moving element connected to the pendulum and, when the element is set in motion by the pendulum, acting on the magnetic field in the stator, thereby inducing a current in the electrical coil. BR-A-8 501 719 describes an inductive power generator with a bipolar permanent magnet as rotor element, which is rotatable in a stator's air gap. An electrical coil is provided around the stator. A pendulum oscillates when subjected to movement stress and transfers the movement to the rotor element via a system of gears. The task of the gears is to shift up rotor element oscillation in the air gap so as to achieve a greater number of alternations in the stator's magnetic field when the pendulum oscillates. The current generated in the electrical coil in this way is rectified and used for powering a watch. The induced current changes direction when the magnetic field alternates. Low gearing produces a smaller number of alternations in the magnetic field for a given pendulum travel, whereas high gearing produces a larger number of alternations in the magnetic field with the same pendulum travel. However, higher gearing also means that heavier movement stress is required to produce the same pendulum travel, i.e. the higher the gearing, the heavier the movement stress needed to induce the pendulum to oscillate. In addition, a design employing a plurality of gears is complex and space-demanding. With a plurality of gears, total friction is greater in the rotor's subelements, and the pendulum, for this reason, is more difficult to set in motion. An inductive power generator, comprising a magnetic circuit with a permanent magnet, is known through WO 84/01041. The magnetic circuit comprises a stator with an air gap, an electrical coil and a rotor in the form of a semicircular pendulum disc with a plurality of permanent magnets located so they pass through the air gap when the pendulum oscillates. Increasing the number of magnets on the pendulum disc increases the number of alternations in the magnetic field and accordingly results in a higher frequency for the induced current. The power generator is designed to generate as much energy as possible from movement stress and, in particular, to generate energy from heavy movement stress, since a small swing of the pendulum only produces a limited change in the magnetic field. The version with a plurality of magnets on the pendulum disc means that the magnets must either be made smaller or the pendulum disc must be made larger. Reducing the size of the magnets also reduces their magnetic field strength, thereby reducing the induced current. Since the pendulum disc only consists of a semicircle, no magnets will be in the air gap for half the rotation cycle when the pendulum oscillates around its oscillation axis. The above-described generators are designed to transform kinetic energy into electrical energy in an efficient way, but they can even serve as motion sensors, since they emit a specific electrical signal for each movement signal. However, it is important for a motion sensor, intended for use e.g. in an implantable medical apparatus in order to detect body movements, to display high sensitivity to small movements and changes in movement. For example, the motion sensor must be able to emit a signal which differentiates between a patient's normal walking and his/hers rapid walking. Sensitivity to an induction-type motion sensor depends on the sensor's ability, on the basis of movement stress, to generate an identifiable signal which can be used as a parameter related to movement stress. Alternations in the direction of the induced current is one appropriate parameter. The generator according to BR-A-8 501 719 produces a large number of alternations in current direction through high gearing in the system of gears. As noted above, however, high gearing simultaneously demands greater movement stress to set the pendulum in motion. Sensitivity to movements is therefore limited to the gearing selected. This can be circumvented with a generator according to WO 84/01041 in which the number of magnets on the rotor disc produces a large number of alternations in current direction without any system of gears. However, a magnet cannot be too small and still retain a field strength sufficient to induct a measurable current. Further, more magnets make the peripheral part of pendulum disc heavier, thereby affecting the disc's natural frequency and the movement stress required to set the pendulum in motion, since the pendulum disc's inertia accordingly increases. The difficulties in using the prior art power generators as motion sensors become more pronounced when implantable medical apparatuses are involved. Space in these apparatuses is limited, and developments are tending to reduce this space even further. Both the system of gears and the pendulum disc with a plurality of magnets are too space-demanding for such applications and insensitive to small movements when designed to be motion sensors. An objective of the invention is to achieve a motion sensor of the above-described type which can be made small but still have a simple, stable construction and be simultaneously capable of detecting small movement stress. The invention intends in particular to produce a motion sensor which is usable in implantable medical apparatuses for sensing body movements. Such a motion sensor is achieved in accordance with the present invention in that the movement sensor, as described above, is devised so the permanent magnet is a fixed part of the stator and the moving element is ferromagnetic and devised so it passes through the air gap when set in motion by the pendulum. According to the invention, a design with one or more moving permanent magnet(s) is not used. With a fixed, bipolar permanent magnet which is a part, or all, of the stator, a specific magnetic flux is applied across the air gap. The permanent magnet can be made as large as space permits, since it does not have to move. The ferromagnetic moving element, which induces an alternating current by its passage in and out of the air gap, can be made very small and designed to produce as many current alternations as possible. This therefore results in a motion sensor which can be made very small and sensitive to small changes in movement. The pendulum's moment of inertia can be directly adapted to the movements to be sensed, since it does not have to be weighed down with extra magnets or gears to increase the number of alternations in the magnetic field. An improvement of the motion sensor is achieved in accordance with the invention in that the moving element comprises a circular disc with a plurality of radial teeth, arranged around the periphery, and when the circular disc is devised to oscillate around an axis in such a way that the teeth pass in and out of the air gap. Movement of the teeth through the air gap has the requisite effect on the magnetic field, thereby giving rise to the induced current in the electrical coil. The teeth are designed so the induced current displays maximum change even when changes in the position of the teeth are small, e.g. by always passing a tooth through the air gap even in small body movement. In conjunction with the design of the teeth, it is an advantage if the air gap can be devised with a predetermined cross-section, the teeth are devised so their radial cross-section is essentially identical to the cross-section of the air gap and the distance between teeth is devised so it is essentially identical to the cross-section of the air gap. This produces the largest difference in alternation in the magnetic field from a state in which an entire tooth is in the air gap to a state in which no tooth is in the air gap. The size of the air gap's cross-section and, accordingly, of the teeth and the inter-tooth distance, is the only limitation on the minimum movements which can thereby be recorded. For example, the stator could be devised so its cross-section declines at the air gap, becoming narrow in the teeth's direction of travel so at least one tooth always passes through the air gap even when movement stress is very small. Another improvement of the motion sensor is achieved in accordance with the invention when a weight is provided on the moving element so the element acquires an eccentric distribution of mass around the axis, the moving element then serving as the pendulum. When the pendulum and moving element consist of a single element, the size of the motion sensor can be reduced even further. In particular when the moving element consists of a toothed circular disc, the weight can advantageously be provided along the periphery over several teeth. The weight will then not prevent alternation of the magnetic field when the weighted teeth pass through the air gap. This can be compared to the generator of WO 84/01041 in which the weights on the semicircular pendulum disc consist of magnets, a current being induced only during the half of the revolution in which the magnets are in the air gap. In the use of the motion sensor in an implantable medical apparatus, it would be an advantage if the pendulum's natural frequency is between 2 and 8 Hz, preferably 4 Hz. The body has a natural frequency of about 4 Hz in normal physical activity. Noise from other sources with a higher frequency, as occurs during car driving or a ride on a bumpy train, would therefore not be recorded by the sensor as body movement. One embodiment of the motion sensor according to the sensor is described below, whereby; FIG. 1 shows one embodiment of the invention in the form of a motion sensor viewed from above and FIG. 2 shows a side view of the motion sensor. The motion sensor shown in FIGS. 1 and 2 comprises a magnetic circuit containing a stator 2 with an air gap 9 and a rotor 1 which moves in the air gap 9. An electrical coil 3 is wound around the stator 2 and connected to a signal conditioning device 10. The stator 2 is magnetic and can consist in whole or part of a permanent magnet or, as shown in FIG. 1, of a permanent magnet 12 and two ferromagnetic pole shoes 13a and 13b which form the air gap 9. The rotor 1 consists of a circular disc 4 with a number of teeth 5 and an eccentric weight 6 to produce a pendulum effect for the rotor 1. The rotor 1 is journaled on a shaft 7 between two bearing housings 8a and 8b, the rotor 1 oscillating on said axis 7 when a movement load is imposed. When the rotor 1 oscillates, the teeth 5 move in an out of the air gap 9. The eccentric weight 6, which provides the rotor with an eccentric distribution of mass around the axle 7, consists of two lead wires 6a and 6b, the mass of the lead wires 6a and 6b being chosen so a specific natural frequency is obtained for the rotor 1. When it is used e.g. in a pacemaker or other implantable medical apparatuses to sense body movements, an appropriate natural frequency of about 4 Hz is selected. Most body movements occur at this frequency. The teeth 5 are devised so every tooth 5 has an axial surface corresponding to the cross-section of the air gap 9. When the motion sensor is subjected to movement stress, the rotor 1 begins oscillating around an equilibrium position at the predetermined natural frequency. One or more teeth 5 will then pass in and out of the air gap 9, depending on the magnitude of movement stress. The magnetic field in the air gap 9 will vary, depending on how much of a tooth 5 is in the air gap 9. A current is induced in the electrical coil 3 as a result of variations in the magnetic field in the air gap 9. Thus, the induced current in the electrical coil 3 thereby becomes directly related to movements around the specific natural frequency, i.e. about 4 Hz, if the motion sensor is to be used to sense body movements. The induced current is sent to the signal conditioning device 10 in which movement stress is determined from the magnitude of the inducted current. The signal conditioning device 10 then emits a signal, via a line 11, corresponding to movement stress. This signal can be used e.g. for controlling the heart rate of a pacemaker patient.

Claims: An induction-type motion sensor comprising a magnetic circuit with a permanent magnet (12), said magnetic circuit comprising a stator (2), a rotor (1) and an electrical coil (3) arranged around the stator (2), characterized in that the rotor (1) comprises a pendulum (4, 6) devised to oscillate when the motion sensor is subjected to movement stress, and a moving element (4, 5) is provided at the stator (2), at least partially in the stator's air gap (9), said moving element being (5) connected to the pendulum (4, 6) and, when the element (5) is set in motion by the pendulum (4, 6), affecting the magnetic field in the stator, thereby inducing a current in the electrical coil (3), the permanent magnet (12) being a fixed part of the stator (2) and the moving element (5) being ferromagnetic and devised so it passes through the air gap (9) when set in motion by the pendulum (4, 6). A motion sensor of claim 1, wherein the moving element (4, 5) consists of a circular disc (4) with a plurality of radial teeth (5), arranged around the periphery, and when the circular disc (4) is devised to oscillate around an axis (7) in such a way that the teeth (5) pass in and out of the air gap (9). A motion sensor of claim 2, wherein the air gap (9) is devised with a predetermined cross-section, the teeth (5) are devised so their radial cross-section is essentially identical to the cross-section of the air gap (9) and the distance between teeth is devised so it is essentially identical to the cross-section of the air gap (9). A motion sensor of either of claims 2 or 3, wherein a weight (6) is provided on the moving element (4, 5) so the element acquires an eccentric distribution of mass around the axis (7), the moving element (4, 5) then serving as the pendulum (4, 6). A motion sensor of any of the above claims, wherein the pendulum's (4, 6) natural frequency is between 2 and 8 Hz, preferably 4 Hz.

Inventors: LJUNGSTROEM KARIN, LJUNGSTROEM, KARIN, Ljungström, Karin
Applicants: PACESETTER AB
Application year: 1993
Main classification: G01P13
IPC codes: G01P13, H02K19, G01P15