FI120988B - Method and apparatus for calibrating a power sensor based on an electret membrane - Google Patents

Description

METHOD AND APPARATUS FOR CALIBRATING A POWER SENSOR BASED ON ELECTRIC FILM

The present invention relates to the calibration of an electret film-based force sensor for monitoring human or animal vital functions.

BACKGROUND

10 Ballistic cardiography (BKG) records the accelerating forces produced by moving blood as the heart contracts over time. The stored forces can be used to determine, for example, the amount of blood that the heart pumps during one heartbeat.

Referring to Figure 1, cardiac forces F2 can be monitored, for example, by placing an electret film transducer 50 on a tray 60, by asking patient 10 to sit on the transducer 50, and by recording a BKG signal. The measured signal can be viewed, for example, by means of a user interface 260, for example a display.

20

In the arrangement of Figure 1, the forces F2 are mainly caused by the blood plummeting from the left ventricle 22 of the shrinking heart 20 to the aorta 25. The blood transferring from the right ventricle 24 to the pulmonary artery (not shown) and venous blood circulation The instantaneous force F2 (t) is the first approximation equal to the product of the instantaneous acceleration a (t) and the accelerating or decelerating mass m: F2 {t) - m a (t) (1) 30

Figure 1 further shows a left atrium 21 and a right atrium 23 of core 20.

Referring to FIG. has at least a first electrode 51 and a second electrode 52 on one side of the diaphragm 53. Compression of the diaphragm 53 displaces electrical charges within the diaphragm, inducing a mirror charge change at electrodes 51 and 52. The change in charge of the mirror, in turn, can be measured as the electric current between electrodes and / or as a change in voltage VEmf between electrodes. Thus, the forces F2 (t) and Fwt applied to the surface A2 of the electret well sensor 50 will cause a local change in the thickness d1 of the compressible electret film 53 which changes the voltage between the electrodes 51, 52 can be amplified by a pre-amplifier or voltage amplifier. charge amplifier. The electrodes 51, 52 may be connected to the amplifier by conductors 55, 56.

The frequency response of a measuring system based on an electret film transducer typically has a lower limit, for example, between 0.1 and 1 Hz. Therefore, the high static force FWT due to the weight of the patient 10 does not affect the amplified signal as a first approximation. The amplified measurement signal is essentially proportional to the weak force F2 (t) caused by the action of the heart 20.

The electret film sensor 50 is primarily a force sensor. The force F2 (t) is transmitted to the sensor surface as a pressure exerted on the sphere A2. The measurement signal is proportional to the pressure and range A2 input. As a first approximation, the measurement signal corresponding to the force 25 F2 is independent of the extent of the area of influence A2 to which said force F2 is applied.

The electret film sensor 50 may be e.g. flexible, slim and / or easily adjustable. The force measuring area of the electrode transducer 50 can easily be larger than the object under investigation, so that, for example, the sensor placed on the carotid artery does not need to be accurately positioned.

Electret film sensors are commercially available, for example, from Emfit Ltd., 35

3

Referring to Figure 3, the vital function examination apparatus 200 based on the electret membrane sensor 50, in particular the ballistogardiographic examination apparatus, may comprise an electrethole sensor 50, an amplifier 210, an AD to digital converter 230, a memory unit 240 for signal signal processing 5 and a signal processing unit 250.

The research apparatus 200 may further comprise an opto-isolator 220 coupled between the amplifier 210 and the AD converter. The opto-isolator allows the electric bowling of the sensor 50 to be optimized, which can improve the system's 200 immunity and / or patient safety.

The measurement signal Semf may be obtained, for example, from the wires of the sensor 50, after the amplifier 210, after the optoisolator 220, or after the AD converter 1530.

The research equipment may comprise a user interface 260 for viewing the results and controlling the equipment 200. The user interface 260 may comprise, for example, a display for graphically displaying the results and / or receiving keystroke commands.

The forces exerted by the patient 10 are transmitted to the electret film transducer 50. The patient 10 thus exerts a time-dependent force F2 on the transducer 50.

The sensor 50 can be mounted on the support 60. The support 60 may be, for example, a chair or a bed.

The apparatus 200 may further comprise means for receiving, for example, an ECG signal (electrocardiography) for scheduling the BKG signal. Said means may be, for example, connectors, sensors or receivers.

Figure 4 shows, by way of example, the BKG signal measured from patient 10 as a function of time, t being the time, and lu-35 readings on the horizontal axis at time instant in seconds. SEMf means the instantaneous value of the sensor signal. The unit of readings on the vertical axis is so far unknown. However, the positive values of the vertical axis are related to the forces 4 applied from the heart 20 of the patient 10 upwards towards the head. Figure 4 illustrates by way of example the features of the BKG signal associated with three consecutive heartbeats. Of these, the local maximum and minimum positions of the signaling period associated with the first heartbeat are denoted by F, G, H, I, J, K, L, M, N, O, and P. The features F, G, and H are related to the heart preconcentration phase. The features I, J and K are related to the contraction phase. Features L, M, and N are associated with cardiac arrest. Shortening of the time intervals of contraction phase features H, I, J and K may be indicative of arteriosclerosis.

10

The signal change ASy between peak J and minimum point I represents the maximum contraction force of the left ventricle 22 of the heart 20. By comparing the measured signal change with ASy reference values, one may seek to diagnose heart disease such as aortic valve disease and coronary artery disease.

In order to meaningfully compare the changes in the amplitude values of the signal Semf with the reference values determined by other measuring equipment, it is necessary to know the proportionality factor k2 between the force change AF2 on the sensor 50 and the corresponding signal change ASemf 20. which is defined as: k2 = - ^ ~ (2) ASemf 25 The proportionality factor k2 can be determined, for example, by quickly lifting off a piece of known mass resting on the membrane 50 and recording the corresponding signal change. In principle, the proportionality factor k2 can also be determined by placing a piece of known mass on the membrane 50 and recording the corresponding signal change 30. When determining the change, the instantaneous impact force due to the deceleration of the part can be eliminated by signal processing and only the static weight of the part can be taken into account.

5

The polarization of the electret film 53 decreases with time. For this reason, there may be a need to monitor the change in the proportionality factor, and there may be a need to redefine the proportionality factor.

US 2001 / 0024712A1 discloses some ways of implementing an electret film sensor.

US2003 / 0233034A1 discloses a measuring chair suitable for monitoring the forces exerted by a patient's vital signs.

10

In "Air mattress sensor system with balancing tube for unconstrained measurement of respiration and heart beat movements", Physiol. Meas. 26 (2005) pages 412-422 discloses an air mattress for monitoring vital signs.

15

SUMMARY

It is an object of the present invention to provide an apparatus and method for determining a proportional coefficient between a change in force applied to an electret film sensor and a change in the signal produced by said electret film sensor.

In an adapted apparatus for measuring human or animal vital functions, an electret film transducer used as a force transducer is calibrated using a dynamic coil speaker coil to generate calibration power. An alternating current of known amplitude is applied to the coil, whereby the change in force generated by the coil in the magnetic field is known. The relationship between the change in the sensor signal and the change in force on the sensor 30 can then be determined by the change in the known force and the corresponding change in the measurement signal.

According to a first aspect of the invention, there is provided a method 35 for calibrating an electret film sensor according to claim 1.

6

According to another aspect of the invention, there is provided a method of calibrating an electret membrane sensor according to claim 2.

According to a third aspect of the invention, there is provided a method of measuring the forces exerted by a patient or animal in accordance with claim 10.

Calibration can be performed, for example, by applying AC current at a frequency between 2 and 10 Hz to the speaker coil. In this case, the frequency used for calibration 10 approximately corresponds to the strongest component of the frequency spectrum of the ballistocardiographic signal.

The force generated by the speaker coil is applied to the surface of the electret film sensor to be calibrated mechanically. Calibration can thus be performed on the back-15 hair, which is substantially below the loudspeaker -10dB lower bound hair. Thus, the speaker does not have to be large in size to provide a sufficiently low calibration frequency. In one embodiment, the diameter of the speaker membrane hanger may be, for example, less than or equal to 250 mm.

20

Serial loudspeakers useful in calibration are available at commercially low prices. In one embodiment, the speaker structure does not need to be modified.

The invention and its essential features, as well as the advantages obtained by the invention, will be more apparent to those skilled in the art from the claims and the following description, in which the invention will be described in more detail by means of a few selected examples.

30

BRIEF DESCRIPTION OF THE IMAGES

Fig. 1 is a three-dimensional view of a patient sitting on an electret film sensor, Fig. 2 is a three-dimensional view of an electret film sensor, Fig. 3 is a side view of a device for investigating vital forces, Fig. 6b shows a side view of a measuring arrangement for measuring speaker coil forces, Fig. 6b shows a side view of a measuring arrangement for measuring speaker coil forces, wherein the coil force is arranged to transfer to a power meter via speaker suspension, Fig. 7 is a side view of a measuring arrangement dynamic speaker, Fig. 8 shows an example of a signal produced by a loudspeaker, Fig. 9a shows a side view of a speaker with k Fig. 9b is a side elevation view of a speaker having a raised portion attached to the frame, Fig. 9c is a side elevation view of a speaker having a raised portion attached to the frame, 30 Fig. 10a is a side elevation view of a Fig. 10b is a side elevational view of a combination of a speaker and an electret film transducer having a coupling agent between the spacer and an electret film transducer; with a calibration speaker attached.

10

DETAILED DESCRIPTION

The proportional coefficient k2 between the change AF2 of the force 15 on the sensor 50 applied to the electret well sensor 50 and the corresponding signal change ASemf is determined using the dynamic speaker 100.

Referring to Figure 5a, the dynamic loudspeaker comprises a coil 110 movable in a magnetic field B1 20 with a diaphragm 120. The current Icoil applied to the coil 110 causes a force Fcoil perpendicular to the magnetic field of the coil to move the coil 110 and its associated diaphragm 120. sound at at least one frequency between 16 and 20,000 Hz when the diaphragm head-25 moves and coil 110 is fed with an alternating current of the same frequency Icon ·

Figure 5b illustrates in more detail the typical structure of dynamic speaker 100. The loudspeaker has a magnet 130, which may comprise a duration 30 magnet 131, a center pin 133, a bottom disk 134, and an aperture disk 132. Between the center pin 133 and the apertured disk 132 is a magnetic field B1. A magnetic coil 110 is disposed within the magnetic field and is mechanically coupled to the diaphragm 120 and the dust shield 150. The coil 110 is also called a voice coil. The film 120 may be, for example, cardboard, plastic, im-35 impregnated fabric, fibreboard or metal foam. The diaphragm 120 is typically in the form of a truncated cone. The dust cover is typically in the form of a dome. The magnet 130 is attached to the frame 140 typically by frame ribs 9 142. The diaphragm 120 is movably secured to the frame 140 by means of a flexible and / or flexible hanger 160 (surround). Suspension 160 is an unidirectional annular well structure that centers the speaker cone 120 and allows the speaker cone 120 to move while producing sound. The Ri-5 body is made of, for example, rubber, plastic or cardboard.

Referring to Fig. 6a, the force FCoil generated by the coil 110 corresponding to the electric current Icoil applied to the coil 110 can be measured by a conventional force meter 910, which may be, for example, a spring scale. The diaphragm 120 of the loudspeaker 100 10 or the dust shield 150 may be positioned to push the sensing element 912 of the force meter. Loudspeaker 100 and power meter 910 may be mounted on frame 920.

The arrangement according to Fig. 6a can be used to determine the proportionality coefficient k ^ kl = ^ ML · (3) between the change in the control current Icoil of the coil 110 and the change AFref of the corresponding force FCOil.

^ REF

20

In determining the proportionality factor, the electric current ICOil may be alternating current, and the instantaneous values of the force FCoil will be stored to determine the force change AFref corresponding to said alternating current. For example, the AC frequency may be in the range of 2 to 10 Hz. The AC frequency may be substantially equal to 6 Hz. The most important frequency component of a ballistic cardiography signal is typically at about 6 Hz.

The measuring arrangement according to Fig. 6a may be implemented in such a way that the movement of the coil from the middle position is not so great that the result of the suspension caused by the suspension to return to the middle position would substantially affect the measurement result.

In the arrangement of Fig. 6a, a direct current Iref can also be used, whereby the proportionality factor ki can be determined by the formula: 10 k, = ^ SL · (4)

* REF

FRef is the force corresponding to the control current Iref. When using formula (4), make sure that the power meter reading is zero when coil current 5 is zero. Thus, formula (3) is reduced to formula (4) when the change reference is zero current and zero force.

Instead of the arrangement of Fig. 6a, the arrangement shown in Fig. 6b may be used to determine the proportionality factor ki, where the power 10 transmitted through the speaker hanger 160 is used to measure the coil force FCoil · The hanger 160 is may also be performed such that no force is applied to the force gauge 910. 15 The proportionality factor ki determined here corresponds in particular to the calibration situation shown in Figure 7.

Referring to Figure 7, the dynamic speaker 100 may be used to apply a variable force Fcoil of known magnitude to the electret 20 membrane sensor 50. The calibration arrangement comprises at least a speaker 100, a loudspeaker control device 180, and power wires 182. The calibration arrangement may further comprise a switch 190 and an interface 185 between the control device 180 and the signal processing unit 250.

25

The speaker 100, the control device 180 and / or the switch 190 may be part of the measuring apparatus 200.

The protruding portion of the hanger 160 of the speaker 100 may be pressed against the electret 30 ladder sensor 50. The contact surface between the suspension 160 and the sensor 50 is typically annular. The pressure applied to the annular contact surface P1 transmits to the transducer 50 the force generated by the coil 110 Fcoil · 11

The sensor 50 may be on a substantially horizontal substrate 60. The magnet 130 of the speaker 100 generally has a considerable mass, so that the weight of the speaker 100 may be utilized to press the speaker 100 against the sensor 50.

5

The control device 180 can produce a variable control current Icoil for the coil 110 of the loudspeaker 100. The known proportional coefficient k1 can now determine the change in the coil force FCoil corresponding to the change in the control current ICoil AICoil.

In particular, the control device 180 may be adapted to provide an alternating current or an alternating voltage having a frequency in the range of 2 to 10 Hz, preferably 6 Hz.

Alternatively, the AC output frequency of the control device 180 may also be between 3 and 10 times the heart rate of the patient 10 being examined. For example, if the heart rate of the patient is 2 Hz (= 120 bpm), the corresponding calibration frequency may be from 6 to 20 Hz. This same calibration frequency can also be used in the determination of the proportionality factor ^ in Figure 6.

20

The control device 180 may be arranged to provide a constant amplitude of constant amplitude that is substantially independent of the impedance of the coil 110. For example, heating of the coil 110 can substantially affect the resistance of the coil.

25

The control device 180 may be adapted to provide an alternating voltage of constant amplitude. The control current then depends on the impedance of the coil 110 and the conductors 182, which may depend on the operating temperature.

The control device 180 may be adapted to provide the same variable control current Icoil as used in the arrangement of Figure 6a or 6b.

Thus, a combination of speaker control device 180 of speaker 100 and 35 can be provided for calibrating the ethereal membrane, said control device 180 being adapted to provide coil 110 of said speaker with a varying control current Icoil having a frequency between 2 and 10 Hz change AFcoil is well known. In addition, said known force change AFCoil may have been measured by a force measuring device 910.

Fig. 8 shows, by way of example, the instantaneous values of the measuring signal SEmf as a function of time for the varying control current Icoil, i.e. the changes in the control current AICoil. Signal changes can be determined from the measurement signal SEmf, such as the difference between maximum and minimum values ASemf · 10

The purpose of the calibration is to determine the proportional factor k2 between the force change AF2 applied to the electret film sensor 50 and the corresponding signal change ASemf. The proportionality factor k2 can be determined by equation (5) if the change in coil force caused by the signal change ASEmf 15 is known AFCoil: k2 = —C-QIL (5) ASemf

The change in coil force AFCoil 20 caused by the control current change AICoil can in turn be determined by equation (6) or (7): AFCoil = kj -ΔΙC0IL (6) 25 mcoil = ^ S2! L & Fref (7)

A1REF

The determined coil force change AFCoil is plotted in formula (5) to determine the proportionality factor k2. Formulas (6) and (7) are based on the linearity assumption.

30

The magnitude of the change AICoil of the control current Icoil when determining the proportionality factor k2 (Figure 7) may be equal to or different from the change of the control current Icon AIref when determining the proportionality factor ^ (Figures 6a and 6b).

13

If it is known that the second change in control current AICoil is still large than the first change in control current AIrEF, then, based on the stability assumption, it can be determined that the second change in coil force 5 AFcoil is equal to the first change in coil force AFref. 6a, 6b and 7, for example, the same control device 180, which is adapted to provide a constant amplitude of alternating current or alternating voltage, may be used.

10

The measurement signal SEMf may be, for example, a digital signal produced by the AD converter 230. It is advantageous if the proportionality factor k2 compensates for as many potential sources of error in the measurement chain as possible. The proportionality factor k2 may be determined by the signal-15 attachment of the sensor 50, for a signal amplified, for example, by an amplifier 210, and / or a signal produced by an opto-separator 220.

Referring back to Figure 7, the switch 190 may be adapted to actuate the control device 180 when the speaker is positioned on the surface of the sensor 50. The switch 20 190 and / or the control device 180 may be adapted to transmit to the signal processing unit 250 information that the speaker is on the surface of the sensor 50 and / or that the control device 180 is in operation. The signal processing unit 250 can then automatically determine the signal range ASEmf · The signal stored in the memory unit 240 may automatically be provided with 25 additional information indicating that it is a calibration signal.

The determined proportionality factor k2 and / or the known coil change AFrEf and / or the determined coil change AFCoil can be stored in the memory 200.

30

The memory can also store at least one parameter which depends on the proportionality factor k2 in a known manner. The proportionality factor k2 and / or said parameter may be subsequently combined with measurements made or to be made using sensor 50. Said parameter 35 may thus be, for example, the determined coil force change AFcoil or its inverse value.

14

Referring to Fig. 9a, the speaker hanger 160 may be protruding so that the hanger 160 protrudes by the amount u1 relative to the frame 140. Referring to Fig. 9b, the protrusion amount u1 may be reduced by attaching a raised member 144. Referring to Fig. 9c extension piece 146.

For example, if it is determined by test measurements that the hanging 160 of the speaker 100 does not transmit the same force F1 as is generated by the coil 110, the arrangement shown in FIG. 10a may be used for example, a machined cylindrical body having a flat end for applying force to the surface of the sensor 50.

15 The diameter of the portion of the loudspeaker 100 moving with the coil, i.e. the outside diameter of the hanger, is dial. If the coil force FCoil is transmitted to the transducer 50 via a suspension 160, the diameter dial should be less than the length and width of the transducer 50. For example, the diameter of the dial may be less than or equal to 250 mm. The diameter of the dial may be less than or equal to 125 mm to make the speaker easy to move and / or store. Even if the control frequency is in the range of 2 to 10 Hz, the speaker 100 used may be considerably smaller than a speaker having a lower limit of 10 Hz for the acoustic frequency response -1QdB.

25 The travel of the reel 110 typically increases as the diameter dial increases. For example, the diameter of the dial may be greater than or equal to 60 mm to provide sufficient space for the coil 110. If the stroke is too small, the coil 110 may be close to its extreme position during calibration, and it may be possible that the previously determined proportional coefficient ki 30 does not apply.

Referring to Figure 10b, there may be a liquid coupling agent 171 between the spacer 170 and the sensor 50. The coupling 171 may be, for example, water or silicone grease. In this case, the connection 35 between the spacer 170 and the sensor 50 can also momentarily transmit the pulling forces.

15 Human or animal vital functions produce forces that can be measured from the surface of the body, and can be monitored or measured by an electret caliper. The electret film transducer can be used in particular to study cardiovascular function. Vascular function may include examination of vascular elasticity. The electret film sensor can be used in particular to measure a ballistocardiographic signal.

The determined proportionality factor k2 can be used to convert arbitrary signal values Semf of an electret caliper into cubic-10 power values, such as Newton (N).

Thanks to the proportionality factor k2, the signals measured with the electret film transducer 50 can be compared with those measured with other equipment or sensors. Thanks to the proportionality factor k2, the values of the sensor signal-15 Iin can be compared with reference values determined by other equipment or sensors. Thanks to the proportionality factor k2, the parameters calculated from the sensor signals can be compared with corresponding parameters determined by other equipment or sensors.

Figure 11 shows a ballistocardiography signal corresponding to Figure 4, in which the values of the sensor signal are converted to force values using a proportional factor k2. The vertical scale of Figure 4 is in Newton (N). The muscular strength of the heart causes a difference in pressure, which causes blood to flow (ejection) from the heart to the aorta and pulmonary artery. For example, the signal change ASu (Fig. 4) multiplied by the proportional factor k2 between the I and J features of the ballistocardiography signal results in the parameter AF2u, expressed in Newton (N), which describes the maximum muscle force in the left ventricle of the heart. The balloon cardiography signal can also be used to determine other cardiac output parameters, such as cardiad output, volume flow (in units of ml / s, for example) and / or pressure rise rate (dp / dt, where p = pressure and t = time). .

By measuring the time difference of pressure pulses transmitted through the vessel wall or vessel walls at different distances from the heart, conclusions can be drawn on the degree of elasticity of the vessels and the prevailing blood pressure level.

16

Alternatively, with reference to Figure 3, the patient 10 or animal may also be in a supine position on an electret film transducer 50 for measuring acceleration forces.

5

Cardiovascular function can also be examined with an electrethine caliper 50, which is positioned to measure pressure pulses transmitted through the artery wall to the neck 11 of the patient 10, or to the extremities.

The electret film transducer 50 may also be used to monitor respiratory variable forces.

The memory 240 of the measuring apparatus 200 may have stored a computer program for determining the proportionality factor k2. Said computer program may also be stored on a computer readable medium, such as a CD-ROM. Assuming that the force change AFref- corresponding to the first change in speaker control current AIREf is known, said computer program may now comprise computer program components executed by a signal processing unit 250 to: said change in sensor signal ASemf, the known force change AFref, the first change in control current AIREF, and said second change in control current Alcoil.

The measured signals SEmf from a variety of patients 10 may be stored and / or processed to create a reference database. The signals can be stored as such in the database, or they can calculate various parameters and parameters which are stored in the database. For example, the database may be stored in memory 240.

The calibrated measurement signal SEmf can be utilized to diagnose a patient's state of health, the actual diagnostic method comprising all of the following steps: connecting patient vital forces to sensor 50, 17 measuring sensor signal Semf, correcting signal values by at least one parameter - selecting at least one relevant reference value; - determining a deviation between said parameter and said reference value; and - determining a patient's health based on said deviation.

10

It should be noted that the selection of at least a relevant reference value and the determination of a medical condition based on the abnormality are typically procedures performed by a physician. Of these, at least the selection of a reference value can in principle also be performed automatically. The selection of a reference value for the relay-15 rim from the database can be made automatically based on, for example, patient gender, age, race (e.g., Caucasian or African American), physical activity, and / or known patient conditions.

20 Calibrator-based calibration can be used to test proper operation of the sensor and / or the entire measuring chain. This can be used for example. simply ensure that the digital coefficients used in signal processing and the operation settings of the analog components are as desired. Sensor aging and change in sensor characteristics can be tested by comparing sensor signal values with previously measured signal values of the same sensor, or by examining changes in the proportionality factor k2.

Referring to Figure 12a, the speaker 100 used for calibration may be connected to the surface of the sensor 50, for example by glue. The speaker may be permanently attached to the center or edge of the transducer membrane. The loudspeaker may be small in size, for example, with a hanging diameter less than 25 mm. The transducer 50 may be placed over the substrate 60 so that the force generated by the coil of the speaker 100 compresses the electret film 53. In order to provide sufficient static-35 compression, the Patient 10 or other object of sufficient weight (not shown)

18

Referring to Figure 12b, the speaker 100 used for calibration may also be a component connected to the substrate 60. The speaker 100 may be located, for example, in the seat portion of the chair such that a sensor placed on the chair contacts the 5 speakers 100. The structure of the speaker 100 may be designed to withstand the weight of a patient sitting on a platform. The speaker 100 may be disposed in an open or closed cavity 61. A patient 10 or other object of sufficient weight (not shown) may be placed over the sensor 50 so that the force generated by the coil of the speaker 100 compresses the electret film 53.

10

The measuring apparatus 200, and in particular its signal processing unit 250, can also be adapted to analyze on the basis of the sensor signal Semf whether the patient 10 applies to the sensor 50 variable forces F2 (t) in the frequency range 2 to 10 Hz as a function of time. The signal processing unit 250 15 can then automatically detect when the BKG signal is gaining or disappearing when the patient 10 settles on the sensor 50 or rises off the sensor 50. The positioning of the patient 10 on the sensor and / or the rise from the sensor may also be observed based on a large change in signal Semf of a major force change due to a momentary change in patient weight. The measuring apparatus may be adapted to perform calibration by loudspeaker 100 automatically when patient 10 rests on sensor 50 and / or patient 10 exits. Automatic calibration can be performed especially when the speaker 100 is connected to the sensor diaphragm 50 (Figs. 12a and 12b).

25

The invention is not limited only to the embodiments described above in the specification or figures. Figures 1-3, 5a-7, 9a-10b, 11-12b are schematic. The invention is intended to be limited only by the scope of the appended claims.

Claims (12)

A method for calibrating an electret film transducer (50) using 5 loudspeakers (100) based on a coil (110) adapted to be movable in a magnetic field (B1), wherein a change in the force generated by the coil (110) corresponding to a change in AFref), said method comprising: applying a force (FCoil) generated by the coil (110) to the surface of the electret film transducer (50), changing the control current (ICoil) applied to the coil (110) to provide said known change (AFREf) coupled force (FCoil), - determining a change in the sensor signal (ASEMf) corresponding to said known power change (AFref), and determining (k2) the relationship between the 15th signal change (ASEmf) and the corresponding force change (AF2), and based on said known force change (AFCoil). A method for calibrating an electret film transducer (50) using a loudspeaker (100) based on a coil (110) movable in a magnetic field (B1), wherein a first change in the force generated by the coil (110) corresponding to a first change in control current (AIref) is ), said method comprising: applying a force (FCoil) generated by the coil (110) to the surface of the electret film-25 Turin (50), - causing a second change in control current (Alcoil) in the control current applied to the coil (110) (AFCoil), - determining a change in sensor signal (ASEMF) corresponding to said second change in force (AFCoil), - determining a second change in said force (AFCoil), and - determining the ratio (k2) between the signal change (ASEMf) and the corresponding force change (AF2). said signal change (ASEmf) and a second change in said force (AFC oil). 35 Method according to claim 2, characterized in that the relationship between the second change in steering current (AIcoil) and the first change in steering current (AIREf) is determined (ΔΙcoil / ΔΙREf), wherein the second change in force (AFcoil) is defined as the first change in force (AFref). times the current change ratio (ΔΙ coil / ΔΙ ref) · 5 Method according to one of claims 1 to 3, characterized in that said signal (SEmf) of the electret film sensor (50) is amplified by an amplifier (210). Method according to one of claims 1 to 4, characterized in that said coil control current (ICoil) is substantially sinusoidal and has a frequency in the range of 2 to 10 Hz. A method according to any one of claims 1 to 5, characterized in that said coupling of the coil force (FCoil) comprises placing the speaker (100) in contact with said electret film transducer (50) such that an oscillating suspension is coupled to the coil (110). (160) contacts the sensor (50). Method according to claim 6, characterized in that said application of a coil force (FCoil) further comprises - placing said electret film transducer (50) on a substantially horizontal support (60), and - positioning said speaker (100) on said transducer (50) such that the weight of the speaker (100) compresses said suspension (160) against the sensor (50). A method according to any one of claims 1 to 7, characterized in that said method comprises storing in a memory (240) a sensor signal (SEmf) corresponding to the force change (AFcoil) generated by the coil. The method according to any one of claims 1 to 8, characterized in that said method further comprises storing at least one parameter 35 such that said at least one parameter can be retrospectively combined with measurements made or to be performed by said sensor, said at least one parameter 50) the ratio (k2) of the signal (SEmf) to the force (F2) applied to said sensor (50). A method of measuring the forces exerted by patient (10) or animal vital functions using an electret film transducer (50), using a speaker (100) based on a coil (110) adapted to move in a magnetic field (B1) to calibrate said electro film transducer (50), a first change (AFref) of force generated by said coil (110) corresponding to a first change (AIref) of the guide current, said method comprising: applying a force (FCoil) generated by the coil (110) to the surface of an electret film sensor (50), causing a change (AICoil) in the control current applied to the coil (110) 15 to provide a second change (AFcoil) of force applied to the surface of the electret film transducer (50), - determining a change in the sensor signal (ASEmf) corresponding to said second change 20th signal change determining the relationship (k2) between it (ASEMF) and the corresponding force change (AF2) based on said signal change (ASEmf) and a second change in said force (AFcoil), - transmitting the variable forces (F2) exerted by the patient (10) or animal 50), and 25) determining the variable powers (F2) caused by the patient (10) or animal vital functions by means of the signal (SEmf) of said electret film transducer (50) and said ratio (k2). The method of claim 10, characterized in that said method comprises measuring a balistocardiographic signal using said electret film transducer (50). A method according to claim 10 or 11, characterized in that said speaker control current is substantially sinusoidal and has a frequency between 3 and 10 times the heart rate of the patient (10) or animal.