WO2005062924A2 - Transducer for converting heartbeat pulsations to electrical signal - Google Patents

Abstract

Transducers for converting mechanical pulsations to an electrical signal employ two membranes, one of which is a follower membrane exposed to the pulsations and the other of which is a piezoelectric membrane coupled to the follower membrane by a slightly pressurized volume of air. The membranes may be part of a single chamber or may be parts of separate chambers coupled by an air column. The invention is useful as a transducer of heartbeat pulsations, for example.

Description

Title: TRANSDUCER FOR CONVERTING HEARTBEAT PULSATIONS TO ELECTRICAL SIGNAL

Inventors: David W. GERDT, Martin C. BARUCH, and Charles M. ADKINS

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of provisional application 60/531,624 filed December 23, 2003, incorporated herein by reference.

BACKGROUND OF THE INVENTION This invention is concerned with transducers for converting mechanical pulsations into an electrical signal. One environment in which the invention is useful is physiological monitoring of the states of health, fatigue and mind of military personnel, emergency rescue personnel, and so-called first responders, for example.

BRIEF DESCRIPTION OF THE INVENTION The invention provides a sensitive device that produces an electrical signal in response to heartbeat pulsations, for, example. In one embodiment, the invention employs a wrist- mounted case or housing in which slightly pressurized gas, preferably air, is confined in a chamber between a pair of flexible walls, one of which is a high compliance follower membrane that senses the heartbeat pulse signature and the other of which is a piezoelectric membrane. Heartbeat pulsations applied to the follower membrane are transferred to the piezoelectric membrane through pressure oscillations in the air, serving as a coupling medium. In another embodiment, two chambers containing gas, preferably air, are wrist-mounted at separate locations. A first chamber has a wall constituted by a piezoelectric membrane and is contained in a case positioned on a wrist, like a watch. A second chamber has a wall constituted by a compliant follower membrane exposed to heartbeat pulsations, and is mounted on a band attached to the case, like a watch band. The two chambers are connected by tubing that transfers heartbeat pulsations from the second chamber to the first chamber via air in the tubing, serving as a coupling medium. Both embodiments preferably employ electronics including, for example, an amplifier with a high input impedance stage to convert the current output of the piezoelectric membrane to a voltage, and a voltage amplifier stage to produce an output signal that can be displayed locally or at a distance.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described in conjunction with the accompanying drawings, which illustrate preferred (best mode) embodiments, and wherein: Fig. 1 is a diagrammatic sectional view of a first embodiment of the invention; Fig. 2 is a circuit diagram of an amplifier for use in the invention; Fig. 3 is a perspective view of a second embodiment of the invention; Fig. 4 is a diagrammatic sectional view of a first unit employed in the second embodiment; Fig. 5 is an exploded view showing components of the first unit; and Fig. 6 is an exploded view showing components of a second unit of the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION The invention will be described in its application to wrist-mounted transducers, but it will become apparent that the invention has broader utility. For example, the transducers can be mounted on other parts of a human or animal body. Certain general aspects of the invention will be described first, before a detailed description of embodiments shown in the drawings. A pulse sensing transducer in accordance with a first embodiment of the invention includes two main components mounted in a suitable case or housing. A first component is a very low durometer high compliance follower membrane, preferably silicone, anchored at its perimeter. The follower membrane is disposed near a radial artery on the wrist, at a pulse palpation point, to sense the heartbeat pulse signature and transfer it to the second component, a piezoelectric membrane, via a gaseous coupling medium, preferably air. The low stiffness of the follower membrane and the low inertia of the air coupling medium allow the palpations of the artery to be transferred efficiently to the piezoelectric membrane. It appears that the air coupling medium acts as an incompressible piston with almost no inertia. Since the velocity of transferred pulsations is very low, there is very low frictional loss and essentially no viscosity and no turbulence in the coupling medium. The acoustic signature of the heartbeat pulse resides in frequencies above its fundamental (near 1 Hz) and has useful content out to about 15 Hz. The energy is mostly contained in the fundamental frequency at the conventional pulse rate. The identification of this frequency bandwidth is useful in determining sample rates and collection times that may be required by various analytical techniques. The displacement of the follower membrane transfers the motions associated with the heartbeat pulses into pressure oscillations in the coupling gas. At the low frequencies contained in the pulse and breathing signatures, there is essentially no distortion due to inertial or acoustic phenomena, and the displacement of the follower membrane directly correlates to pressure at the piezoelectric membrane . The piezoelectric membrane is preferably a piezopolymer film, such as PVDF, having electrodes (e.g., silver) on opposite sides of the film. In the invention, the piezoelectric membrane may be circular with a diameter about the same as a dime, or rectangular, for example. The membrane is anchored at its perimeter and is sensitive whether or not there is a differential pressure between the opposing sides of the membrane. However, it is preferred to establish a differential pressure between the sides of the membrane to put the membrane in elastic tension. The differential pressure is produced by slightly pressurizing the air between the membranes. Once a static elastic deformation of the piezoelectric membrane is reached, electrical charge accumulates on one face of the membrane for deformation caused by increases in the pressure differential. This charge can be used to produce a current in proportion to the stressing of the stretched piezoelectric membrane. Pressure oscillations in the air coupling medium are converted into strain oscillations in the piezoelectric membrane, which are converted to current oscillations that can be measured. In the invention, the volume of gas between the follower membrane and the piezoelectric membrane is preferably pressurized to of the order of 1 to 2 psi, which is sufficient to cause bulging of the membranes (spherical in the case of a circular membrane) . In the absence of pressurization, a membrane is not appropriately stretched and can deform permanently in very small amounts. Pressure oscillations can cause some parts of the membrane to move in one direction while other parts move in another, which can wash out a signal from a piezoelectric membrane. Moreover, slight movements of the hand or body can cause the membrane to flap, creating artifact signals that degrade the signal- to-noise ratio. Although different approaches can be used for measuring charge accumulated on the piezoelectric membrane, or a resultant current, it is preferred to use a device that converts the current to a voltage and then to amplify that voltage. In the invention, it is preferred to use a transimpedance amplifier that inputs current and outputs voltage, followed by a voltage amplifier. The transimpedance configuration clamps the voltage on the piezoelectric membrane to zero and gives an output proportional to the current produced by the movement of the charge produced piezoelectrically. The output is proportional to the rate of strain in the piezoelectric membrane, which is the time derivative of its displacement. There are several benefits to the transimpedance approach. First, when movement stops, the output is zero, regardless of the displacement. Therefore, significant gain can be used in this amplification stage and yet the amplifier rapidly recovers from being overdriven. Second, since there is usually very little power in electrostatic noise sources, the circuit is immune to such noise in the environment . If there is concern that the output is the time derivative of pulses rather than the direct signal itself, a modification of the transimpedance design can include an integrating capacitor in the feedback path of the amplifier. This sets a low frequency break point and allows the integration of signals in the passband of the amplifier. If the break point is set about 1 decade below the pulse fundamental, the output is purely a pressure pulse, but the settling time to sudden inputs is slow (about 10 sec), If the break point is set about 1 decade above the pulse fundamental, the output is purely derivative pulse, and the settling time is rapid (about 0.1 sec). If the break point is set at the pulse rate, both signals are down 3 dB, and the output is a mixture of the two signals. The choice of signal type is a function of its use. If observation of the pressure pulse itself is important, the fully integrated result is preferred at the expense of settling time. If determination of the pulse rate or interbeat interval is the intended use, the differential approach may be the preferred signal because it settles so much faster. Both signals have the same fundamental frequency and the same harmonics . The weighting of the high frequency content is slightly higher in the differential signal . Turning now to the drawings, Fig. 1 shows, somewhat diagrammatically, an example of a transducer 10 in accordance with a first embodiment of the invention. The embodiment shown is intended to be wrist-mounted and includes a case 12 about the size of a watch case and a conventional wrist strap 14 (watch band) attached to the case for securing the case to a pulse palpation point on a wrist. A follower membrane 16, shown bulging outwardly, is typically a silicone film about 0.01 inch thick with a SHORE A durometer of about 10 (available from the Rogers Corp.). A volume of air 20 is contained in a chamber between the follower membrane and a piezoelectric membrane 18 and is pressurized to about 1 to 2 psi, for example, by an air pump 22 that can be a simple thumb-operated pump mounted on the transducer device or a pump connected to the air space chamber via tubing. In either case, a one-way valve can be used to admit air and prevent loss of air pressure. The membranes are anchored in the case at their peripheries . In the embodiment, a printed circuit board 24 is mounted in the case and provides electronics for processing the current output of the piezoelectric membrane. Fig. 2 shows an amplifier circuit 25 useful in the electronics of the invention and including a transimpedance amplifier as a first stage 28 and a voltage amplifier as a second stage 30. The feedback resistor of the first stage should be as high as possible. The output of the amplifier can be converted to a digital signal that can be processed by a conventional digital signal processor, and the electronics may include a transceiver (or merely a transmitter) for remote communication. An output signal of the electronics can be sampled at an appropriate rate to reduce power requirements. In the embodiment shown in Fig. 1, a power supply of the electronics includes a battery 32 mounted on the case, but the battery can be mounted in the case or on the wrist strap, for example. The invention can be used in a single-point blood pressure monitoring system of the type described in co- pending application 10/502,932 filed July 29, 2004, incorporated herein by reference. Other applications of the invention will be apparent to those skilled in the art. Fig. 3 show a second embodiment of the invention, employing two gas-containing chambers connected by tubing. A first chamber is incorporated in a first wrist-mounted unit 34, and a second chamber is incorporated in a second wrist-mounted unit 36. The first unit uses a conventional watch case with certain added components. Fig. 4 shows the complete first unit, and Fig. 5 is an exploded view of the first unit. As shown, the watch case 38 is surmounted by a conventional finger-operated pump 40 that includes a flexible diaphragm 42 with a finger-closed center hole 44 and a one-way check valve 46 for pressurizing a chamber 48, one wall of which is a piezoelectric membrane 50 perimeter- anchored over a center hole 52 of a washer 54. Pressure pulsations are transferred to the chamber 48 of the first unit via tubing 56 connected to the second unit. The first unit 34 may include the valve body 58, a washer/spacer 60, and electronics on a printed circuit board 62, and may have a conventional case back 64. The first unit 34 is wrist- mounted by a watch band strap 66 which also supports the second unit 36. The second unit 36 has a gas-containing chamber connected by the tubing 56 to the first unit. One wall of the chamber is constituted by a high compliance elastomeric follower membrane 68, which, in the form shown, is a silicone membrane of rectangular configuration. The follower membrane is perimeter-mounted between upper and lower rectangular washer/spacers 70,72. The upper spacer 70 abuts a wall 74 with a center opening 76 over which is mounted a sleeve 77 closed by a top wall 78 to form a hollow turret. A stainless steel coupling 80 connects the interior of the hollow turret to the tubing 56 attached to the first unit 34. In a specific construction of the second embodiment, washer 72 is a 1/8 inch thick silicone stand-off. Membrane 68 is a 0.01 inch thick grey Bisco silicone membrane. Washer 70 is a 0.031 inch thick Bisco silicone member. Wall 74 is a 0.031 in thick Bisco silicone member. Sleeve 77 is a 7/16 x 1/16 inch wall silicone tubing. Wall 78 is a 1/16 inch silicone sheet. Tubing 56 is a 1/8 x 1/16 inch wall silicone tubing. In this embodiment (as well as in the first embodiment) components of the transducer device can be assembled using an appropriate adhesive. When the gas (preferably air) in the chamber of the second unit 36 is slightly pressurized by the pump 40 of the first unit 34 via the tubing 56, the follower membrane 68 bulges from the stand-off 72 to contact the skin of the wrist at a palpation point, so that heartbeat pulsations to which the follower membrane is exposed are transferred through the air column in the tubing 56 to pressurized air contained in the chamber in the first unit, and thereby to the piezoelectric membrane 50. A piezoelectric signal can then be processed as in the first embodiment. In both embodiments, the piezoelectric membrane and the electronics are preferably electrically shielded, as in a Faraday cage. The Faraday cage can be constituted from one of the electrodes of the piezoelectric membrane and metallized walls of the chamber containing the electronics. The transducer devices of the invention can be part of a system which may include, for example, a small intermediate transceiver worn on a belt of an individual and a command transceiver with a range of about two miles, for example. Vital physiological variables may be monitored on a periodic (programmed) or queried basis (e.g., from the command transceiver) . The system may include a device for measuring core temperature of an individual and may use GPS for individual location. Primarily, the invention can be used to provide a continuous physiologic monitor of heart rate, breathing rate, systolic blood pressure, inspiratory effort, heart rate variability, sleep cycles, restfulness of sleep, sleep disorder problems, and respiratory problems, for example. Secondarily, during wounding of an individual or mishaps for example, the invention can assist as a triage device, allowing for maximum utilization of medical resources . While preferred embodiments of the invention have been shown and described, it will be apparent that modifications can be made without departing from the principles and spirit of the invention, the scope of which is defined in the accompanying claims .

Claims

WHAT IS CLAIMED IS: 1. A transducer comprising a pair of flexible walls, one of which senses mechanical pulsations and transfers the pulsations to a volume of gas, and the other of which receives the pulsations from the gas and produces an electrical signal therefrom.

2. A transducer according to claim 1, wherein the one flexible wall is a compliant follower membrane and the other flexible wall is a piezoelectric membrane, and wherein the gas is pressurized so that the membranes bulge.

3. A transducer according to claim 2 , wherein the membranes are perimeter-anchored in a case and confine the volume of gas therebetween, and wherein the case is constructed for support on a person's limb with the one membrane exposed to receive heartbeat pulsations. . A transducer according to claim 3 , wherein the case has a strap for attaching the case to the limb.

5. A transducer according to claim 3 , wherein the transducer has electronics that produce an amplified version of the electrical signal.

6. A transducer according to claim 5 , wherein the electronics convert current produced by the piezoelectric membrane to a voltage, and amplify the voltage.

7. A transducer according to claim 6, wherein the electronics include a transimpedance amplifier followed by a voltage amplifier.

8. A transducer according to claim 5, wherein at least a portion of the electronics is surrounded by an electrical shield.

9. A transducer according to claim 5, wherein a transceiver is part of the electronics.

10. A transducer according to claim 5, wherein a digital signal processor is part of the electronics.

11. A transducer according to claim 2, wherein the transducer has a pump for pressurizing the gas .

12. A transducer according to claim 2, wherein the perimeter of each membrane is mounted in a case.

13. A transducer according to claim 2 , wherein the follower membrane is silicone about 0.01 inch thick with a SHORE A durometer of about 10.

14. A transducer according to claim 2, wherein the bulge of the membranes is spherical or rectangular.

15. A transducer according to claim 2, wherein the thickness of the piezoelectric membrane is of the order of 25 mm.

16. A transducer according to claim 2, wherein the gas pressurization is of the order of 1 to 2 psi.

17. A transducer comprising first and second gas- containing chambers connected by gas-containing tubing, the first chamber having an outer wall constituted by a compliant follower membrane adapted to receive mechanical pulsations, the second chamber being disposed in a case and having a wall constituted by a membrane that produces an electrical signal in response to pulsations transferred from the first chamber to the second chamber via the tubing.

18. A transducer according to Claim 17, wherein the first chamber is part of a first wrist-mounted unit and the case is part of a second wrist-mounted unit.

19. A transducer comprising a sensing device adapted to receive mechanical pulsations and a piezoelectric device for producing an output in response to the sensed pulsations coupled to the piezoelectric device via a volume of gas.

20. A transducer comprising a first membrane adapted to receive mechanical pulsations and a second membrane that produces an electrical signal in response to the pulsations transferred from the first membrane via a volume of gas.

21. A transducer according to claim 20, wherein the gas is pressurized to of the order of 1 to 2 psi.

22. A transducer according to claim 20, wherein the membranes are perimeter-anchored and are under tension.