Movement detector and apnea monitor including same

Abstract: A movement detector particularly useful as an apnea monitor includes a piezoelectric crystal transducer (2), a supporting member (6) on one side of the transducer, and a collector member (8) on the opposite side of the transducer and having an effective area, as circumscribed by its outer edges, which is at least fifty times the surface area of the respective side of the transducer, for collecting the forces applied at that side of the transducer and for applying them to the transducer.

Description: The present invention relates to movement detectors such as are used in apnea monitors, and also to an apnea monitor including such a detector. Apnea monitors are used to detect a cessation of breathing (apnea) in infants in order to prevent sudden infant death (SID), which may occur while the infant is asleep. Many types of apnea monitors have been developed for this purpose. One type includes movement detectors, such as piezoelectric crystal transducers, which detect the breathing movements of the infant. Examples of the latter type movement detector are described in US Patents 4,169,462, 4,576,179, 4,657,026 and Re 32,180. In GB 2192460A there is disclosed a movement detector comprising a piezoelectric crystal transducer outputting electrical signals in response to changes in force applied to its opposite faces; a supporting member on one side of the transducer; a collector member on the opposite side of the transducer and having an effective area, as circumscribed by its outer edges, which is at least fifty times the surface area of the respective side of the transducer, for collecting the forces and for applying them to the side of the transducer; and a spacer member between the collector member and the opposite side of the transducer and contacting the opposite side of the transducer over an area less than that side of the transducer, for directing the forces from the collector member to the transducer. In the movement detector described, the one side of the transducer is in direct contact with the supporting member. However, such a movement detector was found to have a high rate of false alarms, which made the detector unreliable. An object of the present invention is to provide a movement detector particularly useful for an apnea monitor having advantages in the above respects. According to the present invention, there is provided a movement detector comprising a piezoelectric crystal transducer outputting electrical signals in response to changes in force applied to its opposite faces; a supporting member on one side of the transducer; a collector member on the opposite side of the transducer and having an effective area, as circumscribed by its outer edges, which is at least fifty times the surface area of the respective side of the transducer, for collecting the forces and for applying them to said side of the transducer; and a spacer member between said collector member and said opposite side of the transducer and contacting said opposite side of the transducer over an area less than that side of the transducer, for directing the forces from said collector member to said transducer; characterized in that said movement detector further includes an annular spacer ring between the supporting member and the transducer; said annular spacer ring having an inner diameter slightly smaller than the diameter of the respective side of the transducer, and an outer diameter slightly larger than the respective side of the transducer; and that said supporting member and said collector member are both of stiff material and are joined together at their outer peripheries outwardly of and around the transducer. It was found that a movement detector constructed in accordance with the foregoing features leads to a significantly lower rate of "false negatives", as compared to the detector in which the transducer was in direct contact with the supporting member. According to a further aspect of the invention, there is provided an apnea monitor including a movement detector as set forth above, an alarm device, and a control circuit receiving the output of the detector and controlling the alarm device. Fig. 1 is a longitudinal sectional view illustrating one form of movement detector constructed in accordance with invention; Fig. 2 is a block diagram of an apnea monitor including the movement detector of Fig. 1; and Fig. 3 illustrates an infant's bed equipped with the apnea monitor of Fig. 2. The movement detector illustrated in Fig. 1 comprises a piezoelectric crystal transducer, generally designated 20, having a metal electrode 20a on one side, and a piezoelectric crystal 20b on the opposite side with its outer face coated with a metal electrode film 20c. Such piezoelectric crystal transducers are well known; they output electrical signals, via their output leads 4, in response to changes in force applied to the opposite faces of the transducer. As shown in Fig. 1, the metal electrode 20a is of substantially larger diameter than the crystal 20b and its electrode film 20c. The motion detector illustrated in Fig. 1 further includes a pair of plate-like members 26, 28 on opposite sides of transducer 20 and of substantially larger dimensions than it. Preferably, the two members 26 and 28 each have an area which is at least 50 times larger than the area of transducer 20. Members 26 and 28 are made of relatively stiff, (i.e., rigid, or semi-rigid) material, such as plastic or wooden sheets. They are joined together at their outer peripheral edges with the transducer 20 located centrally between them. A spacer member 29 is interposed between the small-diameter side 20c of the transducer 20 and its respective plate 28 to space the transducer therefrom. Spacer member 29 is of a yieldable material, preferably a hard elastomeric material such as hard rubber. Member 26 serves as a supporting member on one side of transducer 20, whereas member 28 serves as a collector member on the opposite side of the transducer, for collecting the forces and for applying them to that side of the transducer. Spacer member 29 serves as a directing member for directing the forces from the collector member 28 to the transducer. Being yieldable under force, it does not concentrate but rather distributes the forces applied to the transducer. The movement detector illustrated in Fig. 1 further includes an annular spacer ring 30 interposed between the large-diameter side of transducer 20 and its supporting plate 26. Spacer ring 30 has an inner diameter slightly smaller than the outer diameter of transducer electrode 20a, and an outer diameter slightly larger than that electrode. Preferably, ring 30 is made of rigid material such as plastic or fiber. Both the transducer 20 and the spacer member 29, are in the form of flat discs of circular configuration. Preferably, transducer 20 is of a total thickness of 0.5-1.00 mm, and the spacer member 29 and annular ring 30 are each of a thickness of 0.5-5.00 mm. Particularly good results have been obtained when using a piezoelectric crystal transducer having an overall thickness of 0.53 mmm, with the metal electrode side 20a of a thickness of 0.25 mm and a diameter of 35 mm, and the crystal side 20b of a thickness of 0.28 mm and a diameter of 25 mm; a spacer member 29 having a thickness of 4.0 mm and a diameter of 20 mm; and an annular ring 30 having a thickness of 1.0 mm. In this example, the members 26 and 28 are wooden sheets of rectangular configuration, of a length of 480 mm, a width of 290 mm, and a thickness of 4.0 mm, and are joined together at their outer peripheral edges. Fig. 2 is a block diagram of, and Fig. 3 pictorially illustrates, an apnea monitor system including the movement detector of Fig. 1. As shown in Fig. 2, the apnea monitor includes a movement detector, generally designated 31. The detector's output is fed, via a filter 32 and an amplifier 34, to a control system 36 which controls a local indicator 38 (e.g., a light energized with each detected breathing movement), a local visual alarm 40 (e.g., a flashing red light) which is energized if an apnea condition is detected, and a local audio alarm 42 (e.g., a sounding device) which is also energized if an apnea condition is detected. Control system 36 further controls a corresponding remote indicator 44, remote visual alarm 46, and remote audio alarm 48, all located, for example, in the parent's bedroom. In the system illustrated in Fig. 2, the filter is a lowpass filter and removes high-frequency components, such as may be caused by audio sounds within the room, other noises, and also the heartbeat of the infant. The system thus responds only to the relatively low-frequency sounds produced by the breathing of the infant, so that if such sounds are not detected within a predetermined interval indicating an apnea condition, the above-described alarms are actuated. Fig. 3 illustrates an infant's bed 50 equipped with an apnea monitor system as described above. Preferably, two detector units 51 and 52, each constructed according to Fig. 1, are located between the mattress 53 and the baseboard 54 of the infant's bed. The outputs of the movement detector units 51, 52 are fed to another unit 55 which houses the electrical circuit illustrated in Fig. 2. Unit 55 also supports the local indicator 38 and alarms 40, 42. The remote indicator and alarms may be housed within a unit similar to unit 55 and located at a remote place, e.g., the parent's bedroom. Unit 55 further includes a manual switch 60 which may be depressed to turn-on the system. It will be appreciated that the embodiment described above is set forth merely for purposes of example, and that many variations and modifications may be made. For example, the spacer member 29, and/or the annular ring 30, may be integrally formed with their respective plates 26, 28. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included just for the sole purpose of increasing intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A movement detector comprising a piezoelectric crystal transducer (20) outputting electrical signals in response to changes in force applied to its opposite faces; a supporting member (26) on one side of the transducer; a collector member (28) on the opposite side of the transducer and having an effective area, as circumscribed by its outer edges, which is at least fifty times the surface area of the respective side of the transducer, for collecting the forces and for applying them to said side of the transducer; and a spacer member (29) between said collector member and said opposite side of the transducer and contacting said opposite side of the transducer over an area less than that side of the transducer, for directing the forces from said collector member to said transducer; characterized in that said movement detector further includes an annular spacer ring (30) between the supporting member and the transducer; said annular space ring having an inner diameter slightly smaller than the diameter of the respective side of the transducer, and an outer diameter slightly larger than the respective side of the transducer; and that said supporting member (26) and said collector member (28) are both of stiff sheet material and are joined together at their outer peripheries outwardly of and around the transducer (20). The detector according to Claim 1, wherein said spacer member (29) is yieldable under force to distribute the forces applied to the transducer. The detector according to either of Claims 1 or 2, wherein the area of said opposite side of the transducer is less than the area of said one side of the transducer. The detector according to any one of Claim 1-3, wherein said supporting member (26) and collector member (28) are joined together at their outer peripheries, with the transducer (20) located centrally therebetween. The detector according to any one of Claims 1-4, wherein said annular spacer ring (30) is a separate member located between the supporting member and the transducer. The detector according to Claim 5, wherein said annular spacer ring (30) is of rigid material. The detector according to any one of Claims 1-4, wherein said annular spacer is integrally formed with said supporting member. An apnea monitor including a detector according to any one of Claims 1-7, an alarm device (46, 48), and a control circuit (36) receiving the output of said detector (31) and controlling said alarm device in response thereto.

Inventors: SHTALRYD HAIM, YOTAM VICTOR, SHTALRYD, HAIM, YOTAM, VICTOR
Applicants: HISENSE LTD, HISENSE LTD.
Application year: 1992
Main classification: A61B5
IPC codes: A61B5, A61B10