GB2181251A - Movement transducer - Google Patents

Abstract

Respiration or pulse monitors must be able to detect the very small movements which occur in the chest wall while a person sleeps. The invention provides a movement transducer having a resilient housing 11 containing a coil 10 and permanent magnets 14 mounted for rotation. In use the housing is taped to the body and a tape 15 attached to the magnets 14 is pulled through a slot 16 and attached to the body at some distance from the housing 11. Small surface movements due to respiration in the extended measurement base formed by the tape 15 cause the magnets 14 to rotate and induce an e.m.f. in the coil 10. Respiration can therefore be monitored by monitoring this e.m.f. The tape may be replaced by a rod. The arrangement may also be used to monitor flexible pressure vessels and to provide an alarm. <IMAGE>

Description

SPECIFICATION Movement transducer The present invention relates to a movement transducer particularly, but not exclusively for detecting and/or measuring small movements which occur at the surface ofthe human body; forexamplethesmall movements which occur during respiration and the pulse atthewrists ortemples.

A respiration monitor must be able to detect the very small movements which occur in the chest wall while a person sleeps. Atthe same time any respiration monitor should be of small size, comfortable to wear and easy for nursing staffto attach.

A known type of respiration monitor consists of a girdle forming a strain gauge worn around the body butthisdevicecan be uncomfortableto wear and is somewhat difficult to attach to a seriously ill patient.

Another respiration monitorfor infants depends for operation on changes in chest radius but in adults chest wall excursions are smaller in relation to chest radius.

According to the present invention there is provided a transducer for detecting small movements occurring in the surface ofthe human body comprising a housing capable of being comfortably mounted on the human body, first and second members mounted in the housing for rotation relative to one another, the first member comprising meansforproducing a magneticfield and the second member comprising meansforgene- rating a signal in response to rotation of the magnetic field, and a probe member attached atone point to one ofthe first and second members to cause said relative rotation to occur when movement occurs between another point ofthe probe member and the second member.

The first member may comprise a plurality of per manent magnets and the second member may com prise a coil situated in the magnetic field.

An advantage obtained by using permanent mag nets and a coil isthatthe initial acceleration ofthe relative rotation can be arranged to be such that a useful electrical signal is generated for small move ments.

Preferably the coil is in operation fixed relative to a person's bodywhilethe permanent magnets are arranged to rotate within the coil.

The probe member may be a rod, or a flexibletape which is fixed atone endtothe said one memberand is preferably at least partially wrapped around the said one memberat least undersome conditions of operation. The tape unwinds and winds around the said one member as movements in opposite senses occur. The advantage of using theflexibletape isthat the "base dimension" ofthe respiration transducer can be cnsiderably extended from a small main portion including the first and second members to a point where the tape is fixed to a person's body.Thus for respiration where movements per unit length are small an increase in base dimension provides larger overall movements for input to the transducer without enlarging the said main portion to the point where difficulties in attachment or byway of discomfort occur.

Where permanent magnets are used it is advantag eous to provide a soft iron keeper adjacent to the magnets and then the keeper may also provide a restoring force for the probe member. Alternatively or in addition a resilient member may be used to provide the return force.

In one form the fi rst a nd second mem bers are posi tioned in a dome-shaped member having a base diameter of not more than about 5 cm and a dome height of not more than about 1 cm. Where the first or second member is a coil, the coil may be cast in a resilient material such as silicone rubber and the permanentmagnets mayform an array mounted for rot ation in a recess at the centre of the coil in the resilient material. Aflexibletape may then be fixed at one end to the magnetarrayand, in use, atthe other end to the patient's body usually without moving the patient. Such a transducer can easily be taped to the body and is reasonably comfortabletowear.

The present invention also includes methods corresponding to the transducers mentioned above.

Certain embodiments ofthe invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an inverted plan view of the transducer according to the invention, and Figure2 is a cross-section of the transducer of Figure 1.

In Figures 1 and 2 a respiration monitor shown has a copper wire coil 10 of about 4,000 turns (46 s.w.g.) cast in a dome-shaped body 11 of silicone rubber. At the centre ofthe coil a plastic or aluminium frame 13 is friction fitted into an aperture 12. The frame is roughly cubical with an open top and bottom. A magnet assembly 14 is mounted on a spindleforrot ation within the frame. The magnets may be either elongated or circular (with radially opposite poles) and they are aligned with their north poles together sothatthe assembly has radially opposite north and south poles, as shown, which extend parallel to the spindle. Atape 15 has one end fixed at a point on the assembly 14 and the other free to be fixed, forexample by adhesive tape, to the body of a patient.The diameter of the dome 11 is about 5 cm and its maximum height is about 0.8 of a centimeter. in use the tape is taken outthrough a channel 16 in the dome and the end is taped to the body wall about3 or4cms away from the periphery of the dome. The al u m iniumframe 13also contains a soft iron rod 17 moun ted parallel to the axis ofthe assembly 14to act as a keeperforthe magnets and to provide a restoring force when rotational displacement of the assembly 1 4takes place. As the patient breathes in and out the tape, which is partially wound round the magnet assembly, unwinds from the assembly and rewinds.

Usually the assembly 14 rotates about a tenth of a turn in operation. As the assembly rotates an e.m.f. is induced in the coil, and since the rest position forthe assembly is as shown in Figure 1, that is with the poles ofthe magnet assembly 14 in or nearthe plane of the coil 10, a maximum number of lines offorce are cut inducing a useful signal. The use ofthetape and rotating magnet assembly ensures a high rate of flux cutting.

The coil is centre tapped and therefore, as shown in Figure 1,three conductors 1 8,1 9 and 20, one of which may be a screen, are taken out through a moulding 22 and into a cable 23. The conductors con nected to the ends ofthe coil 10 are connected byway of an unobtrusive detachable connection (not shown) in the cable 23 to the respective inverting and non-inverting input terminals of an operational amplifier (not shown) and the centre tap is taken to the common terminal ofthecircuitcontainingtheop- erational amplifier. The output from the operational amplifier can be recorded on a 24 hourtape recorder, or passed to a monitor at the patient's bedside, or both.Where a mnonitor is used it may include an alarm which sounds and/or gives a visual indication if output from the amplifierfalls below a predetermined level, indicatingforexampleverylowor zero respiration. The above mentioned detachable connection is situated about 1 metre from the dome 11 so that it is not under a patient when in bed and can readily be detached from the monitor to ease nursing procedures.

in an alternative form the tape 15 is replaced by a silicone rubber rod cemented to one point on the assembly 14. The other end of the rod rests on the patient's body, for example atthe wrist, so that in one use a patient's pulse can be monitored. The rotation ofthe assembly 14 is considerably less than a tenth of a turn in this application but monitoring is sufficiently effective to show the dicrotic notch in the pulse wave.

The invention can be put into effect in many other ways than those specifically described; for example the housing need not necessarily be dome-shaped although it should be reasonablythin inthedirection ofthe coil axis and as small as possible in the plane of the coil sothatitcan betaped easily and comfortably to a patient. A small spring may be used to provide a restoring force forthe assembly 14 and when used the keeper 17 may be omitted. Other coil arrangements may be substituted forthe coil 10, for example a coil without a centre tap or a coil assembly.

It is thought that the invention may also have other medical uses. The invention may also have many non--medicaluses,forexampleasa replacementfora strain gauge where small movements are to be detected. It may for instance be used in an alarm monitorfor some flexible pressure vessels, an alarm being given when a dimension orthe amplitude of a varying dimension increases beyond a limit.

Claims (12)

1. Atransducerfordetecting small movements occurring in the surface ofthe human bodycomprising a housing capable of being comfortably mounted on the human body, first and second members mounted in the housing for rotation relative to one another, the first member comprising means for producing a magneticfield and the second member comprising meansforgene rating a signal in response to rotation ofthe magnetic field, and a probe member attached at one point to one ofthe first and second members to cause said relative rotation to occur when movement occurs between another point ofthe probe member and the second member.

2. Atransduceraccording to Claim 1 whereinthe probe member comprises a flexible elongated member attached atone end tothe said onepointto cause the said relative rotation when the flexible member istaut and the said movement occurs.

3. Atransduceraccording to Claim 1 whereinthe probe member comprises a rigid elongated member attached atone end tothe said onepointtocausethe said relative rotation when the said movement occurs.

4. A transducer according to Claim 1, 2 or 3 wherein the first member comprises a plurality of permanent magnets mounted for rotation within the housing.

5. Atransducer according to any preceding claim wherein the second member comprises a coil.

6. Atransducer according to Claim 5 insofar as dependent on Claim 4 wherein the magnets are mounted for rotation at the centre of the coil, with opposite magnet poles situated radially opposite one another with respect to the axis of rotation and with the said axis orthogonal to the axis of the coil.

7. Atransducer according to Claim 6 including a soft iron keeper adjacent to the magnets which provides a restoring force to return the magnets to a rest position when rotation occurs.

8. Atransducer according to any preceding claim wherein the housing is formed by resilient material, is of size easily capable of being held in one adult hand and is ofshapesuitableforcomfortable attach menttothehuman body.

9. Atransducer according to any preceding claim wherein the housing is dome-shaped having a base of diameter not more than 5 cm and a height of not morethan 1 cm.

10. Amovementtransducercomprisingfirstand second members mounted for rotation relative to one another, means for providing an electrical signal which indicatesthesaid relative rotation, and aflexible elongated member attached at one end to the first member to cause the said relative rotation when movement occurs between the other end of the flexible member, when taut, and the first or second member.

11. A transducer having a housing of resilient material of size easily capable of being held by one hand and of shape suitable for comfortable attachmenttothe human body, the transducer comprising first and second members mounted for rotation relative to one another, means for providing an electrical signal which indicates the said relative rotation, and a probe member attached at one point to the first member to cause said relative rotation to occur when movementoccursbetween anotherpointofthe probe member and the first and/or the second member.

12. Atransducersubstantiallyashereinbefore described with reference to the accompanying drawings.