GB2192460A - Movement sensing apparatus - Google Patents

Abstract

A movement sensing apparatus eg a respiration monitor comprises a sensor assembly locatable under a mattress or seat cushion for detecting movement of a patient or seat occupant, the sensor comprising a semi-rigid base portion 7, a substantially planar piezoelectric element (8) secured to the base portion (7), a spacer element (10) holding a backing piece (11) in spaced apart relationship from the piezoelectric element (7), the whole being enclosed in PVC encapsulation (12). Electrical connections to the piezoelectric element (8) are made by way of screened cable (9) connected at its second end to a control device (not shown). The control device contains a filter, amplifiers, and a timer, the signals received from the sensor assembly being filtered and amplified before being used to reset the timer in synchronism with movements detected by the sensor assembly. In the event that the timer times out a predetermined interval, an alarm signal is generated and transmitted to an alarm system. <IMAGE>

Description

SPECIFICATION Movement sensing apparatus This invention relates to movement sensing apparatus.

Movement sensing apparatus of this invention is well suited for use as apparatus for monitoring a baby's respiration, orfor monitoring the time which elderly or geriatric patients have remained in one position without moving.

The monitoring apparatus could also include an optional 'Baby Alarm' facility, which would considerably increase the usefulness of the system in a domestic environment. A number of different methods have been used for sensing respiration movement, one such method being the use of a flexible capacitivetransducer placed under a cot mattress. This cap acitive transducer senses changesthroughthe mattress in the forces and movement caused by respiration. It has been found that the capacitive transducers specially manufactured forthis purpose, are expensive. Another proposed method, is to use a mechanical assembly to bend a piezoceramictrans- ducer, this mechanical assembly has also been found to be relatively expensive to manufacture.

According to the present invention there is provided a movement sensing apparatus having a movement sensor assembly comprising a piezoelectric compression sensor, the sensor mounted on a semirigid body, and having electrical connections made to itto enable electrical signals generated byforces exerted on the compression sensor to betrans- mittable to a control device for monitoring changes of force on the sensor.

Advantageously, this invention provides a cheap and simple sensor, comprising an assemblythat utilisesthe compression of a piezoelectricelement audio generatorelementto detect movement. This element may be of the type used forwarning/alarm buzzers, etc; those commonly available produce sound in the 2 to 5 KHz range when driven by a suitable generator. In most cases these sensors are perfectly adequate forthis purpose which isto produce a usable electrical signal when compressed in re sponsetosubsonicvibrations/movementsapplied to them. This type of sensor qssem bly with the app ropriate electronic signal prn#essing, will sense re- spiration and the cessation of respiration without need for direct attachmentto the body.

According to a second aspect of the invention there is provided a control device for use in conjunction with a movement sensor assembly comprising an input means connected to an amplifier having input and output means, a low pass filter, a trigger means connected to a timer, the trigger means receiving amplified and filtered signals and using them to reset the timer in synchronism with signals received at the input means in order to preventthe timertiming out a predetermined interval and then initiating an output warning signal.

Advantageously a respiration monitoring apparatus utilisesthe compression of a piezoelectrictrans duner for accurately sensing movement, particularly respiration and cessation of respiration.

Another aspect of the invention isto provide an apparatus and method for respiration monitoring that utilises the electrical output from such a piezoceramictransducer assembly, filters and amplifies this signal to activate an audible or visual alarm.

Advantageously another aspect of the invention provides a microphone and the associated processing circuitry, so that an alarm signal for respira toryfailure or a baby's crying can be transmitted to a remote receiver.

Advantageously another application of this invention isto provide a sensor with modified signal processing, suitable for monitoring how long an elderly or geriatric patient has remained in a static seating or lying position.

In a preferred embodiment ofthis invention a pie zoelectrictransducerassembly capable of sensing movements is placed in indirect contact with a patient. The patient's movements are mechanically iinked to the piezoelectric element via a compression linkage. A simple compression type sensor assembly could comprise a sheet of semi-rigid material, with the piezoelectric element mounted on it, and on an opposing surface a suitable spacer; the whole assembly being encapsulated in a PVC enclosure. In operation, the sheet of semi-rigid material would be in contact with a mattress or cushion to act as a 'sounding board'. If the piezoelectric element is mounted in the centre of the sounding board, the downward movement would be concentrated onto the element, producing an optimised maximum signal output.The spacer reduces a loss of sensitivity caused by the edges of the board bending down and acting as a shunt to the movement signal.

The spacer thickness required has been found to depend on the rigidity of the semi-rigid material, and the weightabovethe sensor.

In practice, the sensor described above can be increased in effectiveness in use on uneven or resilient surfaces if a backing load spreader is provided. If it was assumed that an additional external backing would be required for very resilient surfaces such as cot springs orfoam undermattresses,this load spreader could be simply a sheet of thin material such as cardboard, and of the same area as the semirigid base material. Where it is importantto have a sensorthatcan be placed eitherwayup,the backing material can be the same as that used for mounting the piezoelectric element; but in this case it would be desirable to ensure that the two sheets cannot come hard together at any edge or corner after encapsula- tion.This could be achieved by fitting suitableflex- ible spacers at each corner; this sametechnique could be applied to the simple sensorconstruction if required.

The transducer assembly will therefore sense changes in movements and forces acting upon it, for example, respiration movements. The resultant signals can then be amplified and filtered to remove unwanted frequencies, then used to provide an indication that correct movement is taking place, and conversely, and audiolvisual alarm if movement is incorrect.

This invention will now be further described, by way of illustrative and non limiting example, in which: Figure 1 shows a domestic application, involving monitoring the respiration of a baby in a cot; Figure2shows an edge and plan viewofthe pre ferredform ofsensorforthisapplication; Figure 3-is an electrical diagram, showing pre- ferred interface between the sensor and an elec- troniccontrol unit; Figure4 is a block-schematic diagram of a complete monitoring apparatus; and Figure5is an electrical schematic diagram ofthe timer.

Figure 1 shows a cot base 1, comprising a flat base or springs if fitted, placed under a mattress 2, and a sensor assembly 3, placed between the mattress and cot base 1 so that it is roughly below the body of a baby4, the sensor assembly isthen connected to an electronic control unit 6 via a screened cable 5. Alternatively, the sensor assembly 3 could be placed underthe cushion of a bed or chair to detect move mentofa patient or occupant.

Figure 2 shows a typical sensor assembly having a sensor mounting surface in the form of a semi-rigid base portion 7, a substantially planar piezoelectric element 8 having a metal back and a metallized crystal surface with the metal surface of a piezoelectric element 8 glued to the centre of the base portion 7. The base portion may conveniently be made of hardboard, a convenient size has been found to be 170 mm > c-170 mm witha th ickness of 2 m m. A screened cable 9 should preferably have the screen connected to the metal back of the piezoelectric element as this is largerthan a metallised element surface, and will therefore provide a better electrostatic screen. The inner core is connected to the metallised surface.A spacer pad 10 with good sub-sonic frequency trans- fer characteristics is fixed to opposing metallized face of-the sensor. A backing piece 11, which is held in place by PVC encapsulation 12 is of a similararea to the base portion 7, but only~0.7 mm thick. In use, the sensor assembly is placed under a mattress or cushion with the base portion 7 uppermostandthe second backing sheet 11 being in contact with the base ofthe bed or chair. If the second backing sheet 1 is replaced by a sheet of material of similar rigidity to the base portion 7, then there need be no preferred upper or lower-surfaces on the sensor assembly.

In Figure 3, the piezoelectric element 8 of the sensor assembly is connected to the screened cable 9.Atan end ofthecable9, located attheelectronic control unit 6 the screen is connected to system earth, and the inner conductor to the input of an operational-amplifier 16. For optimum signal output, it is advisable to use a high input impedance amplifier such as a field effecttrnnsistor(FET)type,withthe value of a-bias resistor being of value at least 107 ## ohms.A pairofbackto back diodes 13 and 14, pre- vent damage to the amplifierfrom excessive vol tages which can be generated by the sensor if it is compressedabnormally.

In Figure 4, the sensor assembly 3 is electrically connected to a FETpre-amplifier30; this being configured as a low pass filterwith a slow roll off. A pot entiometer32 provides sensitivity adjustment to set the gain to an appropriate level to respond to respiration movement. A second low pass filter34 is a fairly sharp low pass filter, which in conjunction with the pre-amplifier30 will considerably attenuate unwan ted signals, such as heart beats, stray mainsfrequ ency noise and building structural vibrations.This latter point is important as this apparatus may be used upstairs in domestic buildings with predomin entlytimberfloorconstruction andfrequentlyfitted with partition type interior walls; therefore, vibrat- ions from other rooms could easily produce false movement signals that could seriously delay a warn ing from the apparatus. Thissecondfilterwill not completely solve the problem, as some vibrations will be in the frequency range of the wanted signals.

Atypical baby's respiratory rate is in the range of thirty to sixty respirations per minute and a min imum heart rate of 120 beats per minute,forwhich a cut offfrequency of about 1.5 he is thought to be a reasonable compromise. The only signals below 1.5 Hz of interest are instantaneous respiratory move ments, butthese are generally of much greaterampl- itudethan normal respiratory signals and will usu ally still be passed through the filter. The amplified and filtered signal would predominently indicate re spiratory movement, this analogue signal is then applied to a Schmitt trigger circuit 36, to convert itto pulseform suitable for activating logic circuits.This circuit requires a defined analogue signal amplitude to produce an output pulse; therefore, the sensitivity potentiometer 32 can be adjusted for a minimum re spiratory signal to produce an output pulse. This output pulse can then be used to give an indication of each breath via a light emitting diode (Led) 38 and/or an audible clickvia a C/R network 44 an OR gate 48 and a piezo type alarm sounder 50.

Thetimer40 is continuously reset by the pulses from theSchmitttrigger36, thus if respiration ceases, the timer will 'time out' in a selected time, conveniently chosen to be about twenty seconds.

The logic output will then be applied to the OR gate 48andthealarmsounder50.Thealarmcircuitcould then latch permanently on until the apparatus is switched off, but it would be preferable for itto be reset if breathing restarts. In th e case wh ere a numberofmonitorsarein use atthe sametime, such as a hospital, it would be desirable to have an opti onal memory device 52, which is fitted with a Led indicator54; this memory device would be set by the logic outputfrom the timer40 if an alarm is given, and could remain set until the apparatus is switched off The timer output is also applied to an OR gate 82 and then to a remote external sounder 86, via a suit able transmission link 84. This transmission link could be via direct wiring; radio, in which the alarm signal is converted to a radio frequency and is trans mitted by a transmitter device, received by a remote receiver device where it is converted to an alarm signal (audible orvisible); or mains wiring using an RFcarrier,usingsimilarprinciplestothoseindicated above. The timer may be arranged to produce an additional output, this goes via a network 46, OR gate 48 and alarm sounder 50,to give an additional alarm at say, fourteen seconds; this isto reduce the risk of the full alarm being activated bythe baby being picked up and taken away with the apparatus still switched on.

Optionally, a baby alarm may be included in this preferred embodiment and consists of a microphone 60, locatable close to the baby or patient, the electrical signal from this being applied to an audio bandpass amplifier 62, with the sensitivity set by a potentiometer 64. The output of this amplifier is rectified by a diode 66, and the resultant pulsed DC charges a capacitor 68. A resistor 70 allows this charge to leak away when the signal from the amplifier 62 falls. This DC level is now applied to a second Schmitttrigger circuit 72, the function of this being to produce a logic'HI' level when the DC voltage on the capacitor 68 exceeds a defined level.By choosing optimum values for capacitor 68 and resistor70, the logic 'Hl' will be present for the duration that the baby's crying exceeds a present level, set via potentiometer 64; this method ensures that spurious low level gurgles, etc., do not activate the system. The logic 'Hl'from Schmitt trigger 72, will enable oscillator 74, the only requirementofthis being to produce an alarm signal different in character to a respiratory failure alarm. In this preferred embodiment, this is a slow 1 Hz repetitive signal, as distinct from a continuous signal for a respiratory failure. The continuous signal can be more easily used forfurther processing, such as overriding any volume controls on the external sounder.The resultant signal from the oscillator74, is applied to the OR gate 82 and then to the external sounder 86 via the transmission link 84. It should be noted that there is no 'voice' transmission of the baby's crying as in a conventional baby alarm, this is because any RF type voice transmission link can act as a 'bug' if a nearby neighbour has the same system.

Figure 5 shows the timer consists of an oscillatori counter CMOS integrated circuit 90 (type 4060) with the basictiming period setvia a network 92. Normal respiratory movements will cause the counterto be continuously reset via the MR input9l,which will be pulsed 'HI' from the output of Schmitttrigger36 shown in Figure 4. The pre-warning 'beep' activation signal 98, can betakendirectfromanoutputofin- tegrated circuit90 and the main alarm activation signal 100 from the combination oftwo outputs via AND gate 96.

An advantageous circuit feature is that the counter is self-latching when the desired 'time out' count is reached, the diode 94 feeding back the outputfrom ANDgate96tostoptheoscillatorbyclampingthe circuit input, to a 'HI' state in the case shown when using an AND gate; this obviates the expense of a separate latch. if breathing restarts, the counterwill be reset cancelling the alarm.

Claims (15)

1. A movement sensing apparatus having a movement sensor assembly comprising a piezoelectric compression sensor, the sensor mounted on a semi-rigid body, and having electrical connections made to it to enable electrical signals generated by forces exerted on the compression sensor to be transmittable to a control device for monitoring changes of force on the sensor.

2. A control device for use in conjunction with a movement sensor assembly comprising a signal input means connected to an amplifier having input and output means, a low pass filter, a trigger means connected to a timer, the trigger means receiving amplified and filtered signals and using them to reset the timer in synchronism with signals received atthe input means in order to preventthe timer timing out at a predetermined period and then initiating an output warning signal.

3. A sensor assembly according to claim 1, hav ing a second backing sheet contacting a second sur face of the piezoelectric compression sensor and dis posed oppositethesemi-rigid mounting body.

4. A sensor assembly according to either of claim 1 or claim 3, in which the second resilient backing sheet is held spaced apartfrom the compression sensor by a spacer element, and in which the whole sensor assembly is encapsulated in a plastics mat erial.

5. A sensor assembly according to any one of claims 1,3 or 4, in which the sensor is a piezo ceramic material.

6. A sensor assembly according to any one of claims 3 or4, in which the second backing sheet is of resilient material.

7. A sensor assembly according to any one of claims 3,4 or 6, in which the spacer element is integr ally formed with the second backing sheet.

8. A control device according to claim 2, in which the amplifier has a high impedence input and incor porates a low pass filter.

9. A control device according to either claim 2 or claim 8, in which the trigger means is a Schmitt trigger.

10. A control device according to any one of claims 2,8 or9, in which the output warning signal is an audible signal.

11. A control device according to any one of claims 2 or8to 10, inwhichtheoutputwarning signal may be converted into a digital code form and transmitted by a transmitter element to a remote det ectorfor detection and decoding into an alarm signal.

12. A control device according to claim 11, in which the digitial code is transmitted along an elec tric mains system.

13. A control device according to claim 11 in which the transmitter device is a radio frequency transmitter and the remote detector is a radio re ceiverfor receiving radio signals and converting them into an alarm signal.

14. A movement sensing apparatus according to claim 1 having a control device according to claim 2.

15. A movement sensing apparatus substantially as hereinbefore described with reference to the drawings.