Transducer particularly for use in respirator apparatus

Abstract: An electrically conductive diaphragm is sandwiched between a first and second board member. One of said board members has a metallic plate thereon, there being a non conductive spacer having an opening therein separating such board member from the diaphragm. The board members, diaphragm and spacer are joined together to form an integral assembly, the portion of the diaphragm opposite the opening in the spacer being free to move in such opening towards the board member with the metallic plate, motion of the diaphgram away from the spacer being prevented by the other board member. A respiratory gas inlet is provided through the board member with the metallic plate to the opening in the spacer. The diaphragm and metallic plate form a capacitor the capacitance of which is varied in response to the pressure differential between the gas inlet and the atmosphere, the diaphragm only being free to move towards the metallic plate in response to negative pressures (suction) induced during inspiration by a patient.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to capacitive pressure transducers and more to such a transducer for use in respirator apparatus which provides an electrical control signal for controlling the supply of a gas such as oxygen to a patient in response to the inspiratory breathing cycles of such patient. 2.Description of the Related Art Respiratory Apparatus has been developed in the prior art which responds to the breathing cycles of the patient and supplies oxygen in response to such breathing cycles. In such devices, the breathing cycles are generally sensed by a pressure transducer which provides an electrical output in accordance with such breathing. Such an apparatus is described in US-patent no. 4,612,928 issued September 23, 1986 to Tiep et al. Other such devices are described in the various references cited in the Tiep patent. The pressure transducers employed in such prior art devices generally operate in response to both the inspiration and expiration of the patient. As oxygen is to be supplied only in accordance with the inspiration demand of the patient, the expiration input is extraneous and it has been found tends to decrease the accuracy of the control signal for effecting the oxygen supply. This is particularly significant in the case of capacitive transducers in view of the fact that the diaphragm forming one of the capacitive plates of the transducer is driven in response to the expiration of the breathing cycle away from its neutral position and may return late to such position such that it will be seen by the system as an inspiration. GB-A-2149166 discloses an apparatus for monitoring the spontaneous respiration of a patient using a respirator. Two air filled pressure transducers are connected to a differential pressure transducer and arranged such that one transducer gives a negative signal on inspiration, whilst the other transducer gives the positive signal. US-A-4815472 discloses a multi-point pressure sensing catheter system including a capacitive pressure transducer responsive to changes in pressure detected by the diaphragm of the transducer. BRIEF SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a capacitive pressure transducer for a respirator comprising a first capacitor plate and a second capacitor plate arranged to move in a first direction from a datum position in response to a reduction in pressure during the inspiration part of the breathing cycle, whereby the second capacitor plate is restrained to remain substantially in said datum position in response to a pressure increase during the expiration part of the breathing cycle, so as to provide a variation of capacitance only during the inspiration part of the breathing cycle. According to a second aspect of the present invention there is provided a respirator apparatus comprising gas supply means for supplying gas to a patient, a capacitive pressure transducer for sensing breathing cycles of the patient in accordance with the first aspect of the present invention wherein said movement of the second capacitor plate from the datum position during the inspiration part of the breathing cycle is arranged to produce a control signal, the apparatus including control means responsive to said control signal for controlling the supply of said gas to said patient, said control signal varying as a function of pressure variations during the inspiration part of the breathing cycle but remaining at a substantially constant value despite pressure variations during the expiration part of the breathing cycle. The capacitor formed by the transducer is connected in an electrical circuit in series with an oscillatory voltage such that the amplitude of such voltage is varied (modulated) in accordance with the inspiration breathing cycles, this in view of the change of capacitance and capacitive reactance in response to pressure changes induced by the inspiration cycles of the breathing of the patient. It is therefore an object of this invention to provide a more accurate pressure transducer for use in a respirator system. It is another object of this invention to provide a capacitive pressure transducer for use in a respirator system which responds only to the inspiration breathing cycles. Other objects of the invention will become apparent from the following description in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1A is a graphic illustration of the respiratory pressure input generated by breathing; FIG 1B is a graphic illustration of the operation of prior art transducers; FIG 1C is a graphic illustration of the operation of the transducer of the present invention; FIG 2 is an exploded perspective view of a preferred embodiment of the invention; FIG 3 is an exploded elevational view in cross section of the preferred embodiment; FIG 3A is a cross sectional view of the preferred embodiment; FIG 4 is an electrical schematic drawing illustrating the operation of the transducer of the invention; and FIG 5A-5E are a series of waveforms related to the drawing of FIG 4. DETAILED DESCRIPTION OF THE INVENTION Referring now to FIGS 1A-1C, waveforms illustrating the operation of the invention are shown. FIG 1A shows the pressure input to a pressure transducer in response to the breathing of a patient. As can be seen, the variation in pressure is approximately sinusoidal with the inspiration portions of the breathing cycle producing a negative pressure (suction) and the expiration portions of the breathing cycle producing a positive pressure. Referring now to FIG 1B, the variations in capacitance produced in response to the breathing cycles shown in FIG 1A in a prior art pressure transducer are shown. As can be seen, the capacitance of the transducer output increases in response to inspiration and decreases in response to expiration. It is to be noted that this prior art transducer employs an electrically conductive diaphragm responsive to the breathing cycles which is drawn closer to a fixed conductive plate during inspiration and driven away from this fixed plate during expiration, the diaphragm and plate forming the plates of a capacitor. Referring now to FIG 1C, the operation of the invention is illustrated. As can be seen, the device of the invention responds only to the inspiration cycles and not to the expiration cycles. Thus, there is a transducer output reflecting an increase in capacitance and lowering of capacitive reactance only in response to the patient's inspiration. This, as pointed out above, enables the provision of a much more desirable supply of oxygen (or other gas) to the patient. Referring now to FIGS 2, 3 and 3A, a preferred embodiment of the invention is shown. Top board 11 which is of an electrically insulative material has a metal disc 11a formed on the center of the undersurface thereof, this end result typically being achieved by employing a copper clad printed circuit board for board 11 and etching away all but the disk portion. A vent hole 11b is formed through board 11 and disk 11a to maintain the top of diaphragm at atmospheric pressure. Adhesive layer 12 is placed between the undersurface of board 11 and diaphragm 14. Layer 12, which may be formed from no.9491 adhesive available from Minnesota Mining and Manufacturing Corp. has a central circular aperture 12a formed therein which has the same diameter as apertures 18a and 22a. Adhesive layer 12 has the same thickness as disk 11a(typically of the order of .002 inches). Adhesive layer 12 thus acts as both a spacer and a binder for joining the diaphgram 14 to board 11. Spacer member 17 has an aperture 17a formed in the center thereof which is approximately the same diameter as that of disk 11a (typically 1.1 inches). Spacer member 17 which is made of an electrically insulative material such as a polycarbonate plastic typically has a thickness of about .01 inches. Spacer member 17 is bound to the underside of diaphgram 14 by means of adhesive layer 18 which may be of the same material and thickness as adhesive layer 12. Adhesive layer 18 has a circular aperture 18a formed in the central portion thereof which mates with the aperture 17a of spacer 17. Diaphragm 14 is carefully installed between the opposing adhesive layers to assure that it lies flat therebetween. The diaphragm is of a non conductive plastic material suchas polycarbonate with a thickness which is typically about .0005 inches. The top surface of the diaphgram which faces top board 11 is metalized with a material such a nickel to provide one of the plates of the capacitor of the transducer. Bottom board 19 is a printed circuit board similar to top board 11. The copper cladding of this board is etched to form central disk 19a and outer plate 19b with a circular space 19c therebetween. An aperture 20 which runs through board 19 and disk 19a is provided and a pressure inlet tube 25 for receiving the breathing pressure or a patient is installed in this aperture. An electrical connector 21 is attached to board 19, this connector having a pair of electrical leads 21a and 21b which are connected to disk 19a and outer plate 19b, respectively. Board 19 is bound to spacer 17 by means of adhesive layer 22 which is similar to adhesive layers 12 and 18 and has a circular aperture 22a formed in the central portion thereof. It is to be noted that while in the preferred embodiment the device of the invention utilized flat boards, which are joined together by means of adhesive layers, the device can also be fabricated using sheets of electrically insulative material which are joined together by screws, rivets or other fastening means. Referring now to FIGS 4 and 5, an electrical circuit for generating a control signal from the output of the transducer of the invention is shown along with waveforms generated in such circuit. The output of carrier wave generator 30 which may be a square wave generator and typically has a frequency of 200-600 kiloHz is fed to plate 19b of the transducer. A fixed capacitance appears between plate 19b and the opposing portions of diaphragm 14. This capacitance couples the output of the square wave generator to the diaphragm. A variable capacitor is formed between the disk 19a and the opposing central portions of the diaphragm. The central portions of the diaphragm are free to move towards disk 19a in the cavity 17a formed in spacer 17. The diaphragm is prevented from moving from its center position towards plate 11 by metal disk 11a and the opposing portions of adhesive layer 12. Thus, the diaphragm can only move in response to suction pressure as induced during inspiration and is prevented from moving away from its neutral position in response to expiration induced pressure. FIG 5A schematically illustrates the square wave signal generated by generator 30 without there being any pressure input to the transducer to vary the capacitance of the capacitor formed between the diaphragm and disk 19a. FIG 5B schematically illustrates a modulation or variation in amplitude of the square wave signal due to variations in the capacitance of the variable capacitor in response to a breathing inspiration induced input pressure. The individual square waves, as shown in FIG 5D are differentiated to form sharp pulses as shown in FIG 5E. This differentiation is effected by the differentiator formed by the fixed and variable capacitors and resistor 32. This differentiated signal has its negative going portions clipped by diode 31, and this signal is integrated in the integration circuit formed by resistor 33 and capacitor 35 to provide a varying DC output "C" as shown in FIG 5C. In this manner, a DC output signal is generated which is in accordance with the inspiration portions of the breathing cycle only and which has no output in response to expiration. While the invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the scope of the invention being limited only by the terms of the following claims.

Claims: A capacitive pressure transducer for a respirator comprising a first capacitor plate (11) and a second capacitor plate (14) arranged to move in a first direction from a datum position in response to a reduction in pressure during the inspiration part of the breathing cycle, the transducer being characterised in that the second capacitor plate (14) is restrained to remain substantially in said datum position in response to a pressure increase during the expiration part of the breathing cycle, so as to provide a variation of capacitance only during the inspiration part of the breathing cycle. A capacitive pressure transducer according to Claim 1 wherein said first capacitor plate comprises a first board member (11) having a first conductive surface, and said second capacitor plate comprises a diaphragm having a second conductive surface positioned opposite said first conductive surface; means are provided for joining said first board member (11) and said diaphragm (14) together to form an integral assembly, and inlet means (25) are provided for introducing a respiratory pressure signal through said first board member (11) to said diaphragm (14), the diaphragm (14) being restrained from movement away from said first board member (11) during the expiration cycles of said pressure signal and being permitted to move towards said first board member (11) during inspiration cycles of said pressure signal. The capacitive pressure transducer of Claim 2 wherein said means for permitting movement of said diaphragm (14) towards said first board member (11) comprises a spacer board (17) having an opening therein mounted between the first board member (11) and the diaphragm (14). The capacitive pressure transducer of Claim 2 including a second board member (19) arranged such that the diaphragm (14) is sandwiched between the first and second board members (11,14), wherein said means for restraining said diaphragm (14) against movement away from said first board member comprises a surface of said second board member (19) which is mounted flush against said diaphragm (14). The capacitive pressure transducer of Claim 4 wherein the means for joining the board members (11,19), the diaphragm (14), and the spacer board (17) together comprises a first adhesive layer (12) between said first board member (11) and said spacer board (17), a second adhesive layer between said spacer board (17) and said diaphragm (14), and a third adhesive layer (22) between said diaphragm (14) and said second board member (19). The capacitive transducer of Claim 2 wherein said inlet means (25) comprises a tube member extending through said first board member (11) to provide a fluid communication to the diaphragm (14). The capacitive pressure transducer of Claim 3 wherein said first board member (11) has a second conductive surface independent of said first conductive surface on said one flat surface thereof, said second conductive surface forming a fixed capacitor with the portions of said diaphragm (14) which are not opposite the opening in said spacer means (17). Respirator apparatus comprising gas supply means for supplying gas to a patient, a capacitive pressure transducer for sensing breathing cycles of the patient in accordance with any one of Claims 1 to 7 wherein said movement of the second capacitor plate (14) from the datum position during the inspiration part of the breathing cycle is arranged to produce a control signal, the apparatus including control means responsive to said control signal for controlling the supply of said gas to said patient, said control signal varying as a function of pressure variations during the inspiration part of the breathing cycle but remain at a substantially constant value despite pressure variations during the expiration part of the breathing cycle.

Inventors: CHAD THERAPEUTICS INC, CHAD THERAPEUTICS, INC.
Applicants: CHAD THERAPEUTICS INC, CHAD THERAPEUTICS, INC.
Application year: 1992
Main classification: H04R13
IPC codes: A61M16, G01L9