WO2002027277A1

WO2002027277A1 - Apparatus and method for measuring fluid flow

Info

Publication number: WO2002027277A1
Application number: PCT/GB2001/004316
Authority: WO
Inventors: Richard Henry Cooke
Original assignee: Blease Medical Equipment Limited
Priority date: 2000-09-27
Filing date: 2001-09-27
Publication date: 2002-04-04
Also published as: GB0023693D0; GB2368398A

Abstract

Apparatus and corresponding methods for measuring fluid flow are described and claimed. In one aspect, the apparatus includes a flow resistance and pressure sensing means generating a first analogue signal indicative of the pressure difference across the flow resistance. A signal processor receives the first analogue signal and generates a second analogue signal whose magnitude is proportional to the square root of the magnitude of the first. A converter converts the second analogue signal to a digital signal. In preferred embodiments, the signal processor comprises a multiplier and a precision rectifier arranged such that the apparatus measures fluid flow in either direction. A second aspect provides apparatus and a corresponding method in which, during measurement, the pressure sensing means is repeatedly 'short circuited', so that an offset can be adjusted to take into account any drift in the output signal from the pressure sensor.

Description

APPARATUS AND METHOD FOR MEASURING FLUID FLOW

The present invention relates to apparatus and method for measuring fluid flow, and in particular, although not exclusively, to apparatus and methods for measuring gas flow to and from a patient using a ventilator system.

A common method of sensing gas flow (i.e. the volume of gas passing a certain point per unit of time interval, typically measured in litres per minute

(LPM) is to place a resistance in the gas flow path and then measure the pressure difference developed across the resistance as a result of the gas flow through it. The resistance typically takes the form of a tube having a constriction of pre-determined size. When gas flows through the resistance the pressure difference is developed across the constriction. The pressure difference across a simple resistance is, to a first approximation, proportional to the square of the rate of gas flow through it. Piezo-electric transducers have been used to generate electrical signals indicative of pressure differences developed across such resistances. These transducers typically take the form of a wafer or some other substrate on which a Wheatstone bridge array of resistors is screen printed. The wafer is contained in a housing, and seals one half of the housing from another. The pressure from one side of the resistance constriction is applied to one side of the wafer (via suitably arranged tubing) and the pressure at the other side of the constriction is applied to the other side of the wafer. The pressure difference developed across the resistance thus is transferred to the wafer and causes it to deflect/distort which in turn strains the resistors and changes their resistance values.

Piezo-electric transducers such as this are typically arranged so that they produce a differential electrical output signal (an analogue signal) which has a magnitude proportional to the pressure difference across the wafer.

It is clearly desirable to convert the electrical analogue output signal from whichever type of pressure transducer is used into a corresponding digital signal for subsequent digital signal processing. In the past, the analogue signal from the pressure transducer has typically been amplified and then input to an analogue-to-digital (A-D) convertor. An A-D has a pre-determined input range and pre-determined resolution, which is constant over the input range. In the past this has meant that the A-D output has provided a uniform resolution of pressure differences over a range of measurement values, but because of the square law relationship between pressure difference and fluid flow rate, has given a poorer resolution of gas flow rates at the low flow end of the range compared with the resolution at the high flow end.

In fact, the A-D output of previous systems has delivered maximum resolution at peak flow. In ventilator applications, peak flow in the profile of expired gas from a patient is typically in the region of 200 litres per minute (LPM) . In previous systems, resolution of flow rates lower than approximately 3 LPM has not been achieved, and information on the low flow part of the expired gas flow profile has been extrapolated (i.e. estimated) from the measurements at the high flow values. This has, of course, involved making numerous assumptions about the shape of the expired gas flow profile, and is clearly undesirable.

It would clearly be possible to improve resolution of low pressure signals, and hence improve resolution of low flow rates, by using more sophisticated, higher resolution A-D convertors. This will of course increase the complexity and cost of the equipment .

It is an object of the present invention to provide increased resolution measurement of low fluid flow rates without necessitating the use of A-D convertors of improved resolution.

Previously, in ventilator systems the above described resistance-in-the-gas-flow-path measurement technique has only been used to monitor the expired air flow profile. It is an object of preferred embodiments of the present invention to provide improved resolution measurement of low gas flow rates of both expired and inhaled air.

A further problem with gas flow sensing apparatus and methods employing electrical pressure transducers, such as Wheatstone bridge arrays of screen-printed resistors on substrates, has been the drift in the electrical output signal from the transducer, especially as a result of temperature changes of the transducer (i.e. sensor). Such drift is a particular problem with trying to measure small pressure differences, resulting from low gas flow rates.

One previous attempt to solve this problem has been to place the pressure sensing element in a temperature controlled environment, for example an oven having an internal temperature controlled to say 50°c. This clearly adds to the complexity and cost of the gas flow measurement and apparatus leads to increased power consumption, and in ventilator applications in particular has meant that there is a considerable time delay before the output of the pressure transducer can be used (i.e. before it is a reliable indication of flow rate) while the oven reaches operating temperature. Another attempted solution to the drift problem has been to try to measure the temperature of the sensor, and then compensate electrically according to the measured temperature for the resultant drift in output signal. This approach again leads to increased complexity and cost of the apparatus as a whole, and furthermore it is a non-trivial matter to measure the temperature of small, sensitive, low heat capacity sensing elements such as the screen printed resistors of the Wheatstone bridge array type Piezo-electric transducers. The temperatures of the resistors themselves may differ from the substrate temperature, and the currents passing through the array of resistors may themselves cause localised heating.

It is an object of a second aspect of the present invention, and of embodiments of the first aspect, to address the problems of pressure transducer output signal drift in a way which overcomes and/or addresses the problems associated with the prior art.

According to a first aspect of the present invention there is provided apparatus for measuring fluid flow, comprising: resistance means through which the fluid can be arranged to flow; pressure sensing means arranged to sense a pressure difference developed across the resistance means as a result of fluid flow through through the resistance means and to provide a first analogue electrical signal having a magnitude substantially proportional to said pressure difference; signal processing means arranged to receive said first electrical signal and to generate and output a second analogue electrical signal having a magnitude substantially proportional to the square root of the magnitude of the first signal; and an analogue-to-digital converter arranged to receive, as an input, said second signal and to output a corresponding digital signal.

Thus, the A-D converter receives a signal which is substantially proportional to the rate of fluid flow through the resistance means, i.e. a signal which is directly proportional to the quantity being measured, rather than a signal which varies as the square of the fluid flow rate as with the prior art systems. This is advantageous because the A-D convertor has an input range with uniform resolution throughout that range, and by making the input signal proportional to fluid flow, the outputs from the A-D convertor are equally spaced for equally spaced values of fluid flow. The output of the A-D convertor thus provides uniform resolution of fluid flow rates through the resistance means. This is in contrast to prior art systems in which resolution of fluid flow rates was better at high flow rates.

By providing the A-D convertor with a signal whose magnitude is proportional to the rate of fluid flow, the A-D output provides uniform resolution of fluid flow rate through the resistance right down to zero flow rate.

Advantageously, the means by which the second analogue electrical signal is generated from the first signal comprises an operational amplifier (or equivalent device) whose inverting input terminal is arranged to receive the first signal via an input resistor, and whose output is supplied, directly or indirectly, to first and second inputs of a multiplier circuit. The multiplier circuit generates an output signal whose magnitude is proportional to the product of the magnitudes of the signal supplied to its first and second inputs, and this multiplier output signal is fed back to the inverting input of the operational amplifier via a feedback resistor. In embodiments where the pressure sensing means is arranged to sense only fluid flow in one direction, the output of the operational amplifier can be supplied directly to both multiplier inputs, in which case the multiplier output is simply the square of the opamp output. By feeding back this squared signal to the inverting input of the opamp, the opamp output Is therefore constrained to be proportional to the square root of the first electrical signal supplied via the input resistor.

Preferably, the pressure sensing means is arranged to sense fluid flow in both directions through the flow resistance. In such arrangements it is desirable for the signal processing means to output a signal having a magnitude proportional to the square root of the signal from the pressure sensing means, and whose sign reflects the direction of fluid flow. In preferred embodiments this is achieved by supplying the output of the opamp directly to one of the multiplier input terminals, and supplying a rectified version of the opamp output signal to the second multiplier input terminal. Thus, the signal supplied to the second multiplier input terminal always has the same polarity (i.e. a polarity independent of the sign of the first electrical signal, whilst the signal supplied to the first multiplier input terminal reflects the sign of the first electrical signal) . Preferably the rectification is provided by a precision rectifier circuit.

The fluid flow measurement apparatus may preferably include a level shifting circuit arranged to ensure that the second signal supplied to the A-D convertor is centred in the middle of the A-D convertor input range.

Preferably the A-D convertor is a 12 bit convertor and the apparatus is arranged such that the digital signal provides an indication of fluid flow rates in the range 0 to 100 LPM with a resolution of 0.1 LPM.

In preferred embodiments, the fluid whose flow is measured is a gas.

One aspect of the present invention is ventilator apparatus for supplying oxygen and/or other gases to a patient (subject), the ■ventilation apparatus including gas flow measurement apparatus in accordance with the first aspect of the invention.

Preferably, the pressure sensing means comprises a pressure transducer having first and second pressure inputs arranged to receive respective pressure signals from opposite sides (i.e. opposite ends, with respect to fluid flow) of the resistance means, the transducer being arranged to output a differential electrical signal indicative of the difference between the pressure signals at the first and second inputs. The apparatus may further comprise an amplifier arranged to receive the differential electrical signal and to output the first electrical signal, where the amplifier includes means for adjusting the offset of the output first electrical signal. Advantageously, the apparatus further includes means for equalising the pressures at the first and second pressure inputs of the transducer. The pressure equalising means is controllable in response to a control signal from a microprocessor the microprocessor is preferably arranged to receive the digital signal from the A-D convertor and to output the control signal to the pressure equalising means to cause it to equalise the pressures at the transducer inputs, and to output the offset control signal to the amplifier. The microprocessor is preferably arranged to store a value of the digital signal received at a particular time when the pressures at the first and second pressure inputs to the transducer were substantially equal (for example when the flow rate is known to be zero, or at a previous time when the pressure equalisation means was operated) . The microprocessor may be further arranged to output the control signal to the pressure equalisation means periodically, each time for a predetermined time interval, to cause the pressure equalisation means to equalise the pressures. at the transducer inputs, and during these time intervals the microprocessor compares the value of the received digital signal with the previously stored value. According to the results of this comparison, the microprocessor outputs an appropriate offset control signal to adjust the offset of the first electrical signal to compensate for any drift. This drift may, for example, be a result of temperature changes of the pressure transducer. Each time the pressure inputs to the transducer are equalised, the system thus recreates the situation which would pertain if the gas flow through the resistance were truly zero. It does this without affecting the true gas flow through the resistor, and so in ventilator systems does not affect the breathing of the patient. By repeatedly recreating the "zero flow" scenario for the pressure transducer, the system can thus repeatedly correct for drifts simply by adjusting the amplifier offset. It is preferable to use the digital signal from the A-D convertor for making this adjustment, thus by doing so it is possible to correct for drifts originating in components anywhere from the pressure transducer up to the input of the A-D convertor.

According to a second aspect of the present invention there is provided apparatus for measuring fluid flow, comprising: resistance means through which the fluid can be arranged to flow; pressure sensing means arranged to sense a pressure difference developed across the resistance means as a result of fluid flow through the resistance means and to provide a first analogue electrical signal having a magnitude substantially proportional to said pressure difference; and an analogue-to-digital converter arranged to receive, as an input, said first signal or a signal derived from said first signal, and to output a corresponding digital signal, wherein the pressure sensing means comprises pressure transducer means having a first and second pressure inputs arranged to receive respective pressure signals from first and second sides of the resistance means, the transducer being arranged to output a differential electrical signal indicative of the difference between the pressure signals at said first and second inputs, the apparatus further comprising: amplifier means arranged to receive said differential electrical signal and output first electrical signal, the amplifier means including an offset input terminal for receiving an offset control signal to adjust an offset of said first electrical signal; pressure equalising means for equalising the pressures at said first and second pressure inputs in response to a control signal; and microprocessor means arranged to receive said corresponding digital signal and to output said control signal to the pressure equalising means and to output said offset control signal to the amplifier means, the microprocessor means being further arranged to store a value of the digital signal received at a time when the pressures at the first and second pressure inputs are substantially equal, and to output said control signal at a later time and for a predetermined time interval to control the pressure equalisation means to equalise said pressures, and, during said time interval, to compare the value of the received digital signal with said stored value and to set said offset control signal according to the comparison to adjust the offset of the first electrical signal to bring the value of the digital signal received during the predetermined time interval substantially into agreement with the stored value.

Preferably, the amplifier means further comprises a manual offset adjustment for adjusting the offset of the first electrical signal. Thus, initially when gas flow through the resistance is zero, the manual offset can be adjusted so that the first electrical signal gives a true zero reading. Preferably, the digital signal corresponding to this zero position is stored in the microprocessor.

The automatic offset control signal may be supplied to the amplifier means via a D-A convertor. Preferably the pressure equalising means comprises a controllable valve arranged to connect the first and second pressure inputs together. This valve may thus be arranged to provide a short circuit across the first and second pressure inputs such that any pressure difference developed across the resistance means is not conveyed to the pressure transducer. In practice, when the valve is first opened, it takes a little time for the pressures at the first and second pressure inputs of the transducer to equalise, say 20 milliseconds. Preferably, the microprocessor does not begin to compare the stored digital signal with the presently received digital signals until after this settling time.

According to a third aspect of the present invention there is provided apparatus for supplying gaseous anaesthetic to a breathing animal or human subject, comprising: tube means through which the subject can be arranged to breath: gas flow measurement apparatus arranged to measure gas flow to the subject through the tube means and arranged to detect gas flow resulting from an attempt by the subject to inhale through the tube means; gas supply means responsive to the gas flow measurement apparatus detecting said attempt to inhale to supply gaseous anaesthetic and oxygen to the subject via the tube means at a pressure selected to facilitate inhalation of the anaesthetic and oxygen by the subject.

Preferably the gas flow measurement apparatus is arranged to measure gas flow to and from the subject, and advantageously the apparatus may be arranged to measure gas flow rates. in at least the range 0 to 3 LPM, and preferably up to 100 LPM, with 0.1 LPM resolution.

Advantageously the gas flow measurement apparatus will comprise a flow resistance arranged in series with the tube, and pressure measurement apparatus arranged to measure a pressure difference developed across the flow resistance. Advantageously, the flow resistance (such as a tube incorporating a constriction) is arranged close to the subject, for example connected to the tracheal tube. According to a fourth aspect of the present invention there is provided a method for measuring fluid flow, comprising the steps of: arranging the fluid to flow through resistance means; sensing a pressure difference developed across the resistance means as a result of fluid flow through the resistance means and providing a first analogue electrical signal having a magnitude substantially proportional to said pressure difference; receiving said first electrical signal and generating and outputting a second analogue electrical signal having a magnitude substantially proportional to the square root of the magnitude of the first signal; receiving and converting said second signal to a corresponding digital signal; and outputting the digital signal. Advantageously, the step of generating the second signal comprises inputting the first signal via an input resistor the inverting input of an operational amplifier, supplying an output signal from the operational amplifier directly or indirectly to the first and second input terminals of a signal multiplier, supplying an output signal from the signal multiplier via feedback resistor to the inverting input of the operational amplifier. This ties the opamp output to be proportional to the square root of the first electrical signal.

Preferably the method comprises the step of sensing pressure differences developed across the resistance in both directions, and the step of supplying the opamp output to the multiplier comprises the step of rectifying the opamp output signal for supply to the second multiplier input. Preferably, the step of sensing pressure comprises the steps of providing a pressure transducer having first and second pressure inputs arranged to receive respective pressure signals from opposite ends of the flow resistance, the pressure transducer generating a differential electrical signal indicative of the difference between pressure signals at the first and second inputs, and amplifying the differential electrical signal to produce said first electrical signal, the method further comprising the steps of: monitoring the first electrical signal, directly or indirectly; storing a value indicative of the first electrical signal at a first time, when the pressure signals at the first and second pressure inputs are substantially equal; short-circuiting the pressure inputs at a second, later, time to equalise the pressure signals received at the pressure inputs; composing a quantity indicative of said first electrical signal while said pressure inputs are short circuited with said stored value; and according to the result of the comparing step, adjusting an offset of the first electrical signal to compensate for drift between the first and second times .

Accordingly to a fifth aspect of the present invention there is provided a method for measuring fluid flow, comprising the steps of: arranging the fluid to flow through resistance means; sensing a pressure difference developed across the resistance as a result of the flow and outputting a first electrical signal indicative of the sensed pressure difference, wherein the step of sensing pressure comprises the steps of providing a pressure transducer having first and second pressure inputs arranged to receive respective pressure signals from opposite ends of the flow resistance, the pressure transducer generating a differential electrical signal indicative of the difference between pressure signals at the first and second inputs, and amplifying the differential electrical signal to produce said first electrical signal, the method further comprising the steps of: monitoring the first electrical signal, directly or indirectly; storing a value indicative of the first electrical signal at a first time, when the pressure signals at the first and second pressure inputs are substantially equal; short-circuiting the pressure inputs at a second, later, time to equalise the pressure signals received at the pressure inputs; composing a quantity indicative of said first electrical signal while said pressure inputs are short circuited with said stored value; and according to the result of the comparing step, adjusting an offset of the first electrical signal to compensate for drift between the first and second times.

According to a sixth aspect of the present invention there is provided a method of supplying gaseous anaesthetic to a breathing animal or human subject, comprising the steps of: detecting an attempt by the subject to inhale by measuring gas flow towards the patient in a tube system through which the anaesthetic is supplied; and in response to detecting said attempt, providing gaseous anaesthetic and oxygen to the subject via the tube system at a pressure selected to facilitate inhalation of the gaseous mixture by the subject.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram of gas flow apparatus in a ventilator system in accordance with a first embodiment;

Fig. 2 is a partial circuit diagram showing parts of the pressure sensor and pressure signal processing circuits of the first embodiment; and Fig. 3 is a schematic diagram of a pressure support ventilation system in accordance with a second embodiment .

Turning now to Fig. 1, this diagram is a schematic of a ventilator system embodying the present invention. Gas flow to and from a patient is conveyed through tubes Tl and T2. A pneumatic resistance 1 is connected in series with tubes Tl and T2 such that the gas flow passes through the resistance 1 also. The resistance 1 is in the form of a tube having end portions 11 of a first diameter, and a central constriction forming a region 12 of reduced cross section. Gas flow through the resistance 1 causes a pressure difference to be developed across the constriction (P1-P2) . Pressure sensing apparatus is arranged to sense the pressure developed across the pneumatic resistance and to output an electrical signal 29 having a magnitude proportional to the developed pressure difference. This pressure sensing apparatus comprises a Piezo-electric transducer connected via tubes 22 and 23 to communicate with the regions 11 on either side of the constriction in the pneumatic resistance. Thus, via the tubes 22 and 23 the pressures Pi and P2 on either side of the constriction are communicated to either side of the Piezo-electric transducer 21. Typical dimensions for the end portion 11 of the resistance and for the tubes 22 and 23 are 22mm and 3mm respectively.

The pressure difference P1-P2 developed across the Piezo-electric transducer causes it to generate a differential electrical output signal (E1-E2) which is supplied to an amplifier- 2 . This amplifier amplifies the differential input signal and outputs the electrical signal 29 having a magnitude proportion to the pressure difference P1-P2. The pressure sensing apparatus further includes a manual offset adjustment 25 which controls the offset applied to the amplifier 24. This enables manual adjustment of the amplifier output voltage. In normal use, the manual offset adjustment 25 is adjusted to give an amplifier output signal 29 of zero when no gas is flowing through the pneumatic resistance.

The output signal 29 from the amplifier is input to an analogue signal processing circuit 3 which generates an output signal 39 having a magnitude proportional to the square root of the magnitude of the input signal 29 from the amplifier 24. The polarity of the output signal 39 is arranged to be dependent on the polarity of the input signal 29. In this embodiment the differential electrical output signal from the Piezo-electric transducer may be positive or negative, depending on the direction of gas flow through the pneumatic resistance, and hence on the direction of the pressure difference across the constriction. The amplifier output 29 may thus swing positive or negative, and the output signal from the analogue signal processing circuit 3 reflects this. The output signal 39 is input to an A-D convertor 4 which generates a corresponding digital output signal 49. The digital output signal 49 is input to a microprocessor 5. The microprocessor 5 is arranged to store the value of the digital input signal 49 when the offset adjustment 25 has been set to give nominal zero amplifier output 29 for zero gas flow through the resistance. The microprocessor is arranged to generate an output signal 58 with which it controls a valve means 7 connected in parallel with the Piezo-electric transducer 21 across the tubes 22 and 23. In response to the control signal 58, the valve means 7 can open, and via tubes 71 and 72 provides a pneumatic short circuit across the Piezo-electric transducer 21. The microprocessor 5 is arranged to open the controllable valve 7 at regular intervals during the monitoring of the gas flow to and from the patient, each time for a pre-determined time interval. For example, the microprocessor 5 may control the valve 7 to open once every minute, each time for a duration of 100 milliseconds. Even though there may be a pressure difference P1-P2 developed across the resistance as a result of gas flow, when the valve 7 opens the pressure difference seen across the Piezo-electric transducer falls to zero because of the shorting action of tubes 71 and 72 over a period of approximately 5 milliseconds. The effect of periodically opening the valve 7 is thus to periodically set the pressure across the Piezo- electric transducer to zero, i.e. to simulate the conditions when gas flow through the resistance was truly zero. During the period for which the valve 7 is open, the microprocessor 5 is arranged to monitor the output signal 49 from the A-D convertor 4 and repeatedly compare that signal with the stored value mentioned above (i.e. the value stored for the condition when the offset adjustment was set to give nominal zero output from the amplifier 24. According to the results of this comparison the microprocessor 5 outputs a further control signal 59 to a D-A convertor 6 which in turn outputs a corresponding analogue offset control signal 69 to the amplifier. This signal is arranged to further and automatically adjust the amplifier offset to bring its output 29 closer to zero. The microprocessor 5 repeats this comparison and control step repeatedly throughout the period for which the valve 7 is open. Thus, when the valve 7 closes, the amplifier offset has been automatically adjusted to reset the amplifier output 29 to zero. Thus, any temperature fluctuations of the Piezoelectric transducer which cause drifts in the differential electrical signal E1-E2 applied to the amplifier 24 are automatically compensated.

Referring now to Fig. 2, this shows in schematic form part of the electrical circuit of the pressure sensing means 2 and analogue signal processing means 3 of the ventilator system shown in Fig. 1. The pressure sensing means 2 comprises a Piezo-electric transducer 21 in the form of a Wheatstone bridge array of four resistors, screen printed on a distortible substrate. A reference voltage of 4 volts is applied across the array of resistors, and as pressure differences cause the substrate to distort differential electrical signals El and E2 are generated and supplied to amplification means comprising operational amplifiers IC5A and IC5B. The manual offset voltage control means 25 is provided by a variable resistor RV2, connected via resistor R3 to the inverting input of op-amp IC5A. The automatic offset control signal 69 from the D-A convertor 6 is applied to the inverting input of IC5A via a further resistor.

Thus, the operational amplifiers IC5A and IC5B provide amplification means for the electrical signal developed by the Piezo-electric transducer, and the output signal 29 from the amplifier means can swing both positive and negative, in the range +5 volts to -5 volts depending on the direction of the pressure difference causing distortion of the transducer substrate.

This signal 29 from the pressure sensing means is applied to one end of an input resistor R27 of the analogue signal processing means 3. The other end of the input resistor R27 is connected to the inverting input terminal of an operational amplifier 31 (IC7B) . This op-amp 31 has a non-inverting input connected to ground, and an output terminal 7. The analogue signal processing means 3 further comprises a multiplier circuit 32 having a first input terminal Yl directly connected to the op-amp 31 output. The multiplier circuit 32 has a second input terminal Xl . This second input terminal is connected to the op-amp 31 output via precision rectifier circuit 33. The rectifier circuit 33 is connected to the opamp 31 output to receive the signal output therefrom, rectifies this signal, and outputs the rectified signal to the second input XI of the multiplier circuit 32. Thus, depending on the sign of the pressure signal 29 applied to the input resistor R27, the voltage at the op-amp 31 output may be negative or positive, and hence the signal input to the first input terminal of the multiplier circuit 32 can also be positive or negative. The rectifier circuit 33, however, ensures that the signal supplied to the second input terminal XI of the multiplier 32 has a fixed polarity (i.e. a polarity which is independent of the polarity of the pressure signal 29) . The multiplier circuit 32 has an output terminal W which is connected, via a feedback resistor R28 to the inverting input terminal of the op-amp 31 to provide negative feedback thereto. The signal generated at output terminal W has a magnitude which is proportional to the product of the magnitudes of the signals received at the first and second inputs (Yl and XI) of the multiplier circuit 32, and the sign of the signal generated at output terminal W simply corresponds to the sign of the signal input to the first input terminal Yl, because the signal applied to the second input terminal XI is always positive as a result of the rectification circuit 33. By arranging the multiplier circuit 32 to provide negative feedback to the op-amp 31 in this way, the signal at the op-amp 31 output has a magnitude which is proportional to the square root of the magnitude of the pressure signal 29, and a polarity which is opposite that of the pressure signal 29.

The output signal from the op-amp 31 is connected to a further resistor R37, which in turn is connected to the inverting input of another op-amp IC7A. IC7A is part of a level shifting circuit 34 which provides an output signal 39 to the A-D convertor 4, the output signal 39 being centred at approximately 2H volts (i.e. when the pressure signal 29 is zero, the output signal 39 is approximately 2.5 volts. In this embodiment, 2.5 volts corresponds to the middle of the input range of the A-D convertor. In other embodiments, using different A-D convertors, other level shifts may be appropriate, and it will be apparent to the skilled person that the output signal from the op-amp 31 may be inverted and level shifted as appropriate to suit the particular circumstances/requirements .

Moving on now to Fig. 3, this figure shows in schematic form a pressure support ventilation system in accordance with a second embodiment of the present invention. The system comprises a ventilator which includes pressure sensing means and analogue signal processing means similar to that described above with references to figs. 1 and 2. The system has a flow sensor which includes the pneumatic resistance, and tubes 22 and 23 communicate the pressure difference developed across the resistance to the pressure transducer contained in the ventilator. The ventilator contains a 12bit A-D convertor and, as a result of the analogue signal processing circuit, is. able to provide digital output signals corresponding to a range of flow rates from -100 to +100LPM with a resolution of 0.1LPM. Thus, the apparatus provides very sensitive measurement of gas flow to and from the patient, and can be used to measure, for example, differences between the amount of gas supplied to the patient and the amount of gas exhaled. Thus, leaks in the ventilator system can be detected (such measurements were not previously possible because of the poor resolution of low flow rates) . The apparatus is able to detect the very small gas flows associated with the patient attempting to inhale gas through the flow sensor, and when this condition is detected the ventilator can assist (i.e. support) the patient's breathing by triggering, via the vent line, the anaesthetic machine or air supply to provide gas to the patient. The sensitive measurement of low gas flow rates provided by embodiments of the present invention enables the pressure support ventilation system to be implemented with more heavily anaesthetised patients, and patients who are experiencing more difficulties in breathing, than could be supported with previous systems.

Returning now to figs. 1 and 2, a common method of sensing gas flow is to have a resistance in the gas path and measure the differential pressure across it. The flow through a simple resistance has a square law relationship to the pressure developed across the resistor. In numbers this would mean that: a flow of 1 may equal a pressure of 1; a flow of 2 would equal a pressure of 4; and a flow of 10 would equal a pressure of 100 and so on.

In the application to which the first embodiment is directed this relationship was extended to 100 LPM max (i.e. the application necessitated measurement of flow rates up to this value) and a resolution of 0.1 LPM was required. The application also required measurement in both directions, measurement of inspired and expired gas- from a living being i.e. breathing.

The derived pressure (i.e. the pressure developed across the resistance as a result of flow) is applied to a Piezo-electric transducer to create an electrical signal proportional to the pressure. The signal then needs to be read into a microprocessor via an Analogue to Digital (A-D) converter. The fact that ± 100 LPM at 0.1 LPM resolution is required doubles the effective range of the A-D converter. Reading the output of the transducer directly requires a 2,000,000 : 1 range an A-D converter with a

22 bit resolution would be required if 0.1 LPM resolution of ± 100 LPM is to be achieved, because of the square law relationship between pressure and flow. This is uneconomic and is difficult to handle arithmetically in software.

The dynamic range of the flow is only 2000:1 and cost effective converters of 12 bit resolution

(4096:1) are readily available which is twice the flow range. If the electrical signal from the transducer can be processed in some way that would give a square root function the problem is solved.

The J function is an electronic form in the first embodiment is performed with opamps and a four quadrant multiplier. Unfortunately the application here with flow being in both directions implies a need for the J of a negative number, it is impossible to take a J of a negative number.

The solution implemented in the first embodiment is to use a precision rectifier to effectively give the absolute number by changing the sign of the 4 quadrant multiplier.

In the design of the first embodiment a second problem was encountered. This problem in part is exacerbated by the solution to the first (the first being more accurate resolution of low flow rates using existing A-D hardware) . ^• Having produced a suitable bi-directional v⁷ function applied around the instrument amplifier following the transducer, it became apparent that any zero drift or offset in the transducer output would be processed as a flow. Small errors into the J circuit have large effects because of the effective difference in dynamic range. The input dynamic range is as above, 2,000,000:1 as opposed to the output range of 2000:1. The offset becomes indistinguishable from a standing flow. This has two effects, in one direction the offset adds to the flow in the other it subtracts compounding the error.

The Piezo transducers selected were ostensibly corrected for the effects of temperature. On close investigation it transpired that the correction was only applicable to gain and not zero output. This problem is endemic in all transducers made in the same way. It will apply to strain gauges equally. The temperature of the transducer is obviously related to ambient, it also has a self heating effect after switch on. It is the self heating that was most drastic as it takes about 30 minutes to achieve equilibrium. To mitigate the offset when equilibrium has been achieved, it was arranged for the circuit to contain an offset null potentiometer.

A second Offset null device was added in the form of a Digital to Analogue (D-A) converter. This D-A output was summed with the appropriate gain factor into the amplifier such that it could change the output of the A-D over the range commensurate with the expected temperature range and thermal drift.

To implement this solution the unit is calibrated following a warm up period of 1 hour after achieving thermal equilibrium.

Calibration is as follows: Adjust the offset null potentiometer to give zero.

Store the output of the A-D convertor in memory. Use (D-A) converter output to re-adjust the offset back to this stored value every time the flow is known to be zero.

In practice a pneumatic short circuit under control of the processor is applied to the transducer to make the flow appear to be zero, (the short circuit equalises the pressures at the pressure transducer inputs) the processor then varies the output of the D-A to make the A-D output equal the stored value.

This process completely stabilises the flow measurement system against the expected thermal drift. Turning again now to fig. 3, a novel application of the gas flow sensing apparatus described above is made possible as a result of the accuracy achieved above .

The applicant makes anaesthesia systems for use in surgery, as do others.

Hitherto, ventilation of patients under anaesthesia has been either : Spontaneous breathing, which implies a low level of anaesthesia, or Forced mechanical ventilation of deeply anaesthetised patients (i.e. patients unable to breath unassisted) . In other forms of patient ventilation, such as Intensive Care, a mode of assisting the lightly sedated patient to breach is invoked. This mode is known as Pressure Support Ventilation. To apply this mode it is necessary to sense when the patient tries to breath and add some mechanical help. This overcomes the increased difficulty of breathing through the incidental resistance of the associated tubing. In modern anaesthesia much more obstruction to spontaneous breathing is encountered because of the more complex breathing circuitry. This implies that only the most healthy patients can be allowed to spontaneously breath and be anaesthetised to the level whereby pain is obviated but automatic responses are not suppressed. This situation is desirable in modern practice because recovery is much speeded by light anaesthetic doses.

Having produced the above described sensitive flow measuring system it is possible to sense the very small movement of gas towards the patient as an attempt to inhale takes place. In an embodiment of the invention use is made of this, to apply the Pressure Support Mode to an anaesthetic ventilator. This has been achieved during anaesthesia and successfully tested clinically.

To the applicant's knowledge, this is the first product to offer this mode to the anaesthetist.

It will be appreciated that certain preferred embodiments of the invention provide fluid flow measurement which is bi-directional. In previous arrangements, reversing the flow of the fluid producing the pressure signal resulted in the mathematical equivalent of trying to take the square root of a negative number. In preferred embodiments of the invention, a unity gain precision rectifier is inserted in one input path of the multiplier. This allows the square root to take place on flow in either direction.

Claims

Claims :

1. Apparatus for measuring fluid flow, comprising: resistance means through which the fluid can be arranged to flow; pressure sensing means arranged to sense a pressure difference developed across the resistance means as a result of fluid flow through through the resistance means and to provide a first analogue electrical signal having a magnitude substantially proportional to said pressure difference; signal processing means arranged to receive said first electrical signal and to generate and output a second analogue electrical signal having a magnitude substantially proportional to the square root of the magnitude of the first signal; and an analogue-to-digital converter arranged to receive, as an input, said second signal and to output a corresponding digital signal.

2. Apparatus in accordance with claim 1, wherein the signal processing means comprises: operational amplifier means having an inverting input terminal and an output terminal, the inverting input terminal being connected to a first end of an input resistor, the input resistor having a second end connected to receive said first electrical signal; and signal multiplier means having first and second input terminals and an output terminal, the first and second input terminals each being connected, directly or indirectly, to the output terminal of the operational amplifier means to receive a respective signal therefrom, and the signal multiplier means being arranged to generate at its output terminal a signal having a magnitude proportional to the product of the respective magnitudes of the signals received at the first and second input terminals, the output terminal of the signal multiplier means being coupled via a feedback resistor to the inverting input terminal to provide negative feedback thereto, such that the operational amplifier means generates at its output terminal a signal having a magnitude proportional to the square root of the magnitude of the first electrical signal.

3. Apparatus in accordance with claim 1 or claim 2, wherein the pressure sensing means is adapted to sense pressure differences developed across the resistance means as a result of fluid flow through the resistance means in a first direction and in a second direction, opposite said first direction, and the first electrical signal has a polarity dependent on the direction of the fluid flow.

4. Apparatus in accordance with claim 3, as dependent on claim 2, further comprising rectification means connected to the output terminal of the operational amplifier means to receive a signal output therefrom and being arranged to generate a corresponding rectified signal having a magnitude proportional to, and a polarity independent of, the magnitude and polarity, respectively, of the signal received from the operational amplifier means output terminal, and wherein said first input terminal is connected directly to the operational amplifier means output terminal to receive the signal therefrom, and said second input terminal is arranged to receive said rectified signal such that the second input terminal is indirectly coupled to the operational amplifier means output terminal via the rectification means, whereby the signal received at the first input terminal has a polarity dependent on the polarity of the first electrical signal and the signal received at the second input terminal has a polarity independent of the polarity of the first electrical signal.

5. Apparatus in accordance with claim 2 or claim 4, or claim 3 as dependent on claim 2, wherein the analogue-to-digital converter has a predetermined input range and the apparatus further comprises: level shifting means arranged to receive a signal from the output terminal of the operational amplifier means and to output a corresponding level-shifted signal to the A-D converter, centred in said range.

6. Apparatus in accordance with any preceding claim, wherein the pressure sensing means comprises pressure transducer means having first and second pressure inputs arranged to receive respective pressure signals from first and second sides of the resistance means, the transducer being arranged to output a differential electrical signal indicative of the difference between the pressure signals at said first and second inputs, the apparatus further comprising: amplifier means arranged to receive said differential electrical signal and output the first electrical signal, the amplifier means including an offset input terminal for receiving an offset control signal to adjust an offset of said first electrical signal; pressure equalising means for equalising the pressures at said first and second pressure inputs in response to a control signal; and microprocessor means arranged to receive said corresponding digital signal and to output said control signal to the pressure equalising means and to output said offset control signal to the amplifier means, the microprocessor means being further arranged to store a value of the digital signal received at a time when the pressures at the first and second pressure inputs are substantially equal, and to output said control signal at a later time and for a predetermined time interval to control the pressure equalisation means to equalise said pressures, and, during said time interval, to compare the value of the received digital signal with said stored value and to set said offset control signal according to the comparison to adjust the offset of the first electrical signal to bring the value of the digital signal received during the predetermined time interval substantially into agreement with the stored value.

7. Apparatus for measuring fluid flow, comprising: resistance means through which the fluid can be arranged to flow; pressure sensing means arranged to sense a pressure difference developed across the resistance means as a result of fluid flow through through the resistance means and to provide a first analogue electrical signal having a magnitude substantially proportional to said pressure difference; and an analogue-to-digital converter arranged to receive, as an input, said first signal or a signal derived from said first signal, and to output a corresponding digital signal, wherein the pressure sensing means comprises pressure transducer means having first and second pressure inputs arranged to receive respective pressure signals from first and second sides of the resistance means, the transducer being arranged to output a differential electrical signal indicative of the difference between the pressure signals at said first and second inputs, the apparatus further comprising: amplifier means arranged to receive said differential electrical signal and output said first electrical signal, the amplifier means including an offset input terminal for receiving an offset control signal to adjust an offset of said first electrical signal; pressure equalising means for equalising the pressures at said first and second pressure inputs in response to a control signal; and microprocessor means arranged to receive said corresponding digital signal and to output said control signal to the pressure equalising means and to output said offset control signal to the amplifier means, the microprocessor means being further arranged to store a value of the digital signal received at a time when the pressures at the first and second pressure inputs are substantially equal, and to output said control signal at a later time and for a predetermined time interval to control the pressure equalisation means to equalise said pressures, and, during said time interval, to compare the value of the received digital signal with said stored value and to set said offset control signal according to the comparison to adjust the offset of the first electrical signal to bring the value of the digital signal received during the predetermined time interval substantially into agreement with the stored value.

8. Apparatus in accordance with claim 6 or claim 7 wherein the amplifier means further comprises a manual offset adjustment for adjusting the offset of the first electrical signal.

9. Apparatus in accordance with any one of claims 6 to 8 wherein the microprocessor means further comprises a D-A converter arranged to receive a digital offset control signal and to output said offset control signal to the amplifier means.

10. Apparatus in accordance with any of claims 6 to 9 wherein the pressure equalizing means comprises a controllable valve arranged to connect said first and second pressure inputs.

11. Apparatus in accordance with any one of claims 6 to 10 wherein the pressure equalizing means is arranged to provide a short circuit across the first and second pressure inputs such that a pressure difference developed across the resistance means is not conveyed to the pressure transducer means.

12. Apparatus in accordance with any one of claims 6 to 11 wherein the microprocessor means is arranged to monitor said corresponding digital signal and to periodically control the pressure equalising means to equalise said pressures during the monitoring, each time for said predetermined time interval, and to control the offset of the first electrical signal, such that the apparatus periodically and automatically compensates for any drift in the differential electrical signal form the transducer and/or for drift in the amplifier means output.

13. Apparatus in accordance with any preceding claim wherein the A-D converter is a 12 bit converter and the apparatus is arranged such that the digital signal provides an indication of fluid flow rates in the range 0-100 LPM with a resolution of 0.1 LPM.

14. Apparatus in accordance with any preceding claim wherein the fluid is a gas.

15. Ventilation apparatus for supplying oxygen to a breathing subject, the ventilation apparatus comprising apparatus in accordance with any preceding claim arranged to measure gas flow to and/or from the subject during inhalation and/or exhalation.

16. Apparatus for supplying gaseous anaesthetic to a breathing animal or human subject, comprising: tube means through which the subject can be arranged to breath: gas flow measurement apparatus arranged to measure gas flow to the subject through the tube means and arranged to detect gas flow resulting from an attempt by the subject to inhale through the tube means; gas supply means responsive to the gas flow measurement apparatus detecting said attempt to inhale to supply gaseous anaesthetic and oxygen to the subject via the tube means at a pressure selected to facilitate inhalation of the anaesthetic and oxygen by the subject.

17. Apparatus in accordance with claim 16, wherein the gas flow measurement apparatus is arranged to measure gas flow to and from the subject.

18. Apparatus in accordance with claim 16 or claim 17 wherein the gas flow measurement apparatus is arranged to measure gas flow rates at least in the range 0 to 3 LPM and preferably up to 100 LPM with 0.1 LPM resolution.

19. Apparatus in accordance with any one of claims 16 to 18 wherein the gas flow measurement apparatus comprises a flow resistance arranged in series with the tube means, and pressure measurement apparatus arranged to measure a pressure difference developed across the flow resistance as a result of gas flow through it.

20. Apparatus in accordance with any one of claims 16 to 19, and comprising fluid flow measuring apparatus in accordance with any one of claims 1 to 14.

21. A method for measuring fluid flow, comprising the steps of: arranging the fluid to flow through resistance means; sensing a pressure difference developed across the resistance means as a result of fluid flow through the resistance means and providing a first analogue electrical signal having a magnitude substantially proportional to said pressure difference; receiving said first electrical signal and generating and outputting a second analogue electrical signal having a magnitude substantially proportional to the square root of the magnitude of the first signal; receiving and converting said second signal to a corresponding digital signal; and outputting the digital signal.

22. A method in accordance with claim 21, wherein the step of generating the second signal comprises: inputting the first signal via an input resistor to the inverting input of an operational amplifier means; supplying an output signal from the operational amplifier means directly or indirectly to first and second input terminals of a signal multiplier means; supplying an output signal from the signal multiplier means via a feedback resistor to the inverting input of the operational amplifier means.

23. A method in accordance with claim 21 or claim 22, wherein the step of sensing a pressure difference comprises sensing pressure differences developed across the resistance means as a result of fluid flow through the resistance means in a first direction and in a second direction, opposite said first direction, and the first electrical signal has a polarity dependent on the direction of the fluid flow.

24. A method in accordance with claim 22 or claim 23, wherein the step of supplying an output signal from the operational amplifier means comprises the steps of: supplying the output signal directly to the first multiplier input terminal; rectifying the output signal; and supplying the rectified signal to the second multiplier input terminal.

25. A method in accordance with any one of claims 21 to 24, further comprising the step of shifting the level of the second signal to centre the second signal in the input range of an A-D converter.

26. A method in accordance with any one of claims 21 to 25, wherein the steps of sensing pressure comprises the steps of providing a pressure transducer having first and second pressure inputs arranged to receive respective pressure signals from opposite ends of the flow resistance, the pressure transducer generating a differential electrical signal indicative of the difference between pressure signals at the first and second inputs, and amplifying the differential electrical signal to produce said first electrical signal, the method further comprising the steps of: monitoring the first electrical signal, directly or indirectly; storing a value indicative of the first electrical signal at a first time, when the pressure signals at the first and second pressure inputs are substantially equal; short-circuiting the pressure inputs at a second, later time, to equalise the pressure signals received at the pressure inputs; comparing a quantity indicative of said first electrical signal while said pressure inputs are short circuited with said stored value; and according to the result of the comparing step, adjusting an offset of the first electrical signal to compensate for drift between the first and second times .

27. A method for measuring fluid flow, comprising the steps of: arranging the fluid to flow through resistance means; sensing a pressure difference developed across the resistance as a result of the flow and outputting a first electrical signal indicative of the sensed pressure difference, wherein the step of sensing pressure comprises the steps of providing a pressure transducer having first and second pressure inputs arranged to receive respective pressure signals from opposite ends of the flow resistance, the pressure transducer generating a differential electrical signal indicative of the difference between pressure signals at the first and second inputs, and amplifying the differential electrical signal to produce said first electrical signal, the method further comprising the steps of: monitoring the first electrical signal, directly or indirectly; storing a value indicative of the first electrical signal at a first time, when the pressure signals at the first and second pressure inputs are substantially equal; short-circuiting the pressure inputs at a second, later time, to equalise the pressure signals received at the pressure inputs;. comparing a quantity indicative of said first electrical signal while said pressure inputs are short circuited with said stored value; and according to the result of the comparing step, adjusting an offset of the first electrical signal to compensate for drift between the first and second times .

28. A method of ventilating a breathing subject comprising a gas flow measurement method in accordance with any one of claims 21 to 27.

29. A method of supplying gaseous anaesthetic to a breathing animal or human subject, comprising the steps of: detecting an attempt by the subject to inhale, by measuring gas flow towards the subject in a tube system through which the anaesthetic is supplied; and in response to detecting said attempt, providing gaseous anaesthetic and oxygen to the subject via the tube system at a pressure selected to facilitate inhalation of the gaseous mixture by the subject.

30. A method in accordance with claim 29, and including a measurement method in accordance with any one of claims 21 to 27.

31. Apparatus for measuring fluid flow substantially as hereinbefore described with reference to the accompanying drawings.

32. Apparatus for supplying gaseous anaesthetic substantially as hereinbefore described with reference to the accompanying drawings.

33. Ventilator apparatus substantially as hereinbefore described with reference to the accompanying drawings.

34. A method for measuring fluid flow substantially as hereinbefore described with reference to the accompanying drawings.

35. A gaseous anaesthetic supply method substantially as hereinbefore described with reference to the accompanying drawings .

36. A method of ventilating a subject substantially as hereinbefore described with reference to the accompanying drawings .

37. Apparatus in accordance with any one of claims 1 to 6 wherein the pressure sensing means is arranged to sense a pressure difference developed by fluid flow in either direction through the resistance means, the first analogue electrical signal having a magnitude substantially proportional to the pressure difference and having a polarity dependent on the direction of fluid flow, and wherein the signal processing means comprises a rectifier and a multiplier arranged to receive said first analogue electrical signal and generate an analogue signal having a magnitude substantially proportional to the square root of the magnitude of the first signal and a polarity dependent on the polarity of the first signal.

38. Apparatus in accordance with claim 37, wherein the rectifier is a precision rectifier.

39. Apparatus in accordance with claim 37 or claim 38 wherein the rectifier has unity gain.