CN118266879A - Micro differential pressure sensor type biological signal bed - Google Patents
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Abstract
The invention discloses a micro differential pressure sensor type biological signal bed, which comprises a signal acquisition pad, a flexible connecting pipe and a micro differential pressure sensor, wherein the flexible connecting pipe is arranged on the signal acquisition pad; the signal acquisition pad covers the upper surface of the bedstead, the flexible connecting pipe stretches between the bedstead and the signal acquisition pad, one end of the flexible connecting pipe is closed, the other end of the flexible connecting pipe is freely arranged, and the free end of the flexible connecting pipe is connected with the micro differential pressure sensor. The advantages are that: the micro differential pressure sensor detects heart rate, respiration, limb movement and cough action of a subject by a novel noninvasive air pressure vital sign measuring method, and the problems of long saturation recovery time, narrow dynamic range, high noise level, low signal-to-noise ratio and the like caused by adopting a high-sensitivity sensor in the prior art are avoided.
Description
Technical Field
The invention relates to the technical field of biological signal acquisition, in particular to a micro differential pressure sensor type biological signal bed.
Background
In non-medical institutions such as families and hotels of medical centers, vital signs such as heart rate, respiration, turn-over, cough and the like of bedridden patients are monitored remotely, so that medical professionals can find emergency patients conveniently. The equipment used in such telemedicine must be automated and the patient only needs to lie in the bed.
Various sensing methods and systems have been proposed for such bed sensing, including non-invasive monitoring of breathing or apnea, heart rate and sleep stages. Accelerometers have been used for real-time detection of apneas and home health monitoring. Nanofiber strain sensors and fabric sheets have been used for heart rate monitoring and in-bed measurement of physiological and behavioral signals. Sleep stages were assessed using continuous wave doppler radar. The sensitivity to near vital signs is enhanced. Respiratory movements of the chest and abdomen in different prone positions were studied using a flexible tactile sensor array and pressure sensor array smart pad systems for non-interfering sleep monitoring were reported.
At present, an air pressure detection method is available, and can conveniently measure the heart rate, respiration, limb movement, cough, scratching and other actions of bedridden patients. This approach is also non-invasive to electromagnetics. Furthermore, the authors also apply this method to evaluate sleep stages. The method adopts a high-sensitivity pressure sensor developed by improving a capacitive microphone sensor. The sensor can measure sound pressures of 2 x 10 -5 Pa to 1.0 Pa. The actual frequency range is 0.024Hz (the specification ensures a frequency of 0.5 Hz) to 10kHz. The structure and characteristics of a microphone as a sensitive pressure sensor and a preamplifier for effectively operating the sensor have been studied in detail.
The sensitivity of the pressure sensor is weaker than that of the microphone sensor, but the sensitivity thereof causes inherent problems due to the improved use of the microphone sensor. First, the sensitivity is high, saturation is easy, and the dynamic range is narrow. Next, a capacitive microphone sensor is one of the capacitive sensors, having two electrodes that realize a capacitance. One of the electrodes is a permanently charged electret film having a width of about 25 μm, facing the sound pressure. When a person sits on the air tube or cushion, the pressure inside it rises to a few kilopascals, pushing and bending the microphone electrode into contact with the other electrode. The electret film has very small leakage holes to avoid damage caused by abrupt changes in high pressure, the size of the leakage holes determining the cut-off frequency. The cut-off frequency is proportional to the size of the hole. At 0.05Hz, the holes are very small, e.g., 1.88×10 -9m2 cross-sectional areas. The electret film is bent in the air tube or air cushion due to high pressure, and gradually returns to the shape due to the air flowing through the orifice plate, so that the pressure on two sides of the film is equal. For example, when the cutoff frequency is 0.05Hz, the time constant is about 1/(2pi×0.05) =3.2 s, and a long time is required for recovery from the film saturation state. When the pressure exceeds a certain value, the microphone loses pressure sensing for about 3.2 seconds, thereby producing unnecessary biological measurements. Also, the signal-to-noise ratio in such a saturated state is inferior to that used in a general acoustic microphone.
Disclosure of Invention
The object of the present invention is to provide a micro differential pressure sensor type bio-signal bed, which solves the aforementioned problems of the prior art.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
a micro differential pressure sensor type biological signal bed comprises a signal acquisition pad, a flexible connecting pipe and a micro differential pressure sensor; the signal acquisition cushion covers the upper surface of the bedstead, the flexible connecting pipe stretches between the bedstead and the signal acquisition cushion, one end of the flexible connecting pipe is closed, the other end of the flexible connecting pipe is freely arranged, and the free end of the flexible connecting pipe is connected with the micro differential pressure sensor; the micro differential pressure sensor comprises a controller, a shell and a capillary tube which is partially positioned in the shell, wherein both end joints of the capillary tube extend out of the shell, and one end of the capillary tube is communicated with the free end of the flexible connecting tube; the controller is arranged in the shell, a heater and a thermal sensor are arranged in the capillary tube in the shell, and the heater and the thermal sensor are connected with the controller;
the micro differential pressure sensor detects vital signs of a subject through a thermal flow method; the heat flow method is specifically that,
The flow rate of hot air flow in the capillary duct is in direct proportion to the pressure born by the flexible connecting pipe; when a subject lies on the signal acquisition cushion and presses the flexible connecting pipe, the weight of the subject pressurizes air in the flexible connecting pipe, and after a period of transition, the internal and external pressures acting on the flexible connecting pipe reach balance, namely the pressure difference between the internal and external of the flexible connecting pipe is zero; after transition, only the limb movement of a subject impacts the flexible connecting pipe through the vital head positive vibration signal, so that the flexible connecting pipe generates differential pressure near a zero level, and the hot air flow signal in the capillary duct can be obtained through the differential pressure generated by the flexible connecting pipe;
Since the vital signs of the subject vibrate slowly, the flow of hot air through the capillary tube is slow and can be considered to be laminar, the Hagen-Poiseuille equation can be applied to flow phenomena and flow resistance can be derived.
Preferably, the flow resistance of the capillary tube is calculated as follows,
Wherein r f is the flow resistance of the capillary; r is the radius of the capillary; mu is the viscosity coefficient; l is the length of the capillary;
according to the Hagen-Poiseuille equation, obtained,
Wherein q is the flow through the flexible connecting tube; p e is the in-tube pressure of the flexible connecting tube; p o is the pressure outside the flexible connecting tube; Δp is the differential pressure between the inside and outside of the flexible connecting tube;
from the ideal gas law, get
The internal pressure drop time constant is defined as follows,
Wherein A is the cross-sectional area of the flexible connecting pipe; l is the length of the flexible connecting pipe; AL is the volume of the flexible connecting tube; c is the volume of 1mol of air; t a is absolute temperature; t is the internal pressure drop time constant; r is a gas constant;
The transfer functions of the flow q passing through the flexible connecting pipe and the pressure difference delta p between the inside and the outside of the flexible connecting pipe to the pressure p e in the flexible connecting pipe can be obtained by using the internal pressure falling time constant,
Wherein s represents a waveform characteristic index of signal separation, s=i2pi f, i is an imaginary number; f is the waveform frequency;
Since the differential pressure Δp between the inside and the outside of the flexible connection pipe is a step function of the pressure p e in the flexible connection pipe and the internal pressure drop time constant is T, then
Where t represents the time at which the real response is obtained from the signal transfer function.
The beneficial effects of the invention are as follows: 1. the micro differential pressure sensor detects heart rate, respiration, limb movement and cough action of a subject by a novel noninvasive air pressure vital sign measuring method, and the problems of long saturation recovery time, narrow dynamic range, high noise level, low signal-to-noise ratio and the like caused by adopting a high-sensitivity sensor in the prior art are avoided. 2. The micro differential pressure sensor has the characteristics of quick saturation recovery, wide dynamic range, low noise, high signal-to-noise ratio and the like, and can be stably applied to vital sign measurement with high precision and wide range.
Drawings
FIG. 1 is a schematic diagram of the structure of a micro differential pressure sensor type biosignal bed;
FIG. 2 is a schematic illustration of the effect of blood pulsation on a flexible connection pipe;
FIG. 3 is a schematic diagram of the principle of measuring temperature difference by a thermal flow method;
FIG. 4 is a schematic diagram of an experimental apparatus for a comparative experiment in example two;
FIG. 5 is a schematic diagram of the pressure response of the microphone sensor and micro differential pressure sensor when 1cc of air is injected into the tube;
FIG. 6 is a schematic diagram of an experimental setup for simultaneous measurement of a subject's vital signals using an electrocardiogram amplifier, microphone sensor and micro differential pressure sensor;
FIG. 7 is a schematic diagram of the pressure response of a microphone sensor and a micro differential pressure sensor for a subject resting on his back;
FIG. 8 is a schematic of the pressure response of a microphone sensor and a micro differential pressure sensor in a supine bed of a subject;
fig. 9 is a schematic of the pressure response of a microphone sensor and a micro differential pressure sensor for a subject coughing in bed.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the detailed description is presented by way of example only and is not intended to limit the invention.
Example 1
As shown in fig. 1 to 3, in the present embodiment, a micro differential pressure sensor type bio-signal bed is provided, which includes a signal acquisition pad, a flexible connection pipe, and a micro differential pressure sensor; the signal acquisition cushion covers the upper surface of the bedstead, the flexible connecting pipe stretches between the bedstead and the signal acquisition cushion, one end of the flexible connecting pipe is closed, the other end of the flexible connecting pipe is freely arranged, and the free end of the flexible connecting pipe is connected with the micro differential pressure sensor;
the micro differential pressure sensor comprises a controller, a shell and a capillary tube which is partially positioned in the shell, wherein both end joints of the capillary tube extend out of the shell, and one end of the capillary tube is communicated with the free end of the flexible connecting tube; the controller is arranged inside the shell, a heater and a thermal sensor are arranged inside the capillary tube inside the shell, and the heater and the thermal sensor are connected with the controller.
The english definitions appearing in fig. 1 to 3 are as follows:
| Flexible tube with stiffness | rigid flexible connecting pipe |
| Closed terminal | Closed terminal |
| Cushion | Cushion pad |
| Bed frame | Bedstead |
| Thermal flow and/or pressure sensor | Thermal flow and/or pressure sensor |
| Inside of human body | Inside the human body |
| Blood pulsation | Blood pulsation |
| Blood vessel | Blood vessel |
| Cushion | Cushion pad |
| Bed frame | Bedstead |
| Flexible tube | Flexible pipe |
| Thermal flow sensor | Thermal flow sensor |
| Flexible tube with stiffness | Rigid flexible connecting pipe |
| Pressure due to blood pulsation | Pressure generated by blood pulsation |
| Displacement of deformation of tube | Deformation and displacement of pipe |
| Radius | Radius of radius |
| Flow | Flow rate |
| Flow resistance | Flow resistance |
| Heater | Heater |
| Heat sensor | Thermal sensor |
In this embodiment, the micro differential pressure sensor detects vital signs of the subject by a thermal flow method; the heat flow method is specifically that,
The flow rate of hot air flow in the capillary duct is in direct proportion to the pressure born by the flexible connecting pipe; when a subject lies on the signal acquisition cushion and presses the flexible connecting pipe, the weight of the subject pressurizes air in the flexible connecting pipe, and after a period of transition, the internal and external pressures acting on the flexible connecting pipe reach balance, namely the pressure difference between the internal and external of the flexible connecting pipe is zero; after transition, only the limb movement of the subject impacts the flexible connecting pipe through the vital head positive vibration signal, so that the flexible connecting pipe generates differential pressure near the zero level, and the hot air flow signal in the capillary duct can be obtained through the differential pressure generated by the flexible connecting pipe.
The hot air flow signal is a waveform signal of 0.1HZ-10KHZ, and the waveform signal is divided according to the frequency domain, so that signals such as heart rate, respiration, snoring, cough, body movement, BCG and the like can be obtained; the monitoring of vital signs of human body is realized.
In this embodiment, the Hagen-Poiseuille equation can be applied to the flow phenomenon and flow resistance can be derived, since the vital sign of the subject vibrates slowly, the flow of hot air through the capillary tube is slow and can be regarded as laminar.
In this example, the flow resistance of the capillary tube is calculated as follows,
Wherein r f is the flow resistance of the capillary; r is the radius of the capillary; mu is the viscosity coefficient; l is the length of the capillary;
according to the Hagen-Poiseuille equation, obtained,
Wherein q is the flow through the flexible connecting tube; p e is the in-tube pressure of the flexible connecting tube; p o is the pressure outside the flexible connecting tube; Δp is the differential pressure between the inside and outside of the flexible connecting tube;
from the ideal gas law, get
The internal pressure drop time constant is defined as follows,
Wherein A is the cross-sectional area of the flexible connecting pipe; l is the length of the flexible connecting pipe; AL is the volume of the flexible connecting tube; c is the volume of 1mol of air; t a is absolute temperature; t is the internal pressure drop time constant; r is a gas constant;
The transfer functions of the flow q passing through the flexible connecting pipe and the pressure difference delta p between the inside and the outside of the flexible connecting pipe to the pressure p e in the flexible connecting pipe can be obtained by using the internal pressure falling time constant,
Wherein s represents a waveform characteristic index of signal separation, s=i2pi f, i is an imaginary number; f is the waveform frequency;
Since the differential pressure Δp between the inside and the outside of the flexible connection pipe is a step function of the pressure p e in the flexible connection pipe and the internal pressure drop time constant is T, then
Where t represents the time at which the real response is obtained from the signal transfer function.
In this embodiment, the cutoff frequency of the micro differential pressure sensor is calculated as follows,
Example two
In the embodiment, the micro differential pressure sensor is compared with the microphone sensor in the prior art to verify the superiority of the micro differential pressure sensor in measuring vital signs of a subject.
As listed in table 1, three condenser microphone sensors and one micro differential pressure sensor were provided. The following table lists the capacitive microphone sensor gains and frequency ranges. EM183 is a two-way capacitive microphone sensor with two open pressure ports. The two ports may act as capacitive microphone sensors when placed in two separate spaces. At this time, one port is placed inside the tube, and the other port is placed in the atmosphere, functioning as a microphone sensor. ME246 is a low frequency capacitive microphone sensor for use in biometrics. One port is placed within the tube and the other port is placed within the closed microphone sensor cavity. Ensuring that EM246 has a flat frequency response in the range of 0.1Hz to 10 kHz. In fact, the low range is below 0.1 Hz. AOM-5024L-HD-R (AOM) is a medium and high sensitivity microphone sensor that is identical in construction to EM246, but has a larger orifice than EM246, ensuring a frequency range of 20Hz to 1000Hz. In fact, such microphone sensors may work well below 20 Hz. The gains and signal-to-noise ratios of all microphone sensors are listed in table 1. Note that when used for audio purposes, the signal-to-noise ratio of the microphone sensor is defined by a 1kHz, 94dB (1 Pa) input weighted condition, unlike conditions in which high voltages are measured in a closed tube. The signal-to-noise ratio of the micro differential pressure sensor is evaluated according to the specifications, and the pressure in the range of + -500 Pa is given by a 16-bit digital signal.
Table 1 study of microphone sensor and micro differential pressure sensor dynamic range and saturation recovery time constant
As shown in FIG. 4, a microphone sensor or a micro differential pressure sensor was attached to a rubber tube having an inner diameter of 0.4cm, a length of 33cm, and a volume of 4.14 cc. The syringe is inserted into the other end. There is no leakage when connected or inserted, but it can leak through the microphone orifice or capillary tube of the differential pressure sensor.
The syringe piston was pushed by a step function method, and 1cc of air was injected into the tube. The pressure rise in the tube is 24.4kPa, according to the ideal gas law, which is very high for microphone sensors and micro-differential pressure sensors. After air injection, high pressures can complicate phenomena within the tube such as dynamic expansion of the tube, nonlinear states of microphone sensors and micro differential pressure sensors, and the like.
Fig. 5 shows the pressure response and micro-differential pressure sensor output of three different microphones when 1cc of air is injected, filtered by a first order digital low pass filter with a cut-off frequency of 2.21Hz. The microphone is driven by a 4.5V direct current power supply, and output voltage is directly collected without an amplifier or an analog filter so as to avoid the influence of a circuit. An oscilloscope is used as a data recorder to obtain the microphone sensor output voltage. The voltage is converted to pressure by the microphone sensor gains listed in table 1. The differential pressure was obtained by an evaluation kit (EK-P3 sensor).
The English definitions appearing in FIG. 4 are shown in Table 2 below
TABLE 2
The micro differential pressure sensor is more obvious in measuring pressure saturation than the microphone sensor, and the recovery speed is faster. The recovery time constant was 0.012 seconds.
The new definition of the pressure signal-to-noise ratio measured by the microphone sensor or micro differential pressure sensor is as follows:
Wherein noise N and signal S are defined as follows
S=maximum-minimum value of measured pressure
N=standard deviation of steady state pressure filtered with a first order high pass filter with a cut-off frequency of 2.21 Hz.
Table 3 shows the time constant or minimum frequency, saturation pressure range, noise level and signal-to-noise ratio of the microphone sensor and the micro differential pressure sensor. As is clear from Table 2, the differential pressure time constant was 0.012s, which is 4.5% of the AOM-5024-HD-R minimum time constant of 3.5s, and the time constant of the low frequency microphone sensor was 65s. At differential pressure, the noise level was lowest at 0.82Pa, and the saturation pressure range and signal-to-noise ratio were maximum at + -500Pa and 62dB, respectively. The time constant, lowest frequency range, saturation range, noise level and micro differential pressure sensor signal to noise ratio are 0.012s, 13.2Hz, -500Pa to +500Pa, 0.82Pa and 62dB, respectively. The time constant, lowest frequency range, saturation range, noise level, and micro-differential pressure sensor signal-to-noise ratio of the low frequency microphone sensor are 65s, 0.0024Hz, -80Pa to +4Pa, 4.02Pa, and 32dB, respectively.
Table 3 time constants and/or lowest frequency ranges of microphone sensor and micro differential pressure sensor have saturated pressure ranges
Fig. 6 shows an experimental setup for simultaneously measuring vital sign signals of a subject using an electrocardiogram amplifier, a microphone sensor and a micro differential pressure sensor, a sensor tube, which is a circular silicon tube having a length of 1.7m, an outer diameter of 7mm, and an inner diameter of 5mm, is placed between the bed frame and the mattress. Covered with an 80cmx25cmx1mm lid. The tube is closed at one end and connected at the other end to a T-branch. Microphone sensor EM246 is inserted into one end of the T-branch and micro differential pressure sensor SDP610-500Pa is connected to the other end of the T-branch. Both ends have no leakage. Thus, the pressure measured in the sensor tube by both sensors is the same. The microphone sensor output voltage is amplified by a low frequency amplifier with a gain of 20dB and a frequency range of 0.2Hz to 20kHz. The differential pressure was obtained by the evaluation kit (EK-P3). Furthermore, an electrocardiogram is measured as a reference. As shown in fig. 6, three thin copper electrodes (positive, negative and ground) of dimensions 35mm x 30mm x 0.1mm were connected to the chest. According to the recommended circuit given in the specification, an electrocardiogram amplifier with a gain of 40dB and a high pass filter with a cut-off frequency of 0.3Hz were realized using an LT116 instrumentation amplifier (linear technique). The electrodes were fixed to the chest using tap water as a conductive paste.
The English definitions appearing in FIG. 6 are shown in Table 4
TABLE 4 Table 4
The biological sign signal acquisition system is fixed by an iron frame. A50 mm thick pad of memory foam was placed on the bed frame. The sensor tube is placed in the lower half of the pillow to reach the shoulder of the subject. The position of the sensor tube can affect the measurement. Placed directly under the chest of the subject, the respiration reading increases. The pulse can be obviously measured by being placed under the pillow and far away from the chest. Respiration and pulsation can be measured. An oscilloscope is used as a data recorder to collect microphone output voltage and electrocardiogram output voltage. The differential pressure was obtained on a computer by evaluating kit EK-P3. The sampling interval is 10ms and the measurement time interval is 12s. By pressing an electrode on the electrocardiograph, a time stamp indicating the start of the measurement can be obtained. By pressing, both the microphone and the micro differential pressure sensor present electrocardiographic artifacts and high pulse pressures.
The subject was a healthy male 75 years old, weighing 71 kg and 167 cm. Without heart or lung disease.
During the measurement, the subject pressed one of the electrodes about 0.5s after the data recording. The subject lies in the bed in the following posture.
(1) Rest on back
(2) Supine on the bed
(3) Cough lying on bed
The lower graph shows the results of the above three cases. For ease of comparison, all figures show (a) ±3v scaled electrocardiogram output and ±10pa scaled differential pressure output (b) ±5v scaled microphone output and ±500pa scaled differential pressure output.
Figure 7 shows the readings of the subject at rest on his back. Artifacts are evident from the electrocardiogram of 0s-1s, with a sharp negative pulse occurring at about 0.5 s. Which is a pulse of the subject pressing down on the electrode, as a marker of the start time of the measurement. At the same time, the outputs of the microphone sensor and the micro differential pressure sensor will show significant pulses. All measurements are synchronized by time stamping. In terms of synchronization, it is confirmed that the peak occurrence times of the microphone sensor and the differential pressure sensor are the same, and the same as the time at which the T peak occurs in the electrocardiogram.
From the measurement results of fig. 8, the output of the microphone sensor was saturated both before and after the start-up, and the recovery time after the saturation was long, about 3s after the start-up. No respiration was observed. On the other hand, the micro differential pressure sensor immediately returns to the 0Pa level and measures respiration and heart rate. In addition, the micro differential pressure sensor output has a wide dynamic range, and heart rate and respiration can be clearly observed even if the range is changed from ±500Pa to ±10Pa, as shown in fig. 8 (a) and (b). In fact, heart rate and respiration can be observed even in the range of ±0.1 Pa.
Figure 9 shows the measurements of a subject lying on his back in a bed. In this case, there is no time stamp. Four seconds after the bed, the subject was supine. When the subject is in bed, the micro differential pressure sensor is in a saturated state. But after saturation, the sensor quickly reaches a level of 0Pa and accurately measures heart rate. The microphone sensor output saturates, again showing longer transition and drift times.
English definitions appearing in FIGS. 7-9 are shown in Table 5
TABLE 5
In this embodiment, in the above-described subject experiment, the reference electrocardiogram and pressure were measured by the low frequency microphone sensor and the micro differential pressure sensor in one closed tube between the bed frame and the mattress. The three measurements are synchronized. By comparing the pressures of the micro-differential pressure sensor and the microphone sensor, the micro-differential pressure sensor can provide an accurate R-R interval by displaying a sharp and sharp peak, but the microphone sensitivity is too high, so that the noise is large and the peak time is ambiguous. The micro differential pressure sensor has a wider dynamic range than the microphone, and has a short recovery time from saturation, and the biological signal starts to be measured immediately after saturation. On the other hand, microphones require longer transition and recovery times after saturation due to high pressure variations. This slow response does not accurately detect the time interval of the apnea, which is critical for diagnosis of the apnea, resulting in erroneous results and/or measurement interruption problems. As shown in fig. 9, the micro differential pressure sensor showed a maximum pressure of 500Pa at the time of cough, but the measurement result was clear and unsaturated.
The microphone sensor has high sensitivity, but the saturation recovery is slow, the dynamic range is narrow, the noise level is high, and the signal-to-noise ratio is low. The micro differential pressure sensor has high sensitivity, quick saturation recovery, wide dynamic range, low noise and high signal-to-noise ratio.
By adopting the technical scheme disclosed by the invention, the following beneficial effects are obtained:
The invention provides a micro differential pressure sensor type biological signal bed, wherein a micro differential pressure sensor detects heart rate, respiration, limb movement and cough movements of a subject through a novel noninvasive air pressure vital sign measuring method, so that the problems of long saturation recovery time, narrow dynamic range, high noise level, low signal-to-noise ratio and the like caused by adopting a high-sensitivity sensor in the prior art are avoided. The micro differential pressure sensor has the characteristics of quick saturation recovery, wide dynamic range, low noise, high signal-to-noise ratio and the like, and can be stably applied to vital sign measurement with high precision and wide range.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which is also intended to be covered by the present invention.
Claims (2)
1. A micro differential pressure sensor type biological signal bed, which is characterized in that: the device comprises a signal acquisition pad, a flexible connecting pipe and a micro differential pressure sensor; the signal acquisition cushion covers the upper surface of the bedstead, the flexible connecting pipe stretches between the bedstead and the signal acquisition cushion, one end of the flexible connecting pipe is closed, the other end of the flexible connecting pipe is freely arranged, and the free end of the flexible connecting pipe is connected with the micro differential pressure sensor; the micro differential pressure sensor comprises a controller, a shell and a capillary tube which is partially positioned in the shell, wherein both end joints of the capillary tube extend out of the shell, and one end of the capillary tube is communicated with the free end of the flexible connecting tube; the controller is arranged in the shell, a heater and a thermal sensor are arranged in the capillary tube in the shell, and the heater and the thermal sensor are connected with the controller;
the micro differential pressure sensor detects vital signs of a subject through a thermal flow method; the heat flow method is specifically that,
The flow rate of hot air flow in the capillary duct is in direct proportion to the pressure born by the flexible connecting pipe; when a subject lies on the signal acquisition cushion and presses the flexible connecting pipe, the weight of the subject pressurizes air in the flexible connecting pipe, and after a period of transition, the internal and external pressures acting on the flexible connecting pipe reach balance, namely the pressure difference between the internal and external of the flexible connecting pipe is zero; after transition, only the limb movement of a subject impacts the flexible connecting pipe through the vital head positive vibration signal, so that the flexible connecting pipe generates differential pressure near a zero level, and the hot air flow signal in the capillary duct can be obtained through the differential pressure generated by the flexible connecting pipe;
Since the vital signs of the subject vibrate slowly, the flow of hot air through the capillary tube is slow and can be considered to be laminar, the Hagen-Poiseuille equation can be applied to flow phenomena and flow resistance can be derived.
2. The micro differential pressure sensor type biosignal bed according to claim 1, characterized in that: the flow resistance of the capillary tube is calculated as follows,
Wherein r f is the flow resistance of the capillary; r is the radius of the capillary; mu is the viscosity coefficient; l is the length of the capillary;
according to the Hagen-Poiseuille equation, obtained,
Wherein q is the flow through the flexible connecting tube; p e is the in-tube pressure of the flexible connecting tube; p o is the pressure outside the flexible connecting tube; Δp is the differential pressure between the inside and outside of the flexible connecting tube;
from the ideal gas law, get
The internal pressure drop time constant is defined as follows,
Wherein A is the cross-sectional area of the flexible connecting pipe; l is the length of the flexible connecting pipe; AL is the volume of the flexible connecting tube; c is the volume of 1mol of air; t a is absolute temperature; t is the internal pressure drop time constant; r is a gas constant;
The transfer functions of the flow q passing through the flexible connecting pipe and the pressure difference delta p between the inside and the outside of the flexible connecting pipe to the pressure p e in the flexible connecting pipe can be obtained by using the internal pressure falling time constant,
Wherein s represents a waveform characteristic index of signal separation, s=i2pi f, i is an imaginary number; f is the waveform frequency;
Since the differential pressure Δp between the inside and the outside of the flexible connection pipe is a step function of the pressure p e in the flexible connection pipe and the internal pressure drop time constant is T, then
Where t represents the time at which the real response is obtained from the signal transfer function.
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