RU2631364C1 - Telemetric sensor for controlling wakefulness on electrodermal activity signals - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: sensor contains a wrist bracelet for the operator, on the inner surface of which electrodes are fixed for recording the galvanic skin reaction associated with the current source and the microprocessor-based signal processing unit installed in the body that is placed on the outer surface of the bracelet, a radio communication channel with wakefulness facilities. And the electrodes are placed on the opposite surfaces of the bracelet, one of the electrodes is reference, it is partially enclosed by the wrist and is placed on the side of the body with the possibility of abutment to the dorsal surface of the hand and galvanic contact with it, and the other - in the form of a matrix of independent electrodes with the possibility of galvanic contact with the inner surface of the wrist. Independent electrodes are connected via individual current sources and the multiplexer - to a signal processing unit with the possibility of periodically interrogating matrix elements with the allocation of the signal with the greatest amplitude for controlling the wakefulness facilities.

EFFECT: increasing the signal-to-noise ratio.

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Description

The invention relates to medical instrumentation and can be used in the design of wearable elements for monitoring the functional state of the operator.

It is known that electrodermal activity (EDA) is a reliable indicator of the psychoemotional state of a person and has a number of technical applications, in particular, to control the wakefulness of the operator. So, the telemetric system for monitoring the wakefulness of the driver (TSKBM), developed by ZAO NEUROKOM, was put into commercial operation. The telemetric sensor on the bracelet is designed to obtain information about the relative change in the electrical resistance of the skin and digitally transmitted to the receiver via a radio channel (http://www.neurocom.ru/ru2/rail/tskbm.html). The change in electrical resistance occurs as a result of a pulsed ejection of a microscopic amount of sweat by the sweat gland. It is this impulse that is the useful signal that is analyzed in the TSKBM algorithm. Registration of EDA on the arm is carried out by means of two convex oval-shaped electrodes that are adjacent to the wrist and are attached using a bracelet or watch strap.

A device for multichannel registration of specific signals of EDA - skin-galvanic reaction (RAG). RAG on the palm surface of the hand (RU No. 137669 U1, Center TPP, 02.27.2014 - prototype). The device contains a multi-channel sensor block in the nodes of the array with a number of sensors of at least five, a multi-channel ADC, a block for determining the latency of the RAG at different points of the palm, and a signal detection unit with a maximum level, a visualization and registration unit. The clamp of the matrix of sensors placed on the glove is carried out by a pneumatic cuff, the reference electrode is fixed on the wrist of the same hand on which the glove is worn. Skin potential sensors are arranged in a 5-row matrix, 18 skin potential sensors are located on the palm surface in 10 mm increments, 3 more skin potential sensors are located on the proximal phalanx of the fingers from the second to fourth, and the reference electrode 2 is located on the wrist of the same hand. However, this device is intended for research purposes, and the output signal is sensitive to the compressive force of the pneumatic system, and, in addition, the task is not to increase the noise immunity of the output signal.

Closest to patentable is the telemetric sensor described in the patent (RU No. 2376159 C1, NEUROKOM, op. 20.12.2009 - prototype). The sensor for the driver’s wrist contains a bracelet similar to a wristwatch, to which the sensor unit of the telemetry sensor itself is attached - two electrodes for contact with the driver’s skin when put on the hand. The elements of the radio circuit (ADC, transceiver and antenna) are located in the sensor housing.

The disadvantage of this device is that the output value of the RAG signal, characterizing a low signal-to-noise ratio.

The present invention is directed to solving the technical problem of forming a stable RAG signal with an increased signal to noise ratio, which can be successfully used in wakefulness monitoring systems.

Patented telemetry sensor for monitoring wakefulness according to signals of electrodermal activity contains a wristband for the operator’s wrist, on the inner surface of which electrodes for recording the skin-galvanic reaction are fixed, connected to a current source and a microprocessor-based signal processing unit installed in the case, which is located on the outer surface of the bracelet, radio channel of communication with means of maintaining wakefulness.

The difference is that the electrodes are placed on opposite surfaces of the bracelet, one of the electrodes is a reference one, made partially covering the wrist and placed on the side of the body with the ability to adhere to the dorsal surface of the hand and galvanic contact with it, and the other in the form of a matrix of independent electrodes with the possibility of galvanic contact with the inner surface of the wrist. Independent electrodes are connected through individual current sources and a multiplexer to the signal processing unit with the possibility of periodically interrogating the matrix elements with the selection of the signal with the largest amplitude to control the means of maintaining wakefulness.

The sensor can be characterized by the fact that the reference electrode is partially covering the wrist by at least 0.5 perimeters, as well as the fact that the area of the reference electrode is 10-50 square meters. cm.

The sensor can also be characterized by the fact that the matrix of independent electrodes is made on a flexible substrate of dielectric material, the electrodes have a predominantly square section of 1-5 square meters. mm with gaps between the electrodes - 0.5-2 mm with a total number of electrodes of 25-35 pieces, as well as the fact that the ratio of the area of a single electrode of the matrix to the area of the reference electrode is 0.001 or lower.

The technical result is an increase in the signal-to-noise ratio due to optimization of the topology of the measuring electrodes, as well as a decrease in the bit depth of the measuring ADC.

The invention is based on the following provisions established by the applicant. It is known that every sweat gland in the human body consists of a sweat duct, which opens outward by sweat pores. The RAG signal is characterized by an abrupt change in conductivity / electrical resistance, is recorded between the reference electrode in contact with the human body and the measuring electrode located on the skin surface, where sweat pores are present, when an electric current is passed through the electrodes.

Below, we estimate the recording of the RAG signal when using small (m) and large (b) measuring electrodes in the presence of a reference electrode of constant area, when conduction jumps (A) or jumps in electrical resistance (B) are recorded.

A. When comparing the jumps in conductivity when opening the sweat gland for small (m) and large (b) electrodes, the ratio of relative jumps has the form: γ _gm / γ _gb = S _eb / S _em , since:

γ _g = (G _e0 -G _ezh ) / G _e0 ,

G _e0 = g _s ⋅S _e ,

_Ezh G = g _s ⋅ _{(S E} -S _x) + g _standby ⋅S _w,

γ _g = 1-G _ezh / G _e0 ,

γ _g = S _w / S _e ⋅ (1-g _standby / g _s),

where g _s is the specific conductivity of the skin with a closed sweat gland (S / cm ² );

g _aw - conductivity in the open sweat gland (Cm / cm ² );

S _e - electrode area (cm ² );

S _W - the area of the gland (cm ² );

G _e0 - conductivity under the electrode with a closed sweat gland (cm);

G _ezh - conductivity under the electrode with an open sweat gland (cm);

γ _g is the ratio of the change in conductivity to its initial value.

When g _ozh >> g _s , and it always happens, since sweat has a much higher conductivity (salt solution in water) than the epidermis, the ratio of the change in conductivity to its initial value has the form: γ _g = S _w / S _e ⋅ g _oz / g _s .

B. When comparing jumps in resistance when opening the sweat gland for small (m) and large (b) electrodes, the ratio of relative jumps has the form: γ _ρm / γ _ρb = S _eb / S _em , since:

γ _ρ = (R _e0 -R _eog ) / R _e0 ,

R _e0 = ρ _s / S _e ,

1 / R _ezh = (S _e -S _w ) / ρ _s + S _w / ρ _ozh ),

γ _ρ = 1-R _eog / R _e0 ,

γ _ρ = S _w / S _e ⋅ (1-ρ _s / ρ _ozh ),

where ρ _s is the specific resistance of the skin with a closed sweat gland (Ohm / cm ² );

ρ _ozh - resistivity in the open sweat gland (Ohm / cm ² );

S _e - electrode area (cm ² );

S _W - the area of the gland (cm ² );

R _e0 - electrode resistance rel. body with closed sweat gland (Ohm);

R _ezh - the resistance of the electrode rel. open sweat gland bodies (ohms);

γ _ρ is the ratio of the change in resistance to its initial value.

When ρ _s >> ρ _op , and it always happens, since sweat has a much lower resistivity than the epidermis, the ratio of the change in resistance to its initial value has the form: γ _ρ = S _w / S _e ⋅ρ _s / ρ _aw .

The above analysis shows that if there is one sweat gland under the electrode and it is present there when the electrode size is changed, then reducing the electrode area by N times increases the jump in conductivity or resistance by N times as well. The minimum relative magnitude of the jump in measurements of the RGR, which must be recorded, is γ = 10 ^-5 , which requires the use of an ADC with 18 effective discharges. With a reasonable partition of the electrode area, for example 10 into 10 parts, the gain in the jump is 100 times (γ = 10 ^-3 ), and the bit depth of the measuring ADC can be reduced from 18 to 11.

The limitation for reducing the area of the electrode is the thermal noise of the resistance of the electrode with a closed sweat gland, in which the voltage jump is compared with the rms value of the noise. In the case considered above, with a typical value of ρ _s = 100 kΩ / cm ² and an initial electrode size of 1 cm ^{2, the} resistance with a small electrode will increase to 10 MOhm. This corresponds to a rms thermal noise of about 1 μV in the 8 Hz band. With a permissible measuring voltage of 1 V between the electrodes, the voltage jump at γ = 10 ^-3 will be 1 mV, which is three orders of magnitude higher than the level of thermal noise.

In fact, the measuring device will be limited by the noise of the object, which at least does not change with decreasing electrode area, but is actually proportional to it, i.e. decrease with decreasing electrode area. The useful RAG signal (jump) is inversely proportional to the area, i.e. increases with decreasing electrode area, which indicates an obvious gain in signal-to-noise ratio.

Accordingly, with the correct construction of the measurement circuit, it becomes possible to several times increase the accuracy of the fixation of the RGR signal. Such a technical result does not follow explicitly from the recommendations of the prior art to make the electrode for analysis of the electrical properties of biological objects partitioned, since these recommendations are aimed at solving other problems.

The invention is illustrated in the drawings, where:

FIG. 1 is a block diagram of a sensor;

FIG. 2 - topology of the placement of electrodes on the inner surface of the bracelet;

FIG. 3 is a block diagram of a RAG signal recording unit.

In FIG. 1 is a block diagram of a telemetry sensor. The sensor 1 is made in the form of a bracelet 10 worn on the operator’s hand, having an outer 11 and inner 12 surface. On the outer surface 11 of the bracelet in the housing 13 there is a current source, a microprocessor-based signal processing unit, elements of a radio communication channel with wakefulness support means. On the inner 12 surface of the bracelet 10 on opposite surfaces opposite each other, a pair of electrically conductive electrodes 21, 22 for registering the RAG is placed. The electrodes 21 and 22 are placed in such a way that galvanic isolation 23 is provided between them, in the simplest implementation case, these are parts of a plastic strap or insert made of non-conductive material that are free of electrodes. The electrodes 21 and 22 have a different area and design.

The electrode 21 is a reference, made partially covering the wrist at least 0.5 perimeter from the side of the dorsal surface of the hand with the ability to adhere to the skin and provide galvanic contact with it. The area of the electrode 21 can be 25-50 square cm or more with a width of the bracelet 10 of about 3 cm. The electrode 21 is placed symmetrically with respect to the housing 13.

The electrode 22 is made in the form of a matrix 221 of independent electrodes 222 (Fig. 2) connected by individual conductors to the signal processing unit 30. Conductors connecting the electrodes 222 with the registration circuit are located in the body of the bracelet 10.

The signal processing unit 30 is configured to periodically interrogate the matrix electrodes 222 with a signal with the highest amplitude being selected to control the wakefulness support means.

In more detail, the registration scheme of the RAG signals is shown in Fig.3. Each independent electrode 222 is connected to one pole of an individual current generator 31, the other pole of which is connected to a common reference electrode 21. The number of current generators 31 coincides with the number of matrix electrodes 222.

The control inputs of the microcontroller 32 are connected to the current generators 31 and provide a cyclic supply of the measuring current between each electrode 222 of the matrix 221. At the same time, the output signal from each current generator 31, taken from the load resistor and proportional to the RGR, is fed to the inputs of the multiplexer 33. From the output of the multiplexer 33 is fed to the input of the module 34 highlight the largest amplitude signal, which generates a wakeful signal according to the parameters of the RAG in the block 35 maintain wakefulness.

The matrix 221 of independent electrodes 222 can be made on a flexible substrate of dielectric material and structurally formed directly in the body of the wrist bracelet. The area of a single electrode 222 is several square meters. mm, mainly 1-5 square meters. mm The electrodes 222 are conveniently square in size with gaps between the electrodes of 0.5-2 mm and placed in rows. The total number of electrodes 222 may be 25-35 pieces. The electrodes may be arranged, for example, in three rows, as shown in FIG. 2.

For example, if the area of a single electrode 222 is about 5 square meters. mm, and the electrode 21 is about 3000 square meters. mm (length 100 mm, width 30 mm), the ratio is about 10 ^-3 . For a better result, one should strive to further reduce this ratio, but by reducing the area of a single electrode 222, up to the size of the mouth of the sweat gland.

The implementation of a microprocessor-based signal processing unit, a radio communication channel with means of maintaining wakefulness and the means of maintaining wakefulness themselves is known from the prior art and is not described in the present description as not characterizing the invention.

The device operates as follows.

The bracelet is put on the operator’s hand so that the case 13 with the reference electrode 21 is located on the dorsal (outer) surface of the wrist, in this case, the matrix 221 of the electrodes 222 will be on the inner side of the wrist. When the signal processing unit 30 is connected, a cyclic polling of the current RAG signals from the electrodes 222 is carried out by means of a multiplexer 33.

Then, in module 34, the signal with the highest amplitude is selected, according to which, in accordance with the algorithms “RAG signal parameters - wakefulness”, developed by CJSC NEYROKOM and used in mass-produced products (see http://www.neurocom.ru/ ru2 / auto / vigiton.html), the intended purpose of the telemetric sensor is being implemented as part of the “Telemechanical System for Monitoring the Wake of the Driver,” “The System for Maintaining Driver Efficiency”

Increasing the signal-to-noise ratio by optimizing the selection of the topology of the measuring electrodes makes it possible to increase the reliability of monitoring systems of the psychophysiological state of the operator.

Claims (5)

1. A telemetry sensor for monitoring wakefulness according to signals of electrodermal activity, comprising a wrist strap for the operator’s wrist, on the inner surface of which electrodes for recording a skin-galvanic reaction are fixed, connected to a current source and a microprocessor-based signal processing unit mounted in a housing that is located on the outer surface a wristband, a radio channel of communication with means of maintaining wakefulness, characterized in that the electrodes are placed on opposite surfaces of the wristband, one the in of the electrodes is a reference, made partially covering the wrist and placed on the side of the body with the ability to adhere to the dorsal surface of the hand and galvanic contact with it, and the other in the form of a matrix of independent electrodes with the possibility of galvanic contact with the inner surface of the wrist, while the independent electrodes are connected through individual current sources and a multiplexer to the signal processing unit with the possibility of periodic polling of the matrix elements with the selection of the signal with the greatest a Amplitude for controlling wakefulness. 2. The sensor according to claim 1, characterized in that the reference electrode is made partially covering the wrist by at least 0.5 perimeters. 3. The sensor according to claim 1, characterized in that the area of the reference electrode is 10-50 square meters. cm. 4. The sensor according to claim 1, characterized in that the matrix of independent electrodes is made on a flexible substrate of dielectric material, the electrodes have a predominantly square section of 1-5 square meters. mm with gaps between the electrodes - 0.5-2 mm with a total number of electrodes 25-35 pieces. 5. The sensor according to claim 1, characterized in that the ratio of the area of a single electrode of the matrix to the area of the reference electrode is 0.001 or lower.

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