GB2378762A

GB2378762A - Galvanic Skin Response (GSR) sensor with skin contact pressure sensor

Info

Publication number: GB2378762A
Application number: GB0113992A
Authority: GB
Inventors: Clive Goodenough; Martin Bernard Litherland
Original assignee: INNER TEK Ltd
Current assignee: INNER TEK Ltd
Priority date: 2001-06-09
Filing date: 2001-06-09
Publication date: 2003-02-19
Also published as: GB0113992D0

Abstract

A GSR sensor has at least one skin contact electrode and a skin contact pressure sensor to provide a GSR signal that is corrected to take account of the skin contact pressure on the electrode. A handheld biofeedback device has electrodes E1, E2 held against the palm of a users hand to sense skin resistance and a pressure sensor arrangement 2, 3, SG, for sensing the contact pressure on the electrodes. The pressure sensor includes cantilevered strain gauge element SG. A correction signal is generated to account for the electrode skin contact pressure dependance of the GSR signal. The device also has a display 6 for displaying GSR or other stress-related signals and may also incorporate heart-rate and temperature sensors L. The handheld device may be a computer mouse or gaming handset for example.

Description

Galvanic Skin Response Sensor The present invention relates to a galvanic skin response (GSR) sensor. Such sensors are used to measure stress and are used in biofeedback arrangementsfor reducing stress. For example JP 11-169588 (E & C Taiyo) discloses a hand-held games device which has a sensor arrangement which measures changes in pulse rate, skin temperature and GSR and provides biofeedback in order to induce relaxation. WO 9941654 (Jeffrey) discloses a computer mouse with biofeedback skin sensors.

A problem with the above known galvanic skin response sensors is that the measured skin resistance is dependent on the contact pressure exerted by the user's skin on the electrodes.

JP2,177, 93 6A discloses a GSR sensor arrangement comprising skin contactelectrodes mounted on microswitches which standardise the contact pressure when the user's fingers are placed on them by clicking at a defined pressure. However the contact pressure is only crudely controlled by this arrangement and the clicking of the microswitches could be distracting in a relaxation device.

Accordingly the present invention provides a GSR sensor arrangement comprising at least one skin-contact electrode arranged to be applied to the user's skin and pressure sensing means arranged to generate a correction signal which is dependent on the sensed skin-contact pressure.

It has been found that such correction significantly enhances biofeedback-induced stress reduction, for example.

The term GSR is to be construed broadly to cover any signal dependent upon skin response to an applied voltage or current.

Preferably the correction signal varies continuously with the skin contact pressure.

Preferably the pressure-sensing means comprises a strain gauge or a bimorph element mechanically coupled to at least one skin-contact electrode.

Preferably the arrangement includes circuit means for processing a GSR signal in dependence upon the correction signal to output a corrected GSR signal. The circuit

means preferably comprises a microprocessor arranged to process digitised GSR and correction signals.

The arrangement optionally also includes skin temperature and/or heart rate sensors whose outputs are preferably coupled to the circuit means for processing in conjunction with the GSR signal.

In one embodiment the arrangement is incorporated in a hand-held casing having skin-contact electrodes on one surface thereof and having means (eg a strap) for holding the skin-contact electrodes against the palm of the user's hand.

The GSR sensor arrangement may for example be included in a biofeedback device comprising means for indicating (eg on a LCD screen or computer monitor) a GSRdependent condition of the user (eg stress).

In one embodiment the biofeedback device is a computer pointing device (eg a mouse). In another embodiment the biofeedback device is a computer/video gaming handset arranged to utilise the user's GSR as an imput to the game. In another embodiment the biofeedback device is a mobile telephone with a WAP facility and arranged to utilise the user's GSR as an imput input Other preferred features of the invention are defined in the dependent claims.

A preferred embodiment of the invention will now be described by way of example only with reference to Figures 1 to 3 of the accompanying drawings, wherein: Figure 1 is a diagrammatic elevation, partly in section, of a biofeedback device in accordance with the invention; Figure 2 is a plan view showing the underside of the biofeedback device of Figure 1, and Figure 3 is a circuit diagram showing the processing circuitry for the GSR and correction signals in the device of Figure 1.

Referring to Figure 1, the device comprises a generally flat casing C of plastics material having a convex lower surface and which, as shown in Figure 2, is generally

oval in plan view. The casing has a strap S attached to opposite ends thereof which can be fitted around a user's hand to enable the convex surface to be held against the user's palm. This convex lower surface carries two disc-shaped stainless steel skincontact electrodes El and E2 which in use complete an electric circuit through the user's skin. A low voltage is applied between the electrodes and the resistance sensed between them is measured and processed to generate a GSR (Galvanic Skin Response) signal, as will be described in more detail below.

The mechanical arrangement of the electrodes will first be described with reference to electrode E2 in Figure 1. Electrode E1 and its mounting arrangement are similar in all respects to electrode E2 and its mounting arrangement and for this reason the latter is not shown or described in detail.

As shown, electrode E2 is resiliently mounted on a generally top hat-shaped elastomeric washer 1 and carries a tab la which extends through the head portion of washer 1 and is electrically connected to processing circuitry 4. Washer 1 protrudes through an aperture in casing C and has an annular portion which is secured to a support 2 which is in turn carried on a support member 3. Support member 3 is rigidly secured at one edge to the lower end of a generally upright cantilevered strain gauge element SG. The upper end of strain gauge element SG is fixed (by means not shown) and hence pressure exerted by the user's skin on electrode E2 is transferred by washer 1 via support 2 to support member 3, causing strain gauge element SG to flex and to change its resistance to an extent dependent upon the pressure; this resistance is measured by circuit block 5. The flexure approximates to a twisting about an axis P, as shown in both Figure 1 and Figure 2. The movement of the support member 3 and washer 1 in response to applied pressure is indicated (in exaggerated fashion for the sake of clarity) at 3'and I'respectively.

Processing circuitry 4 measures the resistance between electrodes E1 and E2 and modifies the resulting GSR value in dependence upon the pressure signals from circuit blocks 5 (as described in more detail below with reference to Figure 3) and also processes signals from LED-type heart rate and skin-temperature sensors L (Figure 1). The heart rate sensor comprises an LED-photodiode combination H and the skin temperature sensor comprises a thermistor as shown in Figure 2 but as these are conventional they will not be described in detail. The processed signals are displayed on a conventional LCD display 6 to indicate stress so that the user can

mentally modify his or her stress symptoms to reduce his or her stress by biofeedback. The circuitry is powered by a battery (not shown) housed in casing C.

It will be noted that the heart rate and temperature sensors are mounted on a central raised portion 7 between the electrodes El and E2 so as to be substantially flush with the skin-contact surfaces of the latter.

The circuitry in the biofeedback device, in particular the circuitry within block 4, will now be described with reference to Figure 3. An analogue skin resistance signal is derived in conventional fashion by an operational amplifier A configured as a voltage follower. The non-inverting input of amplifier A is connected (via a series rsistorRI) to electrode E2 and is also connected to earth via a resistor R2. electrode El is connected directly to the positive supply voltage + VCC and the skin resistance between electrodes El and E2 and the resistance of R2 act as a voltage divider of VCC. Accordingly the output of amplifier A, which buffers the resulting potential, is proportional to the skin resistance.

Resistor R2 is bypassed by a capacitor Cl which, together with resistor Rl, protects the amplifier A against transient voltages occurring at the electrodes El and E2.

The resulting crude analogue GSR signal is digitised in an analogue-to-digital converter (ADC) 11 and then fed to a microprocessor 12.

An analogue pressure-dependent signal from circuit block 5 (Figure 1) is digitised in a further analogue-to-digital converter (ADC) 10.

Since the digitised GSR signal from ADC 11 will increase with increasing pressure (for a given skin resistance) the microprocessor 12 is programmed to operate on this signal to reduce it to an extent which increases with an increasing output from ADC 10. This processing can be implemented by a stored program (eg held in non-volatile memory, not shown) optionally with the aid of a look-up table (not shown). Suitable parameters for this processing can be determined empirically.

The resulting digital output signal is sent to display 6 (Figure 1), optionally after processing in conjunction with the outputs of the heart rate and skin-temperature sensors L (Figure 1) to generate a composite stress signal. The corrected GSR signal and the heart-rate and skin temperature signals can also be displayed independently.

In other embodiments the electrodes El and E2 are arranged to be mounted separately in contact with eg respective fingers of the user. In other embodiments the skin-contact pressure on the electrodes can be measured indirectly eg by at least one pressure sensor which is mounted adjacent to but separate from the electrodes.

Claims

Claims 1. A GSR sensor arrangement comprising at least one skin-contact electrode arranged to be applied to the user's skin and pressure sensing means arranged to generate a correction signal which is dependent on the sensed skin-contact pressure.
2. A GSR sensor arrangement according to claim 1 wherein the correction signal varies continuously with the skin contact pressure.
3. A GSR sensor arrangement according to claim 1 or claim 2 wherein the pressure-

sensing means comprises a strain gauge element mechanically coupled to at least one ZD skin-contact electrode.
4. A GSR sensor arrangement according to claim 3 wherein the strain gauge element is cantilevered from a support and is coupled near its free end to at least one skincontact electrode.
5. A GSR sensor arrangement according to any preceding claim further comprising circuit means for processing a GSR signal in dependence upon the correction signal to output a corrected GSR signal.
6. A GSR sensor arrangement according to claim 5 wherein the circuit means comprises a microprocessor arranged to process digitised GSR and correction signals.
7. A GSR sensor arrangement according to any preceding claim which is incorporated in a hand-held casing having skin-contact electrodes on one surface thereof and having means for holding the skin-contact electrodes against the palm of the user's hand.
8. A biofeedback device comprising a GSR sensor arrangement as claimed in any 0 preceding claim and having means for indicating a GSR-dependent condition of the user.
9. A GSR sensor arrangement substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawing.