US20140142460A1 - Method for detecting potential falls and a fall detector - Google Patents

Abstract

There is provided a fall detector for detecting a potential fall by a user, the fall detector comprising a sensor for measuring the skin conductivity of the user; and a processor for analyzing the skin conductivity measurements to determine whether the user has had a potential fall, wherein the processor is configured to analyze the skin conductivity measurements by matching the skin conductivity measurements to a predetermined skin conductivity response corresponding to a stressful event.

Description

TECHNICAL FIELD OF THE INVENTION
• The invention relates to a method for detecting potential falls by a user and a fall detector, and in particular relates to a method for detecting a potential fall and a fall detector that provides increased fall detection reliability.
BACKGROUND TO THE INVENTION
• Falls affect millions of people each year and result in significant injuries, particularly among the elderly. In fact, it has been estimated that falls are one of the top three causes of death in elderly people. A fall is defined as a sudden, uncontrolled and unintentional downward displacement of the body to the ground, followed by an impact, after which the body stays down on the ground.
• Personal Help Buttons (PHBs) are available that require the user to push the button to summon help in an emergency. However, if the user suffers a severe fall (for example if they are knocked unconscious), the user might be unable to push the button, which might mean that help doesn't arrive for a significant period of time, particularly if the user lives alone.
• Fall detectors are also available that process the output of one or more movement sensors to determine if the user has suffered a fall. Most existing body-worn fall detectors make use of an accelerometer (usually an accelerometer that measures acceleration in three dimensions) and they try to infer the occurrence of a fall by processing the time series generated by the accelerometer. Some fall detectors can also include an air pressure sensor, for example as described in WO 2004/114245. On detecting a fall, an alarm is triggered by the fall detector.
• Some fall detectors are designed to be worn as a pendant around the neck of the user, whereas others are designed to be worn on the torso or limbs of the user, for example at the wrist. However, the wrist is capable of complex movement patterns and has a large range of movement, which means that existing fall detection methods based on analyzing measurements from an accelerometer do not provide a sufficiently high detection rate while minimizing the number of false alarms for this type of fall detector.
• Therefore, there is a need for an improved method for detecting a potential fall and a fall detector implementing the same.
SUMMARY OF THE INVENTION
• According to a first aspect of the invention, there is provided a fall detector for detecting a potential fall by a user, the fall detector comprising a sensor for measuring the skin conductivity of the user; and a processor for analyzing the skin conductivity measurements to determine whether the user has had a potential fall, wherein the processor is configured to analyze the skin conductivity measurements by matching the skin conductivity measurements to a predetermined skin conductivity response corresponding to a stressful event.
• In a preferred embodiment, the processor is configured to generate skin conductivity coefficients by matching the skin conductivity measurements to a predetermined skin conductivity response corresponding to a stressful event, the value of each coefficient indicating the match to the predetermined skin conductivity response, and wherein the processor is configured to determine whether the response is consistent with a user having had a fall by analyzing the skin conductivity coefficients.
• In an embodiment, the processor is configured to analyze the skin conductivity coefficients to determine whether the response is consistent with a user having had a fall by determining one or more of (i) the maximum value of the coefficients, (ii) the mean of the coefficients, and/or (iii) the median of the coefficients.
• In some embodiments, the processor is configured to filter the skin conductivity measurements to remove noise and/or interference prior to matching the skin conductivity measurements to the predetermined skin conductivity response.
• In those embodiments, the processor is preferably configured to use a median filter to remove the noise and/or interference from the skin conductivity measurements.
• In preferred embodiments, the fall detector further comprises at least one movement sensor for sensing the movement of the user, the processor being further configured to analyze measurements from the at least one movement sensor to determine if the user may have fallen.
• In a preferred embodiment, the processor is configured to analyze the skin conductivity measurements only in the event that the measurements from the at least one movement sensor indicate that the user may have fallen.
• Preferably, the processor is configured to use the skin conductivity sensor to measure the skin conductivity in the event that the measurements from the at least one movement sensor indicate that the user may have fallen.
• In embodiments of the invention, the processor is configured to analyze the measurements from the at least one sensor to identify whether an impact and/or a change in altitude greater than respective threshold values have occurred.
• Preferably, the processor is configured to analyze the skin conductivity measurements obtained during a time window following a time at which the measurements from the at least one sensor indicate that the user may have fallen.
• According to a second aspect of the invention, there is provided a method of detecting a potential fall by a user, the method comprising measuring the skin conductivity of the user and analyzing the skin conductivity measurements to determine whether the user has had a potential fall by matching the skin conductivity measurements to a predetermined skin conductivity response corresponding to a stressful event.
• In a preferred embodiment, the step of analyzing comprises analyzing skin conductivity coefficients generated from matching the skin conductivity measurements to the predetermined skin conductivity response, the value of each coefficient indicating the match to the predetermined skin conductivity response.
• In an embodiment, analyzing the skin conductivity coefficients to determine whether the response is consistent with a user having had a fall comprises determining one or more of (i) the maximum value of the coefficients, (ii) the mean of the coefficients, and/or (iii) the median of the coefficients.
• In some embodiments, the method further comprises the step of filtering the skin conductivity measurements to remove noise and/or interference prior to matching the skin conductivity measurements to the predetermined skin conductivity response.
• In those embodiments, the method preferably comprises using a median filter to filter the skin conductivity measurements to remove the noise and/or interference from the skin conductivity measurements.
• In preferred embodiments, the method further comprises the steps of sensing the movement of the user and analyzing the movement of the user to determine if the user may have fallen.
• Preferably, the step of analyzing the skin conductivity measurements is performed only in the event that the step of analyzing the movement of the user indicates that the user may have fallen.
• In preferred embodiments, the step of measuring the skin conductivity is performed only in the event that the step of analyzing the movement of the user indicates that the user may have fallen.
• In embodiments of the invention, the step of analyzing the movement of the user comprises determining whether an impact and/or a change in altitude greater than respective threshold values have occurred.
• Preferably, the step of analyzing skin conductivity measurements comprises analyzing skin conductivity measurements obtained during a time window following a time at which the movements of the user indicate that the user may have fallen.
• According to a third aspect of the invention, there is provided a computer program product comprising computer readable code embodied therein, the computer readable code being configured such that, upon execution by a suitable computer or processor, the computer or processor performs the method described above.
BRIEF DESCRIPTION OF THE DRAWINGS
• Exemplary embodiments of the invention will now be described, by way of example only, with reference to the following drawings, in which:
• FIG. 1 is a block diagram of a fall detector in accordance with the invention;
• FIG. 2 is a flow chart illustrating a method of detecting falls in accordance with an embodiment of the invention;
• FIG. 3 is a flow chart illustrating the analysis of skin conductivity measurements according to an embodiment of the invention;
• FIGS. 4(a)-(d) are a series of graphs illustrating the skin conductivity measurements and the results of the processing steps shown inFIG. 3;
• FIG. 5 is a graph illustrating the filter used instep1095 of the method shown inFIG. 3; and
• FIG. 6 is a graph illustrating an analysis window for the skin conductivity coefficients determined from the method shown inFIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Afall detector2 according to an embodiment of the invention is shown inFIG. 1. In a preferred embodiment of the invention, thefall detector2 is designed to be worn by a user on their wrist, although it will be appreciated that the invention is not limited to this use, and thefall detector2 could instead be designed to be worn at the user's waist, on their chest or back or as a pendant around their neck or carried in their pocket.
• In this exemplary embodiment, thefall detector2 comprises two movement sensors—anaccelerometer4 andpressure sensor6—which are connected to aprocessor8. Theprocessor8 receives measurements from themovement sensors4,6, and processes the measurements to determine if a user of thefall detector2 has suffered a fall. Although two movement sensors are shown in this embodiment, it will be appreciated that fall detectors according to alternative embodiments may comprise only one movement sensor (for example just the accelerometer4).
• Thefall detector2 further comprises anaudible alarm unit10 that can be activated by theprocessor8 in the event that theprocessor8 determines that the user has suffered a fall. Thefall detector2 may also be provided with a button (not shown inFIG. 1) that allows the user to manually activate theaudible alarm unit10 if they require assistance (or deactivate the alarm if assistance is not required).
• Thefall detector2 also comprises a transmitter unit12 that allows thefall detector2 to transmit an alarm signal to a base station associated with the fall detector2 (which can then issue an alarm or summon help from a healthcare provider or the emergency services) or directly to a remote station (for example located in call centre of a healthcare provider) if a fall is detected, so that assistance can be summoned for the user. In some embodiments, theprocessor8 in thefall detector2 may not execute the algorithm on the data from thesensors4,6 to determine if the user has fallen; instead theprocessor8 and transmitter unit12 may provide the raw data from thesensors4,6 to the base station and a processor in the base station can execute the algorithm on the data from thesensors4,6 to determine if the user has fallen.
• As existing fall detection methods based on analyzing measurements from an accelerometer in a wrist-worn fall detector do not provide a sufficiently high detection rate while limiting the number of false alarms, thefall detector2 according to some aspects of the invention further comprises asensor14 for measuring the conductivity of the user's skin and theprocessor8 is configured to analyze the skin conductivity measurements in conjunction with the measurements from themovement sensors4,6 to provide a more reliable indication of whether the user has suffered a fall. In a wrist-worn fall detector2, theskin conductivity sensor14 is preferably arranged to contact the skin of the user on the volar side of their wrist. In some embodiments, thefall detector2 comprises multipleskin conductivity sensors14 that are to be placed at different positions on the user's body. In this case, at least one of thoseskin conductivity sensors14 can be integrated into a separate housing to the rest of the components of thefall detector2.
• In the illustrated embodiment, all of the components of thefall detector2 are integrated into a single housing that is to be placed in contact with the user's skin. In alternative embodiments, for example where part of the fall detector is in the form of a pendant to be worn around the user's neck (and so might not be in contact with the user's skin at all times), theskin conductivity sensor14 can be provided in a housing that is separate from the pendant (the pendant including theaccelerometer4 and pressure sensor6), so that theskin conductivity sensor14 can be located in contact with the user's skin during use.
• The operation of thefall detector2 will now be described in more detail with reference to the flow chart inFIG. 2.
• In order for theprocessor8 in the fall detector2 (or, in the alternative embodiment described above, for the processor in the base station) to determine if the user has suffered a fall, it is necessary to extract values for various features that are associated with a fall from the movement sensor measurements. Thus, instep101, theaccelerometer4 andair pressure sensor6 measure the accelerations and air pressure changes experienced by thefall detector2 and these measurements are provided to theprocessor8 by thesensors4,6, and theprocessor8 analyses them to determine whether the user might have suffered a fall (step103).
• A fall can be broadly characterized by, for example, a change in altitude of around 0.5 to 1.5 meters (the range may be different depending on the part of the body that thefall detector2 is to be worn), culminating in a significant impact, followed by a period in which the user does not move very much. Thus, conventionally, in order to determine if a fall has taken place, theprocessor8 needs to process the sensor measurements to extract values for features including a change in altitude (which is usually derived from the measurements from thepressure sensor6, but can also or alternatively be derived from the measurements from the accelerometer4), a maximum activity level (i.e. an impact) around the time that the change in altitude occurs (typically derived from the measurements from the accelerometer4) and a period in which the user is relatively inactive following the impact (again typically derived from the measurements from the accelerometer4). It will be appreciated that other features can further improve the detection algorithm. For example, the detection of a change in orientation upon falling can improve the likelihood that the signal is due to a fall.
• The analysis performed by theprocessor8 instep103 will not be described in further detail herein, but those skilled in the art will be aware of various algorithms and techniques that can be applied to determine whether a user may have suffered a fall from accelerometer and/or pressure sensor measurements.
• If theprocessor8 determines that a user may have suffered a fall (step105), i.e. if the analysis of theaccelerometer4 and/orpressure sensor6 measurements indicates that a fall has taken place, then measurements of the skin conductivity of the user are taken by the skin conductivity sensor14 (step107). If theprocessor8 has not detected a possible fall, the process returns to step101 and further measurements are taken and analyzed (step103).
• In some embodiments, theskin conductivity sensor14 is only activated once a possible fall (or simply a change in altitude of at least 0.5 m detected in the measurements from the pressure sensor6) has been detected from the analysis of theaccelerometer4 and/orpressure sensor6 measurements, thus reducing the power consumption of thefall detector2. As the analysis instep103 is performed by theprocessor8 substantially in real time or with only a small delay, theskin conductivity sensor14 will be activated shortly after a fall event has occurred.
• In alternative embodiments, theskin conductivity sensor14 may measure skin conductivity constantly or frequently whenever thefall detector2 is in use (i.e. even when a possible fall has not yet been detected). This way, skin conductivity measurements will be available to theprocessor8 as soon as a possible fall is detected.
• Instep109, theprocessor8 analyses the measurements from theskin conductivity sensor14 for characteristics associated with a fall. In particular, theprocessor8 analyses the measurements to identify whether there has been a measurable skin conductivity response that is consistent with the user having suffered a stressful event, such as an accidental fall. In the alternative embodiment described above where theskin conductivity sensor14 is measuring skin conductivity constantly or frequently, theprocessor8 may analyze the skin conductivity measurements only when a possible fall or change in altitude greater than a predetermined amount, for example 0.5 m, has been detected from the analysis of the measurements from the movement sensor(s)4,6. Alternatively, theprocessor8 may analyze the skin conductivity measurements for a response consistent with the user experiencing a stressful event, such as suffering a fall, regardless of whether a possible fall or change in altitude greater than 0.5 m has been detected yet.
• When a person accidentally falls, the process of falling, from losing balance to the impact with the ground and the (possible) inability to get up after the fall, causes a measureable change in the skin conductivity of the user as a result of the stress suffered by the user. However, a movement, such as sitting down or intentionally ‘falling’ onto a chair, which might appear to be a fall from an analysis of theaccelerometer4 andpressure sensor6 measurements alone, would not cause this stress response, and the skin conductivity measurements can be used to qualify whether an accidental fall has taken place.
• Thus, the analysis by theprocessor8 instep109 aims to identify a skin conductivity response associated with a stressful event (i.e. an accidental fall) from the measurements obtained after (and in some embodiments, during) the possible fall event. The skin conductivity measurements analyzed instep109 may relate to a period of around 15 seconds after the possible fall event has occurred, although those skilled in the art will appreciate that time windows having different lengths can be used.
• Instep111, theprocessor8 uses the result of the analysis of the skin conductivity measurements and the result of the analysis of the acceleration and air pressure measurements to determine if the user has potentially suffered a fall. If the user is determined to have fallen, theprocessor8 can trigger an alarm to obtain help for the user (step113), as described above. After triggering the alarm, the process can return to step101 to continue the monitoring of the user. If it is determined instep111 that the user has not fallen, the process also returns to step101 to continue the monitoring of the user.
• It will be appreciated that the results of the analysis steps (steps103 and109) can be combined in a number of different ways. For example, where theskin conductivity sensor14 is only activated once a possible fall has been detected, a fall may be determined instep111 if the skin conductivity measurements are consistent with a fall having taken place. Alternatively, features (such as an impact, altitude change, skin conductivity response) can be extracted from the sensor measurements, and the features combined (possibly using weightings for each extracted feature) to determine whether a fall has taken place.
• In the event that the measurements from theskin conductivity sensor14 indicate that thesensor14 is not in contact with the user's skin, i.e. the measurements are very low or zero (for example if thefall detector2 is not being worn properly or if thesensor14 has been knocked out of position by the fall), theprocessor8 can take a decision on whether a person has fallen just based on the measurements from theaccelerometer4 and/orpressure sensor6.
• The analysis of the skin conductivity measurements by theprocessor8 instep109 ofFIG. 2, will now be described in more detail with reference to the flow chart inFIG. 3, and the graphs shown inFIGS. 4,5 and6.
• Instep1091, theprocessor8 receives the skin conductivity measurements. An exemplary set of measurements is shown inFIG. 4(a). The skin conductivity is measured in micro Siemens, μLS.
• Firstly, theprocessor8 filters the skin conductivity measurements to remove noise and interference (step1093). The interference may result from electrical interference in the circuitry of thefall detector2, poor contact between theskin conductivity sensor14 and the user's skin, etc.). Preferably step1093 comprises applying a median filter to the sensor measurements. The result of the filtering step is illustrated inFIG. 4(b), and it can be seen that spike in the measurements at around time=550 seconds has been removed.
• Then, instep1095, the filtered skin conductivity measurements are matched to a predetermined pattern corresponding to a skin conductivity response associated with a (any) stressful event, such as an accidental fall, using a matched filter. The output is a set of coefficients that indicate the match of the measurements to the pattern. Each coefficient represents the match of a number of consecutive measurement samples (covering a time period of the same length as the predetermined pattern) to the predetermined pattern. The higher the coefficient, the better the match of the measurements to the pattern (and therefore the greater the chance that a stressful event, including a fall, has occurred).
• In one embodiment, this step is implemented using a matched filter that is DC-free. This means that the DC component in the measurements from theskin conductivity sensor14 will not adversely affect the filtering, which will produce a ‘0’ skin conductivity coefficient for non-variant (or very low variant) skin conductance.
• An exemplary predetermined pattern, i.e. impulse response of the matched filter, that represents the skin conductivity response that would be measured at the wrist of a person who suffers a fall or other sudden stress-inducing event can be given by the function
• 
  match_—filter=A(tⁿ=B)e^−t/τ−DC_remove  (1)
• where t is time (0≦t≦30), A is a scaling factor that determines the height of the initial part of the pattern (i.e. how strong the skin conductivity response will be), n and B are further scaling factors that determine the gradient of the initial part of the pattern (i.e. how quickly the skin conductivity will increase), τ is a scaling factor that determines the gradient of the part of the pattern after the initial peak in the pattern (i.e. how quickly the skin conductivity returns to the initial level) and DC_remove is the DC component excluded from the pattern.
• This function is illustrated inFIG. 5 and shows that there is a large steep increase in the skin conductivity measurement following a stressful event, such as a fall, followed by a slow recovery, with A, n, B, τ and DC_remove taking the values, 1/150, 300, 3, 3 and 2 respectively. It has been found that the recovery of skin conductivity happens relatively quickly after normal emotional arousal, whereas recovery is much longer after an event in which the person is scared or stressed, such as a fall. The recovery or decay in skin conductivity after such an event can occur in two stages after the skin conductivity maximum, with the decay coefficients differing by at least a factor of 10. The y-axis inFIG. 5 has no unit, but it's scale is the same as the skin conductance measurement inFIG. 4(a) (i.e. a difference of 1 unit in the y-axis direction ofFIG. 5 corresponds to a difference of 1 Siemens difference in the y-axis direction inFIG. 4(a). An exemplary output of the matched filtering process is shown inFIG. 4(c). It will be appreciated that the values of A, n, B, τ and DC_remove can be varied from the exemplary values given above depending on available data (perhaps user-specific data) relating to the response of skin conductivity to a fall or other stress-inducing event. An optimization technique, such as higher order degree polynomial regression, can be used to fit the curve of equation (1) to the available data.
• It will also be appreciated by those skilled in the art that the predetermined pattern can be represented by alternative functions to that shown in equation (1). For example, any function that defines a curve having characteristics similar to that shown inFIG. 5 can be used (i.e. a sharp increase at the start, followed by a gradual recovery). Also, different curves can be used, with theprocessor8 in thefall detector2 selecting one for use based on the estimated severity of the fall. For example, where there is a high impact value deduced from the accelerometer data, a response curve (predetermined pattern) corresponding to high tension can be used. In addition or alternatively, the predetermined pattern can be personalized to the user, since different people exhibit different skin conductivity levels and responses to different events (although generally the differences concern the shape, rather than the magnitude).
• The skin conductivity coefficients used in subsequent processing are determined by reversing the magnitude of the coefficients output by the matched filter (seeFIG. 4(d)).
• Theprocessor8 can then analyze the skin conductivity coefficients to provide an indication of whether the user has potentially suffered a fall (step1097). In particular, theprocessor8 can analyze the coefficients within an analysis window that spans the skin conductivity measurements obtained during a predetermined period after an altitude drop of more than a predetermined value has been observed. The predetermined period can be 15 seconds, although any suitable time period can be used. The predetermined value for the altitude drop may be 0.5 meters, although this value can be set based on user-specific parameters, such as the height of the user. An exemplary 15-second analysis window is shown inFIG. 6. It will be appreciated that in the embodiment where skin conductivity measurements are only collected following the detection of a change in altitude of at least 0.5 m or a possible impact, the analysis window will start effectively when the skin conductivity sensor starts to measure the skin conductivity (i.e. there will not be any data to the left of the analysis window inFIG. 6).
• In some embodiments, theprocessor8 analyses the skin conductivity coefficients in the analysis window by determining any one or more of the following: (i) the maximum value of the coefficients in the analysis window (ii) the mean of the coefficients in the analysis window, and/or (iii) the median of the coefficients in the analysis window. Theprocessor8 can analyze individual ones of the features listed above to determine if there has been a skin conductivity response consistent with a stressful event, such as a fall, having taken place, or multiple ones of the features listed above, for example by comparison of the features with respective thresholds. In that case, theprocessor8 can determine whether there has been a skin conductivity response based on a combination or a weighted combination of the derived features.
• It will be appreciated that the final decision by theprocessor8 on whether the user has suffered a fall can be based on a combination or weighted combination of the results of the analysis of the measurements from the movement sensor(s)4,6 and the analysis of the measurements from theskin conductivity sensor14.
• In an alternative embodiment of the invention, a fall detector is provided that determines if the user has potentially suffered a fall based on measurements from a single sensor (specifically a skin conductivity sensor14). In this embodiment, the measurements from theskin conductivity sensor14 can be analyzed as described above to provide an indication of whether the user has potentially suffered a fall. Thus, the measurements of the skin conductivity are matched to a predetermined skin conductivity response corresponding to a stressful event to determine if the user has experienced such an event. Although this fall detector may not necessarily be able to distinguish between a fall and other types of stressful event that might be experienced by the user, the analysis of the skin conductivity measurements described above would still allow stressful events (including falls) to be identified from other types of event (including normal emotional arousal) that lead to a or no significant change in skin conductivity. Although a fall detector according to this embodiment of the invention might provide some false positive indications (because non-fall stressful events are also identified by the analysis of the skin conductivity measurements) this is still a useful embodiment since the rate of false negatives (i.e. where there would be no indication of a fall when in fact the user has fallen) will be quite low.
• There is therefore provided a method for detecting a potential fall and a fall detector that provides increased fall detection reliability compared to conventional techniques.
• While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.
• Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims (15)

1. A fall detector for detecting a potential fall by a user, the fall detector comprising:

a sensor for measuring the skin conductivity of the user; and

a processor for analyzing the skin conductivity measurements to determine whether the user has had a potential fall, wherein the processor is configured to use a matched filter to match the skin conductivity measurements to a predetermined skin conductivity response corresponding to a stressful event to generate skin conductivity coefficients, the value of each coefficient indicating the match of a respective set of consecutive skin conductivity measurements to the predetermined skin conductivity response; and to determine whether the user has had a potential fall by analyzing the skin conductivity coefficients.

2. (canceled)

3. A fall detector as claimed inclaim 1, wherein the processor is configured to analyze the skin conductivity coefficients to determine whether the skin conductivity measurements are consistent with a user having had a fall by determining one or more of (i) the maximum value of the coefficients, (ii) the mean of the coefficients, and/or (iii) the median of the coefficients.

4. A fall detector as claimed inclaim 1, wherein the processor is configured to filter the skin conductivity measurements to remove noise and/or interference prior to matching the skin conductivity measurements to the predetermined skin conductivity response.

5. A fall detector as claimed inclaim 4, wherein the processor is configured to use a median filter to remove the noise and/or interference from the skin conductivity measurements.

6. A fall detector as claimed inclaim 1, further comprising at least one movement sensor for sensing the movement of the user, the processor being further configured to analyze measurements from the at least one movement sensor to determine if the user may have fallen.

7. A fall detector as claimed inclaim 6, wherein the processor is configured to analyze the skin conductivity measurements only in the event that the measurements from the at least one movement sensor indicate that the user may have fallen.

8. A fall detector as claimed inclaim 6, wherein the processor is configured to use the skin conductivity sensor to measure the skin conductivity in the event that the measurements from the at least one movement sensor indicate that the user may have fallen.

9. A fall detector as claimed inclaim 6, wherein the processor is configured to analyze the skin conductivity measurements obtained during a time window following a time at which the measurements from the at least one sensor indicate that the user may have fallen.

10. A method of detecting a potential fall by a user, the method comprising:

measuring the skin conductivity of the user; and

analyzing the skin conductivity measurements to determine whether the user has had a potential fall by using a matched filter to match the skin conductivity measurements to a predetermined skin conductivity response corresponding to a stressful event to generate skin conductivity coefficients, the value of each coefficient indicating the match of a respective set of consecutive skin conductivity measurements to the predetermined skin conductivity response, and determining whether the user has had a potential fall by analyzing the skin conductivity coefficients.

11. (canceled)

12. A method as claimed inclaim 11, wherein analyzing the skin conductivity coefficients to determine whether the response is consistent with a user having had a fall comprises determining one or more of (i) the maximum value of the coefficients, (ii) the mean of the coefficients, and/or (iii) the median of the coefficients.

13. A method as claimed inclaim 10, further comprising the steps of:

sensing the movement of the user; and

analyzing the movement of the user to determine if the user may have fallen.

14. A method as claimed inclaim 13, wherein the step of analyzing the skin conductivity measurements is performed only in the event that the step of analyzing the movement of the user indicates that the user may have fallen.

15. A computer program product comprising computer readable code embodied therein, the computer readable code being configured such that, upon execution by a suitable computer or processor, the computer or processor performs the method claimed ofclaim 10.

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