Sensor assay

Abstract

A method for determining an unworn state of a sensor system comprising a pair of sensors configured for mounting on first and second body parts on either side of a joint is disclosed. The method comprises obtaining one or more measurements from each sensor; calculating a relative angle between the two sensors using one or more of the obtained one or more measurements; calculating a joint angle between the first and second body parts using one or more of the obtained one or more measurements, wherein the joint angle is defined within a normal plane of flexion of the joint; and determining whether either of the sensors is not mounted based on the calculated angles, and if so, determining that the system is in an unworn state. A system for providing information about a joint is also disclosed.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to UK Patent Application No. 1915139.8, filed on October 18, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a method for determining the unworn state of a system for providing information about a joint. Background Technology

[0004] Devices for measuring movement are becoming increasingly popular. These sensing devices can take the form of wearable devices that measure a user's movement, smartphones worn by the user to measure their movement, or mobile devices that can generally sense movement, such as video game controllers or sensors attached to industrial equipment. In particular, wearable devices can be used to track the movement of humans or other animals, and especially to monitor the movement of specific joints.

[0005] These sensing devices may include satellite positioning sensors that can sense the position of the device, and one or more motion sensors that sense the movement and / or orientation of the device. These motion sensors may include one or more of an accelerometer, gyroscope, magnetometer, compass, and barometer. Measurements obtained by the sensors can be used to provide information about the joint.

[0006] When using wearable devices, it may be necessary to use them for extended periods, such as a month or longer, to accumulate data that changes slowly only over time. This means that any sensing devices used may need to be removed for any number of reasons, including but not limited to: the need to recharge the device's power supply, the desire to clean the sensors to remove accumulated dust, dirt, or spills, or the need to wash the area on the person or animal to which the sensors are attached. Such sensing devices may also fall out. Including measurements taken when the device has been removed or otherwise detached from its intended location when providing information could be misleading in monitoring problematic joints.

[0007] Therefore, improvements are expected in how wearable sensors are operated. Summary of the Invention

[0008] According to a first aspect of the disclosure there is provided a method for determining an unworn state of a sensor system, the sensor system comprising a pair of sensors configured for mounting on first and second body parts on either side of a joint, the method comprising: obtaining one or more measurements from each sensor; calculating a relative angle between the two sensors using one or more of the obtained one or more measurements; calculating a joint angle between the first and second body parts using one or more of the obtained one or more measurements, wherein the joint angle is defined within a normal plane of flexion of the joint; determining whether either or both of the sensors are not mounted based on the calculated angles, and if so, determining that the system is in an unworn state.

[0009] In some variations, obtaining one or more measurements from each sensor can be performed a plurality of times over a period of time and the calculating a relative angle and the calculating a joint angle can be performed using measurements obtained at least some of the plurality of times. The method can comprise the further step of, after determining that the sensor system is in an unworn state, adopting a different procedure for calculating the joint angle at subsequent times. The different procedure can comprise any one or more of: not calculating the joint angle at subsequent times; processing the calculated joint angles at subsequent times differently from those calculated at times prior to the determination; not storing the joint angles calculated at subsequent times.

[0010] The method can further comprise associating the joint angles calculated at a plurality of times with the relative angle to provide information about the joint over the period of time, and adopting a different procedure can comprise omitting from the information any data calculated using measurements obtained at subsequent times. The information can comprise any one or more of: a change in joint angle during the period of time; a duration of activity of the joint; a duration for which the joint is under load; a length of the period of time; and, if the joint is a knee joint, a step count over the period of time.

[0011] Adopting a different procedure can comprise performing calculations to provide information about the joint different from the information, wherein the different information comprises any one or more of: unworn time; and, if the first sensor is determined to be on charge, charge time.

[0012] The not-worn state can comprise any one or more of: the first sensor has been at least partially removed from its mounting location; the first sensor has fallen at least to some extent from its mounting location; the first sensor is turned on but not mounted on the first body part; the second sensor is turned on but not mounted on the second body part.

[0013] Determining whether the first sensor is mounted on the first body part can comprise calculating a value of a function of the calculated angle. The method can further comprise processing a plurality of calculated values of the function over a certain time period to determine a moving average value of the function, and wherein the determining can further comprise comparing the determined moving average value to a threshold value. The function can comprise a function of a joint angle penalty and a relative angle penalty. In some implementations, the joint angle penalty can be zero if the joint angle is within a feasible range for the joint and can have a value depending on how far the joint angle is outside the feasible range if the joint angle is not within the feasible range. In some implementations, the relative angle penalty can be zero if the relative angle is less than or equal to a relative threshold value and can depend on how much the relative angle is greater than the relative threshold value if the relative angle penalty is greater than the relative threshold value.

[0014] The relative angle can be a difference in tilt angles of the two body parts, wherein the tilting occurs about an axis that is perpendicular or substantially perpendicular to the plane defining the joint angle.

[0015] The joint angle can be defined in an x-z plane and the tilt angle can be defined in an x-y plane and the tilt angle can not be zero if the first body part and the second body part are relatively side-tilting about an x-axis.

[0016] The method can further comprise, prior to the obtaining, determining whether the first sensor and the second sensor are mounted on the correct respective first and second body parts and / or are substantially mounted in a predetermined orientation relative to the correct respective first and second body parts, and if one or both sensors are determined to be mounted on the wrong body part or in an orientation different from the predetermined orientation, adjusting the calculating step to take into account the actual determined mounting location and / or orientation.

[0017] The method can further comprise, prior to the obtaining, calibrating the first sensor and the second sensor with respect to a predetermined orientation relative to the respective first and second body parts.

[0018] The joint can be a knee joint or an elbow joint. The joint can be another joint such as a shoulder joint or an ankle joint.

[0019] Any of the above methods and features can be used in any feasible combination.

[0020] According to a second aspect of the application, there is provided a system for providing information about a joint, the system comprising: a first sensor unit configured to be mounted on a first body part on a first side of a joint, the first sensor unit comprising: one or more first sensors arranged to take one or more measurements relating to the first body part and one or more measurements relating to the first sensor; and a transmitter arranged to transmit the taken measurements; and a second master sensor unit configured to be mounted on a second body part on a second side of a joint, the second master sensor unit comprising: one or more second sensors arranged to take one or more measurements relating to the second body part and one or more measurements relating to the second sensor; a receiver arranged to receive the measurements transmitted from the first sensor unit transmitter; and a computing unit configured to calculate, using one or more of the measurements, a relative angle between the two sensors and a joint angle between the first body part and the second body part, wherein the joint angle is defined within a normal bending plane of the joint, the computing unit further configured to determine, based on the calculated angles, whether either or both of the sensor units are not mounted, and if so, that the system is in an un-worn state.

[0021] The first and second sensor units can be configured to take measurements a plurality of times over a time period, and the computing unit can be configured to calculate the relative angle and the joint angle using the measurements obtained at each of at least some of the plurality of times. The computing unit can be further configured to employ a different procedure for calculating the joint angle at subsequent times after determining that the system is in an un-worn state. The computing unit can be further configured to correlate the joint angles calculated at the plurality of times with the relative angle to provide information about the joint over the time period, and in some implementations, employing a different procedure can comprise omitting any data relating to measurements taken at subsequent times from the information.

[0022] The second master sensor unit can be configured to provide the information to a mobile device for viewing by a user. The information can be used to assess the health and / or rehabilitation of the joint.

[0023] In some examples, the first and second sensor units can be substantially planar with a front side and a back side, and the back side of the sensor units can be configured to be mounted on the first and second body parts at a side of the joint substantially parallel to the normal bending plane of the joint.

[0024] Any of the above features can be used in any feasible combination. BRIEF DESCRIPTION OF DRAWINGS

[0025] The application will now be described by way of example with reference to the accompanying drawings, in which:

[0026] Figure 1 A diagram of a joint is shown;

[0027] Figure 2 A directional reference frame for a joint is shown;

[0028] Figure 3 A pair of sensors fitted on either side of a joint according to some implementations is shown;

[0029] Figure 4 A schematic diagram of a system for monitoring a joint is shown;

[0030] Figure 5 A diagram showing some functions of a system for monitoring a joint is shown;

[0031] Figure 6 One of the sensors being removed is shown; Figure 3

[0032] Figure 7a shows a plot of knee angle;

[0033] Figure 7b shows a plot of roll angle; and

[0034] Figure 8 A method according to some implementations is shown.

[0035] In the drawings, like reference numerals refer to like parts. DETAILED DESCRIPTION

[0036] ​The current subject matter relates to a method for determining whether a body sensor is in an unworn state and a system for implementing said method. The method relates inter alia to a system having a pair of sensors mounted on either side of a joint, wherein the sensors are used to monitor the angle of the joint over time. For example, if a person has suffered a joint injury or has had surgery on a joint, it can be that the person is unable to bend the joint through the full range of movement of a healthy joint. By monitoring the movement of the joint over time, a picture can be built up of the level of activity of the person and whether the range of movement of the joint has improved.

[0037] If this information is to be useful, it is preferred that the information only includes data obtained when the sensors are mounted in their intended position on the person. If data obtained when not in this position is included in the information, this can bias the information such that it becomes less useful in monitoring the joint over time. Embodiments of the present invention aim to avoid such bias in the information.

[0038] A specific example is described below with respect to the use of sensors in relation to a knee joint on a human. However, the underlying principles are applicable to many different joints such as the ankle joint, elbow joint or wrist joint and can also be applied to joints associated with other animals.

[0039] There is provided Figure 1 and Figure 2 Certain terms used within this specification are provided to allow for simple explanation. Figure 1 A standard leg is shown having a femur 1, a tibia 2 and a fibula 3. These are joined at a knee joint 4. The femur 1 defines a femoral mechanical axis 5 which extends from the knee to a ball joint 6 which forms part of the person's hip. A tibial mechanical axis 7 extends from the knee 4 to the lower end 8 of the tibia 2 itself. The femur and calf (made up of the tibia 2 and fibula 3) can pivot relative to each other about a knee joint axis 9. The femur and calf thus define a plane about which the respective mechanical axes pivot relative to each other. Thus, each mechanical axis will be substantially aligned with the respective part of the leg such that the knee joint axis 9 is perpendicular to the plane about which the axes pivot. The knee angle is thus generally the angle between the two mechanical axes. This is an idealised case which forms the basic geometry considered by the present invention. It is possible that one or more compensation schemes can be applied to account for any misalignment between the axes and the respective parts of the leg.

[0040] Figure 2A coordinate system associated with the knee is helped to be defined, and how the terms pitch and roll are applied to the knee. The following convention exists when discussing the knee: when a person is standing straight, the x-axis points forward, i.e. away from the knee in the direction of walking, parallel to the ground, the y-axis points to the right of the person, and the z-axis points downward to the ground. This convention applies to both the left leg and the right leg, i.e. the positive y-axis always points to the right-hand side of the knee, regardless of the leg. The y-axis is thus similar to the knee joint axis 9.

[0041] The orientation of any sensor associated with the knee has two components in general. The rotation of the sensor around the x-axis is the roll motion, identified by arrow 18, and defines the roll angle. The rotation of the sensor around the y-axis is the pitch motion, identified by arrow 19, and defines the pitch angle. The third component, the rotation of the sensor around the z-axis will be the yaw motion, identified by arrow 20, and will define the yaw angle.

[0042] Figure 3 A pair of sensors 10 attached to the leg 11 is shown. Each sensor contains one or more motion sensing devices that permit (i) to determine the pitch and / or roll of the individual sensor, or (ii) to determine the relative pitch and / or roll between the sensors. These motion sensing devices can be any suitable devices, such as but not limited to accelerometers, gyroscopes, or a pair of strain gauges.

[0043] The upper sensor 10a is placed on the thigh 12, and the lower sensor 10b is placed on the shank 13. The purpose of the sensors is to monitor the bending of the knee at the knee joint, i.e. the pitch angle around the y-axis / knee joint axis 9. If the two sensors 10a, 10b can be aligned such that the z-axes of the sensors are parallel to the respective mechanical axis of the leg, and the sensor y-axes are parallel to the knee joint axis 9, the calculation of the knee angle will be a simple subtraction of the shank pitch angle from the thigh pitch angle. In practice, there can be misalignments with the femoral and tibial mechanical axes that need to be corrected in order to obtain an accurate knee angle measurement.

[0044] In instances where a patient has had a total knee replacement or any other knee surgery that actually results in limited movement of the knee or knee discomfort, it can be helpful for the healthcare professional or even the patient themselves to monitor the knee angle over a longer period of time such as weeks or even months. As a result, additional issues can arise because for a number of reasons it can be necessary to remove the sensor on a regular basis, including but not limited to cleaning the sensor, recharging the sensor, improving patient comfort at night or cleaning the patient at the location of the sensor. When the sensor is removed or actually if the sensor falls off or comes off the location where the knee angle is being monitored, the sensor will be said to be in an unworn state. It would be beneficial to know whether one or both of the sensors goes into an unworn state because this indicates that the sensor system of which the sensor forms a part is in an unworn state. As a result, this can be taken into account when presenting data for monitoring. Systems and methods for detecting an unworn state are described hereinafter.

[0045] Figure 4 The sensor system 400 is schematically shown, including the knee sensors 10, some components of the knee sensors 10 and the operative connections to and from the sensors and components thereof. It will be appreciated that the sensors 10 can each form part of a sensor unit configured to be mounted on a body part on one side of a joint as described with reference to Figure 3 Thus, each sensor 10 can form part of or be attached to a patch which can be applied to the skin or can be used in conjunction with an adhesive or other attachment means to enable the sensor to be mounted to the body part.

[0046] The two sensors 10a, 10b include identical or similar components, which in each case are denoted by the same reference numerals and a or b, respectively. These components are: power receivers / voltage regulators 402, which are connected to battery chargers and monitors 404, which are connected to batteries 406 (e.g., lithium ion batteries or other suitable energy storage devices), which are connected to power supplies and support circuits 408; temperature sensors 410, which can in fact include any of a number of temperature sensors, e.g., an external temperature sensor for sensing the temperature of the skin on which the sensor unit is mounted, one or more internal temperature sensors for sensing the temperature of one or more components of the sensor itself; power control / push switch logic units 412, which are connected to on-off push switches 414; inertial measurement units (IMUs) 416, which include accelerometers 418 and gyroscopes 420; and Bluetooth Low Energy (BLE) modules. All of the components discussed above are connected to a microcontroller 424. The connections on the aforementioned sensors are shown in solid line arrows. It will be appreciated that additional components can be present in the sensors 10, such as magnetometers (for determining absolute movement and orientation of the patient), visual indicators regarding status, etc., but these components are omitted for the sake of clarity.

[0047] The sensors 10 have additional external connections. The power receivers / voltage regulators 402 can be connected to charging units 426. For example, they can be placed in respective docking ports 428 for charging when needed (dashed lines indicate these connections). The two BLE modules can communicate wirelessly. In implementations in which the sensor 10b acts as the master unit, the BLE module 422a of the sensor 10a can transmit data to the BLE module 422b of the sensor 10b. This will be described below with reference to Figure 5 This functionality is described in more detail. The BLE module 422b of the sensor 10b can transmit data to a mobile phone 430, or other device on which information relating to the monitoring of the knee or other joint can be displayed, e.g., by interacting with an app running on the mobile phone 430. Such an app can be downloaded and installed and can be dedicated to interacting with the sensor 10 and enabling viewing of information relating to the joint being monitored. These transmissions are indicated by chain lines.

[0048] In operation, the accelerometers 418a, 418b and gyroscopes 420a, 420b of the IMU 416 can take measurements relating to the orientation and changes in orientation of their respective sensor 10, and the movements experienced by their respective sensor 10 as a result of the movements of the body part to which the sensor 10 is attached. These movements reflect the use of the knee by the person to which the sensor 10 is attached during a certain period of time. If desired, information calculated from these measurements can be provided, along with other sensed information such as readings provided by one or more temperature sensors 410a, 410b, to indicate the functioning of the knee during the period of time. Other desired information can be indicated in conjunction with the knee function information, for example, the knee temperature. In particular, it is possible to calculate the knee angle from the accelerometer and gyroscope measurements, for example by calculating the pitch and roll angles through which the sensor 10 passes, and thus, the changes in knee angle over a certain period of time, such as a day, can be determined. One way of calculating the knee angle using measurements from accelerometers and gyroscopes is discussed in the paper by Madgwick et al. entitled "Estimation of IMU and MARG orientation using a gradient descent algorithm" from the 2011 IEEE International Conference on Robotics, the contents of which are incorporated herein by reference. Those skilled in the art will appreciate that other methods and algorithms can be used for this purpose. Furthermore, the skilled reader will appreciate that other types of measurement devices that can determine roll and pitch between two points can be used in place of the IMU 416.

[0049] Turning to the Figure 5 Some of the functions of the sensor 10 that facilitate the data recording and calculation performance referred to above will now be explained. Figure 5 Some of the functions of the first sensor 10a and the second sensor 10b are shown schematically. It will be understood that the functions shown and described below are not an exhaustive list of all functions that can be implemented by the sensors, and that the function blocks shown are a simplified representation of some of the actual functions that can be performed by the sensors 10a, 10b and the like for the purpose of monitoring a joint. In a similar manner to Figure 4 In a similar manner to

[0050] Both sensors 10a, 10b share some common functionality. Indicated by way of some measurement functionality that can be performed by the one or more temperature sensors 410 and the IMU 416 through the sensors 10a, 10b. Raw measurements from the one or more temperature sensors 410 and the IMU 416 are provided to a data conversion unit 532 which processes the raw measurements. The processed measurements can be passed to a calibration unit 534 which can hold information relating to a previous calibration that can have been performed for the sensor 10, including corrections for any misalignment with the femur and tibia. Such calibration can be updated periodically. The processed and calibrated data can be passed to an orientation estimation unit 536 which determines the orientation of the sensor 10. In some implementations this includes the pitch and roll experienced by the sensor 10. The orientation estimation unit 536 can make use of methods such as the methods discussed above with reference to Figure 4 The calculated parameters such as pitch and roll data can be passed to a transmitter such as the orientation data packing unit 538 for onward transmission. Such onward transmission can make use of a wired connection or a wireless connection such as a Bluetooth transmission or a radio transmission. The data conversion unit 532, the calibration unit 534, the orientation estimation unit 536 and the orientation data packing unit 538 can be considered to provide functionality generally referred to as a sensing algorithm unit 540.

[0051] It will be seen in Figure 5 that the second sensor 10b, which is shown in Figure 3 as being mounted on the lower leg 13 of the patient, has some additional functionality not present in the first sensor 10a, which is shown in Figure 3 as being mounted to the upper leg 12 of the patient. In this implementation the second sensor 10b is the master sensor and is configured for mounting on the patient at a lower position than the first sensor 10a. It will be appreciated that the master sensor could instead be the first sensor 10a applied to the upper leg of the patient and thus configured to be mounted at an upper position to the second sensor 10b. The master sensor can be referred to as a fusion node and the other sensor can be referred to as a source node.

[0052] The further functionality present in the second sensor 10b can generally be referred to as a metrology algorithm unit 542. Within the metrology algorithm unit 542 there are various functions discussed below. There is a receiver such as a first orientation data unpacking unit 544 arranged to receive data from the orientation data packing unit 538a of the first sensor 10a; a receiver such as a second orientation data unpacking unit 546 arranged to receive data from the orientation data packing unit 538b of the second sensor 10b. There is a computation unit 548 comprising a knee angle estimation unit 550 and a wear classification unit 552. Both the knee angle estimation unit 550 and the wear classification unit 552 can receive data relating to the angles of the sensors 10 from the first orientation data unpacking unit 544 and the second orientation data unpacking unit 546. The wear classification unit 552 can additionally receive output from the knee angle estimation unit 550. The IMU 416 is again functionally indicated as providing input to the metrology algorithm unit 542, specifically into a step count unit 554. The knee angle estimation unit 550, the wear classification unit 552 and the step count unit 554 are all functionally connected to an activity of daily living (ADL) unit 556 which can transmit data to the mobile phone 430 or other display device. Such transmission is indicated by the chain line.

[0053] In operation, once installed in place on a person's leg 11, the sensors can begin taking measurements. One option for triggering this process is to use the switch 414 to turn on the sensors 10. Another additional or alternative option is for one or both of the sensors 10 to begin interacting with the above-mentioned app on the mobile phone 430. The temperature sensor(s) 10 can begin taking temperature readings. The accelerometer 418 and gyroscope of the IMU 416 can begin taking orientation information for the sensor 10, which can include information relating to changes in roll, pitch, and yaw (if needed) angles. It is convenient to take these measurements on a regular basis so that multiple measurements can be taken over a period of time that the sensor 10 is worn. For example, measurements can be taken at 50 times per second (i.e. at 50 Hz) or at some other suitable interval. The measured data passes through various functional units common to both sensors 10 until, ultimately, the orientation data packaging unit 538 processes the data into a transmittable form. After passing to the metrology algorithm unit 542 within the second sensor 10b, specifically to the orientation data unpacking unit 544, 546, the various functional blocks of the metrology algorithm unit 548 can process the data along with the temperature data. The knee angle estimation unit 550 can use this data to calculate the knee angle at at least some of the times that measurements are taken. The wear classification unit 552 can calculate the roll and pitch angles of the sensor 10 at at least some of the times that measurements are taken. These calculations are then correlated in the ADL unit 556. This unit stores a summary of the metrics calculated from the obtained measurements, which can include any of the following: knee angle, step count, activity time, load time, and wear time. At each time that a measurement is taken, the knee angle can be stored as a value, but in practice, to provide a more useful output, it can be more relevant to correlate this data over a period of time. For example, the data can be stored as a histogram in 5 degree buckets, to show the time spent in each bucket. The skilled reader will think of other possibilities for presenting the knee angle data. Depending on the storage capacity of the ADL unit 556, it can be preferable to store only the correlated data rather than the raw measurements. The ADL unit 556 can operate to store data during the entire period of time that the sensor 10 is worn, giving an indication of the function of the knee during the patient's daily activities. This data can be uploaded to the mobile phone 130 when required, for example, on request or at the end of a period of time. The data can also be further correlated over a number of periods of time, for example, over a week. If desired, the data can also be correlated over a particular period of time within the period of time that the sensor is worn, for example, during an exercise session. Such shorter bursts of data can be streamed to the mobile phone 130. Any data uploaded to the mobile phone 430 can be processed and viewed via the app as required.

[0054] As mentioned above, it is not desirable to present data based on measurements taken when one or both of the sensors 10 are not mounted in their intended locations, as such data can be misleading in terms of the function of the knee. To address this issue, the present system includes an undoneness detection system. Reference is made to Figure 6 It will be appreciated that if one of the sensors 10 is removed, its orientation will change dramatically. This fact is used in the present system to enable detection of such removal. The systems and methods described hereinafter can be used after it has been previously determined that the sensors are worn, for example after having been operated for any length of time as described above. Such systems and methods can also be used to determine that one or both of the sensors 10 are not mounted, but then determine that both of the sensors are mounted on the leg 11 and thus can be used in some cases instead of providing an on / off switch 414. Alternatively, such systems and methods can be used as an alternative to waiting for interaction with the app on the mobile phone 430. As another alternative, such systems and methods can be used in cases where the on / off switch 414 is used to turn on the sensors before they have been correctly mounted on the patient, so as not to take into account any measurements taken before the mounting is completed. The systems and methods can also be used in cases where one or both of the sensors 10 are dislodged from their intended locations, or fall off completely or to some extent. The systems and methods can be used after initial calibration of the sensors. The systems and methods can also be used after initial setup of the sensors. The systems and methods can also be used after determining whether the sensors are correctly placed or determining that there is a correctable error in the placement of the sensors. For example, as mentioned above, the first and second sensors 10 can include different functionality, and each can be configured to be placed in a specific orientation on a specific side of a joint. Thus, such systems and methods can be used to determine whether the sensors have been mounted on their respective correct body parts. Any of the possibilities discussed here can be used in any combination to determine that one or both of the sensors are not mounted and thus determine that the system is in an undoneness state. Such systems and methods will now be described.

[0055] Referring back to Figure 6exemplary first sensor 10a is being removed from the thigh 12 by peeling, such that the sensor 10a is tilted. The exemplary tilt shown is a roll movement. It will be appreciated that if the second sensor 10b is still mounted on the calf 13, then this movement will be inconsistent with the recorded movement of the other second sensor 10b mounted on the calf 13. This mismatch in the relative orientation of the two sensors 10 can be used to determine that the sensor system 400 is in an unworn state. Even if the second sensor 10b is subsequently removed, the relative orientation of the two sensors will be different from their relative orientation when mounted across the knee of the patient. This will also be the case if the second sensor is removed before the first sensor. It will also be the case if either of the sensors is removed in a different way, for example by peeling via a different tilting motion, such as a yaw motion, or via some combination of roll and pitch motions. In addition, in either of these cases, the knee angle as calculated using the pitch and roll of the sensors 10 will not return a feasible knee angle. In other words, the observed pitch and / or roll angles and / or knee angle can be used to indicate the relative position of the body parts, i.e. in this case the thigh and calf, onto which the sensors 10 are configured to be mounted, is infeasible or "unhumanized".

[0056] An exemplary algorithm for determining the unworn state will now be described. At each time step at which a measurement is taken, the wear classification unit 552 receives the current knee angle and roll angle of both sensors. Using these angles, a "penalty" is calculated, which is a measure of how unhumanized the angles are. This penalty is calculated as follows:

[0057] aKnee, aRoll_F, aRoll_S in degrees, where

[0058] k = aKnee (knee angle)

[0059] aRoll_F = roll angle of second sensor 10b (where F indicates a fusion sensor)

[0060] aRoll_S = roll angle of first sensor 10a (where S indicates a source sensor)

[0061] r = |aRoll_F - aRoll_S|

[0062] p_k = knee angle penalty

[0063] p_r = roll angle penalty

[0064] p = penalty

[0065] The following conditional determination is made:

[0066] If -10 <= k <= 150, then p_k = 0

[0067] = 0.5*((k+10) / 160)^4, if k < -10

[0068] = 0.5*((k-150) / 160)^4, if k > 150

[0069] A feasible human knee angle is any angle between about 10 degrees and about 150 degrees, so if the angle is within this range, the penalty is zero, but if the angle is outside this range, there is a penalty.

[0070] If r <= 40, then p_r = 0

[0071] = 0.5*((r-40) / 140)^4, if r > 40

[0072] Thus, a relative tilt between the thigh 12 and the lower leg 13 of up to 40 degrees is considered to be feasible and the penalty is zero, but above this amount is considered to be infeasible or unhuman and there is a penalty.

[0073] It should be noted that the parameters chosen for the above calculations can affect the sensitivity of the system to unhuman orientations. The applicant has found that the more sensitive the system is set, the greater the risk of false classification of the worn / unworn state. The above parameters are shown by way of example only as parameters that the applicant has found to be advantageous after a large number of experiments. In general, the parameters can be chosen so that there are no false readings, but the unworn state is not missed.

[0074] P = P_k + p_r

[0075] Thus, the combination of the tilt penalty and the knee angle penalty is used to determine whether the sensor system 400 is in the unworn state.

[0076] Figures 7a and 7b show graphs of the knee angle penalty and the tilt angle penalty as a function of the input angle, respectively. In this example implementation, the final penalty is calculated as the sum of these two penalties, but different mathematical combinations of the knee angle penalty and the tilt angle penalty can be used. The final penalty is then processed into a moving average (e.g. as an exponential recursive filter with a forgetting rate of 0.05), which smoothes the value of the final penalty over time. If this moving average exceeds a threshold of 0.005, this indicates the unworn state.

[0077] After detecting an unworn state as described above, the sensor unit (e.g., metric algorithm unit 548) can switch in terms of how it functions for the unworn state. As discussed above, this is because any data collected by the IMU 416 in this case will be "malicious" data that does not indicate that the patient is moving their knee. Thus, in particular, for computing knee angles over time after detecting an unworn state, it is desirable to employ a different procedure than the normal operation described above with respect to Figure 4 and Figure 5 the knee angle over time is an important indicator of knee health, and thus the system is operable to avoid biasing such data because of the use of measurements from one or more unworn sensors. There are various options to exclude data after detection of an unworn state from the information provided by the ADL unit 556 and in particular the associated knee angle information. One option is to prevent the accelerometer 418 and gyroscope 420 from collecting data. Another option is to not pass the data to the data conversion unit 532, but instead to discard the data. Alternatively, the data can not be transmitted from the orientation data packing unit 538, or it can be transmitted but discarded upon receipt by the orientation data unpacking units 544, 546. Other options include not having the knee angle estimation unit 550 compute knee angles, or computing knee angles at subsequent times but having the wear classification unit 552 not pass the data, or pass the data but not include it in its association process by the ADL unit 556. The wear classification unit 552 can indicate the unworn state to the ADL unit 556 in various ways as will occur to the skilled reader.

[0078] One advantage of the present system and method is that it detects the unworn state without human intervention. Thus, it does not rely on a person having to remember to take any action to put the sensor into the unworn state.

[0079] After a non-wearing state of the sensors has been detected, as part of a different procedure, an alternative calculation can be made. For example, the ADL unit 556 can record the duration for which the sensor system 400 is in a non-wearing state. This can be determined from the time for which no data is received, or if data is continuously received, from the time until the wearing classification unit 552 indicates that one or more of the sensors is no longer in a non-wearing state. In some implementations, setting the sensor 10 to charge on the charging unit 426 can trigger a non-wearing state, and thus, similar principles can be used to measure the duration for which the sensor is charged. If the calculated penalty falls below a threshold, the sensor system 400 can return to normal operation. In some implementations, the sensor system 400 is set to wait for a user input before resuming normal operation, e.g. before operating an on / off switch on the sensor 10 or interacting the master sensor 10b with the app on the mobile phone 430. In this way, the sensor system 400 will only resume normal operation when the sensors 10 have been placed in position across the joint. In other implementations, an automatic return to normal operation can be implemented when the calculated penalty falls below a threshold, but it will be appreciated that even if not actually fitted to a joint, it is possible for two sensors 10a, 10b to be arranged relative to each other as if they were fitted across a joint. Thus, in order to prevent resuming normal operation in this case, some additional information needs to be gathered, e.g. a detection of skin contact.

[0080] Figure 8 A flowchart of a method according to an implementation of the present application is shown. At 810, measurement values are obtained from a pair of sensors mounted across a joint. For example, the sensors can be sensors 10 mounted on the thigh and the lower leg across the knee joint of a patient.

[0081] At 812, a relative angle between the sensors is calculated using the obtained measurement values. In the example discussed above, this is performed by the wearing classification unit 552. This can be an absolute angle difference around any of the roll, pitch or yaw axes. In the example described above, the angle is a roll angle. This calculation is simplified by placing both sensors 10 on the same side of the leg 11, so that both lie substantially in the plane of the knee angle.

[0082] At 814, a joint angle is calculated using the obtained measurement values. In the example discussed above, this is performed by the knee angle estimation unit 550. In the example described above, the roll and pitch angles can have been calculated using the measurement values, and these values can be used to calculate the knee angle. Thus, the knee angle is calculated indirectly using the obtained measurement values.

[0083] At 816, the calculated relative angle and knee angle are used to determine if any of the sensors are not worn. In the examples discussed above, such a determination can be made in the wear classification unit 552, which has determined the roll angle at 812 and can receive the knee angle from the knee angle estimation unit 550. The determination can instead be made in the ADL unit 556 or by a separate functional unit.

[0084] As discussed above, if a non-worn state is detected, various actions can be taken by the system to change the procedures subsequently employed with respect to the data provided about the knee. As also discussed above, a return to normal operation can be implemented subsequently.

[0085] It will be appreciated that many changes can be made to the examples discussed above without departing from the principles set forth in the above and in the appended claims. For example, the functional units can be arranged differently, and the calculations can be performed in different orders, or by variations of the exemplary methods described above. Reference is made above to Figure 4 The sensor components described can be of different types or used in different combinations, and some of the components described can be provided as a single component. While many of the examples above use the relative roll angle between two sensors 10, this is not essential and relative pitch or relative yaw can instead be used. Also, as mentioned above, in some implementations, mounting a sensor on the wrong limb can trigger a non-worn state for the system, as doing so would cause the knee angle to indicate that the knee is bent incorrectly. However, as an alternative to triggering a non-worn state in this scenario, a different state can be triggered, the actual mounting position is determined and taken into account during the processing of the data in the further normal operation. In some implementations, the non-worn detection system can be disabled before the normal operation of the sensor, and a different procedure used to determine that the sensor is on the wrong limb, triggering a different state. The non-worn detection system can then be enabled, so that subsequent removal of the sensor, etc. is detected.

[0086] In other variations, more than two sensors can be used. For example, if the joint under surveillance is a ball and socket joint with three degrees of freedom of movement, it can be desirable to use three sensors. In some cases, it can be desirable to use additional sensors on a joint such as a knee joint to assist in determining the orientation of the thigh and shank. For example, two sensors can be placed on one of the user's limbs. Measurements from this third sensor can be processed in a similar manner to the processing described above for two sensors. One possibility is to process the data from the two sensors placed on one limb to obtain a data set for that limb, and then process the data set as described above in conjunction with data from the other limb.

[0087] The functions of the microcontroller 424 described herein can be implemented in digital electronic circuitry, integrated circuitry, specially designed application-specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), computer hardware, firmware, software, and / or combinations thereof. These various aspects or features can include implementation in one or more computer programs that are executable and / or interpretable on a programmable system including at least one programmable processor, such as the microcontroller 424, which can be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, such as a memory stick or the like. Such computer programs (which can be software, firmware, applications, components, or code), include machine instructions for the programmable processor, and can be implemented in a high-level procedural and / or object-oriented programming language, and / or in assembly / machine language. As used herein, the term "machine-readable medium" refers to any computer program product, apparatus and / or device, such as for example magnetic discs, optical disks, memory, and Programmable Logic Devices (PLDs), used to provide machine instructions and / or data to a programmable processor, and which can be received as a machine-readable signal. Such a machine-readable medium can store the instructions permanently or transitorily, such as for example, in the static memory 426, or in a separate memory (not shown) and / or can store the instructions on a carrier wave as described herein. Furthermore, certain aspects of the application can be performed under the control of one or more computer systems configured with tangibly embodied computer programs, firmware, and / or software and / or virtual systems implemented via dedicated hardware as described herein. One skilled in the art will appreciate that the system 400 described herein can be implemented in a number of different configurations including as a stand-alone system, or in a networked system or as part of a more extensive system, and the application is not limited in this regard. Thus, the application can be practiced in a variety of system configurations, including hand-held devices, consumer electronics, general and special purpose computing devices, and the like.

[0088] The applicant hereby discloses all elements described herein and any combination of two or more such elements as if each were individually and expressly disclosed. It should be understood that any feature described herein as a means for performing a function is meant to encompass a corresponding structure for performing the function, and that the functions described herein are intended to encompass the means for performing the functions. The above description is that of current embodiments of the application. Various alterations, modifications and improvements will readily occur to those skilled in the art, and it is the said that the application is intended to include all such alterations, modifications and improvements as fall within the scope of the application. Accordingly, though the application is described with reference to specific on-premises and cloud-based embodiments, it is the applicant's intention to include all such variations and alterations within the scope of the application. The application is not limited to specific embodiments, methods, or implementations described herein. Rather, one skilled in the art can use features described herein to implement or practice the application in combination with other specific embodiments, methods, or implementations.

Claims

1. A method for determining an unworn state of a sensor system, the sensor system comprising a pair of sensors configured for mounting on first and second body parts on either side of a joint, wherein each sensor contains one or more motion sensing devices that permit (i) determination of pitch and / or roll of each sensor, or (ii) determination of relative pitch and / or roll between the sensors, the method comprising: obtaining one or more measurements from each sensor; using one or more of the obtained one or more measurements to calculate a relative angle between the two sensors, the relative angle being a difference in the angle of inclination of the two body parts; using one or more of the obtained one or more measurements to calculate a joint angle between the first and second body parts, wherein the joint angle is defined within a normal plane of flexion of the joint; and determining whether either or both of the sensors are unmounted based on the calculated relative angle and the calculated joint angle, and if so, determining that the system is in an unworn state.

2. The method of claim 1, wherein obtaining one or more measurements from each sensor is performed a plurality of times over a period of time and the calculating a relative angle and the calculating a joint angle are performed using measurements obtained at least some of the plurality of times, the method comprising the further step of, after determining that the sensor system is in an unworn state, employing a different procedure for calculating the joint angle at subsequent times.

3. The method of claim 2, wherein employing a different procedure comprises any one or more of: not calculating the joint angle at subsequent times; processing the calculated joint angles at subsequent times differently from those calculated at times prior to the determining; not storing the joint angles calculated at subsequent times.

4. The method of claim 2 or claim 3, further comprising associating the joint angles calculated at a plurality of times with the relative angles to provide information about the joint over the period of time, and wherein employing a different procedure comprises omitting from the information any data calculated using measurements obtained at subsequent times.

5. The method of claim 4, wherein the information comprises any one or more of: joint angle variation during the period of time; duration of joint activity; duration of loading of the joint; length of the period of time; and, if the joint is a knee joint, step count over the period of time.

6. The method of claim 4, wherein employing a different procedure comprises performing calculations to provide information different from the information about the joint, wherein the different information comprises any one or more of: unworn time; and, if the first sensor of the pair of sensors is determined to be on charge, charge time. ​ 7. The method of claim 6, wherein the non-wearing state comprises any one or more of: the first sensor has been at least partially removed from its mounting location; the first sensor has fallen at least to some extent from its mounting location; the first sensor is on but not mounted on the first body part; a second sensor of the pair of sensors is on but not mounted on the second body part.

8. The method of claim 7, wherein determining whether the first sensor is mounted on the first body part comprises calculating a value of a function of the calculated angle.

9. The method of claim 8, further comprising processing a plurality of calculated values of the function over a period of time to determine a moving average value of the function, and wherein the determining further comprises comparing the determined moving average value to a threshold value.

10. The method of claim 8, wherein the function comprises a function of a joint angle penalty and a relative angle penalty.

11. The method of claim 10, wherein the joint angle penalty is zero if the joint angle is within a feasible range for the joint and has a value depending on how far the joint angle is outside the feasible range if the joint angle is not within the feasible range for the joint.

12. The method of claim 10, wherein the relative angle penalty is zero if the relative angle is less than or equal to a relative threshold and depends on how much greater the relative angle is than the relative threshold if the relative angle penalty is greater than the relative threshold.

13. The method of any one of claims 1-3, wherein the tilt occurs about an axis that is perpendicular or substantially perpendicular to the plane that defines the joint angle.

14. The method of claim 13, wherein the joint angle is defined in an x-z plane and the tilt angle is defined in an x-y plane and the tilt angle is not zero if the first body part and the second body part are relatively laterally tilted about an x-axis.

15. The method of claim 7, further comprising, prior to the obtaining, determining whether the first sensor and the second sensor are mounted on the correct respective first body part and second body part and / or are substantially mounted in a predetermined orientation relative to the correct respective first body part and second body part, and if one or both sensors are determined to be mounted on the wrong body part or in an orientation other than the predetermined orientation, adjusting the calculating step to take into account the actual determined mounting location and / or orientation.

16. The method of claim 7, further comprising, prior to the obtaining, calibrating the first sensor and the second sensor with respect to a predetermined orientation relative to the respective first body part and second body part.

17. The method of any one of claims 1-3, wherein the joint is a knee joint or an elbow joint.

18. A system for providing information about a joint, the system comprising: a first sensor unit configured to be mounted on a first body part on a first side of a joint, the first sensor unit comprising: one or more first sensors arranged to take one or more measurements related to the first body part and one or more measurements related to the first sensor; and a transmitter arranged to transmit the taken measurements; and a second sensor unit configured to be mounted on a second body part on a second side of a joint, the second sensor unit comprising: one or more second sensors arranged to take one or more measurements related to the second body part and one or more measurements related to the second sensor; a receiver arranged to receive the measurements transmitted from the first sensor unit transmitter; and a computing unit configured to calculate, using one or more of the measurements, a relative angle between the two sensors and a joint angle between the first body part and the second body part, wherein the relative angle is a difference in inclination angles of the two body parts and the joint angle is defined within a normal bending plane of the joint, the computing unit further configured to determine, based on the calculated relative angle and the calculated joint angle, whether either or both of the sensor units are not mounted, and if so, that the system is in an un-worn state.

19. The system of claim 18, wherein the first sensor unit and the second sensor unit are configured to take measurements a plurality of times over a time period, and the computing unit is configured to calculate the relative angle and the joint angle using the measurements obtained at each of at least several of the plurality of times, the computing unit further configured to employ a different procedure for calculating the joint angle at subsequent times after determining that the system is in an un-worn state.

20. The system of claim 19, wherein the computing unit is further configured to correlate the joint angles calculated at the plurality of times with the relative angle to provide information about the joint over the time period, and wherein employing a different procedure includes omitting any data related to measurements taken at subsequent times from the information.

21. The system of claim 20, wherein the second sensor unit is configured to provide the information to a mobile device for viewing by a user.

22. The system of claim 20, wherein the information is used to assess the health and / or rehabilitation of the joint.

23. The system of any one of claims 18 to 22, wherein the first sensor unit and the second sensor unit are generally planar with a front and a back surface, and the back surface of the sensor unit is configured to be mounted substantially parallel to the normal bending plane of the joint on the first body portion and the second body portion at one side of the joint.

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