LLDN Health FCL SensingFloor

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The sensing floor (Future Care Floor)

The concept of the “Future Care Floor” is that of an intelligent floor that detects characteristic walking patterns, fall events or other abnormal movement behaviors that would indicate an emergency situation for the user. In case that such an emergency situation is detected the system may contact a relative or professional medical personnel. Thus, users do not have to activate the emergency call themselves, which in a lot of cases is not possible, for example when the person is immobile after the downfall or even lost his conscience. Furthermore, older users with high risk for downfalls have an alternative to portable emergency buttons, which are often perceived to be stigmatizing and have a low compliance. The floor is equipped with a grid of piezoelectric elements. When a force is applied to the piezo it will deform and its atomic structure shifts. This causes a charge transfer and a voltage proportional to the applied force is induced within the piezo. This voltage signal we measure between the two poles of the sensor element. In order to achieve a good resolution, a net of 240 piezo elements was installed under the 20 m2 floor surface of the test lab environment. The underlay structure of the floor is a metal grid consisting of steel sections which form squares of 0,6 x 0,6 m2. At all cross points of this metal grid four piezo elements are installed, they serve as free support for the floor tiles. The floor tiles have a dimension of 0,6 x 0,6 m2 aswell and a wooden upper surface and a metal basis. So in each of the four corners of every tile a sensor is positioned and gives information about the force applied to the tile. In order to guarantee good signal quality and safe bedding, the piezo elements are positioned in a custom made Perspex support structure. The support structure has a height of 5 mm, which makes the actual sensor part very thin and opens the possibility of installing the sensor floor within existing home environments. Due to the geometry of the support structure primarily bending stress is applied to the piezo element when a user walks on the floor, which result in better signal quality. The voltage signal induced by mechanical deformation of the piezo material changes according to the type of load that is applied to the panel, which is the basis for robust fall detection and pattern recognition. All sensors are directly wired to operation amplifiers. We use a setup of 15 operation amplifier boards to connect all 240 sensor units. The operation amplifier circuit consists mainly of a logarithmic unit and a voltage adjustment unit. We use 15 Arduino Mega microcontroller boards with serial interface to carry out the analog-digital conversion of the signals. A 10 Bit resolution at a sampling rate of 370 samples per sensor and second can be achieved in the experimental setup. All further signal processing is done digitally. The data is acquired by a software and gathered in a two dimensional array which represents the structure of the piezos under the floor. This array of raw sensor signals is the basis for the extraction of various features and patterns within the signals. In order to do this, distinct parameters have to be identified and connected to other parameters or sensor information by a superior software entity (context manager). The determined parameters can be for example:

• User enters/leaves the room
• Position of the user within the room
• Pose of the user (standing, sitting, laying)
• Weight of the user

Those parameters combined with the time information provide a relative exact picture of the users movement behavior. For example:

• Velocity of pace
• Movement direction
• User identification

For specific tasks like for example fall detection, the patterns have to be subdivided in different classes, in order to calculate the probability of a fall according to the identified user.