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This paper describes a novel bed sensing method for noninvasive, constraint-free, subliminal detection of biosignals. The sensor system detects the heartbeat, respiration, body movement, position change, and scratching motion of a person lying or sleeping on the bed. These biosignals provide not only basic medical information but also sophisticated details about sleep conditions. Thus, the bed sensing method can be used to monitor the health condition of people sleeping at home, as well as that of patients in the hospital. Furthermore, the bed sensor system can detect emerging changes in the physical condition of a person, whether at home or in the hospital. The basic device used for sensing is piezoceramic bonded to stainless steel plate sandwiched between the floor and the four corners of the bed. Thus, no special bed is required. The device, which detects the biosignals generated as mechanical vibrations, has a wide dynamic range and high signal-to-noise (SN) ratio enabling the detection of microvibrations from the heartbeat by the change in acting force, without saturation from body movements. It accurately detects the person's heartbeat and respiration as well as body movement and even the number of scratching motions. The device is suitable for various health monitoring applications including sleep and medical monitoring for circulatory system disorders, as well as diseases characterized by itching.

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