Abstract
This paper presents a cost-effective sensor system for mattresses that can classify the sleeping posture of an individual and prevent pressure ulcers. This system applies projected capacitive sensing to the field of health care. The charge time (CT) method was used to sensitively and accurately measure the capacitance of the projected electrodes. The required characteristics of the projected capacitor were identified to develop large-area applications for sensory mattresses. The area of the electrodes, the use of shielding, and the increased length of the transmission line were calibrated to more accurately measure the capacitance of the electrodes in large-size applications. To offer the users comfort in the prone position, a flexible substrate was selected and covered with 16 × 20 electrodes. Compared with the static charge sensitive bed (SCSB), our proposed system-flexible projected capacitive-sensing mattress (FPCSM) comes with more electrodes to increase the resolution of posture identification. As for the body pressure system (BPS), the FPCSM has advantages such as lower cost, higher aging-resistance capability, and the ability to sense the capacitance of the covered regions without physical contact. The proposed guard ring design effectively absorbs the noise and interrupts leakage paths. The projected capacitive electrode is suitable for proximity-sensing applications and succeeds at quickly recognizing the sleeping pattern of the user.
Highlights
Diversified health products have been rapidly developed in recent decades
The results show that the transmission line length influences the primary capacitance value: an increase in the transmission line length causes a linear increase in the primary value, except for the first and eighth electrodes
A single electrode capacitance measurement can be performed in only 38 μs, and the capacitance measurement of the entire 32-electrode system can be performed in less than 2 ms with a high resolution of 1 femtofarad
Summary
Diversified health products have been rapidly developed in recent decades. mattresses that influence the sleeping quality of people have not been extensively studied [1,2]. Medical mattresses can measure the patient’s respiration, pressure distribution, decubitus posture [3], and sleeping activities [4,5,6,7,8,9]. These pressure-sensing measurements can be used for other health care purposes such as the prevention of pressure ulcers [10,11] as well as monitoring of stumbling when exiting the bed and sleeping disorders [12,13]. Pressure can be measured from voltage variations to obtain piezo-resistive [15] or piezo-capacitive properties [16]. If the force is only delivered but not received, the sensor cannot detect the correct pressure
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