Improving the reliability of pressure ulcer prevention by sensing human bio-signals: Analysis of using piezoelectric sensors to sense respiration and heart rate

Pressure ulcers, also known as bedsores, are wounds that form when a person sits or lays in the sameposture for an extended period of time. Continued pressure being applied to the same spot causesthe skin to decay, possibly continuing into underlaying tissue if the wounds are not treated. A commonand proven practice to prevent pressure ulcers from forming is to regularly change the posture of thepatient, so that there is not too much pressure on any one part of the skin.Momo Medical is creating a sensor system to assist in preventing pressure ulcers. They alreadyhave a prototype using force-sensing resistor (FSR) and piezoelectric sensors to measure changes inposture of the patient. The prototype is able to detect if a patient has moved enough, whether on theirown or by the nurses, to prevent pressure ulcers. The prototype works, but they need their system tobe more reliable.The research in this thesis focuses on improving the piezoelectric sensors. Momo Medical usesthe piezoelectric sensors to measure human bio-signals, mainly respiration and heart rate, through themattress. Two different printed circuit boards (PCBs) were designed as test set-ups to be able to easilymodify the amplifier to measure the piezoelectric sensors separately. Using these test set-ups twocommon piezoelectric sensors were compared, namely lead zirconate titanate (PZT) and polyvinylidenedifluoride (PVDF) sensors. From this comparison it was concluded that the PVDF sensors consistentlyhave a better signal-to-noise ratio (SNR), but that both the PZT and PVDF sensors were able to measurehuman bio-signals effectively. The PVDF sensors did showed even more promising results whenusing them in a different mechanical configuration.In the test set-up the amplifier was also changed to improve the read-out of the sensors. Multipleiterations on the amplifier design were tested. In the final design a non-inverting amplifier was chosen todecouple the amplification from the input impedance of the amplifier and a input impedance of 100MΩwas chosen. Because of the high input impedance in the final design the signal was dampened lessand less amplification was needed, thus reducing noise.Electrical Engineerin