Wide Range Load Sensor Using Quartz Crystal Resonator for Detection of Biological Signals

High-sensitive, wide-measurement range, and small-sized load sensor was developed using AT-cut quartz crystal resonator (QCR). The quartz crystal generates a charge, which is proportional to the external force. Since it has high sensitivity and excellent temperature stability, it has been used for various sensors. In particular, QCR has superior characteristic for static load sensing in nature. However, QCR is fragile and easily broken by the stress concentration. Moreover, a retention mechanism is required to transmit the load efficiently, and we have to fix the QCR firmly while avoiding off axis force. Miniaturization of the retention mechanism is quite difficult to develop, since fabrication and assembly process is complicated. We have proposed a miniaturized sensor element using microfabrication. The QCR load sensor had enormously wide range of force sensing over 10⁴. However, output of previous sensor changes easily by parasitic capacitance change around QCR. The objective of this paper is to improve the resolution of load measurement and stability of sensor output for detection of biological signals. We fabricated QCR sensor whose sensitivity is 973 Hz/N. We succeeded in detection of multiple biological signals (breath, heartbeat, and posture) using proposed QCR load sensor with high stability.