Development of a novel induction shear force sensor based on phase detection for monitoring bedsore susceptibility

Bedsore is one of the types of wounds that resent a significant clinical challenge due to the compromised health status of affected patients. Consequently, developing effective strategies for both prevention and treatment is of critical importance. One effective method could be monitoring the shear force between the vulnerable area of the patient's skin and the bed surface and take required preventive interventions to avoid these wounds. For this purpose, a novel design for induction shear force sensor is introduced in this paper, whose principle is based on the phase difference of the induced voltage on pickup coils with respect to a reference signal instead of the variations of the amplitude. Thus, pure shear loading could be distinguished from normal (compression) loading, and the variation in amplitude is attributed to normal loading. This sensor takes into account two pickup coil pairs, each for the X and Y shear directions. Finite element method (FEM) simulations and experimental validation indicate minimal crosstalk between the orthogonal channels. However, experimental observations reveal that the sensitivity of one axis is notably influenced by concurrent loading on the other axis. The static calibration testing procedure was implemented on the sensor based on the ANSI1979 standard. According to the calibration test results, the accuracy (X and Y axes, 9.34% full scale), precision (0.19% for X axis and 0.4% for Y axis), hysteresis error (3.7% for X axis and 5.9% for Y axis) and other specifications of the sensor are derived. A test for simulating the real condition of utilizing the sensors was also conducted on elbow joint replica.