Passive Wireless Resonant Galfenol Sensor for Osteosynthesis Plate Bending Measurement

The healing process of bone fractures can be monitored by a measurement of the osteosynthesis plate bending. For this purpose a wireless magnetostrictive bending sensor is proposed. A planar rectangular coil on top of a magnetostrictive Galfenol ( alloy) layer forms an electrical resonant circuit. The sensor is manufactured in thin film technology. Coil turns were electrodeposited by pattern plating. Sensors with overall dimensions of (13 2 0.5) mm were manufactured with varied turn numbers showing self-resonance frequencies from 5 to 50 MHz. Examined sensors possess linear frequency-force characteristic up to 6 N with frequency shifts of 6 kHz . In order to obtain the sensor resonance frequency wirelessly, several measurement techniques were employed using inductively coupled coils. Frequency domain measurements have been carried out by employing a network analyzer with a single detection coil and a lock-in amplifier with separate coils for excitation and detection. In time domain measurements, two coils for transmission and reception are used. In the transmit case a short sine pulse excites the sensor and afterwards its decaying response signal is received. To determine the resonance frequency, a frequency counting, Fourier or wavelet technique can be used. By integrating additional cores of high permeability into sensor and detection coil, measurement ranges can be increased.