Study on Impedance Behavior of a Telemetric System Operating with an Inkjet-Printed Resistive Strain Gauge

Devices that measure the strain applied to a body can find several applications in many fields, especially if they are able to satisfy precise specifications about power supply and transmission of information. Telemetric systems offer a viable solution, since they rely on the inductive coupling between the coils connected to a passive sensor and a readout unit. Therefore, they require neither batteries nor wired connections. The present work illustrates a preliminary analysis on a telemetric device made of Printed Circuit Board (PCB) planar inductors and a resistive strain gauge fabricated through inkjet printing on a flexible substrate. We carried out experimental tests to evaluate system frequency behaviour. Firstly, we measured its impedance phase at readout inductor terminals while varying sensor resistance, with the inductors put at a fixed relative distance. Then, we repeated the same operation when that distance changed, with strain gauge at rest position. Obtained results report a total variation of phase most sensitive point of about 0.12° for a strain equal to 1% of sensor length at rest, suggesting that measuring strain with such a structure is feasible. On the other hand, they highlight that distance variation has a strong influence on that result; thus, it has to be taken into account during measurement process.