Passive LC sensor label with distance-independent contactless interrogation

A passive inductive/capacitive (LC) sensor label based on a coil and a capacitive sensor embedded in a flexible substrate, combined with a novel electronic technique for contactless interrogation is presented. The interrogation technique is based on the time-gated approach via electromagnetic coupling using a primary coil to periodically excite the passive LC sensor label. When the excitation to the primary coil is switched off, the damped response of the passive LC label is detected. As a fundamental advantage, this approach ensures that the readout frequency is to first order independent of the interrogation distance between the primary coil and the coil on the sensor label. The sensor label has been tested with reference capacitors and the experimental results show the possibility to operate with interrogation distances up to few centimeters. In addition, applications of the sensor label coupled with a conductive plane as either thickness or compressive force sensor have been tested. The obtained results show the possibility to detect thickness in a range of few millimeters with sensitivity up to -260 kHz/mm, and force with a sensitivity of 3.57 kHz/N across the 0-10 N range.