Interrogation Techniques and Interface Circuits for Coil-Coupled Passive Sensors

Abstract

Coil-coupled passive sensors can be interrogated without contact, exploiting the magnetic coupling between two coils forming a telemetric proximity link. A primary coil connected to the interface circuit forms the readout unit, while a passive sensor connected to a secondary coil forms the sensor unit. This work is focused on the interrogation of sensor units based on resonance, denoted as resonant sensor units, in which the readout signals are the resonant frequency and, possibly, the quality factor. Specifically, capacitive and electromechanical piezoelectric resonator sensor units are considered. Two interrogation techniques, namely a frequency-domain technique and a time-domain technique, have been analyzed, that are theoretically independent of the coupling between the coils which, in turn, ensure that the sensor readings are not affected by the interrogation distance. However, it is shown that the unavoidable parasitic capacitance in parallel to the readout coil introduces, for both techniques, an undesired dependence of the readings on the interrogation distance. This effect is especially marked for capacitance sensor units. A compensation circuit is innovatively proposed to counteract the effects of the parasitic input capacitance, and advantageously obtain distance-independent readings in real operating conditions. Experimental tests on a coil-coupled capacitance sensor with resonance at 5.45 MHz have shown a deviation within 1.5 kHz, i.e., 300 ppm, for interrogation distances of up to 18 mm. For the same distance range, with a coil-coupled quartz crystal resonator with a mechanical resonant frequency of 4.432 MHz, variations of less than 1.8 Hz, i.e., 0.5 ppm, have been obtained.

Contactless Readout of Passive LC Sensors with Compensation Circuit for Distance-Independent Measurements

“…Approaches theoretically independent of the distance have been proposed both in the time-domain [3,4] and in the frequency-domain [2]. In the latter case, measuring the real part of the impedance at the readout coil allows to obtain the resonant frequency and the quality factor of the LC sensor [5,6]. The present work investigates on the limitations of such technique when considering unavoidable parasitic capacitances in parallel to the readout coil due to the connected electronics and cables.…”

Low-Frequency RFID Signal and Power Transfer Circuitry for Capacitive and Resistive Mixed Sensor Array

“…Similarly, contactless interrogation can be achieved through an electromagnetic link between two coupled coils [17,18]. In [19][20][21][22], a primary coil is connected to the interrogation circuit, while a secondary coil is connected to a resonant sensor; i.e., a mechanical resonator such as a quartz crystal microbalance (QCM), a micro electro-mechanical system (MEMS) or an electrical resonant sensor such as an LC-tank circuit. The measurement principle can exploit both frequency-domain and time-domain techniques [21,22].…”

“…In [19][20][21][22], a primary coil is connected to the interrogation circuit, while a secondary coil is connected to a resonant sensor; i.e., a mechanical resonator such as a quartz crystal microbalance (QCM), a micro electro-mechanical system (MEMS) or an electrical resonant sensor such as an LC-tank circuit. The measurement principle can exploit both frequency-domain and time-domain techniques [21,22]. This approach relies on completely passive sensors; i.e., they do not require any active electronic circuits on board to operate.A different approach exploits RFID technologies in the low frequency (LF), high frequency (HF) and ultra-high frequency (UHF) ranges [23].…”

Editorial for the Special Issue on Interface Circuits for Microsensor Integrated Systems

“…There are 10 papers published in this Special Issue, covering micromachined sensors interfacing circuits [ 1 , 2 , 3 , 4 ], techniques for sensor interrogation and conditioning circuits [ 5 , 6 , 7 ], and sensors and systems design [ 8 , 9 , 10 ].…”

“…to be applied to the sensor field up to be applied on a large scale down to micrometric dimensions in agreement with the technologic ability to shrink the capacitive sensor dimensions [ 6 ]. Demori et al proposed an interrogation techniques and interface circuits for coil-coupled passive sensors: the interrogation of sensor units is based on resonance, denoted as resonant sensor units, in which the readout signals are the resonant frequency and, possibly, the quality factor [ 7 ]. On the sensors and systems design, Wei and Bao presented a low power, energy-efficient precision CMOS temperature sensor based on bipolar junction transistors and a pre-bias circuit and bipolar core [ 8 ].…”