Interview

Abstract

Marco Bau Doctor Marco Bau from the University of Brescia in Italy, talks to Electronics Letters about the paper ‘Magnet-less electromagnetic contactless interrogation technique for unwired conductive resonators’, page 642. Our research group in the Department of Information Engineering operates in the fields of sensors, microsystems, electronics and measuring instrumentation. One of our topics of research is the investigation into the possibility of interrogating sensors without contact by means of an external unit which can be placed in the proximity of the sensor, a few centimetres away. In addition, we focus our efforts on using completely passive sensors, that is devices which do not have on-board active electronics or a power supply. Typically, these devices are mechanical or electromechanical resonating structures vibrating at their own resonant frequency and this frequency can change due to specific physical/chemical quantities. Our work investigates techniques to excite and sense the vibrations of such structures without any physical contact and, at the same time, challenging issues due to unfavourable signal-to-noise ratio. Sensors which can be interrogated without contact with an external unit are attractive for applications in environments and locations that are difficult to access and do not allow for cabled solutions, such as measurements inside sealed packages, enclosures and rotating parts, or even the human body. In these contexts, battery-powered systems are one possible solution, but they demand active electronics and energy sources be located on both the sensing device and the interrogation unit. Here, the main drawback is the necessity for periodical recharge/replacement of the sensor battery. As an alternative, the sensor and associated circuitry can be battery-less and powered by energy transfer from the reader, or by harvesting energy from the environment. But again, active electronics are required. However, sometimes this is not compliant with the application, such as for operation in high-temperature environments, where active electronics are not feasible. In our work we have adopted mechanical resonator structures which are only required to be electrically conductive. The excitation and detection of the vibrations exploit an electromagnetic principle, based on the induction of eddy currents on the surface of the resonator through a dedicated proximate 3-coil arrangement. Using only AC magnetic fields to generate vibrations in the resonator gives rise to non-linear behaviour, that is the mechanical structure vibrates at twice the frequency of the exciting magnetic field. In the phase of detection, this may require sophisticated electronic circuits and techniques. Alternatively, it is possible to use both DC and AC magnetic fields, so that excitation and vibration of the structure result in the same frequency. DC magnetic fields are typically generated by magnets, which in some circumstances can be impractical. In our work, magnets are avoided by exploiting a purposely designed 3-coil arrangement to generate both DC and AC magnetic fields. Our approach is particularly advantageous as, in addition to avoiding poling magnets, it allows the detection of vibrations through the same coil arrangement used for excitation. Indeed, we generate an additional AC probing magnetic field, which does not affect the excitation of the resonator, but is modulated by its movement. We detect the vibration-modulated magnetic field adopting a differential measurement technique which allows for an improvement to the signal-to-noise ratio. We expect that the results we have reported will be the starting point to improving the technique and exploring the limits of where it can be pushed in terms of downscaling of the sensing devices, possibly comprising also MEMS devices. We are confident that the proposed technique can be tested in selected practical applications for which we have received interest from industrial partners. Our focus is to further investigate different possibilities for contactless interrogation sensors limited not only to eddy-current based principles. Indeed, we have proposed, and are investigating, techniques still based on electromagnetic coupling, which extend the possibility of contactless interrogation to other typologies of resonating devices, like quartz crystal resonators (QCR), piezoelectric resonators, MEMS electromechanical resonators or electrical resonators based on inductor-capacitance tank circuits. In all these cases, we aim to adopt measurement techniques which can achieve accurate resonant-frequency readouts independent of the reading distance. The advantages and opportunities afforded by contactless interrogation techniques have been investigated in depth in recent years. We think that research in this field will be pursued extensively in the future with the aim of obtaining devices which will be energy-independent, environment-friendly, unobtrusive, disposable and can be interrogated on-demand and as needed. Maybe interrogating devices will be integrated in portable devices (such as smartphones) and will make this technology available for both industrial applications and every-day life.