Passive Frequency Doubling Antenna Sensor for Wireless Strain Sensing

This paper presents the design, simulation, and preliminary measurement of a passive (battery-free) frequency doubling antenna sensor for strain sensing. Illuminated by a wireless reader, the sensor consists of three components, i.e. a receiving antenna with resonance frequency f0, a transmitting antenna with resonance frequency 2f0, and a matching network between the receiving and transmitting antennas. A Schottky diode is integrated in the matching network. Exploiting nonlinear circuit behavior of the diode, the matching network is able to generate output signal at doubled frequency of the reader interrogation signal. The output signal is then backscattered to the reader through the sensor-side transmitting antenna. Because the backscattered signal has a doubled frequency, it is easily distinguished by the reader from environmental reflections of original interrogation signal. When one of the sensor-side antennas, say receiving antenna, is bonded to a structure that experiences strain/deformation, resonance frequency of the antenna shifts accordingly. Through wireless interrogation, this resonance frequency shift can be measured by the reader and used to derive strain in the structure. Since operation power of the diode is harvested from the reader interrogation signal, no other power source is needed by the sensor. This means the frequency doubling antenna sensor is wireless and passive. Based on simulation results, strain sensitivity of this novel frequency doubling antenna sensor is around -3.84 kHz/με. INTRODUCTION In order to accurately assess deterioration of civil, mechanical, and aerospace structures, a large volume of research in structural health monitoring (SHM) has been inspired over past few decades [1]. Sensors can be used to measure various structural responses and operating conditions, including strain, displacement, acceleration, humidity, temperature, etc. Among the measurements, strain can be an important indicator for stress concentration and damage development. Metal foil strain gages are currently among the most common solutions, due to their low-cost, simple circuitry, and acceptable reliability in many applications. However, when applied to large structures, traditional metal foil strain gages require lengthy cable connections for power and data acquisition, which can significantly increase installation time and system cost [2]. To avoid cabling difficulty associated with metal foil strain gages, wireless strain sensors have recently been developed. For example, a wireless strain sensor is proposed based on inductive coupling principle involving two adjacent inductors [3-5]. However, the interrogation distance achieved by inductive coupling is usually limited to several inches, which is inconvenient for practical applications. In order to increase interrogation distance, electromagnetic backscattering techniques have been exploited for wireless strain sensing [6]. Since the electromagnetic resonance frequency of a planar antenna is related to the antenna’s physical dimension, the resonance frequency changes when the antenna is under strain. This relationship between resonance frequency and strain can be used for stress/strain measurement of a structure to which the planar antenna is bonded. For example, a patch antenna has been designed for wireless strain sensing [7], where a phototransistor is adopted for signal modulation of the RF Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/02/2016 Terms of Use: http://www.asme.org/about-asme/terms-of-use 2 Copyright © 2012 by ASME signal backscattered from the antenna sensor. As a result, signal backscattered from the sensor can be distinguished from environmental reflections. However, the light-switching mechanism is not practical for outdoor application, where light intensity is usually so strong that the phototransistor is constantly activated and thus, loses ability of switching. To avoid this difficulty, a low-cost off-the-shelf radiofrequency identification (RFID) chip is adopted as a simple mechanism for signal modulation [8]. Since the RFID chip is powered by wireless interrogation signal, the RFID-based strain sensor is wireless and passive (battery-free). The prototype RFID antenna sensor has shown a strain measurement resolution of 20 με in laboratory experiments, and can measure large strains up to 10,000 με [9]. Previous studies demonstrated that if operating frequency of the wireless strain sensor is increased, strain sensitivity can be improved and sensor size can be reduced. However, the RFID chip only functions in the frequency band of 860-960MHz. Alternative approaches need to be exploited in order to operate at higher frequencies. This paper presents a novel wireless strain sensor design that adopts a frequency doubling scheme to enable sensor operation at a high frequency. The basic concept is to let the sensor double the frequency of reader interrogation signal (f) and backscatter signal at the doubled frequency (2f). Because environmental reflections to reader interrogation signal are concentrated at f, the reader only receives signal at 2f backscattered from the sensor. The frequency doubling operation is implemented through a Schottky diode, which is a nonlinear circuit device and can generate output signal with frequencies at multiples of input frequency. Diode frequency multipliers have been adopted for energy harvesting [10], insect tracking [11], among others. In [12], a high efficiency diode frequency doubling device is designed using a GaAs Schottky diode that provides 1% conversion efficiency at -30 dBm input power. Nevertheless, the authors are not aware of other literature on using Schottky diode to enable frequency doubling for wireless passive strain sensing. In our study, the diode-enabled frequency doubling mechanism is investigated and incorporated with two patch antennas to form a wireless strain sensor. A low-cost Schottky diode (SMS7621-079LF) from Skyworks Solutions, Inc. is adopted. A patch antenna with resonance frequency at 2.9GHz is designed as a receiving antenna of the wireless strain sensor. Meanwhile, another patch antenna with resonance frequency at 5.8GHz is designed to serve as a transmitting antenna of the wireless strain sensor. For connection between these two patch antennas, the simulation model of a Schottky diode is first verified through experimental measurement, and then used to design the matching network between the two patch antennas. Finally, the three components, i.e. the receiving and transmitting antennas and matching network, are combined together to form a frequency doubling antenna sensor. Since operation power of the diode is harvested from wireless interrogation signal, the frequency doubling antenna sensor is wireless and passive (battery-free). Strain sensing simulation shows that the proposed frequency doubling sensor can achieve a strain sensitivity of -3.84 kHz/με. The rest of this paper is organized as follows. Strain sensing mechanism of the antenna sensor is first presented. Designs of the 2.9 GHz and 5.8 GHz patch antennas are then described. Next, verification experiment for the Schottky diode simulation model is presented, followed by the diode-integrated matching network design. Combining all three components, simulation results are presented to demonstrate strain sensing performance of the proposed frequency doubling antenna sensor. Finally, a summary and discussion is provided. STRAIN SENSING MECHANISM OF FREQUENCY DOUBLING ANTENNA SENSOR Fig. 1 illustrates the operation mechanism of a frequency doubling antenna sensor. The sensor consists of three main components, i.e., a receiving antenna (with resonance frequency f0), a transmitting antenna (with resonance frequency 2f0), and a diode-integrated matching network between receiving and transmitting antennas. During operation, a wireless interrogation signal is emitted from the reader side by a function generator and through a transmitting reader antenna. If interrogation frequency f is in the neighborhood of f0, resonance frequency of the receiving patch antenna at sensor side, interrogation power is captured by the sensor-side receiving patch antenna and transferred to the matching network. The diode then generates output signal at doubled frequency 2f. The output signal at 2f is backscattered to reader through sensor-side transmitting patch antenna (resonance frequency at 2f0). A spectrum analyzer finally measures the backscattered signal at reader side. Frequency of backscattered sensor signal is at 2f, and the unwanted environmental reflections to original reader interrogation signal remains at f. Therefore, it is easy for the Function generator