A review of wireless SAW sensors

Wireless measurement systems with passive surface acoustic wave (SAW) sensors offer new and exciting perspectives for remote monitoring and control of moving parts, even in harsh environments. This review paper gives a comprehensive survey of the present state of the measurement systems and should help a designer to find the parameters required to achieve a specified accuracy or uncertainty of measurement. Delay lines and resonators have been used, and two principles have been employed: SAW one-port devices that are directly affected by the measurand and SAW two-port devices that are electrically loaded by a conventional sensor and, therefore, indirectly affected by the measurand. For radio frequency (RF) interrogation, time domain sampling (TDS) and frequency domain sampling (FDS) have been investigated theoretically and experimentally; the methods of measurement are described. For an evaluation of the effects caused by the radio interrogation, we discuss the errors caused by noise, interference, bandwidth, manufacturing, and hardware tuning. The system parameters, distance range, and measurement uncertainty are given numerically for actual applications. Combinations of SAW sensors and special signal processing techniques to enhance accuracy, dynamic range, read out distance, and measurement repetition rate (measurement bandwidth) are presented. In conclusion, an overview of SAW sensor applications is given.     

Detecting and evaluating the signals of wirelessly interrogational passive SAW resonator sensors

The wireless sensing signal of a passive surface acoustic wave (SAW) resonator sensor is the response of the SAW resonator in a passive circuit to wireless radio frequency interrogation. The response is produced only in the case that the interrogation covers the operational frequency band of the resonator. The wireless response is transient and can only be detectable in a proximity after switching off the interrogation. Due to the fact that, while used as a sensor, the resonant frequency of the resonator is related to and varying with the measurand, the interrogation to a passive SAW resonator sensor has to trace and follow the correspondent variation of the frequency band of the device. The energy evaluation of the response is applied to detect the availability of the sensing response and is used as a feedback argument to roughly localize the operational frequency range of the sensor. A modified frequency estimation is employed to estimate the sensing characteristic frequency in the transient wireless sensing signal with a low signal-to-noise ratio. The estimation is used to further adjust the interrogation frequency to follow the frequency variation of the sensor until the response becomes optimal. The evaluation of signal energy along with the statistical quantity of frequency estimation gives a reference for the confidence of the estimated frequency.     

A low-cost high-definition wireless sensor system utilizing intersymbol interference

A new method for wireless interrogation of a passive SAW sensor with multiple reflectors, utilizing the signal interference during overlap of the sensor's response bursts, is introduced. Within this time interval, the amplitude or the phase, respectively, is measured. By varying the frequency of the relatively long interrogation bursts, the amount of interference and destructive interferences (notches) are searched, respectively. So the measurand influencing the sensor or its identification information can be gained. The principle yields high resolution with a low cost interrogating system. The basic principle, calculations of sensitivity, and experimental results for temperature measurements are presented. The hardware effort and the performance of the system are discussed.     

RF Cavity Passive Wireless Sensors With Time-Domain Gating-Based Interrogation for SHM of Civil Structures

Many existing sensing technologies for application to the monitoring of civil structures have a serious deficiency in that they require some type of wired physical connection to the outside world. This causes significant problems in the installation and long-term use of these sensors. This paper describes a new type of passive wireless sensor that is based on resonant RF cavities, where the resonant frequency is modulated by a measurand. In the case of a strain sensor, the electrical length of the cavity directly modulates its resonant frequency. A probe inside the cavity couples RF signals from the cavity to an externally attached antenna. The sensor can then be interrogated remotely using microwave pulse-echo techniques. Such a system has the advantage of requiring no permanent physical connection between the sensor and the data acquisition system. In this type of sensor, the RF interrogation signal is transmitted to the sensor and then reradiated back to the interrogator from the sensor resulting in a signal strength that decreases with the forth power of distance. This places an upper limit on the distance over which the sensor can be interrogated. Theoretical estimates show that these sensors can be interrogated with sufficient SNR at distances exceeding 10 m for radiated powers of less than 1 mW. We present results for a strain sensor and a displacement sensor that can be interrogated at a distance of 8 m with a strain resolution of less than 10 ppm and displacement resolution of 0.01 mm, respectively.     

An interrogation unit for passive wireless SAW sensors based on fourier transform

The application of surface acoustic wave (SAW) resonators as sensor elements for different physical parameters such as temperature, pressure, and force has been well-known for several years. The energy storage in the SAW and the direct conversion from physical parameter to a parameter of the wave, such as frequency or phase, enables the construction of a passive sensor that can be interrogated wireless. This paper presents a temperature-measurement system based on passive wireless SAW sensors. The principle of SAW sensors and SAW sensor interrogation is discussed briefly. A new measurement device developed for analyzing the sensor signals is introduced. Compared to former interrogation units that detect resonance frequency of the SAW resonator by comparing amplitudes of sensor response signals related to different stimulating frequencies, the new equipment is able to measure the resonance frequency directly by calculating a Fourier transformation of the resonator response signal. Measurement results of an experimental setup and field tests are presented and discussed.     

Wireless sensing using oscillator circuits locked to remote high-Q SAW resonators

This paper introduces a method of wireless read out of high Q surface acoustic wave (SAW) resonator sensors. The resonator is excited by a short RF pulse and decays after switching off the interrogating signal. In the measurement system, a gated phase locked loop (GPLL) locks to the resonance frequency of the SAW resonator within a few bursts. Then the frequency of the GPLL oscillator is synchronized to the resonance of the sensor and can be measured easily. The concept is intended to yield an alternative to interrogators with expensive signal processing. Considering the inherent limitations, the proposed system presents a low cost solution for temperature, force, torque, etc. measurements. We describe the sensors, the signals, and the implemented system. Results of temperature measurements using quartz resonators are presented, and merits and disadvantages are discussed.     

A base-band NDS electrode configuration for HACT devices

Heterostructure Acoustic Charge Transport (HACT) devices have been fabricated with a new nondestructive sense (NDS) electrode structure that provides for the recovery of base-band signals without the use of an integrating capacitor. This electrode structure provides an output signal comprising an RF carrier at the SAW frequency, amplitude modulated by the sampled input signal which has been delayed by a period proportional to the output electrodes distance from the input diode. The output of the NDS electrode structure is subsequently demodulated to provide the base-band signal     

Mode selection in a multimode SAW oscillator using FM chirp mixing signal injection

Mode-selection control of a multimode surface-acoustic-wave (SAW) oscillator has been obtained using SAW linear FM chirp signal injection. The prototype 60-100-MHz SAW oscillator design employed a single-phase unidirectional transducer (SPUDT) low-loss comb filter in the feedback loop, with minimum insertion loss of approximately 3.7 dB. Mode selection was achieved using an injection signal derived from the mixed output of two 27.5-52.5-MHz up- and down-chirp SAW filters. Mode switching times of less, similar2 mus were obtained. The device could be useful as a local oscillator on frequency-agile radars, where hopping is required over a moderate number of frequencies.     

An RF-powered, wireless CMOS temperature sensor

We present a wireless, fully integrated CMOS temperature sensor that recovers power from a radio frequency (RF) signal, and returns data as a frequency-modulated 2.3-GHz signal to a base station. Power is recovered from a 450-MHz incident signal with the help of a low-threshold, high-efficiency, voltage rectifier-multiplier circuit. This technique decreases the minimum incident RF power required, compared to state-of-the-art wirelessly powered telemetry systems. The rectifier-multiplier can collect energy from a base station placed up to 18 m away. To further increase the range from the base, the device collects energy in a low power standby/charging mode. A mode selector circuit monitors the amount stored energy and decides if the system is transmitting data or is in the standby/charging mode. A bootstrapped reference generates a complementary to absolute temperature (CTAT) voltage with an R-squared regression of 0.9995 to a linear fit. This reference is used as the temperature sensor of the system, controlling a low-power, integrated, voltage-controlled LC oscillator (VCO). The oscillation frequency of the VCO is modulated by ambient temperature changes. The modulated carrier is transmitted by a fully integrated power amplifier. A temperature sensitivity of 126 ppm//spl deg/C is achieved and the entire sensor consumes 1.1 mA while transmitting data.     

Novel Signal Conditioning Circuit for Push-Pull Type Capacitive Transducers

A novel signal conditioning circuit suitable for push-pull type capacitive transducers is proposed. The circuit developed is capable of providing a linear output over a wide range of values of the physical quantity being measured even when the transducer output has an inverse relationship with the measurand. The push-pull type capacitive transducer becomes an integral part of a relaxation oscillator, the duty cycle ratio of the output of which becomes proportional to the measurand. The various sources of error in the circuit are analyzed, and quantitative expressions to estimate such errors are derived. Results obtained from tests on a prototype indicate that the circuit possesses very low errors     

Extremely low-phase-noise SAW resonators and oscillators: design and performance

The authors describe prototype low-noise SAW (surface acoustic wave) resonator oscillators which have demonstrated state-of-the-art phase-noise performance not only at their fundamental operating frequencies in the 400- to 600-MHz range but also after 16x frequency multiplication to X-band as well. SAW resonator designs with overmoded cavities, very wide apertures, and dual apertures, as well as modified fabrication techniques, have been used to realize an overall reduction in an oscillator's phase-noise spectrum, i.e. white phiM, flicker FM, and random-walk FM. The S resonators can typically handle incident RF power in excess of +20 dBm, a key requirement to achieving an extremely low oscillator-phase-noise floor. A novel burn-in procedure at relatively high incident-RF-power levels (>27 dBm) was used to reduce both the flicker FM and random-walk FM phase-noise levels. Using these various techniques, a 5- to 15-dB improvement in the overall phase-noise spectrum for several prototype oscillators was demonstrated.     

Surface-acoustic-wave (SAW) flow sensor

The use of a surface-acoustic-wave (SAW) device to measure the rate of gas flow is described. A SAW oscillator heated to a suitable temperature above ambient is placed in the path of a flowing gas. Convective cooling caused by the gas flow results in a change in the oscillator frequency. A 73-MHz oscillator fabricated on 128 degrees rotated Y-cut lithium niobate substrate and heated to 55 degrees C above ambient shows a frequency variation greater than 142 kHz for flow-rate variation from 0 to 1000 cm(3)/min. The output of the sensor can be calibrated to provide a measurement of volume flow rate, pressure differential across channel ports, or mass flow rate. High sensitivity, wide dynamic range, and direct digital output are among the attractive features of this sensor. Theoretical expressions for the sensitivity and response time of the sensor are derived. It is shown that by using ultrasonic Lamb waves, propagating in thin membranes, a flow sensor with faster response than a SAW sensor can be realized.     

Orthogonal frequency coding for SAW tagging and sensors

Surface acoustic wave (SAW)-based sensors can offer wireless, passive operation in numerous environments, and various device embodiments are used for retrieval of the sensed data information. Single sensor systems typically can use a single carrier frequency and a simple device embodiment because tagging is not required. In a multisensor environment, it is necessary to both identify the sensor and retrieve the sensed information. This paper presents the concept of orthogonal frequency coding (OFC) for applications to SAW sensor technology. The OFC offers all advantages inherent to spread spectrum communications, including enhanced processing gain and lower interrogation power spectral density (PSD). It is shown that the time ambiguity in the OFC compressed pulse is significantly reduced as compared with a single frequency tag having the same code length, and additional coding can be added using a pseudo-noise (PN) sequence. The OFC approach is general and should be applicable to many differing SAW sensors for temperature, pressure, liquid, gases, etc. Device embodiments are shown, and a potential transceiver is described. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance. Devices then are used in computer simulations of the proposed transceiver design, and the results of an OFC sensor system are discussed.     

Broad-Band Remote-Sensing Magnetometer

A broad-band remote-sensing magnetometer system has been developed for measuring fast-rising pulsed magnetic fields. Broad frequency response is achieved by frequency modulating a 1.5-GHz microwave carrier for transmission from the sensor to a remotely located receiver. The primary field probe is a ferrite-loaded coil, which produces a current proportional to the magnetic-field intensity. This current is used to frequency modulate a YIG-tuned transistor oscillator in the sensor. A traveling-wave amplifier in the receiver provides carrier amplification and amplitude limiting because it is operated in saturation mode. A new type of FM discriminator, which offers multiple-octave bandwidth potential, converts the frequency-modulated carrier to an amplitude-modulated carrier. The signal is finally detected and amplified for oscilloscope display. While the present system achieves a 50-MHz bandwidth, the system concept should be capable of extension to several times this bandwidth.     

A direct digital synthesis system for acoustic wave sensors

Current designs for acoustic wave sensor system electronics are typically based on surface acoustic wave (SAW) oscillators, phase detectors, or phase-locked loops to measure changes in SAW velocity. The advantage of oscillators is a high resolution frequency output, as compared to phase detection systems which are more stable and can more easily provide amplitude information. Phase-locked loops (PLL) offer advantages of both the oscillator and phase detection systems but have the disadvantages of a fixed frequency range and the need for frequency counting circuitry. The objectives of this work were to study the performance of a direct digital synthesis (DDS) based PLL system with the advantages of a programmable frequency range, elimination of the need for frequency counting circuitry, and tolerance of large SAW sensor insertion losses. The DDS system tested had a resolution of 4 Hz and a range of 80 to 120 MHz in SAW humidity and temperature sensing applications indicating that the DDS based PLL is a practical electronic system for SAW sensors.     

Theory and application of passive SAW radio transponders as sensors

Surface acoustic wave (SAW) radio transponders make it possible to read identification codes or measurement values from a remote location. The decisive advantage of these SAW transponders lies in their passive operation (i.e., no power-supply), and in the possibility of wireless installation at particularly inaccessible locations. The passive SAW transponders are maintenance free. Identification marks respond to an interrogation signal with their nonchanging identification pattern. In wireless SAW sensors the physical or chemical properties to be detected change the propagation characteristics of the SAW. SAW radio transponders are advantageously placed on moving or rotating parts and in hazardous environments such as contaminated or high voltage areas. They also can be used for contactless measurements in high vacuum process chambers, under concrete, extreme heat, or strong radioactive radiation, where the use of conventional sensors is complicated, dangerous, or expensive. In this paper we discuss the principles of wireless passive SAW transponders and present a radio frequency interrogation unit and several passive radio SAW sensors developed for noncontact measurements of temperatures, pressures, torques, and currents.     

Overtone Oscillator for SAW Gas Detectors

A design of an overtone oscillator a with surface-acoustic-wave (SAW) resonator is described in this paper. The circuit works stably on the frequency 4.710 GHz (29th harmonic of loaded resonator) at about the -2 dBm level. In the construction, distributed-constant circuits have been applied. Commercially available SAW sensors usually work within the range of frequency from a few dozen to a few hundred megahertz. On the other hand, it is a well-known fact that the mass sensitivity of such devices is directly proportional to the square of its operating frequency, and SAW sensors for organic vapors, for instance, are usually mass sensitive. For this reason, an increase in the SAW sensors' operating frequency seems to be useful. The circuit described in this paper shows the possibility of a dramatic rise in SAW operating frequency by exerting its operation through a specific high overtone (harmonic frequency) of the SAW resonator. The overtone frequency in such a solution then plays the role of basic mode. The oscillator proposed in this paper seems to be a good tool for chemisensitive-SAW-coating investigation     

基于PSO-ESPRIT算法的SAW温度传感器解调方法

目的 为了提高声表面波(Surface acoustic wave,SAW)温度传感器的测量精度,设计一种基于PSO-ESPRIT算法的高精度SAW温度传感器解调方法。方法 以ESPRIT谱估计方法为基础,把Hankel矩阵的时间窗长度与计算噪声方差时的K值作为粒子群优化(Particle swarm optimization, PSO)算法的输入变量,并以频率估计标准差作为粒子的适应度函数,利用PSO对ESPRIT算法中的参数进行优化,以改善频率估计精度,从而提高SAW回波信号频率估计的分辨率,实现SAW温度传感器的高精度解调。结果 仿真和实验结果表明,所设计的方法与其他谱估计算法相比,其对SAW回波信号估计的频率误差最小,标准差小于0.66kHz。把设计的算法用于SAW温度传感器的温度解调,得到的温度值与实际温度的误差小于0.4℃。结论 测试结果说明,设计的温度解调方法提高了SAW回波信号频率解调精度,可用于SAW温度传感器的解调,实现了对食品包装储运过程中温度的实时监测。     

Wireless measurement of temperature using surface acoustic waves sensors

Surface acoustic wave (SAW) devices can be used as wireless sensor elements, called SAW transponders, for measuring physical quantities such as temperature that do not need any power supply and may be accessed wirelessly. A complete wireless sensor system consists of one or more such SAW transponders and a local radar transceiver. The SAW transponder receives an RF burst in the VHF/UHF band transmitted by the radar transceiver. The reader unit performs a radar measurement of the impulse response of the SAW transponder via a high-frequency electromagnetic radio link. A temperature variation changes the SAW velocity and thereby the response pattern of the SAW device. By analyzing the time delay between backscattered pulses with different time delays we get a rough estimation of the temperature of the SAW transponder. By using this information the ambiguity of +/-2pi in the phase differences between the pulses can be eliminated, which provides an overall and unambiguous temperature resolution of +/-0.2 degrees C.     

Sensitive and simultaneous detection of cardiac markers in human serum using surface acoustic wave immunosensor

We present a rapid and sensitive surface acoustic wave (SAW) immunosensor that utilizes gold staining as a signal enhancement method. A sandwich immunoassay was performed on sensing area of the SAW sensor, which could specifically capture and detect cardiac markers (cardiac troponin I (cTnI), creatine kinase (CK)-MB, and myoglobin). The analytes in human serum were captured on gold nanoparticles (AuNPs) that were conjugated in advance with detection antibodies. Introduction of these complexes to the capture antibody-immobilized sensor surface resulted in a classic AuNP-based sandwich immunoassay format that has been used for signal amplification. In order to achieve further signal enhancement, a gold staining method was performed, which demonstrated that it is possible to obtain gold staining-mediated signal augmentation on a mass-sensitive device. The sensor response due to gold staining varied as a function of cardiac marker concentration. We also investigated effects of increasing operating frequency on sensor responses. Results showed that detection limit of the SAW sensor could be further improved by increasing the operating frequency.