The application of Surface Acoustic Wave (SAW) resonators as sensor elements for different physical parameters such as temperature, pressure and force is well known since several years. The energy storage in the surface acoustic wave 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.
The article 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 shown. 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 a field test are presented and discussed.
I. Introduction
Surface Acoustic Wave (SAW) devices are used in electronic circuits since many years. Main applications of SAW devices are resonators and filters for high frequencies. With the high volume production of electronic devices operating at high frequencies (such as mobile telephones) SAW devices are fabricated in mass production. These devices have to be stable in their properties under changing environmental conditions as temperature or mechanical stress. Because the effects of environmental influences to the surface acoustic wave were analyzed very well the idea of using these dependencies to build up sensing elements based on SAW devices was created. Using SAW sensors leads to several advantages. First the environmental parameter is converted directly to a change in frequency, phase or delay time. Because the frequencies that can be handled by SAW devices are quite high, the sensor signal can be transmitted wireless. Due to the energy storage in surface acoustic waves wireless transmission of the sensor signal can be done passively. That means, no power supply and no battery is necessary for the sensor. Further on there is no need for additional electronic components because the conversion from the parameter to be measured to the change of the sensor response signal is done directly by the change of the substrate where the SAW is propagating along. That overrides the restriction of maximum temperature that commonly used wireless sensors with semiconductor elements are exposed. That enables passive wireless SAW sensors to be applied in hot and aggressive environments. Another advantage of the sensors is a small design that features low thermal resistance and thermal inertia. The small dimensions enable application on rotating parts.
The intention of this paper is to present a measurement system based on SAW resonators. This includes the principle of SAW sensors, the description of the interrogation process by an instrument as well as the presentation of the hardware that was developed to analyze the sensor signals and to process the measurement results. Measurement results of a field test are presented and discussed.
II. Principle of Operation
A SAW resonator consists of a piezoelectric substrate, an interdigital transducer (IDT) and two reflectors in the direction of the propagating wave. The interdigital transducer is a structure of overlapping metal fingers that is fabricated on the substrate by a photolithographic process.The IDT is connected to an antenna. It receives energy for the excitation of the SAW by an electromagnetic wave coming from the interrogation unit. The IDT converts electrical energy to mechanical energy of the surface acoustic wave. The two reflector gratings form a resonating cavity where a standing wave is generated in the case of resonance. A portion of the stimulating electromagnetic energy is stored in this standing wave. After the stimulating signal is switched off energy is still present in form of the surface acoustic wave. The IDT converts a portion of the mechanical energy back to electrical energy because the process of energy conversion is partially reversible. The electrical energy is transmitted to the interrogation unit as an electromagnetic wave and can be analyzed.The state of resonance depends on several environmental influences the resonator is exposed to. Figure 1 shows some environmental influences to the sensor and the effects of it.
Fig. 1: Environmental influences to the SAW resonator and its effects.
In the case of temperature influence a change in material parameters of the substrate leads to a change in the phase velocity of the SAW resulting in a change of resonance frequency. A measuring device can detect the change in frequency. If the correlation between change of temperature and variation of frequency is known and can be expressed in a mathematical formula, the temperature can be calculated.
Fig. 2: Example of correlation between temperature and resonance frequency of a SAW resonator.
Figure 2 shows the correlation of a SAW resonator used as a temperature sensor. The presented curve was taken for calibration issues in a heating oven. Temperature was measured by a conventional temperature sensor (PT100) while sensor frequency was measured by the interrogation unit. The sensor was encapsulated to decrease other environmental influences and connected to the interrogation unit by cable. The projected application is presented together with the measurement results at the end of this article.
Fig. 3: Sensor interrogation.
Figure 3 illustrates the principle of sensor interrogation. In order to receive the resonance frequency energy has to be transmitted to the SAW resonator initially. That has to be done because the resonator does not have own sources of energy. Therefore the interrogation unit (the device that stimulates the sensor and analyzes the response) transmits an electromagnetic sine wave to deliver the energy. The frequency should be near the resonance frequency of the SAW resonator in order to transmit as much energy as possible. The SAW resonator stores the electromagnetic energy as mechanical oscillation energy. After switching off the stimulating signal the resonator utilizes a portion of the stored energy to generate a short decaying electromagnetic wave that can be received by the interrogation unit. After receiving the signal the resonance frequency has to be calculated. Figure 4 visualizes the procedure of an interrogation cycle from the point of view of the interrogation unit.
Fig. 4: Operational sequence of an interrogation cycle.
After the stimulating signal is transmitted the receiver has to amplify and digitize the signal coming back from the sensor. The digitized signal is analyzed by an internal processing unit. In order to detect the resonance frequency the signal has to be transformed from time to frequency domain, this is done by Fourier Transform. After calculating the Fourier Transform of the received signal the resonance frequency is detected. Based on the resonance frequency physical parameters are calculated. Further on some error detection and corrections are performed. This includes the increase of accuracy by statistical calculations. If the measurement result becomes available it is displayed or transmitted to a central computer.
III. Hardware of the Interrogation Unit
The interrogation unit consists of two main parts. The radio frequency module (RF-module) generates the necessary RF signals and handles the response of the SAW resonator. The main board controls the RF-module, analyzes the received signals, computes the results, displays the values and communicates with central computers.
A. RF-module of the Interrogation Unit
The block diagram of the RF-module is presented in Figure 5 and contains the main components.
Fig. 5: Block diagram of RF-module.
The transmitter consists of a phase locked loop (PLL) that generates the transmit signal (TF) based on a very stable clock reference followed by an amplifier that boosts the signal to about 10 dBm. After the signal passed the RF-switch it is transmitted to the SAW sensor via the antenna. Now the transmitter is disabled and the RF-switch is toggled. The response of the resonator (RF) is amplified and mixed down to an intermediate frequency (IF). The local frequency (LO) is generated by another PLL that is based on the same clock reference as the first PLL. The requirement to generate an intermediate frequency is caused by the analog to digital conversion. It is more effective (technical and monetary) to sample a low rather than a high frequency. The intermediate frequency is amplified again and passes a filter. The filtered signal is digitized and can be analyzed by the main board of the interrogation unit. Figure 6 shows the different signals that occur in the RF-module.
Fig. 6: Sensor response signals.
The lower signal (number 3) is the switching signal that toggles between transmit and receive. In this diagram the RF-module is switched from transmit to receive after 1 µs. The upper curve (number 1) shows the decaying wave of the resonator response with a frequency of about 434 MHz. The signal is measured at the output of the receive amplifier. The middle curve (number 2) is the signal mixed down in the receiver. The intermediate frequency is about 6 MHz. This signal is digitized and analyzed by the main board. The transmit signal is not displayed.
B. Main Board
The main board contains all digital components that are required to implement a SAW sensor interrogation. The block diagram presented in figure 7 contains the most important components of the main board.
Fig. 7: Block diagram of main board.
The core is an embedded computer equipped with 128 MB of RAM and 64 MB of flash memory. The computer is implemented as a standard industrial embedded module (ETX standard). Currently a module with a 300 MHz National Geode processor is used. The embedded module is supported by an FPGA (field programmable gate array) that controls the RF-module, buffers the data coming from the A/D converter and generates the clock signals from a stable clock reference. In short it handles all tasks that have to be executed in real time. The embedded computer module features interfaces to communicate with its environment. These interfaces include Ethernet, an insulated RS232 and USB (universal serial bus).The entire hardware is controlled by a monitoring circuit. The circuit monitors several temperatures and voltages as well as program execution of the embedded computer module. If something is beyond the specification it has the capability to reset hardware or to switch off the power supply to prevent serious damage. The interrogation process from the perspective of the main board proceeds as follows. The embedded module sends a message to the FPGA. The FPGA initiates the stimulating signal, switches to receiving mode and buffers data coming from the A/D converter. After the SAW resonator response is stored the embedded module gets back a message of completion. The sampled values, which are the time signal of the sensor response, are now read from the FPGA. The processor of the computer module calculates a Fourier transform based on this values. Figure 8 shows an example.
Fig. 8: Fourier transformation based on sensor response.
After the transformation is calculated the resonance frequency can be detected. Based on the resonance frequency the measured physical parameter (for example temperature in the projected application) can be computed. The result can be displayed or transmitted to a central computer via Ethernet or any of the other interfaces.
IV. Measurement Results in Industrial Environment
The interrogation unit was field tested in different industrial applications. One example for the use of a SAW sensor system is the temperature measurement inside a furnace for rotational moulding of plastics. The method of rotational moulding is performed using metallic moulds with a completely irregular movement in the furnace. Figure 14 shows a metallic mould for manufacturing plastic parts inside the open furnace with the senor mounted on it. The antenna for the SAW sensor interrogation is pictured in the foreground.
Fig. 9: Sensor on metallic mould for manufacturing plastic parts inside the open furnace.
The next plot (figure 15) presents the results of a measurement inside the furnace. In addition to the wireless SAW sensor two wired comparison temperature sensors are diagrammed. The first curve shows the temperature of the SAW resonator, the second the temperature of the comparison sensor inside the mould and the third the temperature of the comparison sensor outside the mould. The temperature outside the mould rises and goes down earlier as inside caused by temperature inertia of the mould. The SAW resonators temperature moves between inside and outside temperature because it is thermally connected to the mould itself. The results are overlaid by noise because of the mould movement. That noise can be decreased by statistical calculations.
Fig. 10: Results of measurement inside the furnace.
V. Summary
Sensors based on surface acoustic waves can be applied for wireless remote measurements of physical quantities such as temperature or pressure. The energy storage in SAW together with the possibility to handle high frequencies enable the construction of passive sensors with the ability to be interrogated by RF signals.A measuring device for the interrogation of SAW based sensors was shown. The usability of an SAW sensor measurement system in an aggressive industrial environment has been proofed. The measurement system is installed in different industrial facilities and ready for further applications.
