AU701577B2

AU701577B2 - Scanner for passive resonators as frequency-analog sensors with radio control

Info

Publication number: AU701577B2
Application number: AU58112/96A
Authority: AU
Inventors: Rainer Grossmann; Jurgen Michel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1995-06-06
Filing date: 1996-05-31
Publication date: 1999-02-04
Anticipated expiration: 2016-05-31
Also published as: EP0830617A1; WO1996039639A1; ATE175781T1; EP0830617B1; AU5811296A

Abstract

Disclosed is a scanner for frequency-analog passive resonators that are excited to oscillate at their natural frequency by electromagnetic, inductive or capacitive coupling or directly via a line, the frequency of the excitation signal following the natural frequency of the resonator in such a way that the difference between excitation frequency and natural frequency of the sensor is constant.

Description

In a further known measurement method by radio, surface acoustic wave resonators, as frequency-analog sensors, are excited by short RF pulses. Once the excitation is switched off, the resonator continues to oscillate at its natural frequency.

Owing to the losses in the resonant circuit, including the emission by the antenna, the amplitude of the natural oscillation decays exponentially. Depending on the level of damping, the signal may be available only for a very short time, until it is no longer higher than the noise level. The resolution of the measured variable is thus severely constrained.

The object of the present invention is to evaluate signals from resonators which are used as radio sensors, in a simple manner, quickly and with high resolution.

According to one aspect of the present invention there is provided an apparatus for evaluating resonators which are used as frequency-analog sensors, said apparatus including: a sensor excitable to oscillate at its natural frequency via electromagnetic, inductive or capacitive coupling or directly via a line from an excitation signal; and a control loop configured such that the frequency of the excitation signal is slaved to the natural frequency of said sensor using said control loop, such that the difference between the excitation frequency and the natural frequency of said sensor is constant.

If one wishes to increase the resolution by averaging over a number of transmission periods and in this way reducing the noise, one cannot avoid digital processing and the signals can be evaluated only very slowly.

Feasible digital evaluation methods without phase control use, for example, the (discrete) Fourier transformation or regression analysis in order to determine the 25 natural frequency from the response signal. Such methods require a high level of complexity in terms of circuit technology and resources.

In order to make the sensor signal available continuously, the frequency of the transmitting stage is slaved, according to the invention, as the frequency for the sensor response. This is done using the principle known per se of the phase-locked loop (PLL): the transmitting oscillator is designed as a voltage-controlled oscillator (VCO) which is controlled by a phase or frequency comparator via a control filter. the corresponding comparator produces a link between the transmission frequency and the received sensor frequency.

[n:\libppl 1 GR 96 P 8029 3 In the case of the phase comparator, the transmission frequency is identical, after lock on, to the response frequency (possibly except for a phase angle). It is also possible to use a frequency comparator to control the transmission frequency to provide a fixed difference frequency from the response signal.

In order to evaluate the measured variable, it is now possible either to determine the slaved transmission frequency or to measure the control variable (voltage at the output of the control filter) directly. It is recommended that the pulse width of the excitation signal be varied as a function of the control difference. For example, the smaller the frequency difference between the excitation and the natural frequency of the sensor, the longer may the excitation pulse be, in order to supply the sensor with more energy. Nevertheless, the sensor can still reliably be excited.

Although the measurement provides high precision since the sensor response is evaluated over any required number of excitation frequencies for a steadystate measurement there is no need for any complex design and it is possible to dispense with complex digital signal processing. The part of the design on the sensor side is uncomplicated and thus highly reliable. In comparison to a digital evaluation method, higher evaluation speeds are achieved using the method described here. A higher signal-to-noise ratio is achieved by modulation of the pulse width, which does not make sense unless frequency slaving is used.

The resonator used as the sensor should advantageously have a high Q-factor. Crystals or crystal ceramics are particularly suitable for this purpose. At least two such sensors are advantageously used, physically combined with one another, in a measurement system.

This allows elements of the sensor signal caused by external interference factors to be compensated for in the evaluation unit. An antenna is advantageously provided which is connected to the resonator and via which the signal GR 96 P 8029 4 is emitted. Instead of using a conventional antenna, a narrowband, directional antenna may be used. The distance over which the emitted signal can reliably be received is apart from the antenna dependent on the electrical characteristics of the environment as well as the material and geometric properties of the sensor.

The evaluation unit has to have a controlled oscillator which can follow all the possible natural frequency changes of the sensor. The transmitted signal is regularly interrupted for example being switched to a different frequency when a reference sensor is provided in order to allow the sensor to oscillate at its natural frequency. When the control loop locks on, the difference between the transmission frequency and the received frequency is equal to a fixed, predetermined frequency.

Figure 1 shows a clock 1, an interrogation signal 2 and an exponentially decaying response 3 from a SAW resonator as a strain sensor which can be interrogated by radio.

When the mechanical load on the sensor changes, then its resonant frequency changes, and thus the frequency of the response 3.

Figure 2 shows the block diagram of the measurement signal in the version using an SAW resonator as the sensor. The carrier frequency, which is produced by a voltage-controlled oscillator (VCO) 4, is first of all amplified in an amplifier 5 and is modulated by a switch 6. An antenna 8 is thus fed via an output stage and a transmission/reception duplexer or filter 7. The sensor receives the transmitted signal via its own antenna 9 and, as the response, reflects an exponentially decaying natural oscillation. This is received again by the antenna 8 and passes via the transmission/reception duplexer or filter 7 to a bandpass filter 10. The signal is then amplified in an amplifier 11. It is then converted by a mixer 12, which is controlled by the VCO, into a low-pass signal 13 and is supplied to a frequency discriminator 14. This supplies an output voltage which is proportional to the frequency difference between the low-pass signal and the reference signal and passes via a loop filter 15 in order to control the VCO. If the resonant frequency of the resonator changes, then it follows the frequency of the VCO, which is measured by a frequency meter.

The basic design of a SAW resonator as a strain sensor is illustrated in Figure 3. The illustration shows the actual sensor, which comprises a crystal substrate 16, an interdigital converter 17 and reflectors 18 without an antenna, and bonded onto a measurement object 19. As the measurement object is expanded or compressed, then bonding 20 transmits the strain to the sensor, which changes its resonant frequency.

10 If a SAW resonator on a crystal substrate (ST crystal substrate) is used as the .*see: strain sensor, then the resonant frequency changes linearly with the strain. Figure 4 shows the characteristic of such a passive radio sensor.

0% 0 ••o ooooo [n:\libpp]Ol 1 The claims defining the invention are as follows: 1. An apparatus for evaluating resonators which are used as frequencyanalog sensors, said apparatus including: a sensor excitable to oscillate at its natural frequency via electromagnetic, inductive or capacitive coupling or directly via a line from an excitation signal; and a control loop configured such that the frequency of the excitation signal is slaved to the natural frequency of said sensor using said control loop, such that the difference between the excitation frequency and the natural frequency of said sensor is constant.

2. The apparatus as claimed in claim 1, having at least one sensor element and one reference element for the reference function.

3. The apparatus as claimed in any one of claims 1 or 2, using surface acoustic wave components as sensors.

4. The apparatus as claimed in any one of claims 1 or 2, using volume o ••wave (crystal) elements as sensors.

o• S 20 5. The apparatus as claimed in any one of claims 1 to 4, the sensors being bonded onto the component to be measured.

6. The apparatus as claimed in any one of claims 1 to 4, the sensors being bonded into a mechanical clamping means in order to transmit forces.

7. The apparatus as claimed in any one of claims 1 to 6, the controlled frequency of the excitation signal being evaluated.

[n:\libpp]Ol 185:IAD

Claims

8. The apparatus as claimed in any one of claims 1 to 6, the manipulated variable of the control loop being evaluated.
9. The apparatus as claimed in any one of claims 1 to 8, having a plurality of excitation frequency generators and the same number of sensors, the slaving being carried out via a changeover means for only one excitation frequency generator at any time. The apparatus as claimed in any one of claims 1 to 9, the temperature influence on the sensors being compensated for.
11. The apparatus as claimed in any one of claims 1 to 9, the excitation signal comprising alternating signal pulses of different lengths.
12. An apparatus for evaluating resonators which are used as frequency analog sensors substantially as herein described with reference to Figs. 2 to 4. DATED this Twenty-third Day of October 1998 o• Siemens Aktiengesellschaft 20 Patent Attorneys for the Applicant :o SPRUSON FERGUSON o *o* [n:\libppOl 185:IAD GR 96 P 8029 Abstract Interrogator for passive resonators as radio-controlled frequency-analog sensors Interrogator for frequency-analog passive resonators which are excited to oscillate at their natural frequency via electromagnetic, inductive or capacitive coupling or directly via a line, the frequency of the excitation signal being slaved to the natural frequency of the resonator in such a manner that the difference between the excitation frequency and the sensor natural frequency is constant. Figure 2