FIELD

[0001] The invention relates to a method for measuring an object to be measured with radio- or microwaves by using a resonator. The invention also relates to a measuring device applying the method.

BACKGROUND

[0002] Characteristics of an object to be measured, such as moisture or the dimension parallel with the thickness, may be measured using microwave measurement in which the resonance frequency of the measurement resonator depends on the characteristic of the piece to be measured. Conventionally, the resonance frequency of the resonator can be found by scanning the frequency of an oscillator producing microwave radiation over a measurement band. The characteristic of the piece to be measured can then be determined as a function of the found resonance frequency. [0003] However, frequency scanning involves a number of problems. A measuring device based on frequency scanning is complicated and expensive, because it requires scanning electronics that change the oscillator frequency. In addition, frequency scanning is time-consuming, because the measurement must be carried out at all measurement band frequencies and thus it takes time before the measurement results are obtained. In an oscillator based on feedback, the feedback loop must be phased, which means that the feedback delay must be suitable to enable a positive feedback and to fulfil the oscillation conditions. This limits the measuring range to be achieved. Since the resonator comprises a number of resonance frequencies, which are in rela- tion to one another through multiples of the wavelength half, it is uncertain whether the resonance frequency on the measurement band will be achieved.

BRIEF DESCRIPTION

[0004] It is an object of the invention to provide an improved method for measuring with microwaves, and a measuring device. This is achieved by a method for measuring an object to be measured by means of the resonance frequency of microwave frequency or radiofrequency radiation, the method comprising supplying microwave frequency or radiofrequency radiation to a resonator which uses the object to be measured as a functional part of the resonator in such a manner that the object to be measured affects the reso-

nance frequency of the resonator. The method further comprises providing resonance in the resonator by supplying a noise-like signal to the resonator, the band of the signal being predetermined and having the resonance frequency of the resonator when the object to be measured acts as a functional part of each resonator.

[0005] The invention further relates to a measuring device intended for measuring an object to be measured by means of microwave frequency or radiofrequency resonance, the measuring device comprising at least one resonator, which uses the object to be measured as a functional part of the resona- tor in such a manner that the object to be measured affects the resonance frequency of the resonator. The measuring device comprises a signal source, which, in order to provide resonance, is arranged to supply a noise-like signal to said at least one resonator, the band of the signal being predetermined and having the resonance frequency of the resonator when the object to be meas- ured acts as a functional part of the resonator.

[0006] Preferred embodiments of the invention are disclosed in the dependent claims.

[0007] The method and measuring device of the invention provide a number of advantages. The resonance frequency of a resonator can be deter- mined without frequency scanning, which renders the solution fast, simple and cost-effective in this respect. In addition, the measurement can be used for measuring a characteristic of a shaking plate. By using the solution, the problem of achieving a resonance frequency outside the measuring range can be avoided. Furthermore, without a phased feedback loop the coupling does not limit the achievable measuring range.

LIST OF FIGURES

[0008] The invention will be described in greater detail with reference to preferred embodiments and the accompanying drawings, in which

Figure 1 illustrates a resonator, to which a noise-like microwave sig- nal on a desired band is supplied,

Figure 2A illustrates a resonator, in which the measurement is carried out through an object to be measured,

Figure 2B illustrates a resonator whose near field includes an object to be measured that affects the resonance frequency of the resonator,

Figure 3 illustrates a signal to be supplied to the resonator, and an amplified signal from the resonator,

Figure 4 illustrates a measurement solution using a supply filter, Figure 5 illustrates a measurement solution using a receiving filter, Figure 6 illustrates a signal source,

Figure 7 illustrates a signal source, and

Figure 8 illustrates the flow diagram of the method.

DESCRIPTION OF EMBODIMENTS

[0009] The disclosed solution is applicable to measurements gener- ally performed by microwave resonators. An application is measurement of objects having an electrically conductive surface. In this case, applications may include, although are not restricted to, metal pieces, such as steel, copper and aluminium sheets or insulating sheets coated with an electrically conductive substance. In addition, the solution may be applied to measurement of charac- teristics of insulating substances, for example. Instead of microwaves, radiof- requency radiation may also be used. In the disclosed solution the frequency band of electromagnetic radiation varies approximately from 100 MHz to 30 GHz.

[0010] Figure 1 illustrates a coupling suitable for microwave measurement. A noise-like microwave signal is supplied from a signal source 202 via a transmission line 206 to a resonator 200, the signal being directed at an object to be measured 210 by means of a resonator mirror 208. Microwave radiation is reflected back from the object to be measured 210, and a resonant stationary wave depending on the distance d between the resonator mirror 208 and the upper surface 212 of the object to be measured 210 is formed in the resonator 200. The distance between the resonator mirror 208 and the upper surface 212 of the object to be measured 210 is a multiple of the wavelength half of the microwave signal, mathematically expressed as d = n- — , where n is an integer 1 , 2, ... and λ is the wavelength. The shape of the mirror may be a curved spherical surface, although a paraboloid or some other surface shape directing microwave radiation to the object to be measured is also possible.

[0011] When the distance between the resonator mirror 208 and the upper surface 212 of the object to be measured 210 changes due to changes in the thickness of the object to be measured 210, for example, also the wave- length λ of the resonant microwave radiation changes without outside meas-

ures. Also, the object to be measured may be subjected to a force pulling the object further away from (or closer to) the resonator mirror. If the distance between the object to be measured and the resonator mirror changes, a physical characteristic of the object to be measured or a characteristic affecting the ob- ject to be measured, such as force, can be determined. In this solution resonance frequency does not need to be searched for by means of scanning or any other way either, but the resonator 200 directly determines the resonance frequency. The open resonator 200 may thus be a Fabry-Perot type resonator. The resonant microwave radiation can be received on a transmission line 214, from which the received microwave radiation is coupled to an amplifier 100. The amplifier 100 amplifies the received signal and forwards it via a transmission line 216, for example, to a measurement part 220. The measurement part 220 may comprise a signal processing unit 222 and a computer 224. The signal processing unit 222 may comprise a frequency divider, which is used for reducing the frequency of a microwave frequency or radiofrequency signal to a frequency suitable for other signal processing parts of the signal processing unit. Measurement data that the signal processing unit 222 has received from the amplifier 100 may be input to the computer 224 for further processing.

[0012] As shown in Figure 1 , the open resonator 200 directs micro- wave radiation to a direction in which the object to be measured 210 and reflecting microwave radiation is meant to act as a functional part of the resonator at the moment of the measurement. The open resonator 200 thus determines the resonance frequency according to the location of the surface 212 of the object to be measured 210 automatically. In general a measuring device may also comprise more than one open resonator.

[0013] Figure 2A illustrates another example of a coupling suitable for microwave measurement. In this solution microwave radiation is transmitted through the object to be measured 210. The resonator consists of a transmitter part 250 and a receiver part 252, and the resonance frequency of the resona- tor is affected by the dielectricity of the substance between the transmitter part 250 and the receiver part, i.e. the object to be measured 210. In the solution, the signal source 202 supplies a noise-like microwave signal to the resonator's transmitter part 250, which transmits the radiation further through the object to be measured 210 to the receiver part 252. The resonator centres the micro- wave radiation at its resonance frequency, which depends on the characteristics of the object to be measured. From the receiver part 252 the received mi-

crowave radiation is transferred to the amplifier 200 and from there further to the measurement part 220.

[0014] Figure 2B shows a solution in which microwave radiation travels through the object to be measured. In this example, the moisture con- tent or consistency of a paper machine web 210 is measured, but generally different characteristics of a variety of non-conducting substances can be measured. In this example water which affects the measurement is included in both a wire 260 and the object to be measured 210, which may be a web in this example. The electric near field of the resonator 200 extends through both the wire 260 and the object to be measured 210 at the wavelength used. In the far field, the resonator 200 does not radiate outwards at all or radiates only a little. The boundary B between the near field and the far field may be determined by means of a dimension D (e.g. diameter) of the aperture of the resonator and the wavelength λ for example in the following manner: B = (2D2)/Λ . In this case the electric field, the distance of which from the radiator is the same as or smaller than B, is in the near field. Accordingly, the electric field whose distance from the radiator is bigger than B is in the far field. The electric near field of the microwave radiation is thus in interaction with the wire 260 and the object to be measured 210, wherefore the resonance frequency depends on the wire 260 and the object to be measured 210, in which the amount of water may vary. The resonator 200 automatically tends to have its resonance frequency because of its characteristics, which are affected by the object to be measured.

[0015] In the solutions according to Figures 2A and 2B, the wave- length of the resonant microwave radiation changes according to changes in the dielectricity of the object to be measured when the microwave radiation travels through the object to be measured. In general terms, the wavelength of the resonant microwave radiation changes when a characteristic of the object to be measured or a characteristic affecting the object to be measured changes.

[0016] The measurement part 220 may employ a computer program or a hardware solution. The hardware solution may be implemented, for example, as one or more application-specific integrated circuits (ASIC) or as a functional logic composed of discrete components. [0017] Figure 3 illustrates a noise-like signal 300 to be supplied to the resonator and an amplified signal 302 from the resonator. The horizontal

axis represents frequency on a freely selected scale, and the vertical axis represents signal strength on a freely selected scale. When a wideband signal 300, which may be a deterministic signal, noise-like signal or combination thereof, is supplied to the resonator, the energy of the microwave radiation in the resonator will be centred at resonance frequencies 304 to 308, which depend on the size of the resonator in such a manner that the distance between the resonator mirror 208 and the upper surface 212 of the object to be measured 210 is a multiple of the wavelength half.

[0018] In the disclosed solution the signal source 202 supplies a noise-like signal 312 on a microwave band to the resonator 200, the band 310 of which is predetermined and has the resonance frequency 306 of the resonator when the object to be measured acts as a functional part of the resonator. In this way, creation of new resonance frequencies in the resonator is avoided. [0019] In an application, the signal source 202 supplies noise, which may be bandwidth-limited, amplitude-limited or both, to the resonator. The noise-like microwave signal may be white noise or pseudorandom noise, and the signal may consist of noise completely or partly.

[0020] Figure 4 illustrates a solution in which a signal with a predetermined band is formed by using a supply filter 400. A signal to be supplied from the signal source 202 to the resonator 202 may be filtered by means of the supply filter 400, which may be a band-pass filter, in order to have a band that is narrower than the distance between two successive resonance frequencies (e.g. the distance between resonance frequencies 306 and 308) of the resonator. Before being supplied to the resonator, the microwave frequency signal may also be amplified in an amplifier 402.

[0021] Figure 5 illustrates a solution in which a receiving filter 500, which may be a band-pass filter, may filter the signal from the resonator 202 so that it has a band that is narrower than the distance between two successive resonance frequencies (e.g. the distance between resonance frequencies 306 and 308) of the resonator. Before filtering the received microwave frequency signal may also be amplified by a preamplifier 502.

[0022] Figure 6 illustrates an alternative for the signal source 202 of a signal with a predetermined band. The signal source 202 may comprise a noise source 600, the noise signal of which may be amplified and filtered in a modifier 602. After this, the noise signal may be mixed to a desired frequency

band by multiplying the noise signal by the signal from the oscillator 604 in a multiplier 606.

[0023] Figure 7 illustrates another alternative for the signal source 202 of a signal with a predetermined band. The signal source 202 may com- prise a noise source 700, the noise signal of which may be modified in a modifier 702, which may comprise an amplifier and a filter (not shown in Figure 6), for instance. After this, the noise signal may be transmitted to a desired frequency band by multiplying the noise signal in a multiplier 704, which may comprise a diode multiplier and an amplifier, for instance. [0024] Next, the disclosed solution will be further examined with reference to the flow diagram of Figure 8. Microwave radiation may be supplied to the resonator, which uses the object to be measured 210 as a functional part of the resonator in such a manner that the object to be measured 210 affects the resonance frequency of the resonator. Thus, in step 800 resonance is pro- vided in at least one resonator 200 by supplying a noise-like signal to said at least one resonator 200, the band of which is predetermined and has the resonance frequency of each resonator when the object to be measured 210 acts as a functional part of each resonator 200.

[0025] Step 800 may be carried out for example by means of a computer program containing routines for executing the method steps. For sales purposes, for example, the computer program may be stored on a computer readable memory, such as a CD-ROM (Compact Disc Read Only Memory). The computer program may also be included into a telecommunications signal downloadable from a server (across the Internet, for example) into a measuring device.

[0026] Although the invention is disclosed above with reference to the examples of the accompanying drawings, it is obvious that the invention is not restricted thereto but may be varied in many ways within the scope of the accompanying claims.