GB1560591A - Apparatus for observing an abrupt change in the radio frequency energy reflected or passed by timber - Google Patents

Description

(54) APPARATUS FOR OBSERVING AN ABRUPT CHANGE IN THE RADIO FREQUENCY ENERGY REFLECTED OR PASSED BY TIMBER (71) We, INSINÖÖRITOIMISTO INNOTEC OY., a Finnish Company organized under the laws of Finland, of Luoteisrinne 4 E, 02270 Espoo 27, Finland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an apparatus for detecting knots and other defects in timber, particularly in sawn timber, in order to enable its quality to be determined and to classify it.

When it is desired to sort or classify timber, the main task is to identify the knots present in the timber. This has been accomplished by a variety of methods in known systems. In some systems the surface of the sawn timber has been scanned optically so that the different colour of the knot may be detected by a photometer registering the amount of light. In certain other systems the mechanical differences of the knot from the rest of the timber have been employed to detect them.

It has now been found that a knot also induces changes in the passage of radio frequency energy at the respective point.

The radiation passing through the timber as well as that reflected by it, especially in the microwave range, shows changes of its amplitude and phase in the vicinity of the knot. These changes may be observed by comparing it with radiation that has normally passed through or been reflected by the timber. and whenever herein a sudden change occurs, this can with considerable confidence be attributed to a knot.

When radio frequency energy passed through the timber is used to detect knots, it is possible to determine the average size of the knots, whereas with reflected radiation it is possible to measure in particular the location and size of the visible knots present in the timber on a given surface. It is thus possible by performing the same measurement on all faces of the timber, to perform a sorting by external appearance and not only by the average knot content.

According to the present invention there is provided an apparatus for observing an abrupt change in radio frequency energy reflected from or transmitted through timber, comprising at least one transmitting antenna for transmitting high frequency radio energy to the timber, at least one pair of receiving antennae located close to one another for detecting radio energy reflected from or transmitted through the timber, and a detector for receiving the sum of the radio frequency energy signal received by one receiving antenna plus the radio frequency energy signal received by the other receiving antenna with 1800 phase shift between the two signals, whereby a null output is received from the detector except when a knot occurs in front of one or other of the receiving antennae.

The apparatus is capable of observing abrupt changes with a satisfactory accuracy, and can be made of a small size favourable to the measuring of timber.

A timber board may be classified immedately after it has passed through the antenna field, or it is possible to carry out an automatic cross-cutting operation by which the more valuable knot-free portion of the board is separated and recovered.

When high frequency radio energy, such as microwave energy for instance, is propagated through a medium, it is affected by the dielectric constant and the resistivity of the substance. It has been found that in timber, and in sawn timber in particular, the knots have in this respect properties clearly differing from other parts of the timber, so that when the radiation is specifically confined to the area of a knot and when it moves therefrom into another part of the timber or vice versa, there occurs a distinct change in the properties of radiation passing through or reflected radiation therefrom.

This change is observable by employing a suitable antenna structure, by means of which the intensity andssor phase of the passing or the reflected energy is measured.

The wood in the area around a knot differs to such extent from the normal characteristics and values elsewhere in the timber that a difference is obsenable in the amount andlor phase of radiation that has passed through the timber or has been reflected by it close to a knot, compared with the equivalent value measured in a knot-free area of the same timber.

The apparatus of the invention performs this measurement by comparing the radia- tion intensities andior phases in mutually adjacent areas of the timber, such as a board for instance, and the output of the apparatus consists of the differences between these areas, that is in the equilibrium condition with no knots there iis no output or the output is minimal. Thus a null output reveals that the board is uniform in the area in question.

In one embodiment of the invention the 180 degree phase difference is achieved by dividing the wave guide into two parts at its initial end by a partition, so that h the wave guide becomes in effect two separate wave guides, which then come together before the detector part. In one of the two wave guides a body is placed which has such electrical properties and such a length that it delays the wave travelling in the wave guide with respect to the wave travelling in the other wave guide by 180 degrees or by an odd multiple thereof.

In another embodiment of the invention the 180 degree phase difference is produced by connecting the pair of receiving antennae to the common detector using wave guides, coaxial leads or other transfer leads of different lengths so that the difference in electrical length between the leads at the measuring frequency is an odd multiple of half wavelengths. It is thus possible to vary the spacing of the antennae with reference to each other. This is also possible in another embodiment in which the 180 degree phase difference is produced by connecting the pair of receiving antennae by rectangular wave guides to a common detector in a series branch of a T junction in the E plane in such a manner that the wave guide lengths from antenna to junction are electrically equal or their difference is an integral multiple of wavelengths.

The transmitter antenna and the receiving antennae may be located on the same side of the timber, with the receiving antennae symmetrically disposed on either side of the transmitting antenna. Using such an arrangement the transmitting and receiving antennae can be combined in one apparatus. This makes it possible to use a simple construction which is cheap to manufacture.

The device is specifically intended to carry out measurement involving reflection from the timber.

Means may be provided for exercising electronic control of the transmitter frequency so that 180 degree phase difference will be achieved when measuring a piece of uniform timber. This makes it possible to compensate for slightly different antennas lengths or a change of the electrical length of the leads e.g. owing to changing temperature.

In an embodiment of the invention the antennae constituting the antenna pair are placed with reference to the third antenna of the set, as well as to the major dimension of the timber, in a mutually symmetrical oblique position, whereby the phase differences and//or amplitude differences of the signals arriving from the specimen reveal, as a result of the difference in the direction of polarisation, deviations of the timber grain direction from the said major dimension direction. It is thus possible to identity the direction of grain of the timber and to classify the timber according to it. It may be noted that the angle of symmetrical oblique mounting of the antenna pair of the set with reference to the direction of the board and of the third antenna is preferably 45.

The piece of timber, such as a board for instance, may either be classified immediately after it has traversed the antenna field or arrangements may be made for automatic crosscut sawing by which the more valuable defect-free part of the board is separated and recovered. Null output indication of the apparatus implies that the board is uniform and straight-grained in the respective area.

The invention is further described below with reference to the accompanying draw mugs, wherein: Figures 1 and 2 show an embodiment of the receiving antenna of the apparatus of the invention in elevation and perspective views respectively; Figures 3 and 4 show another embodiment of the receiving antenna of the apparatus of the invention is plan and perspective views respectively, Figures 5 to 7 are plan views of various embodiments of the receiving antenna; Figure 8 is a perspective view of an apparatus of the invention, in which the transmitting and receiving antennae are accommodated in one unit.

Figure 9 is a perspective view of an apparatus according to the invention ap plied to measure the grain direction.

All the embodiments described below are used in association with an appropriate conveyor, it being understood that either the timber is carried past a fixed measuring point or, alternatively, a movable measuring device is traversed with reference to the timber. It is immaterial whether the conveyor is one moving the timber past a fixed measuring station or whether the movement is brought about in the opposite manner.

When referring to movement of the timber, therefore, this is to be taken as meaning relative movement of the timber with reference to the apparatus, regardless of which of the two parts is actually moved.

The apparatus of Figures 1 and 2 comprises a receiving wave guide 11, adjacent one end of which a radiation-detecting receiver is disposed, in this instance a diode detector 12. The aim is that when a uniform or regular board having no knots is under measurement no output is obtained on the diode 12. This balancing is accomplished in the case of the embodiment shown in Figures 1 and 2, by dividing the wave guide 11 longitudinally in two at its initial, or entry, end where the radio frequency waves arriving from one single transmitter reach the wave guide either after passing through the sawn timber or after reflection from it.

This division is accomplished by a partition 13 in the H plane whereby this wave guide becomes in effect two separate wave guides which join before the detector part 12. This division does not in itself affect the radiation passing through the wave guide, but it is now possible to place in one half of these wave guides obtained by division, a body 14 of a suitable material with electrical properties and length selected to be such that a wave travelling in the wave guide is delayed therein, compared with the other lead, by 180 degrees or an odd multiple thereof.

When the waves arriving through the two wave guides combine after the partition 13 ends these components cancel out if the radio frequency energy arriving at the entrance ends in the divisional parts has the same phase and amplitude in both halves.

The measuring instrument will then indicate equilibrium condition, and thus uniformity of the sawn timber.

Only when a knot or other defect causing an equivalent change occurs exactly at the end of the wave guide, so that the amounts of radiant energy incident on the different halves of the wave guide are unequal or differ in phase, is an output obtained from the detector. If the emitted radiation originates in one antenna on the opposite side of the sawn timber, unbalancing of the two channels reveals a sudden change in the average radio energy transmittance values of the sawn timber. This may be interpreted as a knot, as has been observed.In like manner, if a transmitting antenna on the same side of the saw timber is used and the radiation reflected from the timber is monitored with a pair of antennae, as in this embodiment, then unbalancing of the pair of antennae indicates a sudden change in the reflecting properties, which may have their origin even very close to the surface of the sawn timber, and thus the unbalance condition reveals the presence of a knot area with a considerable influence on the appearance of the sawn timber on the side concerned.

The dimensions of the body 14 are naturally determined by the frequency employed in the measurement. The length of this body 14 in the direction of the wave guide is chosen so that the radio energy travelling in this part of the wave guide suffers a phase shift of as exactly 180 degrees as possible at the said frequency or an odd multiple of 180 degrees. As will be appreciated this is specific to a certain given frequency, but this does not impede the measurement, since the receiver is tuned to a given frequency.

Furthermore, the characteristic curve of the responses obtained with different frequencies in the equilibrium condition is quite flat enough to give the desired accuracy.

Figures 3 and 4 illustrate a second embodiment of the present invention, in which the receiving antennae are arranged to constitute separate arms 15 and 16 of a wave guide, although their outer ends may be placed as close together as desired by bending the wave guide. In the figure a situation has been shown, in the interests of clarity and simplicity, without any extra bends, whereby reception takes place into the open ends 15 and 16 of the wave guides, or into horn antennae connected thereto, positioned at a given distance from each other. From these points the wave guide arms are curved towards each other and joined to form a horse shoe configuration.

The energy arriving from the arm 15 is met halfway along the horseshoe by energy from the arm 16.

If these amounts of energy are equal to one another they will cancel each other out at the detector 12 placed in the middle of the horsehose body. The same result is obtained if the detector is so positioned that the distance from it to one end of the wave guide exceeds the distance to the other end by an integral multiple of the wavelength.

In order to elucidate the process, the vectors of the electrical field have also been shown in Figure 3, as well as the turning of their direction in the bends of the wave guide, from which it is evident how the fields are mutually opposed in balanced condition at the middle of the horseshoe body. The apparatus operates in the same way as that of Figures 1 and 2, since the ends 15 and 16 of the wave guides may furthermore be displaced to be as close together as may be required, as has been mentioned above.

Figure 5 shows another way in which the energies received at the two wave guide ends or with horn antennae connected therewith can be combined with 1800 phase shift. The wave guide illustrated is bent in the plane of the broad side of the tube (in the H plane) and the detector is placed at a point in the wave guide where the distance to one end is a half-waves and that to the other end is b half-waves so that b-a is an odd integer. In a manner known from microwave technology a null output is obtained from the detector 12 in the case of uniform board. In Figure 6 the wave guide is replaced by a coaxial line or another equivalent transfer lead and with suitable antennas 17 and 18 connected to its ends. In this case too, a null output is obtained from the detector 12 in the case of a uniform board if b-a is an odd integer.

In Figure 7 the wave guide- has also been bent in the plane of the broad side (in the H plane), but the energies arriving from the ends of the wave guide are detected by means of a detector placed in a series branch of a T junction in the E plane. In a manner known from microwave technology a null output is obtained in the case of a uniform board if m-n is an integer.

The embodiment example of Figure 8 is particularly intended for use in measurements made by reflected radio energy. In this apparatus the transmitting and receiving antennae are assembled to form a single unit in a manner similar to that used in printed circuits. It is thus possible to provide a unit of simple construction, which is cheap to manufacture and of a small size.

Although Figure 8 shows an apparatus in which the transmitting antenna is located on the same side of the timber as the receiving antennae, that is the apparatus is specifically designed to carry out measurement involving reflection from the timber, it is obvious that this embodiment may be modified by detaching the transmitter from its connection and moving it over to the other side of the sawn timber, so that the apparatus can be used for measurements through the timber.

The transmitting antenna 21 is located in the middle of the apparatus. The receiving antennae 22 and 23 are located symmetrically with respect to the lead of the transmitting antenna in the configuration of this printed circuit. It is then simple to obtain in both receiving antennae normally equal intensities if the reflecting properties in the vicinity of the transmitting antenna on both sides thereof are homogeneous. The receiving antennae 22 and 23 are connected to each other in such manner that at the detector 12 the quantities obtained from different antenna leads have a mutual phase shift of 1800 and therefore in the case of uniform reflected radiation cancel each other out. This is achieved when b-a is an odd integer. The transmitting antenna 21 as well as the receiving antennae 22 and 23 are encircled by an earth component 25 to minimize interference.The supply power for the antenna is obtained from a suitable generator 24.

When the distances from the transmitting antenna 24 to the receiving antenna 22 and 23 on either side thereof are comparatively small and with the output to the detector 12 equal to the difference of the radiation quantities arriving at the antennae, an output is obtained from the detector 12 only if a reflector with abruptly changing properties is present close to the antenna and receiver conductors. It has been found that a knot in timber constitutes such a difference in the reflective properties, especially if a substantial amount of moisture is present in the timber. Thus when the timber is conveyed past the antenna and the generator 24 supplies radio frequency energy to the transmitting antenna, each and every pair of points of unbalance recorded by the detector 12 indicates the passage of the edge of a knot past the antenna.

The apparatus of Figure 9 is similar in its operation to the above described devices, but this apparatus indicates the deviations of the grain direction in the specimen, i.e., in the wood. The antennae 29 and 30 are located close together and orientated at an angle to each other which has a magnitude determined according to the principles described below. The antennae 29 and 30 operate as receiving antennae, whereas the antenna 31 is a transmitting antenna. The antennae 29 and 30 are symmetrically disposed with reference to the diametral plane of the antenna 31, or the symmetry plane, on both sides thereof, either on one side of the timber under measurement (reflection measurement) or on opposite sides of the timber 32 (measurement of transmitted radiation). The angles of antennae 29 and 30 are symmetrical with respect to the said diametral plane in such a manner that their planes parallel to the electrical field (the so-called E planes) are at angles a1 and a2 such that a1 = a2. The situation in the measuring of sawn timber is at its most favourable when the angles are t450.

Using a 45-degree angle one ensures the possibility of measuring the maximum obliqueness of the grain, since in the case of slanting grain the phases and amplitudes of the different signals change between themselves so that the differential signal obtained increases with increasing inclination of the grain progressively until it reaches its maximum approximately when the grain is parallel to the electric field of one of the two antennae, that is at the same time as the grain is perpendicular to the electric field of the other antenna.

The measuring signals of the antennae 29 and 30 are shifted to be 1800 out of phase with each other by connecting the antennae 29 and 30 with rectangular wave guides through a T junction in the E plane to a matcher/detector 33 or by arranging that between the matcher 33 and the antennae 29 and 30 there is in these two transfer distances an electrical difference of path length equal to one half wavelength or an odd multiple thereof. In that case a null output is obtained from the receiver (apart from noise) when the grain of the timber constituting the specimen runs with reference to the fields of the antennae symmetrically at equal angles because the intensities of the different signals arriving from the specimen then cancel each other out owing to the phase difference mentioned.

The transmitter is indicated in the drawing merely by a box 34.

When the timber 32 is moved longitudinally with respect to the detector each deviation of the signal received by the pairs of antennae 29 and 30 indicates a change in the quality of the timber. Depending on what one wishes to indicate, each point of unbalance may then be recorded as a point of defect or, in addition, one may separate the knot responses from the other responses, whereby the residual signals are consistent with points where the grain orientation of the sawn timber is warped.

The radio energy transmitters which are used in the apparatus of the invention, and likewise the common detector 12 to be used, are means which are familiar to a person skilled in the art of radio frequency devices and therefore no particular description thereof is necessary in connection with the present invention.

Slightly different distances of the antennae in the pair of antennae or a change of the electrical length of the leads e.g. owing to a temperature change may cause disturbance of the equilibrium even in the case of a uniform board. If the leads connecting the pair of antennae to the detector are electrically of unequal length, a new equilibrium or balanced position is found by slightly changing the transmitting frequency. This may be used in the measurements e.g. in that the transmitting frequency is continuously controlled e.g. by electronic means in such manner that a balanced condition is normally achieved. If the time constant of the control circuit is suitably chosen, any knot will manifest itself in an abrupt increase of the control voltage or, in the case of a short time constant, in a sudden change of frequency.

In the examples only one pair of receiving antenna has been shown, but it is obvious that by placing several such pairs one above the other so that they cover the whole width of the specimen, or of the timber, a closely specified indication can be obtained of the point where a knot or deviant grain orientation is located.

WHAT WE CLAIM IS: 1. An apparatus for observing an abrupt change in radio frequency energy reflected from or transmitted through timber, comprising at least one transmitting antenna for transmitting high frequency radio energy to the timber, at least one pair of receiving antennae located close to one another for detecting radio energy reflected from or transmitted through the timber, and a detector for receiving the sum of the radio frequency energy signal received by one receiving antenna plus the radio frequency energy signal received by the other receiving antenna with 1800 phase shift between the two signals, whereby a null output is received from the detector except when a knot occurs in front of one or other of the receiving antennae.

2. An apparatus according to Claim 1, wherein the said phase shift is accomplished by dividing a wave guide at its upstream end in two in the H plane by a partition in such manner that the wave guide becomes effectively two separate wave guides, which then join before the detector a body being located in one wave guide whose electrical properties and length are such as to delay a wave travelling in that wave guide by 1800 or an odd multiple thereof with respect to a wave travelling in the other wave guide.

3. An apparatus according to Claim 1, wherein the said phase shift is accomplished by connecting the pair of receiving antennae to a common detector by wave guides, coaxial lines or other transfer leads of different lengths so that the difference between the electrical lengths of the leads at the measuring frequency is an odd number of half wavelengths.

4. An apparatus according to Claim 1, wherein the said phase shift is accomplished by connecting the pair of receiving antennae by rectangular wave guides to a common detector in a series branch of a T junction in the E plane in such a manner that the wave guide lengths from the antenna to the junction are electrically equal in length or the electrical lengths differ by an integral number of wavelengths.

5. An apparatus according to Claim 1, 3 or 4, wherein the transmitting antenna and the pair of receiving antennae are located on the same side of the timber under measurement and the receiving antennae are symmetrically on either side of the transmitting antenna.

6. An apparatus according to any pre

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. antennae, that is at the same time as the grain is perpendicular to the electric field of the other antenna. The measuring signals of the antennae 29 and 30 are shifted to be 1800 out of phase with each other by connecting the antennae 29 and 30 with rectangular wave guides through a T junction in the E plane to a matcher/detector 33 or by arranging that between the matcher 33 and the antennae 29 and 30 there is in these two transfer distances an electrical difference of path length equal to one half wavelength or an odd multiple thereof. In that case a null output is obtained from the receiver (apart from noise) when the grain of the timber constituting the specimen runs with reference to the fields of the antennae symmetrically at equal angles because the intensities of the different signals arriving from the specimen then cancel each other out owing to the phase difference mentioned. The transmitter is indicated in the drawing merely by a box 34. When the timber 32 is moved longitudinally with respect to the detector each deviation of the signal received by the pairs of antennae 29 and 30 indicates a change in the quality of the timber. Depending on what one wishes to indicate, each point of unbalance may then be recorded as a point of defect or, in addition, one may separate the knot responses from the other responses, whereby the residual signals are consistent with points where the grain orientation of the sawn timber is warped. The radio energy transmitters which are used in the apparatus of the invention, and likewise the common detector 12 to be used, are means which are familiar to a person skilled in the art of radio frequency devices and therefore no particular description thereof is necessary in connection with the present invention. Slightly different distances of the antennae in the pair of antennae or a change of the electrical length of the leads e.g. owing to a temperature change may cause disturbance of the equilibrium even in the case of a uniform board. If the leads connecting the pair of antennae to the detector are electrically of unequal length, a new equilibrium or balanced position is found by slightly changing the transmitting frequency. This may be used in the measurements e.g. in that the transmitting frequency is continuously controlled e.g. by electronic means in such manner that a balanced condition is normally achieved. If the time constant of the control circuit is suitably chosen, any knot will manifest itself in an abrupt increase of the control voltage or, in the case of a short time constant, in a sudden change of frequency. In the examples only one pair of receiving antenna has been shown, but it is obvious that by placing several such pairs one above the other so that they cover the whole width of the specimen, or of the timber, a closely specified indication can be obtained of the point where a knot or deviant grain orientation is located. WHAT WE CLAIM IS:

1. An apparatus for observing an abrupt change in radio frequency energy reflected from or transmitted through timber, comprising at least one transmitting antenna for transmitting high frequency radio energy to the timber, at least one pair of receiving antennae located close to one another for detecting radio energy reflected from or transmitted through the timber, and a detector for receiving the sum of the radio frequency energy signal received by one receiving antenna plus the radio frequency energy signal received by the other receiving antenna with 1800 phase shift between the two signals, whereby a null output is received from the detector except when a knot occurs in front of one or other of the receiving antennae.

2. An apparatus according to Claim 1, wherein the said phase shift is accomplished by dividing a wave guide at its upstream end in two in the H plane by a partition in such manner that the wave guide becomes effectively two separate wave guides, which then join before the detector a body being located in one wave guide whose electrical properties and length are such as to delay a wave travelling in that wave guide by 1800 or an odd multiple thereof with respect to a wave travelling in the other wave guide.

3. An apparatus according to Claim 1, wherein the said phase shift is accomplished by connecting the pair of receiving antennae to a common detector by wave guides, coaxial lines or other transfer leads of different lengths so that the difference between the electrical lengths of the leads at the measuring frequency is an odd number of half wavelengths.

4. An apparatus according to Claim 1, wherein the said phase shift is accomplished by connecting the pair of receiving antennae by rectangular wave guides to a common detector in a series branch of a T junction in the E plane in such a manner that the wave guide lengths from the antenna to the junction are electrically equal in length or the electrical lengths differ by an integral number of wavelengths.

5. An apparatus according to Claim 1, 3 or 4, wherein the transmitting antenna and the pair of receiving antennae are located on the same side of the timber under measurement and the receiving antennae are symmetrically on either side of the transmitting antenna.

6. An apparatus according to any pre

ceding claim, wherein means are provided for electronically controlling the transmitter frequency so that the 1800 phase shift is provided in the case of uniform timber.

7. An apparatus according to any preceding claim wherein the or each pair of receiving antennae are disposed mutually symmetrically in an oblique position with respect to the or the respective transmitting antenna as well as to the major dimension of the timber, whereby the phase differences and/or amplitude difference of the direction of polarisation reveal deviations of the wood grain direction from the above-mentioned direction of the major dimension.

8. An apparatus according to Claim 7, wherein the angle of symmetric oblique mounting of the or each pair of receiving antennae with respect to the direction of the specimen and to the or the respctive transmitting antenna is 45".

9. An apparatus according to Claim 7 or 8 wherein the antennae are horn antennae.

10. An apparatus according to any preceding claim wherein the radio frequency energy is microwave energy.

11. An apparatus for observing an abrupt change in radio frequency energy reflected from or transmitted by timber, substantially as herein described with reference to Figures 1 and 2, or 3 and 4, or any one of Figures 5 to 9, of the accompanying drawings.