GB1564194A

GB1564194A - Apparatus for observing an abprupt change in the radio frequency energy reflected or passed by timber

Info

Publication number: GB1564194A
Application number: GB3848478A
Authority: GB
Original assignee: INSINOORITOIMISTO INNOTEC Oy
Current assignee: INSINOORITOIMISTO INNOTEC Oy
Priority date: 1976-10-06
Filing date: 1977-03-22
Publication date: 1980-04-02

Description

(54) APPARATUS FOR OBSERVING AN ABRUPT CHANGE IN THE RADIO FREQUENCY ENERGY REFLECTED OR PASSED BY TIMBER (71) We, INSINOORITOIMISTO IN NOTEC OY, a Finnish company organised 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 is to be performed, to be particularly described in and by the follow mg 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 to 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 sawm 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 pair of transmitting antennae located close to one another for transmitting high frequency radio energy to the timber, the signals transmitted by these being shifted 1800 out of phase with one another, at least one receiving antenna for detecting radio energy reflected from or transmitted through the timber, and a detector for receiving the radio frequency energy received by the receiving antenna, whereby a null output is received from the detector except when a knot occurs in front of one or other of the transmitting 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 immediately 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 verse, 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 and/or 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 observable in the amount and/or 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 radiation intensities and/or 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 is no output or the output is minimal. Thus a null output reveals that the board is uniform in the area in question.

If the mode of measurement with radiation passing through the sawn timber is used, the transmitter antennae can therefore be placed as close as is desired to each other on one side of the timber and the receiving antenna on the other side at their halfway point. When measurement by reflection of the radiation from the timber is employed the receiving antenna is naturally positioned between the transmitting antennas in a suitable way. Since the radiation emitted by the transmitting antennae is arranged to have 180 degrees phase shift, a null output is obtained at the receiving antenna in the case of uniform timber.

The initial radiation supplied by a generator or the equivalent is distributed to the transmitting antennae and a transfer route causing a phase shift of either 180 degrees or of an add multiple thereof between the different antennas is used. Alternatively the phase difference is provided between the radiations emitted by the pair of antennae.

In one embodiment of the invention the 180 degree phase difference is achieved by dividing a wave guide into two parts at its downstream end by a partition, so that the wave guide becomes in effect two separate wave guides, which then come together before the transmitter 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 transmitting antennae to a common transmitter 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 transmitting antennae by rectangular wave guides to a common transmitter 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 antannae and the receiving antenna may be located on the same side of the timber, with the transmitting antennae symmetrically disposed on either side of the receiving 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 length or a change of the electrical length of the leads e.g. owing to changing temperature.

In an embodiment of the invention the pair of transmitting antennae are placed with reference to the receiving antenna, 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 identify 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 desribed below with reference to the accompanying drawings, wherein: Figure 1 is a perspective view of an apparatus of the invention, in which the transmitting and receiving antennae are accomodated in one unit; and Figure 2 is a perspective view of an apparatus according to the invention, applied 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 embodiment of Figure 1 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 small size.

Although Figure 1 shows an apparatus in which the transmitting antennae are located on the same side of the timber as the receiving antenna, 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.

A receiving antenna 28 is located in the middle of the apparatus. Transmitting antennae 26 and 27 are located symmetrically with respect to the receiving antenna in the configuration of this printed circuit. The transmitting antennae 26 and 27 are connected to a generator 24 in such a manner that the radiation emitted by the transmitting antennae will have a mutal phase shift of 1800 and therefore in the case of uniform reflected radiation, the signals received by the antenna 28 from the antennae 26 and 27 will cancel one another out. The transmitting antennae 26 and 27 as well as the receiving antenna 28 are encircled by an earth component 25 to minimize interference.

When the distances from the transmitting antennae 26 and 27 to the receiving antenna 28 are comparatively small, an output from a detector 12 connected to the antenna 28 is obtained only if a reflector with abruptly changing properties is present close to the transmitting and receiving antennae. 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 antennae and generator 24 supplied radio frequency energy to the transmitting antennae each 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 2 is similar in its operation to the above described device, but this apparatus indicats the deviations of the grain direction in the specimen, i.e., in the wood. Transmitting antennae 29 and 3f) 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 are symmetrically disposed with reference to the diametral plane of a receiving 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 filed (the so-called E planes) are at angles a1 and a2 such that al = - 0:2. The situation in the measuring of sawn timber is at its most favourable when the angles are + 45 .

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 signals from 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/ transmitter 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 consti tuting 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 detector is indicated in the drawing by a box 34.

When the timber 32 is moved longitudinally with respect to the detector each deviation of the signal generated 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 a 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 receiving antenna has been shown, but it is obvious that by placing several such 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 pair of transmitting antennae located close to one another for transmitting high frequency radio energy to the timber, the signals transmitted by these being shifted 1800 out of phase with one another at least one receiving antenna for detecting radio energy reflected from or transmitted through the timber, and a detector for receiving the radio frequency energy received by the receiving antenna, whereby a null output is received from the detector except when a knot occurs in front of one or other of the transmitting antennae.

2. An apparatus according to claim 1, whereby the said phase shift is accomplished by dividing the wave guide at its downstream end in two with a partition in the H plane so that the wave guide becomes effectively two separate wave guides, which then join before the transmitter part, 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 by180" 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 transmitting antennae to a common generator by wave guides, coaxial lines or other transfer leads of different length so that the difference in electrical length 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 transmitting antennae with rectangular wave guides to a generator via a T junction in the E plane in such a manner that the wave guide lengths from antenna to the junction are electrically equal or differ by an integral number of wavelengths.

5. An apparatus according to claim 1, 2 or 4, wherein the or each pair of transmitting antennae and the or each receiving antenna are located on the same side of the timber under measurement and the transmitting antennae are symmetrically on either side of the receiving antenna.

6. An apparatus according to any preceding claim, wherein means are provided for electronically controlling the transmitter frequency so that the 180 phase shift is provided in the case of uniform timber.

7. An apparatus according to any preceding claim wherein the or each pair of transmitting antennae are disposed mutually symmetrically in an oblique position with respect to the or the respective receiving antenna as well as to the major dimension of the timber, whereby the phase differences and/or amplitude differences of the signals arriving from the timber due to the 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,

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tuting 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 detector is indicated in the drawing by a box 34. When the timber 32 is moved longitudinally with respect to the detector each deviation of the signal generated 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 a 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 receiving antenna has been shown, but it is obvious that by placing several such 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 pair of transmitting antennae located close to one another for transmitting high frequency radio energy to the timber, the signals transmitted by these being shifted 1800 out of phase with one another at least one receiving antenna for detecting radio energy reflected from or transmitted through the timber, and a detector for receiving the radio frequency energy received by the receiving antenna, whereby a null output is received from the detector except when a knot occurs in front of one or other of the transmitting antennae.

8. An apparatus according to claim 7,

wherein the angle of symmetric oblique mounting of the or each pair of transmitting antennae with respect to the direction of the specimen and to the or the respective receiving antenna is 45 .

9. An apparatus according to claim 7 or 8, wherein the antenna 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 Figure 1 or 2 of the accompanying drawings.