WO1998050785A1

WO1998050785A1 - Device for measuring the humidity of a material, e.g. wood

Info

Publication number: WO1998050785A1
Application number: PCT/NL1998/000103
Authority: WO
Inventors: Arnold Hendrik Brookhuis; Pieter Rozema
Original assignee: Arnold Brookhuis Textieltechniek B.V.
Priority date: 1997-05-02
Filing date: 1998-02-20
Publication date: 1998-11-12
Also published as: NL1005956C2; AU6124398A

Abstract

A device for contact-free measurement of the humidity of a material, for instance wood, is intended to be added to transport means for transporting elements of the relevant material, for instance wooden planks, through the device in a chosen direction along a chosen path, and comprises two units for respectively performing and controlling measurements, each with an individual housing, which are separate of each other and connected by means of the first and the second cable, of which two cables at least one is embodied as an electrical connecting cable which is provided with an electrically conducting jacket connected to both housings; which connecting cable has as a coil a number of windings such that the cable thus possesses an increased common-mode self-induction.

Description

DEVICE FOR MEASURING THE HUMIDITY OF A MATERIAL, E.G. WOOD

The invention relates to the measuring of the humidity of a material, for instance wood.

Various devices of this type are known, inter alia from US-A-3 209 247, US-A-3 339 137, US-A-3 523 243, US-A-3 959 723, US-A-4 377 783, US-A-4 563 635, US-A-4 683 418, US-A-4 733 166, US-A-5 315 258, US-A-5 402 075, EP-A-0 616 209, WO-A-91/12518 , EP-A-0 287 725 and EP-A-0 166 707.

Relative to the prior art it is an object of the invention to comply with the highest possible norms of suppression of emitted radiation, a practically complete insensitivity to external spurious signals and measurement precision.

It is a further object of the invention to comply with the highest norms which can be required in respect of the suppression of emitted radiation, wherein this radiation emission must be independent of the quality with which installing of the device has taken place. In respect of the stated objectives, the invention provides a device for contact-free measurement of the humidity of a material, for instance wood, which device is intended to be added to transport means for transporting elements of the relevant material, for instance wooden planks, through the device in a chosen direction along a chosen path, and which device comprises: a first unit, which comprises: - a signal generator which is adapted to generate a first, substantially harmonic signal with a chosen frequency and a second signal which is equal to the first signal but which is in counter-phase relative thereto; - at least one first electrically conducting plate for placing along the path and to which the first signal can be supplied;

- at least one second electrically conducting plate for placing along the path and to which the second signal can be supplied;

- at least one third electrically conducting plate for placing along the path and which is connected to an input of control means; - control means and optional further electrical units, such as an auxiliary electrical supply, for providing the signal generator with suitable direct voltages, and

- a first electrically conducting housing in which at least the signal generator, the control means and said optional further electrical units are accommodated; a second unit, which comprises:

- operating means (for instance a PC or PLC) for controlling the control means via a first cable; and

- an electrical power supply, for instance a battery, a mains connection and/or a stabilized power supply, which is connected via a second cable to at least one of the electrical units of the first unit; and - a second electrically conducting housing in which at least the operating means and the main power supply are accommodated; which first and which second unit are separate of each other and connected by means of the first and the second cable, of which two cables at least one is embodied as an electrical connecting cable which is provided with an electrically conducting jacket connected to both housings; which connecting cable has as a coil a number of windings such that the cable thus possesses an increased common-mode self-induction, whereby a filter action is obtained which ensures an effective suppression of the passage of said frequency, which coil is received in the first housing.

Said increased self-induction ensures with its filter action an effective suppression of the undesired emission of electro-magnetic signals which could be transmitted via said cables, while the filter action also ensures that signals received via the cables are not transferred at all or only to a negligible degree.

A usual measurement frequency lies in the order of 4 MHz.

In a test arrangement a substantial suppression of said undesired transmission was obtained with a self- induction caused by the coil in the order of 0.1 - 1 mH. It will be apparent that the effective suppression is also determined on the basis of the ratio between the effective impedance of the coil at said frequency and the terminal impedance of the coil at said frequency and the terminal impedance on the side of the coil defined as output of the filter. This can possess a real or complex character, '.'he relevant nature and value of the impedance in question at said frequency can be taken into account in the design of the coil.

Each of the two units can comprise a power supply. The second unit can for instance be provided with a main power supply which supplies an auxiliary power supply in the first unit.

In order to allow the coil to be as small as possible, the embodiment is recommended in which the coil comprises a ferromagnetic core, for instance a ferrite core.

A specific embodiment has the special feature that the material of the core displays at said frequency its maximum relative magnetic permeability (μ,,) or μ, varying only to a slight extent therefrom, which μ₁ has a value of at least 500.

In said test arrangement use is made of a ferrite ring core from the Dutch Philips concern. The relevant core was of the 3F3 type and the realized self- induction had a value at 4 MHz in the order of magnitude of 0.2 H. The ferrite material in question is of soft character and has only a negligible remanence. Said material has a relative magnetic permeability of 1000 at the applied frequency of 4 MHz. Use could also be made of the Philips ferrite material 4A11. At said frequency of 4 MHz this has a μ, of about 650. In order to obtain the same self-induction it is then necessary to chose in known manner a higher number of windings for the coil.

An embodiment in which the ferrite core is a ring core has the advantage of less scattering of the magnetic alternating field, since the ring encloses the magnetic field within itself. A greater effectiveness can be obtained with an embodiment in which the ring core takes a multiple form in axial direction. In this embodiment the dimensions of the coil nevertheless remain relatively limited.

In order to achieve a higher self-induction use can also be made of a variant in which a number of coils are connected in series.

A greatly increased immunity to both receiving and emitting said radiation is ensured with an embodiment in which a coil is also received in the second housing. Yet another embodiment has the feature that the first and the second plates are fed via respective amplifiers, the amplification of which is adjusted by the control means. This embodiment has the advantage that the strength of the first and of the second signal can be adjusted such that use is always made of the minimal signal strength which provides measurement results within a set norm, for instance measurement accuracy or signal emission. During the described starting phase the control means can ensure that the amplification factors of the two amplifiers are held temporarily at the value zero.

A preferred embodiment comprises a programmable central control unit of which the control means form part. Such a control unit can comprise input means and read means, in particular a keyboard and a display, and are provided with input and output units for communication with external units, for instance a central computer for controlling larger units of which the device according to the invention forms part.

As is known, for different types of material a determined humidity causes different electrical behaviour. Within the scope of the present invention this manifests itself in different signal strengths sensed by the third plate. To now make the device suitable for diverse types of material, the device can preferably be embodied such that calibration data for the types of material for measuring can be entered in the central control unit. Said calibration data can take the form of "calibration lines". Such data can be included in one or more memories in the form of for instance a table or an analytical function with associated algorithm.

A specific embodiment has the special feature that the frequency is adjustable. This frequency ctn be adjusted in order to optimize the measurement precision and the emission of electro-magnetic signals of said frequency.

The adjustment of the frequency can take place manually or, if desired, automatically in the manner described below.

A preferred embodiment has in this respect the special feature that the frequency is adjustable by the control means and the first signal may or may not be supplied as required to the first electrically conducting plate and the second signal accordingly may or may not be supplied as required to the second electrically conducting plate.

During starting of the device the first and the second signal are not supplied to the first and the second plate. The third plate, which serves as detection plate, receives interfering radiation from the environment which could affect the operation of the device. The thus received signals are transferred from the third plate to the control means. These latter are adapted to determine the received electro-magnetic spectrum. On the basis of the spectrum a spectral band is defined in which the interference density is minimal, and the frequency of the first and the second signal is chosen in this band. The device is then in principle ready to operate. In the described manner the effect of external electro-magnetic radiation on the measurement results can now be reduced to completely negligible proportions.

In order to make the radiation emission as small as possible, the device comprises the described first and second plates which are fed with signals which are equal but in counter-phase. As a result of this choice, the emitted radiation is considerably reduced.

Before measuring can begin, the device must be calibrated. In its simplest form this calibration can for instance comprise two measuring points, i.e. a measuring point corresponding to a humidity of zero, i.e. the absence of material. Use can further be made of a calibrating plate of wood or other material, the humidity or equivalent humidity of which has been accurately determined in other manner to obtain a second calibration point. With the above described data the device can operate and is suitable for measuring the humidity of the relevant type of material, usually expressed as a percentage.

The device preferably has the special feature that the third plates are connected via signal processing means to said input of the control means.

A specific embodiment has the feature that the control means comprise at least one of the following units: a safety circuit against electrostatic charge; a band-pass filter; an amplifier; a demodulator; and a low-frequency band-pass amplifier.

A preferred embodiment is embodied such that the or at least one unit is connected for control of its setting to an output of the control means. This latter variant can be embodied such that during the above described start-up procedure a first setting of the relevant units is already chosen. The described device preferably comprises a demodulator which is supplied with a block signal, the frequency of which is equal to the frequency of the first and the second signal. The block signal serves as control signal for the synchronous detection performed by the demodulator.

The invention will now be elucidated with reference to the annexed drawings. Therein: figure 1 shows a highly schematic representation of a device according to the invention, partly in the form of a block diagram; figures 2, 3 and 4 show flow charts elucidating the invention; figure 5 shows a schematic representation of a coil in its simplest form; figure 6 shows a coil in which ferromagnetic coil is arranged in order to increase the self-induction; figure 7 shows a schematic view of a coil with a ring core; figure 8 is a side view of a coil according to the invention; figure 9 shows the cross-section IX-IX according to figure 8 ; figure 10 shows a cross-section corresponding with figure 9 through an alternative coil, in which two ring cores are stacked on each other in axial direction; figure 11 shows in the form of a simple block diagram a device according to the invention with transport means in which the invention is applied; and figure 12 shows an even more greatly simplified schematic view of an alternative.

Figure 1 shows a device 1 for contact-free measurement of the humidity of wooden planks 3 fed through as according to arrow 2 at a determined speed. Two driven transport rollers 4, 5 are drawn schematically to indicate that means are present to transport wooden planks through the device 1 along the shown path according to arrow 2, a direction opposite thereto, a transverse direction or other direction at a determined speed which is optionally adjustable as required.

The device comprises a central control unit 6 on the basis of a local micro-controller. This controls a signal generator 7 of the type MAX038 from the firm Maxim Integrated Products. Via respective amplifiers 8, 9 of the type LM7171 from National Semiconductor, the signal generator supplies mutually connected first plates 10, 11 and mutually connected second plates 12, 13 with respectively a sine-shaped first signal and a sine-shaped second signal, which signals are in mutual counter-phase. Plates 10 and 11 respectively 12 and 13 are deployed in this embodiment on either side of the path of wooden planks 3.

The invention further comprises mutually connected third plates 14, 15 which serve as detection plates and are connected to a safety circuit 16 against electrostatic charge, which supplies an analog input signal to central control unit 6 via a band-pass filter 17, an input amplifier 18, a demodulator 19 and a low- frequency band-pass amplifier 20. Amplifier 18 is in this embodiment of the type HA-2840 from Harris Semiconductor and the demodulator or synchronous detector 19 is in this embodiment the integrated circuit LM1496 from National Semiconductor. The central control unit 6 is the SAB 80C166 from Siemens. An RS485 driver 21 is connected to central control unit 6 is for communication with external devices. Reference numerals 22 and 23 refer to respectively a keyboard and a display.

Filter 17 is connected for control to the unit 6. /Amplifier 18 comprises two control inputs with the unit 6, one for the amplification factor and one for the offset. The demodulator receives a square wave control signal with a frequency which is equal to that of said first and second signals from signal generator 7. It is further connected for control to the unit 6. Amplifier 20 comprises two control inputs, one for control of the pass-band and one for control of the amplification factor.

Unit 6 further serves for control of the amplification factor of amplifiers 8, 9. In order to define the concept, it is noted that the output voltage of filter 17 lies in the order of a few millivolts. The output voltage of amplifier 18 lies in the order of 500 mV; the output voltage of demodulator

19 lies in the order of 500 mV and the output of amplifier 20, which can for instance possess an amplification factor in the order of 20 dB, lies in the order of 5 V.

With reference to the schematic representation of the arrangement of plates 10, 11; 12, 13; 14, 15, it is noted that configurations other than that described are also possible. Figure 1 shows only a random example. Figure 2 shows a flow chart relating to the adjustment of the sensors. The used blocks 24-42 have the following functions: 24: Start. Load with the last known setting for starting or comparison purposes.

25: Adjustment required?

26: Adjustment possible.

27: Auto-0 possible 28: Reset adjustments.

29: Finished.

30: Initiate sensor hardware, for instance adjustment to an oscillation frequency of 4 MHz.

31: Wood present? 32: Reset adjustments.

33: Give error/warning.

34: Finished.

35: Measure interference density. 36: Choose optimal frequency. 37: Increase control levels. 38: Results correct? 39: Change settings low-pass filter. 40: Results optimal? 41: Save settings. 42: Finished.

The flow chart according to figure 3 comprises the blocks 42-52 with the following functional significance:

42: Reset or start at switch-on.

43: Initiate all, Initiate all hardware/software. Read last-known settings for starting purposes. 44: Initiate sensors and/or readjust them, see flow chart according to figure 1. The described calibration data for different types of material can optionally be entered here. 45: Measure. 46: Present results. 47: Check sensor. See flow chart according to figure 1. 48: User input? Here another calibration line can be chosen, the dimensions of the wood can be adapted and alarm settings can be entered. 49: Obtain input. 50: Execution command. Controls user events, calibration, zero setting, optional automatic zero setting and so on. 51: Execute control command.

52 : Treat host in accordance with host protocol . The flow chart of figure 4 comprises functional blocks 53-60 with the following functional significance:

53: Start.

54: Request: - name of wood for identification

- type of wood - dimensions

- number of calibration samples 55: Place (new) sample in the sensor. In this step the sample with known percentage of relative humidity is placed between the third plates 14, 15. 56: Measure sample. 57 : More samples?

58: Calculate all necessary data. 59: Save all results. 60: Finished.

The procedure according to figure 4 is a calibration for adjusting the device to a determined sensitivity known in advance. This calibration must take place for all types of wood and dimensions.

The user can himself adjust the device in the shown manner for each given parameter, in particular type of wood and dimensions.

The number of samples with known relative humidity which must be applied depends on the desired accuracy and the desired resolution.

It will be apparent from the above that the invention is not limited to the illustration according to the drawings and the associated flow charts. What is essential is that use be made of transmission plates which respectively receive a signal in phase and in counter-phase, whereby an effective emission suppression is obtained. It is further essential that, prior to a measurement or when starting the device, an optimal setting of the various adjustable units is realized. In this respect attention is drawn to the fact that the relevant measurement could be repeated from time to time, since it is possible for the electromagnetic conditions in the environment to be subject to change during a measurement.

The description given above relates to a specific aspect of the invention, i.e. optional automatic control of the frequency adjustment of the device according to the invention. The manner in which the basic aspect of the invention is realized will be discussed hereinbelow. This is an effective suppression of the mutual transmission of spurious signals which have a frequency of the signals generated by the signal generator and which can on the one hand influence the accuracy of the device and on the other give rise to undesired emission. Figure 5 shows a coil 61 consisting of a shielded cable 66, the core of which consists of a number of mutually insulated sub-cores, which sub-cores are collectively enclosed by an electrically conducting jacket. The coil operation according to the invention concentrates on the increased common-mode self-induction which the cable acquires by winding a coil from the total cable in the manner indicated in figure 5.

Figure 6 shows the coil 62 which differs from coil 61 according to figure 5 in the sense that a rod of ferromagnetic material is received in coil 62 in order to increase self-induction. It is per se known that in this manner the self-induction of the coil can be considerably increased. It should however be taken into account that the material of the ferromagnetic coil must be suitable for said frequencies, which lie for instance in the order of 3-5 MHz.

Figure 7 shows a coil 64 with a ferrite ring core 65. Such a configuration is recommended according to the invention. Figure 8 shows coil 64 in more detail. Cable 66 comprises a number of cores 67 which are enclosed by an electrically conducting jacket 68. Via an earth conductor 69 this jacket 68 is connected to the first housing 70, as shown in figure 11. Figure 9 shows coil 64 in cross-section. The ferrite core 65 has a generally rectangular cross- section. For practical reasons the sharp corners are rounded off and the core is provided with an insulating sheath. This bears no relation per se to the principle according to the invention.

Figure 10 shows that alternatively two ring cores 70, 71 can be stacked axially on top of one another. An effective increase in the self-induction is hereby obtained, and therewith in the activity of the filter. It is noted that in the embodiment of figure 10 the cross-section through each of the two cores 70, 71 is round. This choice is made only to make clear that forms other than the more or less rectangular ones shown in figure 9 are also possible. Attention is also drawn to the fact that core 65 according to figure 9 does not have a rectangular form with sharp corners and that the relevant corners are rounded for practical reasons. It is noted that a number of axially stacked ring cores other than two can also be used.

Figure 11 shows a block diagram representation of the device 1, to which the special features according to the invention have been added. Housing 70 is connected in the shown manner via earth wire 69 to the jacket 68 of cable 66. This cable 66 connects units 21, 22, 23 in second housing 71 to auxiliary power supply 72 and local micro-controller 6. Cores 67 are only shown by way of orientation. The number of cores is dependent on the supply current and signals to be transferred. The auxiliary power supply 72 receives its supply current via a main power supply 73. This latter receives its power supply from the mains electricity via a mains connection 75. Jacket 68 of cable 66 is connected in housing 71 via an earth wire 74 to the second housing 71.

Figure 11 shows schematically that filter 64 is connected via earth wire 69 to housing 70. It is important to ensure that earth wire 69 is as short as possible. An embodiment is even recommended in which the jacket of cable 66 is in direct contact with housing 70. Figure 12 shows an even more greatly simplified schematic view of a variant in which both housings 70 and 71 are provided with two filters 64 connected in series. A further increased immunity against the above described undesired phenomenon is hereby obtained, both radiation emission and the receiving and processing of undesired signals, whereby the measurement accuracy is improved even more. It has already been described that earth wire 69 is only shown in figure 11 with a substantial length for the clarity of the drawing. In preference the earth wire 69 is very short or even omitted. A condition is that jacket 68 of cable 66 is connected to housing 70. Figure 12 shows that earth connections 69, 74 must be connected to housings 70, 71 on that side of the relevant filter which is remote from the outward extending side. Figures 1-4 relate to an embodiment wherein an automatic adjustment of the oscillator frequency is realized. For this purpose a signal is temporarily not supplied to the first and to the second plate during calibration. The signal received at the third plates is used to control the frequency such that a search is made for a frequency range with minimal interference.

Alternatively, the received spurious signals can be measured during the calibration procedure and, without modifying the frequency of the signal generator once it has been adjusted, the measurement results can be corrected for the spurious signal. It has been found in practice that even this simple procedure can give a considerable improvement in the measurement results relative to the prior art.

Claims

1. A device for contact-free measurement of the humidity of a material, for instance wood, which device is intended to be added to transport means for transporting elements of the relevant material, for instance wooden planks, through the device in a chosen direction along a chosen path, and which device comprises: a first unit, which comprises:

- a signal generator which is adapted to generate a first, substantially harmonic signal with a chosen frequency and a second signal which is equal to the first signal but which is in counter-phase relative thereto;

- at least one first electrically conducting plate for placing along the path and to which the first signal can be supplied;

- at least one second electrically conducting plate for placing along the path and to which the second signal can be supplied; - at least one third electrically conducting plate for placing along the path and which is connected to an input of control means;

- control means and optional further electrical units, such as an auxiliary electrical supply, for providing the signal generator with suitable direct voltages , and

- operating means (for instance a PC or PLC) for controlling the control means via a first cable; and - an electrical power supply, for instance a battery, a mains connection and/or a stabilized power supply, which is connected via a second cable to at least one of the electrical units of the first unit; and - a second electrically conducting housing in which at least the operating means and the main power supply are accommodated; which first and which second unit are separate of each other and connected by means of the first and the second cable, of which two cables at least one is embodied as an electrical connecting cable which is provided with an electrically conducting jacket connected to both housings; which connecting cable has as a coil a number of windings such that the cable thus possesses an increased common-mode self-induction, whereby a filter action is obtained which ensures an effective suppression of the passage of said frequency, which coil is received in the first housing.

2. Device as claimed in claim 1, wherein the coil comprises a ferromagnetic core, for instance a ferrite core.

3. Device as claimed in claim 1, wherein the material of the core displays at said frequency its maximum relative magnetic permeability (╬╝,) or ╬╝₁ varying only to a slight extent therefrom, which ╬╝₁ has a value of at least 500.

4. Device as claimed in claim 2, wherein the core is a ring core.

5. Device as claimed in claim 3, wherein the ring core takes a multiple form in axial direction.

6. Device as claimed in claim 1, wherein a number of coils are connected in series.

7. Device as claimed in claim 1, wherein a coil is also received in the second housing.

8. Device as claimed in claim 1, wherein the first and the second plates are fed via respective amplifiers, the amplification of which is adjusted by the control means.

9. Device as claimed in claim 1, comprising a programmable central control unit of which the control means form part.

10. Device as claimed in claim 1, wherein calibration data for the types of material for measuring can be entered in the central control unit.

11. Device as claimed in claim 1, wherein the frequency is adjustable.

12. Device as claimed in claim 1, wherein the frequency is adjustable by the control means and as required the first signal may or may not be supplied to the first electrically conducting plates and as required the second signal accordingly may or may not be supplied to the second electrically conducting plates.

13. Device as claimed in claim 12, wherein the third plates are connected via signal processing means to said input of the control means.

14. Device as claimed in claim 12, wherein the control means comprise at least one of the following units: a safety circuit against electrostatic charge; a band-pass filter; an amplifier; a demodulator; and a low- frequency band-pass amplifier.

15. Device as claimed in claim 14, wherein the or at least one unit is connected for control of its setting to an output of the control means.

16. Device as claimed in claim 14, comprising a demodulator which is supplied with a block signal, the frequency of which is equal to the frequency of the first and the second signal .

17. Device as claimed in claim 1, wherein the first and the second cable are combined to form a single connecting cable.