Patent Application: US-93142611-A

Abstract:
a system and method employing a general impedance as a cost - effective detector to measure dielectric properties of a material at both high and low frequencies , using combinations of resistance , capacitance , and inductance . the method may be applied in a wood drying installation including a kiln as coaxial cable element for a data transmission , wherein metal walls and doors of the kiln grounded to the soil include the outer coaxial conductor relative to a substantially isolated axial central conductor . the method is further suitable for determining the fiber saturation point of wood by measuring the loss tangent of the wood and determining the point where the time derivative of the loss - tangent equals zero , which corresponds to the fiber saturation point of the wood .

Description:
the particulars herein are by way of example and for purpose of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention . in this regard , no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention , the description making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice . there are three preferred but not exhaustive embodiments of the present invention . referring to fig7 and , fig8 in more detail , reference numeral 10 indicates a wood drying kiln in which there is a stack timber 12 . the timber 12 is arranged in layers which are spaced by means of spacers . the environment inside the kiln is controlled to facilitate drying with minimum defect . to determine the dielectric properties of the wood accurately , the kiln is provided with a measuring means comprising of an electronics module 14 outside or inside the kiln , a pair of electrodes 16 inside the kiln coupled to the module 14 by means of electric conductors 18 , and a remote data processor 20 which is connected to the electronics module 14 by means of a data link 22 . where there are a number of kilns 10 , each with it &# 39 ; s own electronics module 14 , the various electronic modules may all be connected to the same data processor 20 . the electrodes 16 are in the form of metal plates and are simply inserted into the spaces between layers in the stack 12 . it is an important feature of the invention that the electrodes 16 do not need to be cleaned prior to insertion into the stack , as operation of the apparatus is not affected by the degree of physical contact with the timber , the size and exact separation of the electrodes are also not important since if the size of the electrodes are changed , then a factor in the data processor can be adjusted to compensate . the electronics module 14 serves to measure the values which are required to determine the capacitance , resistance and inductance of the complex impedance between the electrodes 16 . the electronics module further comprises of a very fast comparator 32 , which is connected via a precision buffer 36 to one of the electrodes , the other electrode being connected to instrumentation ground . both electrodes 16 are electrically isolated from the kiln . the waveform of the applied voltage ( i . e . the output of the driver 28 ) is indicated at 38 , whereas the waveform of the voltage across the electrodes 16 ( i . e . after the general series impedance z s , 24 ) is indicated by reference 40 in fig7 . in fig1 , the applied voltage 38 is indicated by the phasor v 1 and the voltage across the electrodes by phasor v 2 . because the impedance 30 is complex , there is a phase difference between the voltages v 1 and v 2 , this being indicated by the angle φ . v r is the voltage ac - cross the series impedance element z s . the comparator 32 serves to convert the sinusoidal voltage 38 and 40 to square wave voltages 42 and 44 respectively . the electronics module 14 further comprises an exclusive - or ( xor ) circuit 46 whose output is indicated at 48 . the peak and the rms value of the output 48 varies in proportion to the phase difference between the voltages 38 and 40 . the outputs of the buffers 34 and 36 and the output of the xor circuit 46 are fed into a multiplexer 50 , which is used to select one of v 1 , v 2 or phase angle φ to be measured by the rmstopc converter 52 . output 54 of the rms - to - dc converter 52 is relayed to the data processor 20 , via the data link 22 . the dielectric components of wood is that of a resistance r x , capacitance c x in parallel as in fig2 . wood has no inductance , but the scope of the invention also includes the case of an inductance l x in parallel with r x and c x for dielectric applications with inductive dielectrics . in the event that the complex impedance includes an inductance ( l x ) in parallel with c x , both c x and l x can be obtained independently by measuring the complex impedance z x at two different excitation frequencies . it is important to note that if z x comprises of either r x and c x in parallel or r x and l x in parallel , then one excitation frequency is sufficient to determine both the components in parallel at the same time with one measurement . two frequencies are only necessary for the case of z x containing both a c x and a l x component . the capacitive and resistive load ( z x ), as depicted in fig2 , is obtained rigorously by means of the following procedure . element “ 30 ” in the drawing fig8 will be referred to as z x in sequel , meaning the parallel combination of the resistive and capacitive properties of the wood sample . the impedance z x ( 30 ) is driven through the impedance z s ( 24 ) and the voltage v 2 is measured across the load . the principle measurement is then the comparison between the applied voltage v 1 and the load voltage v 2 which also involves a phase detection . in what follows we do not include calculations for the case of z x containing an inductance . as will be shown , that can be obtained by a simple correspondence rule between c x and l x to obtain the equations for l x . it is now shown how to express the impedance in terms of the dielectric elements c x and r x and the angular frequency ω . the impedance of the dielectric medium can be obtained in terms of r x and c x as z x = r x ⁢ z c z c + r x ( 10 ) z x = r x 1 + ω 2 ⁢ c x 2 ⁢ r x 2 - j ⁢ ⁢ r x 2 ⁢ c x ⁢ ω 1 + ω 2 ⁢ c x 2 ⁢ r x 2 ( 11 ) rewriting this in terms of magnitude and angular using the euler description yields ,  z x  = r x ( 1 + ω 2 ⁢ c x 2 ⁢ r x 2 ) ⁢ 1 + r x 2 ⁢ c x 2 ⁢ ω 2 ( 13 ) and ( 14 ) it is now shown how to express the impedance z x in terms of the voltages v 1 , v 2 and a series impedance z s z x = z s ⁢ v _ 2 v _ 1 - v _ 2 ( 16 ) since v 1 is the reference voltage ( applied wave ) the phase angle is zero . the load wave obtained at v 2 will display some amplitude decrease due to the dielectric loss of the medium ( wood ) and also display a phase difference φ due to the 80 | 1 polarization of the h 2 o molecules and the cell - wall structure in the presence of the electromagnetic field . v 1 = v 1 , v 2 = v 2 e jφ and z s = ue jτ . by substitution of 17 into 16 and after expanding into real and complex parts and finally rewriting in euler form , z x =∥ z x ∥ e j ( φ + δ + τ ) , the following is obtained .  z x  = uv 2 v 1 2 - 2 ⁢ ⁢ v 1 ⁢ v 2 ⁢ cos ⁡ ( ϕ ) + v 2 2 * cos ⁡ ( ϕ ) 2 ) + v 2 2 ⁢ sin ⁡ ( ϕ ) 2 ( 18 ) since the impedances 18 and 12 are the same in magnitude and phase angle it follows that , where the absolute values of r x and c x are taken as they are generally defined as positive values . although wood has negligible inductance , we can obtain an equation for inductance l x , for any material having inductance by the impedance correspondence the following improvements due to this method will be obvious to anyone skilled in the art . it is clear form and equations 22 , 23 and 24 that by only measuring the magnitudes v 1 , v 2 and the phase angle between these two sinusoids , that the capacitance c x and the resistance r x or the resistance and inductance l x can be obtained simultaneously within the resolution of the measurement . it must be stressed that c x ( respectively l x ) is obtained independently form r x and that variations of one do not influence the other . if an impedance z x is comprised of all three of r x , l x and c x , then all can be determined by using data from two frequencies . both c x and r x can be obtained simultaneously by evaluating the equations 22 and 23 by only measuring v 1 , v 2 and the phase angle ( φ ) once . furthermore the loss - tangent can be constructed from c x and r x by the equation tan , where r x and c x is described by 23 and 22 . the signal voltage v 1 and the voltage across the load z x is related as any variation in v 1 will also create a proportional variation in v 2 . therefore , would be largely constant for any power supply variation , it is clear to see that the result of the determination of z x and therefore also l x , c x and r x in 24 , 22 and 23 will be invariant under any variations and as a consequence also tan δ as it is constructed uniquely from c x and r x . since the measurement principle can detect r x and c x independently , the resistance and capacitance of the probe wiring can be established once and accurately . since capacitances adds in parallel , the probe wire capacitance can simply be subtracted from the capacitance measured with a load attached in order to obtain the capacitance of the wood sample as c l = c t − c c , where c l , c t and c c are the load , total and cable capacitances respectively . this is not the case with prior art . resistance of probe wiring is cancelled by choosing relatively thick conductors or too be more accurate measure the short circuit resistance and obtain the load resistance form where r t is the total resistance measured , r l is the load resistance and r c is the cable resistance . the instantaneous measurement of capacitance and resistance by the method of this invention therefore easily systematically and clearly removes the cable dielectric properties in order to obtain the dielectric properties of the medium independent from cable dielectric influences . a similar procedure can be followed to cancel the wire inductance in the case that z x contains an additional inductance l x in addition to c x and r x by taking the measurements at two different frequencies . for the case of z x comprising of either r x and l x or r x and c x , measurement at only one frequency is necessary . the above described system measures the capacitance , resistance and inductance of a reactive load represented by a stack of wood produced between two plates as probes . in accordance with the invention , the reactive load is connected in a series circuit with a known impedance and the voltage across the reactive load is measured as well as the excitation voltage feeding into the series impedance . the phase angle φ between the two voltages is determined and from the values of the known impedance , the magnitudes of the voltages and the phase - angle between them , the capacitance , resistance and inductance of the reactive load are determined according to equations expressing the capacitance , resistance and inductance in terms of the known voltages , known impedance and known phase - angle . it is appreciated that the equivalent thevenin - norton descriptions of both the known and unknown impedances can also be used to obtain the same result and that such descriptions are equivalent . a benefit of using a capacitance or a capacitance in parallel with a resistance as the impedance element z s is that at higher frequencies z s decreases in value . this makes operation at high excitation frequencies possible . it also makes it possible to construct a z s which has more constant magnitude with varying frequency . z s as resistance is ideal for working at low excitation frequencies where z s as capacitance is better for higher frequencies . one of the major problems facing installation of dielectric moisture meters in a lumber drying kiln , is all the probe wiring and feed through cables necessary to measure the moisture content of various stacks of wood inside the kiln . normally systems will employ probe wire and or power cables or coaxial cable running on electrical isolators terminated to a service point where connections to the plates in the wood can be made . this involves long lengths of cabling inside the kiln which creates maintenance and expensive installation procedures . in order to solve these problems , the method described in this invention , involves using the kiln itself as a coaxial cable element , thereby using the kiln as co - axial cable to suppress common mode noise during data transmission due to the well - known inherent properties of a co - axial cable element . the same co - axial cable arrangement can be used to transmit data and power to instrumentation inside the kiln . fig5 and fig6 displays the configuration as discussed above . in fig5 a classical co - axial cable is displayed , where the tubular axial metal shield ( cc - 1 ) wraps around a dielectric ( cc - 3 ) usually made of e . g . ptfe or similar . the center of the dielectric is pierced by a conductor ( cc - 2 ) which is co - axial to the shield cc - 1 . a typical metal drying kiln consists of a frame ( ck - 1 ) cladded by e . g . aluminum closed on both side by aluminum doors ( ck - 4 ), forming a square cross - sectional cylinder . the metal kiln body is grounded by means of a 2 - 3 m metal grounding rod or obtains it &# 39 ; s ground from the electrical motor grounding grid . the soil on which the kiln is built therefore completes a sufficiently conductive bottom plane of typically 5 - 20 . this conductive layer competes the shield to form a closed cylinder . this shield formed by ( ck - 1 ) and the soil , has it &# 39 ; s analog in the classical co - axial cable as ( cc - 1 ). most high volume metal drying kilns are equipped with rails ( ck - 2 ) resembling train - tracks for the loaded stacks to be pushed into the kiln . these metal tracks are fastened onto a continuous concrete slab ( ck - 3 ) running through the kiln . the concrete slab is a very good isolator , isolating the rails ( ck - 2 ) from the soil . this metal track naturally becomes the isolated central conductor of the coaxial cable with analog ( cc - 2 ) of the classical co - axial cable ). as a result , the kiln is therefore configured as a coaxial cable element of which the dielectric in the kiln , equivalent to ( cc - 3 ), is a combination of air and wood . fig6 shows a cross section of a loaded kiln with the measurement instrumentation showed as connected to the stack of wood ( ck - 5 ) to measure for example , the dielectric properties of the wood between the plates ( ck - 6 ). in more detail , a measuring device ( ck - 8 ) which can be any dielectric moisture meter . the measuring device is connected directly to an upper metal plate extending through the wood . the measuring device is further connected to the remaining bottom plate of the plate pair ( ck - 6 ) with a conductor ( ck - 10 ). the measuring device obtains data communication and power supply through , one conductor connected to the rail tracks ( ck - 2 ) or the metal cart ( ck - 11 ), while another conductor ( ck - 7 ) is connected to the metal kiln wall ( ck - 1 ). the measuring device is now connected to a coaxial cable element . data and power are supplied through cables ( ck - 14 ) connecting to the rails ( ck - 11 ) and cable ( ck - 12 ) connecting to the kiln wall ( ck - 1 ). a power and data transmission system ( ck - 13 ) connected to cables ( ck - 12 ) and ( ck - 14 ) is mounted inside or on the outside of the kiln . the data transmission protocol ideally should be a differential protocol such as rs - 422 / 485 or similar . the use of a differential protocol combined with the kiln as coaxial cable element ensures reasonably high data rates with low failure rates . the power and data transmission system ( ck - 13 ) is then connected by either a data cable or by rf parabolic dish to the processing unit in the control room displaying the measured dielectric properties correlated to e . g . moisture content and fiber saturation point . using the loss tangent as a detector for fiber saturation point ( f . s . p .) fiber saturation point detection is a crucial property to detect in wood - drying . it will now be described how fiber saturation point detection employing the loss - tangent can present a unambiguous clear detection of f . s . p . traditional methods of measuring f . s . p . involve direct current conductivity measurements and determining when the conductivity desaturates and starts to decrease as salts starts to deposit on the cell - walls as free water is lost and the remainder of water when the wood is below f . s . p . becomes chemically bounded . the weakness of this method lies in that it can take considerable amounts of time to establish that the conductivity is dropping out of saturation . in the present invention , it has been found that determining the f . s . p . via the loss tangent is a superior method in comparison to the to conductivity method for detecting f . s . p . the loss - tangent of a dielectric medium against moisture content resembles the shape of a “ bell - curve ”, whereby the maximum of the bell curve occurs at f . s . p . in the case of wood as medium , the f . s . p . is accurately dependent on the density of the wood which in turn influences the amount of free water in the wood . the higher the density , naturally the fiber saturation point occurs at higher moisture contents . this is conclusive , since higher density causes less space for free water in a fixed volume of wood as medium . the bell curve shape of the loss - tangent , creates a unique and easy way to determine the f . s . p . compared to guessing when conductivity is out of saturation . the method of the present invention only requires to obtain the condition the table shown in fig3 , contains data for the loss - tangent tan δ , at fixed frequency , for several wood - densities and temperatures . if plotting the loss - tangent tan δ against moisture content at any given temperature for different densities , it can be seen that the higher the density the higher the moisture content where f . s . p ⁡ ( ∂ tan ⁢ ( δ ) ∂ t = 0 ) occurs . for example , f . s . p . occurs at about 30 % at density 0 . 5 , temperature 90 ° c . and an excitation frequency of 10 khz . the data shown in this table are not high resolution but sufficient to demonstrate the method . another benefit of detecting f . s . p via the loss - tangent is , that the moisture content of the material does not need to be known at all . therefore , no calibration is necessary . the condition tells exactly when f . s . p occurs , independent of any knowledge of the moisture content . the data in fig3 is relatively low resolution . when a high resolution model is used to determine r x and c x to in turn determine the loss - tangent tan δ = a high resolution detection of f . s . p . by means of δ 1 , δ 2 , δ 3 , δ 4 , results in something similar as depicted in the graph fig4 where c x ( correlated with moisture content ) μ 1 , μ 2 , μ 3 , μ 4 , e − σ x and tan δ is plotted . 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