Patent Application
20100171513
The present invention is directed to calibrating measurements of signals received from sensors for measuring the moisture content of drying lumber, and in particular, to determining adjustments of such measurements to compensate for transmission induced variations in the signals, and more particularly, to determining such adjustment due to varying and/or long lengths of cabling used to transmit the signals.
Final moisture content of kiln-dried lumber is extremely critical to ensure optimal performance of the lumber in its end-use. Lumber that is too wet (under-dried) is prone to mold growth that can significantly compromise the lumber's strength, durability, and appearance. Under-dried lumber will also experience shrinkage as it dries to equilibrium, resulting in dimensional problems. Lumber that is too dry, or over-dried, will tend to warp and crack, also causing dimensional problems and rendering it less useful in most applications. Over-drying also results in lost productivity, and increased energy costs.
Both under drying and over drying will significantly reduce the desirability and market value of the lumber. For these reasons, in-situ monitoring of kiln dried lumber is highly advantageous.
It is established in prior art that it is possible to position steel plates spaced apart in a lumber stack, so that an electrical signal can be applied while drying of the lumber is in progress. The capacitance response between the plates can then be used as an indicator of moisture content, since the main dipolar constituent between the plates is the water in the lumber. Prior art systems of this type have been commercially available for two decades, from a variety of vendors. Such prior art systems (e.g., by the manufacturers Wagner, Wellons, and Accudry) have been described in significant detail in various public disclosures, including the following U.S. Pat. Nos.: U.S. 4,389,578 by Wagner; U.S. Pat. No. 4,580,233 by Parker, et. al.; U.S. Pat. No. 6,703,847 by Venter et. al.; and U.S. Pat. No. 6,989,678 by Venter et. al. each of which is fully incorporated herein by reference. Additionally, the following U.S. Patent Nos. are fully incorporated herein by reference: U.S. Pat. No. 3,807,055 by Kraxberger; U.S. Pat. No. 4,107,599 by Preikschat; U.S. Pat. No. 6,124,584 by Blaker et. al.; U.S. Pat. No. 6,281,801 by Cherry et. al.; U.S. Pat. No. 6,784,671 by Steele, et. al.; U.S. Pat. No. 6,784,672 by Steele, et. al.; U.S. Pat. No. 7,068,050 by Steele; U.S. Pat. No. 7,068,051 by Anderson; and U.S. Patent Application Publication No. US 2004/0187341 by Studd, et. al. is also fully incorporated herein by reference.
Such prior art systems use electrical metering devices to measure the moisture content of lumber as it dries in a kiln. In most (if not all) cases, these electrical metering arrangements involve placement of steel plates in the lumber to form a capacitor. The various systems then employ some form of cabling to carry the electrical signal from the plates back to such a metering device. The metering device measures the electrical capacitance of the circuit formed by the steel plates and the lumber. The capacitance is closely related to the moisture content in the lumber, so the metering device output can be appropriately scaled to read out in terms of moisture content. Typically the meter provides the capacitance values from the plates to either a PC (i.e., personal computer) or a PLC-based controller (i.e., programmable logic controller). The transform from capacitance to moisture content is performed either in the meter or in the PLC/PC, and the moisture content values are then used by a kiln controller (a separate control system) to identify when no further drying of the lumber is desired.
Typically, prior art systems include multiple metering devices (each also referred to herein as a capacitive meter, or evaluator) mounted on either the kiln interior or the kiln exterior. In this manner, the electrical signals to and from the steel plates, inserted in the kiln stacks (i.e., lumber stacks within the kiln), travel only a very short distance, e.g., less than 50 linear feet via carefully isolated conductive wires.
The placement of the metering devices close to the kiln stack is primarily due to difficulties with calibration of the signals from the capacitor plates to such a metering device. In particular, the wiring (also referred to as cabling herein) from the capacitor plates to the metering device is a significant design issue in establishing a reliable and accurate metering circuit. For example, numerous issues must be considered in the design of the cabling, including the following:
Most prior art kiln moisture measurement systems attempt to address the calibration issue by minimizing the length of the cable length to the metering device. As indicated above, the metering devices are typically mounted on the exterior of the kiln, and as close as possible to the points where the moisture measurements are taken. Thus, by keeping the length of the cable short, the signal losses in the cable are reduced, and most of the cable signal can be attributed to the capacitance of the plates. Additionally, the relatively small losses incurred can be adjusted out with empirical tuning and zeroing of the circuits to normalize their response, as one skilled in the art will understand.
However, locating the meters on the kiln exterior makes them susceptible to weather conditions, which can be extraordinarily harsh. Many North American kilns are located in the Northern U.S. and Canada where temperatures are extremely low in the winter. Kilns vent a high amount of moisture, which turns to ice on the kiln exterior. In some cases, the high weight of ice on the meter is enough to cause structural damage to the meter. Meters are sealed, but the sealant is not foolproof and in some cases the ever-present moisture in the kiln-environment can get inside the meter, shorting it out. Thus, the reduced lifetime and reliability of the meters is a serious shortcoming in the prior art systems.
Ancillary problems also result from prior art cabling approaches. In particular, prior art calibration methods are only well suited to very simple wiring methods, such as a bare stranded wire or an insulated stranded wire, and with very short cable lengths. Moreover, when non-insulated cables are used in a kiln, the cables are kept electrically isolated via insulating standoffs. That is, for each cable, there is an electrically isolated swing-arm for supporting the cable in a tensioned state between a capacitor plate and a meter. However, electrically isolated standoffs and their cables can be awkward to kiln workers to manipulate, and falling lumber in the kiln can cause the non-insulated cables to short circuit.
Moreover, both non-insulated stranded wire and insulated stranded wire are susceptible to electrical noise which can further compromise the validity of the capacitance signal. Additionally, although coaxial cable is far more resistant to electrical noise than non-insulated stranded wire and stranded wire and communicates a superior signal (e.g., a higher signal to noise ratio), the use of coaxial cable can be difficult for prior art kiln moisture monitoring systems in that there are high capacitance levels generated in coaxial cable that can not be easily removed by the simple prior art calibration methods.
In summary, the above described prior art systems for drying lumber results in a variety of problems:
For all the aforementioned reasons, it is more advantageous to install the metering equipment in a centralized, environmentally-controlled equipment space, such as a control room, wherein humidity, moisture, and ice is prevented from damaging the electronics in the metering equipment. Further, it is advantageous to run the wiring with insulated coaxial cable rather than with standard stranded wire since coaxial cable offers superior noise rejection and a high degree of resistance to electrical faults, such as an inadvertently grounded cable. Moreover, use of these insulated coaxial cables makes it unnecessary to mount the cabling on complicated swing arms that are susceptible to breakage and electrical faults. These advantages are provided by the kiln moisture monitoring system disclosed hereinbelow.
Meter: A set of electronics (and associated software and/or firmware) used to measure the capacitance (or other electrical properties, e.g., resistance, admittance, reactance, impedance, etc.) in a kiln lumber stack, and from such measurements, determine the moisture content of the lumber stack being dried in the kiln. The set of electronics typically includes a signal generator that excites a pair of capacitor plates, a set of amplifiers that measure the excitation and the response voltages from the capacitor plates, a comparator that evaluates the phase and amplitude of the voltage responses from the capacitor plates, a demodulation component that converts the electrical data to capacitance (more generally, impedance) and finally to moisture content in the lumber being dried, and a power supply circuit that supplies power to the aforementioned circuit components. In the case of prior art systems, such electronics typically includes an electronics device (for receiving lumber capacitance measurements) installed on the kiln exterior or kiln interior, near the portions of the lumber from which the capacitance measurements are taken. In at least one preferred embodiment of the present disclosure, the meter is located in a centralized location in an environmentally controlled room.
Metering Point: The lumber between a pair of capacitor plates from which capacitance measurements are obtained.
Channel: A channel refers to the a pair of plates forming a capacitor within a kiln lumber stack, and the cabling for communicating, to a meter, capacitance readings generated by the pair of plates. Typically, most kilns for drying lumber employ 8 channels (i.e., 8 pairs of capacitor plates) per lumber stack in a kiln, wherein each pair measures the capacitance of the drying lumber at a different localized area within the lumber stack. In the case of prior art systems, a meter located on the exterior of the kiln normally serves 1 or 2 channels. Therefore with a prior art system, 4 to 8 meters are required to serve a single kiln with 8 metering points. In the case of the present novel disclosure, the meter, located in a control room, may be coupled with a multiplexer thereby allowing the meter to serve up to 40 such channels. Therefore in the present novel disclosure, a single meter can serve up to 5 kilns, each with 8 metering points (i.e., eight capacitor plate pairs).
Terminal Impedance: The ratio of complex voltage to complex current at the input terminals of an amplifier.
The present disclosure describes a lumber moisture measurement system (also referred to herein as a “moisture measurement system”, and a “meter” that includes highly accurate calibration methods to standardize the signal response in each of one or more channels (i.e., a channel as described hereinabove) so that the output (i.e., capacitance measurements and/or lumber moisture content values) from the meter is substantially independent of signal transmission peculiarities or differences that occur on one channel and not another channel. More specifically, for each channel, capacitances generated in cables, such as coaxial cables, can be effectively cancelled. In particular, calibration methods disclosed herein allow for long lengths of coaxial cable (e.g., up to 1000 linear feet) to be effectively used in the circuitry for monitoring lumber drying in a kiln without the cable length adversely affecting the lumber moisture content measurements determined by the meter. Moreover, the long cable lengths facilitate the centralizing of kiln monitoring electronics, e.g., such as metering devices, in an enclosure satisfying NEMA (National Electrical Manufacturers Association) standards, and in particular, such an enclosure may be in a wood drying monitoring control room, wherein such electronics are not subject to weather variations and are easily accessible for maintenance.
The moisture measurement system disclosed herein also includes shielded coaxial cabling, encased in conduit, for making the connection to the capacitor plates in the kiln. This offers a high degree of electrical noise resistance, and the insulation precludes the possibility of electrical shorts. Since the cabling is now insulated, a tensioned swing-arm device is no longer required, removing the mechanical complications associated with this device. Installation of insulated electrical stand-offs is also no longer required.
In one embodiment, coaxial cabling, encased in conduit, is used to transport signals between a wood drying monitoring meter, and a connection to a pair of capacitor plates in a kiln.
The present disclosure describes three embodiments by which metering (i.e., monitoring) of moisture in the lumber within a kiln can be performed, wherein each one of the three embodiments is suitable for one or more of the calibration methods being described. The methods described in the three embodiments can include changes in the metering electronics and cabling, primarily including changes to the sensor measurement portion of the moisture measurement system. However, to a user, whichever embodiment is employed is substantially transparent in that each embodiment provides substantially identical functionality and user interaction appearance. Briefly, the three metering embodiments are:
The present disclosure further describes three alternate/optional calibration methods by which the cabling (e.g., the length thereof) for a channel can be calibrated so that the electrical properties of the cabling (and cabling variations between channels) do not detrimentally affect a determination of the moisture in the lumber being dried in a kiln. These three calibration methods can be described as follows:
Moreover, the novel moisture measurement system disclosed herein includes centralized metering electronics (e.g., in a NEMA enclosure of a control room) instead of distributing the meters adjacent their corresponding kilns as provided by prior art systems. Accordingly, external installation and servicing of meters is no longer required. Any service required to the metering equipment can be done within the control room. Thus, since the electronics are now enclosed in a controlled environment, substantially all weather-related meter failures caused by ice, water, wind, etc. are prevented. Moreover, moisture measurement system installation and maintenance access becomes far safer and much easier. Additionally, a reduced amount of moisture monitoring electronics is required because most (if not all) of the weather sensitive electronics can be centralized.
The moisture measurement system disclosed herein additionally includes conduit and cabling that can be installed external to the kiln while the kiln is still operating, thus making kiln down time very short for the installation. All the conduit, cabling, and connectors may be standard electrical equipment that can be installed by any electrician, so highly specialized knowledge and equipment (such as electrical insulating stand offs and swing arms) are not needed.
Additional benefits and features of the moisture measurement system and methods for calibrating such a system will become evident from the accompanying figures, and the description herein below.
Each of the
Note that the multiplexer unit 70 typically serves as the switching device between the kiln moisture system 11 and sensor plates (of capacitors 17) in a plurality of kilns 15. That is, the multiplexer unit 70 performs standard multiplexing functions for switching the signal communications between the channels, and in particular, the multiplexer unit rapidly switches signal communications between the kiln moisture system 11 and each of a plurality of the capacitors 17.
The cabling from the kiln moisture system 11 to the multiplexer unit 70 and then on to each of the sensor plates (e.g., 20, 40) includes: (a) a single shielded coaxial cable 78a between the moisture system 11 and multiplexer unit 70, and (b) at least one coaxial cable 78b between the multiplexer unit 70 and each one of capacitors 17. In particular, for each coaxial cable 78b contacting a corresponding one of the sensor plates 20 or 40, the cable's coaxial cable center conductor 80 is attached to this sensor plate (e.g., 20 or 40), and the coaxial cable shield 90 (of this cable 78b) is either excited or grounded depending on the position of a switch 100 (
The grounding switch 100 allows for two different configurations of the coaxial cables 78a and 78b. In the “excited” configuration (i.e., in-line), the switch 100 excites all coaxial shields 90 with an excitation voltage. When the switch 100 is in an “excited” configuration for exciting the all coaxial cable shields 90, the capacitance between the center wire 80 and the surrounding coaxial shield 90 adds impedance in parallel with the load resistor 140. Note that when modeling a circuit including: (a) the kiln moisture system 11, (b) the multiplexer unit 70, (c) the switch 100, (d) the coaxial cabling 78a and 78b, (e) one of the pairs of sensor plates (e.g., the pair 20, 30, or the pair 40, 50) for one of the capacitors 17, and (f) the corresponding lumber stack between the pair of sensors (e.g., lumber stacks 110, or 120), this impedance in parallel with the load resistor 140 must be factored into the circuit model if calibration of capacitance signals obtained from the capacitor 17 is to be highly accurate.
Alternatively, when the switch 100 is in a “grounded” configuration, the switch 100 grounds all the coaxial shields 90. In the grounded configuration, the switch 100 adds capacitance from each center wire 80 to ground into the circuit, e.g., the circuit of the kiln moisture system 11, the multiplexer unit 70, the switch 100, the coaxial cabling 78a and 78b, one of the pairs of sensor plates (e.g., the pair 20, 30, or the pair 40, 50) for one of the capacitors 17, and the corresponding lumber stack between the pair of sensors (e.g., lumber stacks 110, or 120). Thus, the capacitance from each center wire 80 to ground must be factored into a model of the circuit for accurately calibrating the capacitance signals obtained from the capacitor 17.
In each of the
Each of the three embodiments of the kiln moisture system is further described in the three sections immediately following.
In
The computer 160 of the moisture measurement system 10 includes a data acquisition device or card 170 to generate an excitation signal waveform for transmitting to each of the various capacitors 17. Subsequently, for measuring a response from any one of the capacitors 17, the computer 160 utilizes the data acquisition card 170 for digitizing the voltage waveform across the load resistor 140, via the amplifiers 145 in the sensor measurement unit 190, as one of ordinary skill in the art will understand. Subsequently, values corresponding to attenuation measurement, phase measurement, demodulation, etc. can be computed digitally in a demodulation software component 200, and from such values, this component 200 then computes corresponding impedance data (specifically, the parallel capacitance of the lumber). In particular, the demodulation component 200 uses values (denoted “calibration parameters” herein) stored in the calibration parameter storage 204 for adjusting “raw” or uncorrected impedance values that include cabling 78a,b impedances. More specifically, such calibration parameters are retrieved or fetched from the calibration parameter storage 204, and used to adjust such uncorrected impedance values so that the resulting adjusted impedances are substantially independent of variations in the cabling 78a,b lengths between channels, and more preferably so that such adjusted impedances have substantially all impedance due to the cabling 78a,b removed.
Depending on the electrical configuration of the impedance sensor circuit 125, the calibration parameter storage 204 may store different sets of calibration parameters, wherein such parameters are computed in a calibration process prior to initiation of a lumber stack moisture monitoring process. Various calibration techniques/methods are disclosed hereinbelow for generating the sets of calibration parameters. For example, for the moisture measurement system 10 of
Moreover, as described hereinbelow, for alternative embodiments of the moisture measurement system 10 such as those shown in
Returning now to the demodulation component 200, the corrected or adjusted impedance data is computed, and this component outputs the corrected or adjusted impedance data to a kiln moisture analysis component 210. Accordingly, the moisture analysis component 210 translates the impedance data into moisture content. The translation from capacitance to moisture content is facilitated via a moisture data table 220 which includes capacitance-moisture correspondences. The moisture content is then displayed on a user interface 230 and transmitted via a serial communication device 240 to the kiln controller 250, which uses the moisture level to control, e.g., the kiln temperature, kiln dampers, and when to shut down the kiln lumber drying process.
An alternative second embodiment of the moisture measurement system 10 is shown in
A calibration component 205 is also provided for receiving impedance load data (via the data acquisition device/card 170) output by the impedance sensor circuit 125. The calibration component 205 is, in one embodiment, a software module or subsystem that decomposes the impedance load data into its parallel reactive and resistive components as one of ordinary skill in the art will understand. Further, the calibration component 205 removes impedances and inaccuracies in the impedance readings in the form of impedances from cabling (Ccable, etc). Thus, the main purpose of the calibration component 205 processing is to ensure that the final output impedances (especially the capacitance) accurately represents the moisture content in the drying lumber.
The present embodiment of the moisture measurement system 10 works by continually providing a signal (denoted the “null signal”) from the data acquisition card 170 to the variable impedance component 149. The variable impedance component 149 may be an electronically controlled capacitor. One such electronically controlled capacitor is the Intersil X90100 electronically programmed capacitor, produced by the Intersil Corporation, 1001 Murphy Ranch Road, Milipitas, Calif. 95035. However, other similarly controlled capacitors may also be used as one of ordinary in the art will understand. The null signal is used to continually adjust the output (Z3) of the variable impedance component 149 in order to maintain a balanced bridge, i.e., to maintain the voltage across the load resistor Z5 at or near zero volts (e.g., within ±0.1 volts). A parallel electronically programmed resistor (also known as a potentiometer) can also be used as part of the component 149 to further balance the bridge and accurately determine the resistance at the circuit point where impedance Z4 is measured. The present embodiment of the moisture measurement system 10, the data acquisition card 170 determines an accurate (or corrected) capacitance of a capacitor 17 by using feedback signals from the amplifiers 252 for determining what capacitance level (at component 149) is required to balance the bridge, and then multiplying this capacitance level by the ratio of Z2/Z1. In the circuit configuration of
For the bridge circuit 125 of
A third alternative embodiment of the moisture measurement system 10 is shown in
A calibration component 205 is also provided for receiving impedance load data (via the LCR data acquisition device 171) output by the impedance sensor circuit 125. The calibration component 205 is, in one embodiment, a software module or subsystem that decomposes the impedance load data into its parallel reactive and resistive components as one of ordinary skill in the art will understand. Further, the calibration component 205 removes impedances and inaccuracies in the impedance readings in the form of impedances from cabling (Ccable, etc). Thus, the main purpose of the calibration component 205 processing is to ensure that the final output impedances (especially the capacitance) accurately represents the moisture content in the drying lumber.
Note that this third embodiment of the moisture measurement system 10 is similar to the previous embodiments of the moisture measurement system 10 described in
As in the previous embodiments of
For the embodiment of
The three embodiments of the moisture measurement system 10 described in
A key technical challenge in effectively operating an embodiment of the moisture measurement system 10 is to appropriately compensate for signal anomalies that can be generated within potentially long lengths of the coaxial cables 78a and/or 78b (e.g., lengths greater than 50 linear feet, more particularly from 51 linear feet to 1,000 linear feet or more). In particular, calibration techniques/methods are herein disclosed for compensating or factoring out such signal anomalies. The calibration methods described hereinbelow are suitable for use with one or more of the above described embodiments of the moisture measurement system 10. Note that each calibration method provided below is directed to calibrating based on a particular circuit model. Accordingly, such circuit models are also described so that the parameters for their corresponding calibration methods can be identified.
The first calibration method is for calibrating the moisture measurement system 10 response (from each of the capacitors 17) as a response from a high precision resistor-capacitor network circuit. This first calibration method is most readily applicable to the voltage divider configuration of
(1.1) RCN Calibration Method for Voltage Divider Circuit with Excited Shield (
Referring to the circuit model 204 shown in
Once Zterm is determined, the impedance load (Zload hereinbelow) corresponding to the impedance of the capacitor 17 (e.g.,
Zload=(Zterm*V0)/(V1−V0),
wherein V1 and V0 are voltages as described with reference to the description of
Proper determination of the terminal impedance Zterm is the most critical step in the present calibration method. If performed properly, the determination of an accurate terminal impedance will make the Zload value very accurate. Note that the computed Zload value will be effectively linear to the actual impedance/capacitance load placed on the end of the cable 78b connected to the capacitor 17 (or alternatively, a calibration device as described hereinbelow). That is, Zload is proportional to the actual impedance/capacitance, and independent of the length of the cabling for the channel. However, other minor electrical effects in the modeled circuitry can also be further corrected by subsequently performing a two-point calibration. Such two-point calibration applies small gain and offset corrections to further correct the output of the modeled circuitry, as one of ordinary skill in the art will understand. These other minor electrical effects may include a small amount of cable resistance, cable inductance, and/or an apparent phase inaccuracy caused by digital sampling delays, as one of ordinary skill in the art will understand.
A flowchart of the steps performed for the resistor-capacitor network calibration method is provided in
During the calibration of the circuit (as in
When the calibration routine is started for determining or calibrating the capacitance generated by the cables 78a and 78b for a particular capacitor 17 circuit, the routine prompts the user to connect the BNC connector on the end of the cable 78b end (that would otherwise connect to a sensor plate) to a first of the mating BNC connectors (708a) of the calibration box for determining the terminal impedance (Zterm). A description of the steps of the flowchart of
Step 301: The present step determines the terminal impedance, Zterm. In substep 301a, the coaxial cables 78a and 78b to be calibrated are attached (via cable 78b) to the first BNC connector 708a of the calibration box 704. Subsequently, the calibration box 704 (via BNC connector 708a) provides a first capacitive or impedance load of the standardized loads to the cables as a Zload (
Zterm=Zload*(V1−V0)/V0 (Equation A).
At the conclusion of step 301, an accurate terminal impedance is stored in a data file DF for later use (note, in at least one embodiment, DF may be the calibration parameter storage 204). Note that the value of Zterm is used with subsequently determined values of Zload as described hereinbelow.
Step 302 The present step applies a predetermined high impedance load to the circuit being calibrated, and then computes a load value (“Zload1”) for this high impedance load using the Zterm value determined above. In substep 302a, the user is prompted to connect the cable 78b end (for connecting to the to the calibration box) to a second BNC connection 708b of the calibration box 704 for applying a high impedance standard load to the circuit of
Zload1(uncorrected)=(Zterm*V0)/(V1−V0). (Equation B)
Note that Zload1(uncorrected) is an uncorrected load measurement of the high impedance load; i.e., Zload1(uncorrected) is a measurement of the total impedance of generated by: the calibration box 704, the connected cables 78a,b, and the multiplexer unit 70. Typically, for a value of 10 Mohm load (provided by the calibration box 704), an uncorrected load may be roughly 10.5 Mohms, as one of ordinary skill in the art will understand. At the conclusion of step 302, the uncorrected Zload1 is stored to the data file DF.
Step 303 The present step applies a predetermined low impedance load to the circuit being calibrated, and then computes a value for this impedance load using the Zterm value determined above. In substep 303a, the user is prompted to connect the appropriate cable 78b end to a third BNC cable connection 708c for obtaining a next impedance standard load from the calibration box 704. Such a predetermined low impedance load may be approximately a nominal 1 Mohm resistance (although loads in the range of 0.1 to 2.0 Mohms may be used). Subsequently in substep 303b, when the cable 78b is connected to the predetermined low impedance load of the calibration box 704, the voltage is then determined across the load resistor 140, again, in the manner of substep 301b. In substep 303c, the value “Zload2(uncorrected)” is determined through the equation:
Zload2(uncorrected)=(Zterm*V0)/(V1−V0). (Equation C)
Note that Zload2(uncorrected) is an uncorrected load measurement of the low impedance load; i.e., Zload2(uncorrected) is a measurement of the total impedance generated by the calibration box 704, the connected cables 78a,b, and the multiplexer unit 70. Typically, for a value of a 1 Mohm load (provided by the calibration box), an uncorrected load may be approximately 1.2 Mohms. At the conclusion of step 303, the uncorrected Zload2 is stored to the data file DF.
Step 304 The present step determines a set of at least one signal gain adjustment, and at least one signal offset adjustment to correct the uncorrected impedance values of “Zload1(uncorrected)” and “Zload2(uncorrected)”. That is, the equations for these corrections are as follows:
Zgain=[Zload1(corrected)−Zload2(corrected)]/[Zload1(uncorrected)−Zload2(uncorrected)] (Equation D)
Zoffset=Zload1(corrected)−[Zload1(uncorrected)*Zgain], (Equation E)
where: “Zgain” is the impedance gain to be applied to uncorrected impedance measurements,
“Zload1(corrected)” is the known predetermined impedance load provided by the calibration box 704 in step 302,
“Zload2(corrected)” is the known predetermined impedance load provided by the calibration box in step 303, and
“Zoffset” is the offset to be applied to uncorrected impedance measurements.
For example, if the value of Zload1(uncorrected) is 10.5 Mohms, as estimated above, for an actual load of 10 Mohms (i.e., “Zload1(corrected)” here), and if the value of Zload2(uncorrected) is 1.2 Mohms for an actual load of 1 Mohms (i.e., “Zload2(corrected)” here), then typical gains and offsets may be computed as follows:
Zgain=(10.0−1.0)/(10.5−1.2)=0.97, and
Zoffset=(10.0)−(10.5*0.97)=−0.19 Mohms.
At the conclusion of step 304 of
Step 305 The present step applies a high phase load (e.g., a nominal 100 pF capacitance with a nominal −90 degree phase angle) to the circuit being calibrated, and then computes a value for this load using the Zterm value determined above. In substep 305a, the user is prompted to connect the cable 78b end (for connecting to the to the calibration box 704) to a fourth BNC cable connection 708d for obtaining a fourth impedance standard load from the calibration box 704. Such a predetermined high phase load may be in the range of 1 to 1000 pF. Subsequently, when the cable 78b is connected to the predetermined high phase load of the calibration box 704, in substep 305b, the voltage is determined across the load resistor 140 as V1−V0, again, in the manner of substep 301b. The uncorrected phase angle of the load (identified as “Phaseload3(uncorrected)” herein) is then computed in substep 305c by measuring how many degrees of phase difference exist between the differential voltage signal V1−V0, and the excitation voltage V1. The uncorrected phase angle determination can be performed through a wide variety of methods, including trigonometric methods, Fourier Transform, zero crossing method, etc., as one of ordinary skill in the art will understand.
Note that since Phaseload3(uncorrected) is an uncorrected measurement of the high phase load, Phaseload3(uncorrected) is a measurement of the total phase angle of generated by the calibration box 704, the connected cables 78a,b, and the multiplexer unit 70. Also note that the corrected phase angle (“Phaseload3(corrected)” herein) is −90 degrees since the load is a capacitance, as is well known in the art. Typically, an uncorrected value of an actual nominal 100 pF capacitance with a nominal −90 degree phase angle may be approximately −88 degrees. At the conclusion of step 305, the value of “Phaseload3(uncorrected)” is stored to the data file DF, or the calibration parameter storage 204.
Step 306 The present step applies a low phase load (e.g., a load of 1 Mohm with a nominal 0 degree phase angle). In substep 306a, the user is prompted connect the appropriate cable 78b end to a fifth BNC cable connection 708e for obtaining a fifth impedance standard load from the calibration box 704. Such a predetermined low phase load may be in the range of 1 to 10 Mohms. Subsequently, when the cable 78b is connected to the predetermined low phase load of the calibration box 704, in substep 306b, the voltage is then measured across the load resistor 140 as V1−V0 in the manner of substep 301b. The phase angle of the load (identified as “Phaseload4(uncorrected)” herein) is then computed in Substep 306c by measuring how many degrees of phase difference exist between the differential voltage signal V1−V0 and the excitation voltage V1.
Note that “Phaseload4(uncorrected)” is an uncorrected measurement of the low phase load; i.e., Phaseload4(uncorrected) is a measurement of the total phase angle of generated by the calibration box 704, the connected cables 78a,b, and the multiplexer unit 70. Also note that the corrected phase load (“Phasesload4(corrected)” herein) is 0 degrees since the load is a resistance. Typically, an uncorrected value of a load of 1 Mohm with a nominal 0 degree phase angle for the low phase load may be roughly −1.5 degrees. At the conclusion of step 306, the value of “Phaseload4(uncorrected)” is stored to the data file DF.
Step 307 The present step determines a set of values, including a phase gain and a phase offset to correct or adjust uncorrected phase loads received via the channel cabling 78a,b, e.g., Phaseload3(uncorrected) and Phaseload4(uncorrected). In particular, the set includes a phase gain (denoted “Phasegain” herein), and a phase offset (denoted “Phaseoffset” herein). Note that, as indicated above, Phaseload3(corrected) is −90 degrees since the load is a capacitance, and Phasesload4(corrected) is 0 degrees since the load is a resistance. Accordingly, the equations for correcting Phaseload3(uncorrected) and Phaseload4(uncorrected) are as follows:
Phasegain=[Phaseload3(corrected)−Phaseload4(corrected)]/[Phaseload3(uncorrected)−Phaseload4(uncorrected)] (Equation F)
Phaseoffset=Phaseload3(corrected)−[Phaseload3(uncorrected)*Phasegain], (Equation G)
wherein Phaseload3(corrected) is the known predetermined phase angle provided by the calibration box 704 in step 305, and Phaseload4(corrected) is the known predetermined capacitance load phase angle provided by the calibration box in step 306.
For example, if the value of Phaseload3(uncorrected) is −88 degrees, as indicated above, for an actual value of a nominal 100 pF capacitance with a nominal −90 degree phase angle (i.e., “Phaseload3(corrected)” here), and if the value of Phaseload4(uncorrected) is −1.5 degrees, as estimated above, for an actual load of 1 Mohm with a nominal 0 degree phase angle (i.e., “Phaseload4(corrected)” here), then typical gains and offsets may be computed as follows:
Phasegain=(−90−(0)/(−88−(−1.5))=1.04,
Phaseoffset=(−90)−(−88)*1.04=1:5 degrees
At the conclusion of step 307, the computed values for Phasegain and Phaseoffset are stored to the data file DF.
Step 308 In the present step, the calibration parameters Zterm, Zgain, Zoffset, Phasegain, and Phaseoffset are tested to determine if they can be used to appropriately calibrate signals arriving at the amplifiers 145 so that signal characteristics generated by the cables 78a and 78b (and other electrical components) can be effectively removed, thereby leaving substantially only signal characteristics obtained from the calibration box 704. In particular, one or more tests are conducted using the calibration box 704, wherein for each known impedance load (Zload,
In substep 308a, the user is prompted to attach the sensor connection end of the cable 78b to a sixth BNC connection 708f on the calibration box 704 for applying a sixth predetermined load (denoted Zload5 herein) to the cables 78a and 78b, and the load resistor 140. The sixth BNC connection 708f may provide a load from a parallel combination of, e.g., a nominal 100 pF capacitor and a nominal 1 Mohm resistor (
In substep 308d, the gains and offsets (Zgain, Zoffset, Phasegain, and Phaseoffset in DF) are applied to Zload5(uncorrected) and Phaseload5(uncorrected) for obtaining the corrected impedance and phase values, respectively, Zload5(corrected) and Phaseload5(corrected), according to the following equations:
(a) Zload5(corrected)=Zload5(uncorrected)*Zgain+Zoffset,
and
(b)Phaseload5(corrected)=Phaseload5(uncorrected)*Phasegain+Phaseoffset.
In substep 308e, a determination is made as to whether the readings of the known impedance load provided by the calibration box 704, and the impedance load values computed in substeps 308b through 308d (using the calibration parameters Zterm, Zgain, Zoffset, Phasegain, and Phaseoffset) are sufficiently close to one another. In particular, based on a tolerance specified by, e.g., the user (such as within 7% of one another, and more preferably within a range of approximately 5%), a determination is made as to whether or not the calibration of the circuit of
If substep 308e does not determine capacitance load values within a specified tolerance of the actual predetermined capacitive load provided by the calibration box 704, then the calibration parameters determined in steps 301 through 307 are not sufficiently effective, and accordingly the steps 301 through 308 are repeated. Note that the connections of the cables 78a,b to their various electrical components should be checked to assure that appropriate electrical connections are provided prior to repeating the steps 301 through 308. If the computed capacitance load value is within the specified tolerance, then the calibration may be determined to be successful.
Step 309: In the present step, the following values are stored in the calibration parameter storage 204 (e.g., a file or database) (
(309.1a) Zterm,
(309.2a) Zgain,
(309.3a) Zoffset,
(309.4a) Phasegain, and
(309.5a) Phaseoffset.
Step 312: In the present step, a determination is made as to whether there is another channel to calibrate. If so, then the steps commencing with step 301 are again performed. If not, then the calibration process terminates.
Thus, since steps of
Accordingly, during operation of the moisture measurement system 10 configured as a voltage divider with an excited shield 90, for each channel, the following steps are performed:
(Step A-1) The data acquisition card 170 determines a voltage difference (V1−V0), e.g., across the load resistor 140 for the channel being monitored.
(Step A-2) The demodulation component 200 determines an (uncorrected) impedance load for the channel, through application of the equation:
Zload(uncorrected)=(Zterm*V0)/(V1−V0).
(Step A-3) The demodulation component 200 determines an (uncorrected) phase of the load across the load resistor 140 by comparing the phase angle of V0 and V1, using trigonometric methods, Fourier transform, or zero crossing methods, as one of ordinary skill in the art will understand.
(Step A-4) The demodulation component 200 uses the calibration parameters (gains and offsets) stored in the calibration parameter storage 204 (i.e., in step 309) to remove errors generated by electrical components other than the capacitor 17 by performing corrections to the total impedance and phase. Equations to perform these corrections are:
(a) Zload(corrected)=Zload(uncorrected)*Zgain+Zoffset,
and
(b) Phaseload(corrected)=Phaseload(uncorrected)*Phasegain+Phaseoffset.
(Step A-5) With accurate determination of the phase angle and total impedance of the load from step A-4, the demodulation component 200 resolves this corrected impedance into its rectangular components, i.e., the parallel impedances Cload (a capacitance load) and Rload (a resistance load), as one of ordinary skill in the art will understand.
(Step A-6) Since Cload and Rload are substantially independent of the length of the cabling 78a,b, the moisture content of the lumber being monitored (e.g., lumber stack 110 or 120) can be more accurately, determined. Accordingly, the impedances Cload and Rload are provided to the kiln moisture analysis 210 (
(1.2) RCN Calibration Method for Voltage Divider (VD) Circuit with Grounded Shield (
Note that for an embodiment of the voltage divider metering of
Ccable=(Apparent Capacitance at BNC connector 708a)−(known Capacitance at BNC connector 708a).
The remainder of the calibration parameters (gains and offsets) are then determined in the same fashion and steps as described previously in steps 301-312 using the known value of Zterm. In particular, various impedance loads (e.g., Zload1, Zload2, Zload5) according to the equation impedance load equation: (Zterm*V0)/(V1−V0) for voltages V0 and V1 as shown in
Accordingly, for the grounded shield 90 embodiment, the calibration parameter storage 204 is provided with the calibration parameter values (309.1b) through (309.6b) below at successful completion of the steps corresponding to the steps 301 through 312.
(309.1b) Zterm, which is approximately equal to the impedance of the load resistor 140.
(309.2b) Cabling capacitance to ground, Ccable (i.e., the capacitance between grounded shield 90 and the corresponding center wire 80 surrounded by the shield),
(309.3b) Zgain,
(309.4b) Zoffset,
(309.5b) Phasegain, and
(309.6b) Phaseoffset,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
Accordingly, during operation of the moisture measurement system 10 configured as a voltage divider with a grounded shield 90, for each channel, the following steps are performed:
(Step B-1) The data acquisition card 170 determines a voltage difference (V1−V0), e.g., across the load resistor 140 for the channel being monitored.
(Step B-2) The demodulation component 200 fetches the calibration parameter Zterm from the calibration parameter storage 204, and determines an (uncorrected) impedance load, Zload(uncorrected) for the channel, through application of the equation:
Zload(uncorrected)=(Zterm*V0)/(V1−V0).
(Step B-3) The demodulation component 200 decomposes Zload(uncorrected) into its parallel reactive and resistive components, Cload(uncorrected) and Rload(uncorrected).
(Step B-4) The demodulation component 200 fetches the cable capacitance (Ccable) from the calibration parameter storage 204, and subtracts this value from the Cload(uncorrected), thereby obtaining a value denoted Cload(uncorrected)1.
(Step B-5) The demodulation component 200 combines the values
Cload(uncorrected)1 and Rload(uncorrected) (by transformation to polar coordinates) to obtain a new impedance value denoted Zload(uncorrected)1 that has the cable capacitance removed from it.
(Step B-6) The demodulation component 200 determines an (uncorrected) phase of the load across the load resistor 140 by comparing the phase angle of V0 and V1, using trigonometric methods, Fourier transform, or zero crossing methods, as one of ordinary skill in the art will understand.
(Step B-7) The demodulation component 200 fetches the calibration parameters (e.g., gains and offsets) in the calibration parameter storage 204 (e.g., those identified in (309.1b) through (309.6b) above), and uses these values to remove errors generated by electrical components other than the capacitor 17 by performing corrections to the total impedance and phase. Equations to perform these corrections are:
(a) Zload(corrected)=Zload(uncorrected)1*Zgain+Zoffset,
and
(b) Phaseload(corrected)=Phaseload(uncorrected)*Phasegain+Phaseoffset.
(Step B-8) With accurate determination of the phase angle and total impedance of the load across the load resistor 140 from step B-7, the demodulation component 200 resolves the impedance into its rectangular components, i.e., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand.
(Step B-9) Since Cload and Rload are substantially independent of the length of the cabling 78a,b, the moisture content of the lumber being monitored (e.g., lumber stack 110 or 120) can be more accurately determined. Accordingly, the impedances Cload and Rload are provided to the kiln moisture analysis 210 (
(1.3) RCN Calibration Method for Bridge Circuit with Grounded Shield (
Regarding an embodiment of the bridge metering of
However, the cabling 78a,b capacitance, Ccable, is needed, wherein Ccable is related to the length of the cabling 78a,b. Note that variations to steps 301 through 312 of
Since the first load of the calibration box 704 (e.g., from BNC connector 708a) is a known capacitance, for example a capacitor in the range of 100 to 5000 pF, the Ccable value in the circuit model of
Ccable=(Apparent Capacitance at BNC connector 708a)−(known Capacitance at BNC connector 708a).
The remainder of the calibration parameters (gains and offsets) are then determined in the same fashion and steps as described in
Accordingly, for the bridge metering embodiment with a grounded shield 90, the calibration parameter storage 204 is provided with the following calibration parameter values at successful completion of the steps corresponding to the steps 301 through 312.
(309.1c) Cabling capacitance to ground, Ccable (i.e., the capacitance between grounded shield 90 and the corresponding center wire 80 surrounded by the shield), wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
(309.2c) Zgain,
(309.3c) Zoffset,
(309.4c) Phasegain, and
(309.5c) Phaseoffset,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
Accordingly, during operation of the moisture measurement system 10 configured as a bridge circuit with a grounded shield 90, for each channel, the following steps are performed by the data acquisition card 170:
(Step C-1) A null signal is transmitted to adjust impedance Z3 (
(Step C-2) The values of the balancing impedance (Z3) as well as the impedance values of Z2 and Z1 are recorded by the data acquisition card 170.
(Step C-3) Z4 is calculated according to the equation:
Z4=Z3*Z2/Z1.
The impedance Z4 is transmitted to the calibration component 205.
(Step C-4) Z4 is decomposed into its parallel reactive and resistive components, Cload(uncorrected) and Rload(uncorrected) by the calibration component 205.
(Step C-5) The cable capacitance (Ccable) is fetched from the calibration parameter storage 204, and is subtracted from Cload(uncorrected) by the calibration component 205. This provides a new value denoted Cload(uncorrected)1 herein.
(Step C-6) Cload(uncorrected)1 and the Rload(uncorrected) are combined by the calibration component 205 to yield a new impedance value denoted Zload(uncorrected)1 that has the cabling 78a,b capacitance removed from it.
(Step C-7) The calibration component 205 determines an uncorrected phase (“Phaseload(uncorrected)”) of the impedance load (e.g., for the lumber 110 or 120) by evaluation of the real and imaginary portions of the complex impedance Z4, i.e., by converting the rectangular components of Z4's resistance and reactance into polar coordinates, the phase of which can be readily determined.
(Step C-8) The calibration parameters (gains and offsets) in the calibration parameter storage 204 (i.e., identified in (309.7b) through (309.11b) above) are used, by the calibration component 205, to remove errors generated by electrical components other than the capacitor 17 by performing corrections to the total impedance and phase. Equations to perform these corrections are:
(a) Zload(corrected)=Zload(uncorrected)1*Zgain+Zoffset,
and
(bPhaseload(corrected)=Phaseload(uncorrected)*Phasegain+Phaseoffset.
(Step C-9) With accurate determination of the phase angle and total impedance of the load, the calibration component 205 resolves the impedance into rectangular components, i.e., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand.
(Step C-10) Since Cload and Rload are substantially independent of the length of the cabling 78a,b, the moisture content of the lumber being monitored (e.g., lumber stack 110 or 120) can be more accurately determined. Accordingly, the impedances Cload and Rload are provided to the kiln moisture analysis 210 (
Note that for an embodiment of the LCR data acquisition circuit 125 of
Ccable=(Apparent Capacitance at BNC connector 708a)−(known Capacitance at BNC connector 708a).
A key difference between the present calibration method and the previous calibration methods of (1.1) and (1.2) is that in the present calibration method there is no longer any calculation of uncorrected impedances using voltages and the terminal impedance Zterm. The impedance of the circuit 218 (
The remainder of the calibration parameters (i.e., Zgain, Phasegain, Zoffset, and Phaseoffset) are then determined in the same fashion and steps as described previously in steps 301-312.
Accordingly, for the LCR data acquisition circuit 125 embodiment with a grounded shield 90 (
(309.1d) Cabling capacitance to ground, Ccable (i.e., the capacitance between grounded shield 90 and the corresponding center wire 80 surrounded by the shield 90), wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
(309.2d) Zgain,
(309.3d) Zoffset,
(309.4d) Phasegain, and
(309.5d) Phaseoffset,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
Accordingly, during operation of the moisture measurement system 10 configured as an LCR data acquisition circuit with a grounded shield 90, for each channel, the following steps are performed:
(Step D-1) The impedance load Z4 (e.g., for the lumber 110 or 120) is measured by the LCR data acquisition card 171. The impedance Z4 is transmitted to the calibration component 205.
(Step D-2) The impedance load is broken into its parallel reactive and resistive components, Cload(uncorrected) and Rload(uncorrected) in the calibration component 205.
(Step D-3) The cable capacitance (Ccable) from the calibration storage 204 is subtracted from Cload(uncorrected) in the calibration component 205. This provides a new value denoted Cload(uncorrected)1 herein which is performed in the calibration component 205.
(Step D-4) The values Cload(uncorrected)1 and Rload(uncorrected) are then combined to specify a new impedance value Zload(uncorrected)1 that has the cable capacitance removed from it.
(Step D-5) An (uncorrected) phase of the load is then calculated by the calibration component 205.
(Step D-6) The calibration parameters (gains and offsets) in the calibration parameter storage 204 (i.e., identified in (309.1d) through (309.5d) above) are used, by the calibration component 205, to remove errors generated by electrical components other than the capacitor 17 by performing corrections to the total impedance and phase. Equations to perform these corrections are: which is performed in the calibration component 205.
(a) Zload(corrected)=Zload(uncorrected)1*Zgain+Zoffset,
and
(b) Phaseload(corrected)=Phaseload(uncorrected)*Phasegain+Phaseoffset.
(Step D-7) With accurate determination of the phase angle and total impedance of the load, the impedance is resolved into rectangular components, i.e., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand which is performed in the calibration component 205.
(Step D-8) The impedances Cload and Rload are provided to the kiln moisture analysis 210 (
The impedance measurement calibration method uses a high precision LCR meter or impedance bridge (not shown in the figures) together with the steps described in the flowchart of
(2.1) IM Calibration Method for Voltage Divider Circuit with Excited Shield (
For example, the impedance measurement calibration method may be used in the case of an electrically excited shield 90 and according to the circuit model 204 of
In the case of a grounded shield 90, the circuit model shown in
As shown above, the impedance measurement calibration method is suitable for a circuit model involving either an electrically excited coaxial shield 90, or a grounded coaxial shield 90. Moreover, the impedance measurement calibration method offers an advantage over the above described resistor-capacitor network calibration methods in that all tasks of the impedance measurement calibration method can be performed by an operator in a control room where the range in temperature is, e.g., 65° to 75° F., and with humidity below, e.g., 20%. In particular, the impedance measurement calibration method does not require an operator to connect a calibration box 704 at the end of the coaxial cable 78b in the kiln 15 which may be a considerable distance from the control room (e.g., up to 1000 linear feet or more).
A description of the steps of
In step 404 of
In the case of an excited shield 90 (i.e.,
Zterm0=[−i/(2π*f*Ccable)*Rload]/[−i/(2π*f*Ccable)+Rload] (Equation H)
where:
Zterm0 is the complex representation of the load impedance between the leads of the LCR meter,
π=3.14159 (approx.),
f=frequency of the excitation signal for exciting the shield,
Ccable=measured cable capacitance for the channel cables 78a and 78b,
“i” identifies the imaginary terms of Equation H, and
Rload is the resistance value of the load resistor 140. Note that Rload may be known from a previous resistance measure of the load resistor 140.
Subsequently, in steps 410 and 412, all the gains (i.e., the impedance gain (Zgain), and the phase gain (Phasegain)) are simply set to 1, and all the offsets (i.e., the impedance offset (Zoffset), and the phase offset (Phaseoffset)) are simply set to 0. Depending on whether the shield 90 is excited or grounded (step 416), a respective one of the steps 420 and 424 is performed, wherein the data file DF, or the calibration parameter storage 204 (or another data container such as a relational database) for the channel is populated as shown in
(420.1a) Zterm0,
(420.2a) Zgain=1,
(420.3a) Zoffset=0,
(420.4a) Phasegain=1, and
(420.5a) Phaseoffset=0.
Of course the steps 401 through 424 may be iteratively performed for each channel.
Alternatively, for the case where the shield 90 is grounded (i.e.,
(420.1b) Zterm, which, indicated above, is approximated by Rterm since the shield 90 is grounded.
(420.2b) Ccable, wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
(420.3b) Zgain=1,
(420.4b) Zoffset=0,
(420.5b) Phasegain =1, and
(420.6b) Phaseoffset =0.
Note that the calibration parameters whose values are determined with the impedance measurement calibration method for the two voltage divider circuit embodiments are the same calibration parameters whose values are also determined with the resistor-capacitor network calibration method. Accordingly, these two calibration methods can be used interchangeably or together within the moisture measurement system 10. For example, the corresponding parameter values from the calibration parameter set for the impedance measurement calibration method, and from the calibration parameter set for the resistor-capacitor network calibration method may be averaged, or combined in other ways to obtain more reliable calibration parameter values.
During operation of the moisture measurement system 10, the calibration parameters immediately above (i.e., (420.1a) through (420.5a), and/or (420.1b) through (420.6b)) are used to calibrate or adjust the signals obtained from their channel and capacitor 17 according to the following steps. Note that these steps are described in terms of Zterm0 for an excited shield 90. However, for the grounded shield embodiment, Zterm0 can be replaced with Zterm (approximated by Rterm) as described above.
(Step E-1) Since Zterm0 defines the impedance across the terminals of the amplifiers 145 (
(Step E-2) A differential voltage V1−V0 is measured across these amplifier terminals 208, 212 (
(Step E-3) Subsequently, a total (uncorrected) value of the channel circuit impedance (denoted “Zload(uncorrected)” herein) is determined by the equation:
Zload(uncorrected)=Zterm0*(V0)/(V1−V0).
(Step E-4) Then, a corrected value for circuit impedance (denoted “Zload(corrected)” herein) is calculated, wherein at least the impedance induced by the channel cabling 78a,b is substantially factored out or made independent of the length of the cabling. In particular, Zload(corrected) is determined by applying the Zgain and Zoffset values in the data file DF, or the calibration parameter storage 204. That is,
Zload(corrected)=[Zload(uncorrected)*Zgain] +Zoffset. Note that since the gains and offsets are typically set to 1 and 0, respectively, the Zload(corrected) is equal to the Zload(uncorrected).
(Step E-5) Additionally, a value for the total signal phase (denoted
“Phaseload(uncorrected)” herein) for the channel circuit is determined through application of one of the following methods: a trigonometric method, a Fourier transform method, a zero crossing method, etc, as one of ordinary skill in the art will understand.
(Step E-6) Subsequently, a corrected value for the phase of the circuit signals (denoted “Phaseload(corrected)” herein) is calculated by applying the Phasegain and Phaseoffset from the data file DF. That is, Phaseload(corrected)=[Phaseload(uncorrected)*Phasegain]+Phaseoffset.
Note that since the gains and offsets are typically set to 1 and 0, respectively, the Phaseload(corrected) is equal to the Phaseload(uncorrected).
(Step E-7) With accurate determination of the phase angle (i.e., Phaseload(corrected)) and total impedance of the load (i.e., Zload(corrected)), the impedance can be resolved into rectangular components, i.e., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand. Note that for the grounded shield configuration, Ccable is subtracted from Cload to obtain a new value for Cload which is then used in the following step.
(Step E-8) Since Cload and Rload are substantially independent of the length of the cabling 78a,b, the moisture content of the lumber being monitored (e.g., lumber stack 110 or 120) can be more accurately determined. Accordingly, the impedances Cload and Rload are provided to the kiln moisture analysis 210 (
(2.2) IM Calibration Method for Bridge Circuit Embodiment (
Note that for the bridge metering embodiment of
(420.1c) Cabling capacitance to ground, Ccable (i.e., the capacitance between grounded shield 90 and the corresponding center wire 80 surrounded by the shield 90), wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
(420.2c) Zgain,
(420.3c) Zoffset,
(420.4c) Phasegain, and
(420.5c) Phaseoffset,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
Accordingly, during operation of the moisture measurement system 10 configured as in
(Step F-1) A null signal is transmitted to adjust impedance Z3 such that the bridge is balanced, i.e., such that the voltage across Z5 is maintained near zero volts.
(Step F-2) The value of the balancing impedance (Z3) is recorded by the data acquisition card 170.
(Step F-3) Z4 is then calculated according to the equation:
Z4=Z3*Z2/Z1.
The impedance Z4 is transmitted to the calibration component 205.
(Step F-4) Z4 is broken into its parallel reactive and resistive components,
Cload(uncorrected) and Rload(uncorrected), respectively, by the calibration component 205.
(Step F-5) The cable capacitance (Ccable) is subtracted from the
Cload(uncorrected) by the calibration component 205. This provides a new Cload(uncorrected)0.
(Step F-6) The Cload(uncorrected)0 and the Rload(uncorrected) are then combined by the calibration component 205 to specify a new Zload(uncorrected)0 that has the cable capacitance removed from it.
(Step F-7) The calibration component 205 determines an uncorrected phase of the impedance load (e.g., for the lumber 110 or 120) by evaluation of the real and imaginary portions of the complex impedance Z4, i.e., by converting the rectangular components of Z4's resistance and reactance into polar coordinates, the phase can be readily determined.
(Step F-8) The calibration parameters (gains and offsets) stored above (i.e., in step 424) are used, by the calibration component 205, to remove errors generated by electrical components other than the capacitor 17 by performing corrections to the total impedance and phase. Equations to perform these corrections are:
a. Zload(corrected)=Zload(uncorrected)0*Zgain+Zoffset,
and
b. Phaseload(corrected)=Phaseload(uncorrected)*Phasegain+Phaseoffset.
Note that since the gains (i.e., Zgain and Phasegain) and offsets (i.e., Zoffset and Phaseoffset) are typically set to 1 and 0, respectively, the Zload(corrected) is equal to the Zload(uncorrected), and the Phaseload(corrected) is equal to the Phaseload(uncorrected).
(Step F-9) With accurate determination of the phase angle and the total impedance of the load, the calibration component 205 resolves the impedance into rectangular components, e.g., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand.
(Step F-10) The impedances Cload and Rload are provided to the kiln moisture analysis 210 (
Note that for an embodiment of the LCR data acquisition circuit of
Accordingly, for the LCR data acquisition circuit embodiment with a grounded shield 90, the calibration parameter storage 204 is provided with the following calibration parameter values (420.1d) through (420.5d) below when a variation of the steps 401 through 424 of
(420.1d) Cabling capacitance to ground, Ccable (i.e., the capacitance between grounded shield 90 and the corresponding center wire 80 surrounded by the shield), wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated, (420.2d) Zgain, (420.3d) Zoffset, (420.4d) Phasegain, and (420.5d) Phaseoffset,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
Accordingly, during operation of the moisture measurement system 10 configured as in
(Step G-1) Z4, the total impedance of the circuit model of
(Step G-2) Z4 is broken into its parallel reactive and resistive components, Cload(uncorrected) and Rload(uncorrected), in the calibration component 205.
(Step G-3) The cable capacitance (Ccable) is subtracted from the Cload(uncorrected) in the calibration component 205. This provides a new Cload(uncorrected)0.
(Step G-4) The Cload(uncorrected)0 and the Rload(uncorrected) are then combined to specify a new Zload(uncorrected)′ that has the cable capacitance removed from it.
(Step G-5) An (uncorrected) phase of the load is generated in the calibration component 205.
(Step G-6) The calibration parameters (gains and offsets) stored above (i.e., in step 424) are used, by the calibration component 205, to remove errors generated by electrical components other than the capacitor 17 by performing corrections to the total impedance and phase. Equations to perform these corrections are:
a. Zload(corrected)=Zload(uncorrected)0*Zgain+Zoffset, and
b. Phaseload(corrected)=Phaseload(uncorrected)*Phasegain+Phaseoffset.
Note that since the gains (i.e., Zgain and Phasegain) and offsets (i.e., Zoffset and Phaseoffset) are typically set to 1 and 0, respectively, the Zload(corrected) is equal to the Zload(uncorrected), and the Phaseload(corrected) is equal to the Phaseload(uncorrected).
This step is also performed by the calibration component 205.
(Step G-7) With accurate determination of the phase angle and total impedance of the load, the impedance can be resolved into rectangular components, the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand. This step is also performed by the calibration component 205.
(Step G-8) The impedances Cload and Rload are provided to the kiln moisture analysis 210 (
The third calibration method uses a “short-open” procedure as described in the flowchart of
(3.1) S-O) Calibration Method for Voltage Divider Circuit with Grounded shield (
Referring now to the flowchart of
a. Impedance load of the cabling (i.e., Zcable) is determined as (Zterm*V0)/(V1−V0),
wherein V1 is the generated voltage of the excitation signal, and V0 is the voltage of the excitation signal for the center wire 80 at the point indicated in
b. Ccable=1/(2*π*f*Zcable),
where π=approximately 3.14159, and f is the excitation frequency. Note that Ccable is dependent on the length of the channel cabling 78a,b.
Subsequently, the cable capacitance to ground (Ccable) is stored in, e.g., the data file DF, or the calibration parameter storage 204.
In step 605, the end of the cable 78b in the kiln is shorted, which can be done with a jumper cable from the coaxial center wire 80 to any grounded structure in the kiln 15, such as a ground lug on the kiln wall or the conduit through which the coaxial cabling 78b extends. The voltage across the load resistor 140 (Rterm) is then measured to obtain the “short” circuit reading, as described in step 606. In this shorted configuration, the impedance of the lumber Zload is then removed from the circuit (of
Subsequently, the following calibration parameters are stored in, e.g., the calibration file DF, or the calibration parameter storage 204:
(607.1a) Zterm (Rterm),
(607.2a) Ccable, wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
(607.3a) Lcable, and
(607.4a) Rcable.
Of course the steps 601 through 607 may be iteratively performed for each channel.
During operation of the moisture measurement system 10, the four calibration parameters immediately above are used to calibrate or adjust the signals from the channel and capacitor 17 as follows:
(Step H-1) Zterm (Rterm) is known (due to the described measurement of step 601), and the differential voltage V1−V0 can be measured with the amplifier 145.
(Step H-2) During the lumber moisture monitoring operation, I*, the total current across the load resistor 140, can be determined as follows:
I*=(V1−V0)/Zterm,
(Step H-3) Since Ccable is known (as determined in calibration step 604), and since V0 is known, it is possible to determine the impedance of the left leg 226 of the circuit model of
ZCcable=1/(j*ω*Ccable),
where ZCcable is the impedance of the channel cabling 78a,b,
j indicates that this impedance is reactive, and
ω is the radial frequency of the excitation voltage.
(Step H-4) Accordingly, the current across the capacitance of the cabling of the left leg 226 is:
I1*=V0/ZCcable (FIG. 6A).
(Step H-5) Since I* and I1* are now known, the current (I2*) running down the right circuit leg of
I2*=I*−I1*.
(Step H-6) Further, since I2* is now known, V0 is known, and both Lcable and Rcable are retrieved from the calibration storage DF or 204, it is possible to calculate Zload* (i.e., the total impedance of the capacitor 17 formed in the kiln stack) as follows. Since (Zload*+[j*ω*Lcable]+Rcable)*I2*=V0, upon rearrangement, the following equation is obtained for determining Zload*:
Zload*=(V0/I2*)−[j*ω*Lcable]−Rcable.
(Step H-7) Resolve Zload* into its rectangular components, i.e., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand. The impedances Cload and Rload are provided to the kiln moisture analysis 210 (e.g.,
The phase of the load of the capacitor 17 (denoted “PHASE” hereinbelow) can be determined by comparing the phase of the voltage at the load and the current at the load, using any of a variety of methods including trigonometric methods, zero crossing method, Fourier transform, etc. as one of ordinary skill in the art will understand. It is then straightforward to resolve the impedance into rectangular components, the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand.
(3.2) S-O Calibration Method for Bridge Circuit with Grounded Shield (
Note that for an embodiment of the bridge circuit of
Accordingly, for the bridge metering embodiment with a grounded shield 90, the calibration parameter storage 204 is provided with the calibration parameter values (607.1b) through (607.3b) following when the steps of
(607.1b) Ccable (i.e., Z4 in the open configuration), wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
607.2b) Lcable, and
(607.3b) Rcable,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
During operation of the moisture measurement system 10, the three calibration parameters immediately above are used to calibrate or adjust the signals from the channel and capacitor 17 as follows:
(Step I-1) A null signal is transmitted to adjust impedance Z3 such that the bridge is balanced, i.e., such that the voltage across Z5 is maintained near zero volts.
(Step I-2) The value of the balancing impedance (Z3) is recorded by the data acquisition card 170.
(Step I-3) Z4 is then calculated according to the equation: Z4=Z3*Z2/Z1. The impedance Z4 is transmitted to the calibration component 205.
(Step I-4) Z4 is broken into its parallel reactive and resistive components, Cload(uncorrected) and Rload(uncorrected), respectively, in the calibration component 205.
(Step I-5) The cable capacitance (Ccable) stored in DF or the calibration parameter storage 204 is subtracted from the Cload(uncorrected) by the calibration component 205. This provides a new corrected Cload, denoted Cload(corrected).
(Step I-6) The cable resistance (Rcable) and inductance (Lcable) are subtracted from the Rload(uncorrected) by the calibration component 205. This provides a new corrected Rload (denoted Rload(corrected)), which is a complex number.
(Step I-7) The Cload(corrected) and the Rload(corrected) are then combined, by the calibration component 205, to specify Zload* that has the cable 78a,b capacitance, inductance, and resistance removed from it. Note that Zload* is a complex number which can be readily broken down into rectangular components, i.e., the parallel impedances Cload and Rload, as one of ordinary skill in the art will understand.
(Step I-8) The impedances Cload and Rload are provided to the kiln moisture analysis 210 (
(3.3) S-O Calibration Method for LCR Data Acquisition Circuit with
Grounded Shield Note that for an embodiment of the LCR data acquisition of
In the open configuration, Z4 is approximately equal to the impedance provided by Ccable, and in the short configuration Z4 is approximately equal to the parallel combination of (a) the impedance provided by Rcable and Lcable, and (b) the impedance provided by Ccable. Moreover, each of Rcable and Lcable can be readily determined by decomposing the complex impedance Z4 (in the short configuration) into its real and imaginary components, as one of ordinary skill in the art will understand. Accordingly, for the present LCR data acquisition circuit embodiment with a grounded shield 90, the calibration parameter storage 204 is provided with the following calibration parameter values (607.1c) through 607.3c) when the steps of
(607.1c) Ccable (i.e., Z4 in the open configuration), wherein Ccable is dependent on the length of the cabling 78a,b for the channel being calibrated,
(607.2c) Lcable, and
(607.3c) Rcable,
wherein each term identified here has the same meaning as the (any) corresponding identical term used in describing
During operation of the moisture measurement system 10, the three calibration parameters immediately above are used to calibrate or adjust the signals from the channel and capacitor 17 as follows:
(Step J-1) The value of the complex impedance Z4 of the circuit model 224 is determined and recorded by the LCR data acquisition card 171. The impedance Z4 is transmitted to the calibration component 205.
(Step J-2) Z4 is broken into its parallel reactive and resistive components, Cload(uncorrected) and Rload(uncorrected) in the calibration component 205.
(Step J-3) The cable capacitance (Ccable) stored in DF or the calibration parameter storage 204 is subtracted from Cload(uncorrected) in the calibration component 205. This provides a new corrected Cload, denoted Cload(corrected).
(Step J-4) The cable resistance (Rcable) and inductance (Lcable) are subtracted from the Rload(uncorrected) in the calibration component 205. This provides a new corrected Rload (denoted Rload(corrected)), which is a complex number.
(Step J-5) The Cload(corrected) and the Rload(corrected) are then combined, by the calibration component 205, to specify a new value of Z4 (denoted Z4(corrected)) that has the cable 78a,b capacitance, inductance, and resistance removed from it.
(Step J-6) The phase of the impedance load (e.g., lumber 110 or 120) is determined in the calibration component 205 by conversion of the real and imaginary representation of the complex impedance Z4(corrected) into polar coordinates, i.e., by converting the rectangular components of Z4(corrected)'s resistance and reactance into polar coordinates, the magnitude of Z4(corrected) and the corresponding phase angle for Z4(corrected) are determined.
(Step J-7) With accurate determination of the phase angle (Step J-6) and the total impedance Z4(corrected) of the kiln lumber stack, resolution of the impedance Z4(corrected) into rectangular, i.e., the parallel impedances Cload and Rload, components can be performed by the calibration component 205, as one of ordinary skill in the art will understand.
(Step J-8) The impedances Cload and Rload are provided to the kiln moisture analysis 210 (
The use of any of the aforementioned calibration methods ensures a response from each of the channels that does not depend (or not substantially so) on the length of the channel's cabling. Additionally, no “tuning” is required for the moisture measurement system 10 to accurately determine the capacitance induced by moisture in the kiln dried lumber. That is, no “tuning” is required wherein physical adjustments (e.g., extra electronics, changes in cable lengths, etc.) to the various channels is required in order to make each channel appear to have an output independent of channel cabling length.
Additionally, note that the various sets of steps disclosed above for using calibration parameter values (in order to adjust or correct measurements of signals from the capacitor(s) 17 so that portions of such measurements related to cabling 87a,b length is at least reduced) may be performed by software or firmware. Alternatively, such sets of steps may be performed via a hardware implementation as one of ordinary skill in the art will understand.
Moreover, it is within the scope of the present disclosure that the disclosed steps for obtaining values for the calibration parameters, and/or for adjusting or correcting the signal measurements from a capacitor(s) 17 so that there is a reduced a dependency on a length of cabling 78a,b should not be interpreted so narrowly that the entire length of the cabling 78a,b must be used. In particular, it is within the scope of the present disclosure that a particular length of cabling could be calibrated for use that may not include the entire length of the cable in use. For example, in some embodiments, calibration that compensates for a majority of the length of installed cabling (e.g., from the multiplexer 70 to the plates of a capacitor 17) may be effective without performing the calibration on, e.g., a short length of cabling between the kiln moisture system 11 and the multiplexer. In some embodiments, it is believed that calibrating for at least approximately 90% of the entire cabling 78a,b length is effective for appropriately estimating moisture content values in the drying lumber Additionally, if some portion of the cabling 78a,b has a trivial total capacitance regardless of the cause, then the length of this portion of the cabling 78a,b may not be used in determining the calibration parameter values, and/or in adjusting capacitor 17 signal measurements to reduce cabling 78a,b length dependencies.
Additionally, the present disclosure should not be interpreted so narrowly that the entire length of the cabling 78a,b must be coaxial cable rather than another type of cabling. For example, in some embodiments, small lengths of regular conducting wire may be substituted for coaxial cable without substantially impacting the lumber moisture content determining methods disclosed herein. For instance, a short section of stainless steel wire rope or standard copper wire may be attached between the plate of a capacitor 17, and the coaxial center wire without substantially impacting the effectiveness of the calibration parameters, and without substantially impacting the effectiveness of the adjusting/correcting of the signal measurements from a capacitor(s) 17 so that there is a reduced a dependency on the length of the cabling 78a,b.
While various embodiments of the present invention have been described in detail, modifications and adaptations of these embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims. In particular, the calibration methods disclosed hereinabove can be employed in a variety of moisture measurement system 10 embodiments, including but limited to a voltage divider circuit, a bridge circuit, or an LCR data acquisition circuit.
The present application claims the benefit of U.S. patent application Ser. No. 11/617,910 filed on Dec. 29, 2006; the entire disclosure of which is hereby fully incorporated by reference as part of the present application.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11617910 | Dec 2006 | US |
| Child | 12683023 | US |
