Abstract:
An in-kiln moisture measurement system using in-kiln measurement electronics to produce wood moisture content readings virtually unaffected by temperature variations. The system comprises electrodes in communication with wood in a kiln, a per kiln unit (PKU) containing signal processing circuitry, and a sending unit with a circuit comprised of redundant half-circuits that compensate for the effects of temperature variations in the electronic components. One half-circuit measures moisture content of the wood; the other half-circuit measures a reference load. Matched characteristics of the transistors in each circuit ensure that each half-circuit&#39;s readings drift at about the same rate and in the same direction when experiencing temperature changes. An automatic tuning unit can be used to automatically adjust properties of the PKU&#39;s circuitry and compensate for other capacitances in the system.

Description:
TECHNICAL FIELD  
       [0001]     The present application relates to in-kiln moisture measurement systems.  
       BACKGROUND  
       [0002]     Lumber is often dried in a kiln after it is milled in order to remove moisture from the wood and prepare it for use. When drying wood in a kiln, it is important to know how much moisture remains in the wood. Lumber that is not dried long enough and retains excess moisture may split or warp. Conversely, lumber that is overdried, or dried too quickly, may also split or develop other defects. Additionally, overdrying incurs unnecessary energy costs. Accurate lumber moisture content information also allows kiln operators to: adjust the kiln schedule according to drying needs; shut down the kiln when the lumber reaches a specified condition; and perform zone control.  
         [0003]     One method of measuring and monitoring lumber moisture content involves contacting the lumber with a pair of electrodes and calculating the impedance or resistance of the wood (which varies with the moisture content) using a moisture detection circuit. This can be done, for example, with a handheld meter that has two pins that serve as electrodes. Another type of meter features metal plates which are placed very close to the wood. One example of a moisture detection circuit is described in Wagner, “Moisture Detection Circuit,” U.S. Pat. No. 5,486,815, which is incorporated herein by reference.  
         [0004]     In-the-kiln instrumentation automates obtaining moisture content readings, thus saving manpower and time. Sensors (electrodes) are placed in constant contact with (or very near to) the wood while it is in the kiln, and the measurements are sent to a computer outside of the kiln.  
         [0005]     However, in-the-kiln instrumentation must withstand the extreme environment of the kiln. Temperatures in kilns may vary widely, ranging from about 70 degrees to 300 degrees Fahrenheit. This temperature fluctuation complicates the electronic measurement of moisture content because the properties of electronic components change or “drift” as the temperature changes. For example, the base-emitter voltage of a transistor may decrease as the operating environment temperature increases, thus affecting the precision of analog circuits.  
         [0006]     Additional impedances introduced by the measuring system complicate obtaining an accurate reading. For example, cables used to connect probes to a reader have a given capacitance which must be taken into account. This is complicated by the fact that cable capacitance is partially a function of cable length; thus cables of different lengths can have different capacitances.  
         [0007]     It is common for a moisture sensor circuit to be tuned after installation. This typically involves simultaneously adjusting the zero offset and the gain of the circuit. In some systems the zero offset and gain are each controlled by a potentiometer, and a human being uses the potentiometers to adjust the circuit against a known, stable impedance. This process may require several iterations before the sensor is tuned.  
       SUMMARY  
       [0008]     An in-kiln moisture measurement system described herein uses in-kiln measurement electronics to produce moisture content readings virtually unaffected by temperature variations. The system comprises a personal computer which receives data from a per kiln unit (PKU). The PKU may be mounted above the kiln, on the outfeed side, for example. The PKU features one or more probe boards, which contain electronics for receiving signals from a sending unit. The sending unit receives signals from probes that are in contact with wood in the kiln.  
         [0009]     The sending unit contains a circuit comprised of redundant half-circuits that compensate for the effects of temperature variations in the electronic components, including cables. One half-circuit acts as a moisture detector, and the other acts as a reference circuit. The two largely identical half-circuits each have matched transistor pairs, which ensure that the circuit readings drift by the same amount and in the same direction when exposed to temperature changes.  
         [0010]     The moisture detector half-circuit reads a signal from the probes. The load of the reference half-circuit comes from a fixed reference capacitor. The reference capacitor is chosen for its low susceptibility to temperature drift. Signals generated by each half-circuit are sent to the PKU and processed in the probe board. Redundant circuitry is also found in the probe board. Calibration of the system to moisture content is then accomplished through software.  
         [0011]     The system may be tuned using an Automatic Tuning Unit (ATU). The ATU attaches to a sending unit inside the kiln and interacts with the PKU to adjust the zero offset and gain of the system. This allows the system to provide normalized sensor value output. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  provides an overview of a moisture measuring system, depicting components inside a kiln.  
         [0013]      FIG. 1B  provides an overview of a moisture measuring system, depicting components both inside and outside the kiln.  
         [0014]      FIG. 2  shows a schematic diagram of circuitry inside the sending unit.  
         [0015]      FIG. 3  is a block diagram of a probe board contained within the PKU.  
         [0016]      FIGS. 4A and 4B  show a schematic diagram of probe board circuitry.  
         [0017]      FIG. 5  shows an exemplary embodiment of a handheld Automatic Tuning Unit.  
         [0018]      FIG. 6  shows a block diagram of the main circuit of an Automatic Tuning Unit.  
         [0019]      FIG. 7  shows a schematic diagram of a relay driver circuit.  
         [0020]      FIG. 8  shows a schematic diagram of a detector circuit.  
         [0021]      FIG. 9  shows a schematic diagram of a transmitter circuit.  
         [0022]      FIG. 10  shows a flowchart diagram of the automatic tuning process.  
     
    
     DETAILED DESCRIPTION  
       [0023]     One embodiment of a moisture measuring system  100  is shown in  FIG. 1A . This figure depicts the partial interior of a kiln  110 , including the system components inside the kiln  110 . A stickered lumber unit  115  sits inside the kiln  110 . Probe strips  118 , preferably made of stainless steel, are in contact with wood of the stickered lumber unit  115 . Attached to the probe strips  118  are probe clamps  130 . Mounted on the wall of the kiln  110  is a sending unit  120 , and the probe clamps  130  are connected to the sending unit  120  through wires  135 . The wires  135  may attach to the sending unit  120  through studs (not shown) on the sending unit  120 . These can allow for the wires  135  to detach easily from the sending unit  120  if, for example, a piece of lumber or other object falls on the wires  135 . The components inside the kiln  110  are made of materials that can withstand the extreme temperatures (up to 300 degrees F.) that occur during the kiln&#39;s operation. A number of suitable materials exist, but by way of example, the probe clamps  130  may have a body of heavy duty stainless steel or aluminum, and a stainless steel spring and teeth; the sending units  120  may be housed in containers made of 16 gauge stainless steel; and the wires  135  may also be made of stainless steel. The kiln  110  may feature a walkway  112  to allow for easy access to the lumber unit  115  or the sending unit  120 .  
         [0024]     A fuller view of the components of the moisture measuring system  100  is shown in  FIG. 1B . In this figure the kiln  110  is represented by a broken line. If desired, the kiln  110  can include multiple sending units  120 ( a - c ), each with corresponding wires  135 ( a - c ), probe clamps  130 ( a - c ) and probe strips  118  (not shown in this view).  
         [0025]     Components outside the kiln  110  can include a per kiln unit (PKU)  140  which is connected to a computer  150 , possibly via an RS-422 serial port. Signals travel between the PKU  140  and the sending units  120  via sensor cables  137 ( a - c ). The sensor cables  137  may be protected while in the kiln  110  by conduits or protective channels. The computer  150  can execute software for analyzing or storing data recorded inside the kiln  110 . The system  100  may also include means, such as an alarm and a relay, for shutting down the kiln  110  upon satisfying certain conditions, for example, when lumber drying in the kiln  110  reaches a specified moisture content level.  
         [0026]      FIG. 2  depicts a circuit  200  found in each sending unit  120 . The circuit  200  comprises two half-circuits  210  and  230  which are more or less identical. These identical halves allow for measuring the moisture in the wood in the kiln  110  while compensating for temperature-induced instability of electronic parts in the circuit  200 . Half-circuit  210  serves as a moisture detector (by measuring the impedance of the wood), while half-circuit  230  aids in compensating for temperature-induced drift in half-circuit  210  (by measuring a fixed capacitance). Each half-circuit  210  and  230  features dual transistors,  212 ( a - b ) and  232 ( a - b ), respectively. The dual transistors  212  and  232  are “matched pairs,” i.e., the individual transistors have very nearly the same electrical properties. Ideally, each pair of transistors  212  and  232  is in a solitary package, which helps ensure that the transistors are matched. Generally uniform properties among the transistors  212  and  232  ensure that when the transistors drift due to changes in temperature, the half-circuits both drift in the same direction and by the same amount. Each transistor opposes and negates the temperature-induced drift of the other transistor in the pair. This arrangement also helps cancel drift in the wires  135 , as well. The dual transistors  212  and  232  are selected in part for their ability to withstand the high temperatures of the kiln  110 . Each transistor  212  and  232  is wired with its base tied to its collector, allowing the transistor to function as a diode.  
         [0027]     The load for the half-circuit  210  is the signal PTX  240 , which is provided by one of the probe strips  118  contacting wood inside the kiln  110 . Half-circuit  210  measures the moisture content of the wood using PTX  240  and the signal PGND  250 , which serves as a ground signal for the circuit  200  and is also provided by a probe strip  118 . The load for half-circuit  230  is provided by a reference capacitor  260 . This capacitor is selected for its electrical stability over a given temperature range, allowing it to provide a consistent capacitive load during operation of the kiln  110 . In one embodiment, resistor  211  is of a smaller value than the corresponding resistor  213 . This helps ensure that half-circuit  210  drifts the same amount as half-circuit  230  (which has the capacitive load). Both half-circuits  210  and  230  are fed by the signal TX  270 , which is provided by the PKU  140 . TX  270  is an AC signal that gives the circuit  200  an inherent potential through excitation. TX  270  may vary in amplitude and frequency, but in one embodiment the signal has a frequency of 1 MHz and an amplitude of about 18 V. Analog DC signals R  265  (a reference signal) and M  267  (a response to moisture content in the wood) are sent to the PKU  140  for processing. While both R  265  and M  267  change with temperature, the nature of the circuit  200  ensures that they drift in the same direction and at about the same rate.  
         [0028]     Another element of the circuit  200  is a temperature sensor  280 . In one embodiment the sensor  280  is a current-loop-type sensor where the output current T  282  is proportional to the temperature of its case. Supply voltage +V  284  (in one embodiment, about 15 V) is provided by the PKU  140 .  
         [0029]      FIG. 3  depicts a block diagram of a probe board  300 . The probe board  300  contains circuitry for processing signals from the sending unit  120 . One or more probe boards  300  are contained within the PKU  140 . Through a bus  310 , signals TX  270  and +V  284  travel to the sending unit circuit  200 , and signals M  267 , R  265  and T  282  are received from the sending unit circuit  200 . Signals TX  270  and +V  284  are generated by clock and cable driver circuitry  320 . Signals from the sending unit circuit  200  are fed into buffers  330 ,  335  and  337  which eliminate coupling artifacts. The probe board  300  is further comprised of: divider circuits  340  and  345 ; a microcontroller  350 ; inverter circuits  360  and  365  with zero adjust; gain adjust circuits  370  and  375 ; and a scaling resistor  380 .  
         [0030]     Signals obtained by the sending unit circuit  200  are processed in the probe board  300  using the microcontroller  350 . The microcontroller  350  may be one such as the PIC16C773 from Microchip Technologies, Inc., which includes a 12-bit A/D converter. After digitizing signals M  267  and R  265 , the microcontroller  350  can calculate the difference between them.  
         [0031]     As seen in  FIG. 3 , the principle of redundancy is also applied in the probe board  300 , where M  267  and R  265  are processed in a similar manner. This helps compensate for temperature drift in the probe board  300 . M  267  enters a buffer  330 , which has a very high input impedance. This allows M  267  to be sampled without disturbing it. M  267  then enters a voltage divider circuit  340 . Because the amplitude of M  267  varies with the length of the wire  135 , it is useful to be able to adjust M  267  by means of the voltage divider  340 . M  267  enters an inverting unity gain amplifier  360  with zero adjust. A higher moisture content in the lumber causes a weaker signal M  267 . Inverting M  267  means that the amplitude of inverted M  267  increases as the moisture content increases. Before entering the microcontroller  350 , M  267  also passes through a gain adjust amplifier  370 , which is controlled by a digital potentiometer.  
         [0032]     The signal R  265  travels a similar path in the probe board  300 , passing through a buffer  335 , a voltage divider  345 , an inverting unity gain amplifier  365  with zero adjust, and a gain adjust amplifier  375 , which is controlled by a digital potentiometer. T  282  is coupled to a scaling resistor  380  and passes through a buffer  337  before reaching the microcontroller  350 .  
         [0033]      FIGS. 4A and 4B  together display a detailed schematic of a possible implementation of the block diagram of  FIG. 3 . Signal connections that should be considered continuous between  FIGS. 4A and 4B  (e.g., M′) are indicated with a triangle and a signal name at the point of common connection. Signals M  267  and R  265  are coupled to filter circuits  402  and  424 , respectively, possibly consisting of a capacitor and a resistor in parallel. The output current T  282  is coupled to a voltage conversion resistor  442  and a filter capacitor  444 . Buffers  330 ,  335  and  337  are implemented with op amps. Also shown in  FIG. 4  are additional buffers  412  and  432  between the voltage divider and inverting amplifier stages. These isolate the signal processing stages of the circuit  300 . M  267 , R  265  and T  282  may be measured at ports  406 ,  428  and  448  respectively.  
         [0034]     Voltage dividers  340  and  345  are implemented with digitally controlled potentiometers  407  and  415 , respectively. The present embodiment uses digital potentiometers with 256 possible positions, which allow for a fine level of tuning. Although the connections are not shown in  FIGS. 4A and 4B , the digital potentiometers in this embodiment are controlled by the microcontroller  350 . Inverting amplifiers  360  and  365  are implemented with op amps. The zero adjust for each amplifier is comprised, respectively, of buffer  418  and digital potentiometer  421 ; and of buffer  436  and digital potentiometer  425 . As the amplitudes of R  265  and M  267  are already adjusted by the voltage dividers, adjustments using the zero adjusts are generally very fine. Gain adjust amplifiers  370  and  375  are implemented, respectively, by op amp  422  and digital potentiometer  471 ; and by op amp  438  and digital potentiometer  472 .  
         [0035]     Additional features in  FIG. 4B  include diagnostic LEDs  410  a clock generator  420 , and an amplifier transistor  429 .  
         [0036]     The system  100  may also be tuned, perhaps after it is installed, for example. The tuning process allows for normalization of a circuit  200  in one or more sending units  120 , enabling the system  100  to provide normalized sensor value output. In this case, “normalization” means that, regardless of installation details such as cable length, and regardless of manufacturing details such as component value tolerances, the circuits  200  in various sending units  120  will return essentially the same reading when subjected to the same load of moisture. The tuning process allows for the output of the electronics of the system  100  (sometimes called the “overall gain” of the system) to be scaled such that this output can represent the entire possible range of moisture values. Additionally, the tuning process compensates for the “inherent gain” of the system  100 , which may be influenced by capacitances in the cables  137  of  FIG. 1 , for example.  
         [0037]     Tuning is carried out by means of an Automatic Tuning Unit (ATU)  500  shown in  FIG. 5 , which may be implemented as a device separate from any other element of the system  100 , possibly as a handheld unit  505 . The ATU  500  uses electrodes  510  to attach to studs (not shown) on a sending unit  120 . This allows the ATU  500  to communicate with the probe board  300  in the PKU  140 . It may also feature a set of status indicators  520 .  
         [0038]      FIG. 6  depicts the main circuit  600  of the ATU  500 . A microcontroller  620  controls three relay driver circuits  640 ( a - c ), which in turn control relays  630 ( a - c ). A low-impedance load is provided by capacitor  603 , and a high impedance load is provided by capacitor  607 . Sample values for these capacitors may be 82 pF and 270 pF, respectively. The difference between the high impedance load and the low impedance load is knows as the “span.” Capacitors  603  and  607  are selected in part for their accurate tolerances and their temperature stability. The main circuit  600  is electrically connected to the sending unit  120  through TX  610  and GND  611 , which are physically attached to the sending unit  120  through electrodes  510 . A transmitter circuit  604  and a detector circuit  605  allow the ATU  500  to communicate with the probe board  300 . In one embodiment, the status indicators  520  are comprised of LEDs and include a done indicator  652 , a battery low indicator  654 , and a power indicator  656 .  
         [0039]     Via the relay driver circuits  640 ( a - c ) (working with their corresponding relays  630 ( a - c ), respectively), the microcontroller  620  controls the input and output of the main circuit  600 . For example, the microcontroller  620  uses relay  630 ( a ) and relay driver circuit  640 ( a ) to switch to the high-impedance load provided by capacitor  607 ; or, it uses relay  630 ( b ) and relay driver circuit  640 ( b ) to switch to the low-impedance load provided by capacitor  603 . The microcontroller  620  activates relay control circuit  640 ( c ) and relay  630 ( c ) to connect the transmitter circuit  604  to the sending unit circuit  200 . Control circuit  640 ( c ) is usually not activated unless the ATU  500  is sending a message to the probe board  300 .  
         [0040]     The detector circuit  605  allows the ATU  500  to receive messages from the probe board  300 . In one embodiment, the detector circuit  605  outputs a voltage level corresponding to a logic ‘1’ or ‘0’ whenever the probe board  300  sends a signal through TX  610 . This voltage is converted to a logic level in the microcontroller  620 , which may contain an integrated A/D-converter. The microcontroller  620  may be programmed to recognize a signal longer than a predetermined length and assume that the long signal is not part of a message. By accumulating I&#39;s and O&#39;s, the ATU  500  can decipher various commands. A similar detector is employed in the probe board  300  to detect messages from the ATU  500 .  
         [0041]      FIG. 7  shows a schematic diagram of a relay driver circuit  640 .  FIG. 8  shows a schematic diagram of a detector circuit  605 .  FIG. 9  shows a schematic diagram of a transmitter circuit  604 .  
         [0042]     A flowchart of the automatic tuning process  1000  appears in  FIG. 10 . Step  1010 , in which the ATU  500  sends a “hello” message (i.e., a signal or code signifying initial contact) to the probe board  300 , occurs after the ATU  500  is connected to the sending unit  120  and turned on. In step  1020 , the probe board  300  deciphers the “hello” message and then sends a “set low impedance” command (step  1030 ). Accordingly, the main circuit  600  uses relay  630 ( b ), relay driver circuit  640 ( b ) and microcontroller  620  to switch to the low-impedance load provided by capacitor  603 . With this low-impedance load on the sending unit  120 , in step  1040  the probe board  300  adjusts the zero and gain of elements in the probe board  300  using digital potentiometers, for example. For example, inverter circuits  360  and  365  and gain adjust circuits  370  and  375  of  FIG. 3  may be adjusted according to a predetermined specification. In step  1050 , the probe board  300  sends a “set high impedance” command to the ATU  500 . The main circuit  600  uses relay  630 ( a ), relay driver circuit  640 ( a ) and microcontroller  620  to switch to the high-impedance load provided by capacitor  607 . With a high-impedance load on the sending unit  120 , in step  1060  the probe board  300  again adjusts the zero and gain of elements in the probe board  300 . Steps  1030  through  1060  are repeated (step  1070 ) until the system  100  is tuned within a desired set of parameters. At this point, the probe board  300  sends a “done” command to the ATU  500  (step  1080 ). The ATU  500  then activates the done indicator  652  (step  1090 ).  
         [0043]     The communications protocol used by the probe board  300  and the ATU  500  may include a means by which the ATU  500  echoes back to the probe board  300  a command that the ATU  500  receives. This allows the probe board  300  to confirm that a command has been received and executed. The protocol may also include a means for the probe board  300  to determine that the ATU  500  is not functioning properly or that an error has occurred. A message indicating such a state may be sent from the PKU  140  to the computer  150 , which may notify a human operator of the malfunction. The error condition may also be indicated using the diagnostic LEDs  410  of the probe board  300  shown in  FIG. 4B . One of the LEDs  410  may be dedicated to indicating that the system  100  is properly tuned. Additionally, the protocol may include means by which the ATU  500  may determine that an error has occurred. For example, if the ATU  500  does not receive a message from the probe board  300  within a predetermined time interval, the ATU  500  will indicate an error status, possibly by displaying a blinking pattern on the battery low indicator  654 .  
         [0044]     The process  1000  allows the moisture response curve of the circuit  200  to generally match a desired moisture response curve. Additionally, at the time of tuning, reference values for the signals R  265  and M  267  may be stored so that drift may later be accounted for by comparing present values with the reference values.  
         [0045]     Once the system  100  has been calibrated to provide normalized sensor value output, calibration for moisture content in the wood can be accomplished by software, such as the MC4000 Software available from Wagner Electronic Products, Inc., running in the computer  150 .  
         [0046]     Having described and illustrated the principles of the system with reference to a preferred embodiment thereof, it will be apparent that the system can be modified in arrangement and detail without departing from such principles. In view of the many possible embodiments to which the principles of the system may be put, it should be recognized that the detailed embodiment is illustrative only and should not be taken as limiting the scope of the system. Accordingly, I claim as the invention all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.