Abstract:
In many applications, where the level of a liquid needs to be monitored (and possibly controlled), it is not necessary to provide a continuous (analog) liquid level signal, but limit sensing to one (or two) discrete levels over a small level range. In this case, a ceramic substrate sensor with discrete thermistor/heater pairs provides necessary information to control the level of the fluid. Protection of the substrate from liquid (and potential contaminants) is accomplished by coating the surface with an inert glass (“glaze”) layer and/or polymer layer (e.g., “Parylene”). Packaging of the coated substrate is accomplished by protective base, epoxy, and slosh shield. Cost can be significantly reduced over a prior art multiple thermocouple based sensor design, since discrete electronic components are avoided or reduced. Also, the number of pins, and therefore electrical connections, can be significantly reduced.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    The present invention relates generally to devices used to measure the level of a liquid within a vessel or container and more specifically to such a device formed of a thick film (e.g., ceramic) substrate bearing (e.g., screen printed) heaters and thermistors. 
         [0002]    There exists a wide variety of applications in which it is desirable to monitor to some degree the level of a liquid within a vessel or the like. Such applications may range from monitoring various fluid levels within a motor vehicle or internal combustion engine or fuel tank to monitoring oil levels within a pump or compressor or even water levels within a tank such as for a recreational vehicle or boat. In each of these applications it is desirable that the liquid level sensor be capable of providing a reliable accurate indication of the liquid level over an extended period of time without requiring periodic maintenance. In many applications the level sensor must be capable of enduring various levels of vibration, heat or other hostile environmental elements as well as space limitations. Additionally, in some applications utilizing sealed vessels such as hermetic compressors, it is desirable to minimize the number of penetrations such as electrical leads through the walls of the vessel to reduce the potential for leakage. 
         [0003]    Various types of devices have been developed over the years for sensing such levels of liquids. Such devices range from the extremely simple float arrangement commonly employed in fuel tanks to more complex electrical capacitance type sensors as well as microprocessor based thermocouple sensors. While operable, these various types of sensors have had varying drawbacks depending upon the particular application such as excessive space requirements for accommodating moveable floats and associated linkage, relatively high costs to manufacture, complexity of circuitry required to generate a level indicating signal, susceptibility to errors from extended or extraneous electrical noise, lack of resistance to hostile environmental elements, etc. 
         [0004]    In many applications, where the level of a liquid needs to be monitored (and possibly controlled), it is not necessary to provide a continuous (analog) liquid level signal, but limit sensing to one (or two) discrete levels over a small level range. In this case, a ceramic substrate sensor with discrete thermistor/heater pairs provides necessary information to control the level of the fluid. Protection of the substrate from liquid (and potential contaminants) is accomplished by coating the surface with an inert glass (“glaze”) layer and/or polymer layer (e.g., “Parylene”). Packaging of the coated substrate is accomplished by protective base, epoxy, and slosh shield. Cost can be significantly reduced over a prior art multiple thermocouple based sensor design, since discrete electronic components are avoided. Also, the number of pins, and therefore electrical connections, can be significantly reduced. 
         [0005]    Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a block diagram illustrating a liquid level sensor. 
           [0007]      FIG. 2  is an exploded perspective view illustrating a liquid level sensor probe. 
           [0008]      FIG. 3  is a plan view illustrating a layout of liquid level sensor on a ceramic substrate. 
           [0009]      FIG. 4  is a plan view similar to  FIG. 3 , but illustrating an alternative embodiment of a liquid level sensor. 
           [0010]      FIG. 5  is a plan view illustrating the liquid level sensor of  FIG. 3  with a RAST connector subassembly. 
           [0011]      FIG. 6  is a circuit diagram illustrating an electrical circuit for a dual heater/thermistor pair liquid level sensor with ambient temperature sensor. 
           [0012]      FIG. 7  is a perspective view of a base of a liquid level sensor probe switch case. 
           [0013]      FIG. 8  is a perspective view of a cap of a liquid level sensor probe switch case. 
           [0014]      FIG. 9  is a graphical representation illustrating liquid level sensor response at twenty degrees Celsius liquid temperature. 
           [0015]      FIG. 10  is a graphical representation illustrating liquid level sensor response at fifty degrees Celsius liquid temperature. 
           [0016]      FIG. 11  is a graphical representation illustrating liquid level sensor response at seventy-five degrees Celsius liquid temperature. 
           [0017]      FIG. 12  is a set of flow diagrams, including  FIGS. 12A ,  12 B, and  12 C, illustrating a method of operation for a liquid level sensor. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring now to the drawings, the liquid level sensor can be formed of discrete heaters and thermistors that are screen-printed onto a ceramic (Al 2 O 3 ) substrate, such that uniform heating is accomplished on its surface. Since the ceramic material is very thermally conductive, localized fluid temperature monitoring (and compensation) may not be possible. Therefore, fluid temperature is monitored at a location on the substrate which is always immersed in the fluid (such as at the bottom of a reservoir), and interpreted as being representative of average ambient temperature. 
         [0019]      FIG. 1  generally depicts the major components of a liquid level sensor. The sensor  50  generally includes a probe  52  coupled with a control module  54 . An I/O module can also be included in the sensor  50 . As used in the description, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinatorial logic circuit, and/or other suitable components that provide the described functionality. 
         [0020]    The probe  52  has no moving parts. The probe  52  is typically disposed in a fluid environment  62  for detecting level of a liquid  64 . When it is subjected to presence of the fluid  64 , the probe  52  experiences changes in its condition represented by signal  58  (i.e., a voltage). 
         [0021]    The signal  58  from the probe  52  is continuously monitored by the control module  54 . The control module  54  processes the signal  58 , and generates an output  60  that is indicative of a fluid level. The I/O module  56  provides a means by which the sensor  50  can communicate its output  60  to other device(s) or a user. 
         [0022]    It is presently contemplated that the liquid level sensor  50  can be employed in a household appliance, such as a washing machine, to monitor liquid level in a clothes washing or dishwashing container. 
         [0023]    One embodiment of the probe  52  for use in the sensor  50  is shown in  FIG. 2 . The probe  52  generally comprises a circuit board formed of a ceramic substrate  200  and connector assembly having interconnect P 1 , both of which are inserted in a switch case formed of a polygonal base  300  and a cap  306 . The base  300  has threads  304 , and an o-ring  302  for facilitating installation in the bottom of a liquid container without causing a leak in the container. The circuit board and connector assembly is housed at one end in the switch case. The cap  306  covers the opposite end of the circuit board and connector assembly. Cap  306  serves as a protective shield for preventing sloshing fluid from causing inaccurate liquid level readings. Additionally, cap  306  prevents damage to the ceramic substrate during installation and operation. The ceramic substrate  200  of the circuit board and connector assembly can either be insert-molded into the probe base  300 , or potted using epoxy. The base  300  has one or more holes through which pins of interconnect P 1  are exposed. 
         [0024]    Turning now to  FIG. 3 , the ceramic substrate  200  with 5-pin interconnect P 1  has heaters  202 A and  202 B arranged on a solder side  200 A of the substrate according to the dimensions shown: L 204 ; L 206 ; L 208 ; L 210 ; L 212 ; L 214 ; and L 216 . As shown at  200 B, the substrate  200  has a thickness L 218 . Located on an opposite side  200 C of substrate  200  are thermistors NTC 1 , NTC 2 , and NTC 3 . The leads of interconnect P 1  and the thermistors NTC 1 , NTC 2 , and NTC 3  are arranged according to the dimensions shown: L 220 ; L 222 ; L 224 ; and L 228 . 
         [0025]    It should be noted that a suitable ceramic material for the substrate  200  is Coorstek&#39;s 96% Al 2 O 3  ceramic or equivalent, and that a suitable NTC thermistor paste for the thermistors NTC 1 , NTC 2  and NTC 3  is Electro Science Laboratories&#39; ESL-2115 or equivalent. 
         [0026]    A protective coating covers the entirety of the side  200 C and  200 A of the substrate  200 , except the portion defined by L 226 . The coating is electrically insulating and thermally conductive. The coating may comprise layer(s) of glaze and Parylene. Parylene, which has water shedding properties similar to Teflon, protects the ceramic substrate, and may be applied by vacuum deposition over the glaze. 
         [0027]    In some embodiments, conductive pass-thrus (e.g., plated holes and/or “vias”) (not shown) are formed by perforating the substrate  200  to allow for electrical communication between both sides of the substrate  200 . 
         [0028]    Turning now to  FIG. 4 , in an additional or alternative embodiment, the sensor substrate  200  is similar to the embodiment described above with reference to  FIG. 3 . In particular, heaters  202 A and  202 B are arranged on solder side  200 A of the substrate  200  and thermistors NTC 1 , NTC 2 , and NTC 3  are arranged on opposite side  200 C of the substrate  200  according to the dimensions L 204 -L 216 , L 220 , L 224 , and L 228 . In addition, as shown at  200 B, pass-thrus  400  are formed in the substrate  200  to enable electrical communication between both sides of the substrate  200 . In particular, the pass-thrus  400  are arranged according to the dimensions shown: L 401 ; L 402 ; L 404 ; L 406 ; L 408 ; and L 410 . 
         [0029]    A protective coating is also applied to the substrate  200 . One skilled in the art will readily appreciate that, given the arrangement of the pass-thrus  400  described above, the protective coating should be applied to a region of the substrate  200  in such a manner that the coating does not cover the pass-thrus  400 . For example, the coating can be applied to the substrate  200  in order to avoid covering a pass-thru region  416  defined according to dimension L 412 . 
         [0030]    Turning now to  FIG. 5 , the substrate  200  described above with reference to  FIG. 4  is suitable for connection with a rigid (e.g., “RAST” standard) connector sub assembly  500 A-C, which connects to the substrate through the pass-thrus  400 . The resulting assembled substrate and connector system has the following dimensions: L 212 ; L 502 ; L 504 ; L 506 ; L 508 ; L 510 ; L 512 ; L 514 ; and L 516 . It should be noted that a suitable RAST connector is Molex&#39;s p/n 90861-1001. 
         [0031]    Referring now to  FIG. 6 , a liquid level sensor circuit  100  includes two heater/thermistor pairs. Circuit  100  also includes one ambient liquid temperature measuring thermistor NTC 1 . In some embodiments, the thermistors may use a screen-printable paste such as Electro Science Laboratories&#39; ESL-2115 or equivalent. Alternately, NTC 1 -NTC 3  could be discrete SMT components soldered to the substrate. The heater/thermistor pairs are composed in part of heaters, including heater HTR 1  and heater HTR 2 , each of which can have a resistance of about fifty ohms (+/−1%). In some embodiments the heaters may use a screen-printable paste such as Sumitomo&#39;s R-U sr. or equivalent. Alternately, the heaters could be discrete SMT components soldered to the substrate. Heater HTR 1  is positioned to operably heat a lower liquid temperature measuring threshold thermistor NTC 2 . Similarly, heater HTR 2  is positioned to operably heat an upper liquid temperature measuring threshold thermistor NTC 3 . Each of the thermistors can have a resistance of about fifty kilo ohms (+/−2%). Five pins P 1  are connected to the circuit components, with a series connection of the heaters being connected in parallel with each of the thermistors. 
         [0032]    In operation, sensed readings from the thermistors are compared to assess liquid level. If the liquid level is too high or too low, then the readings from the thermistors are all the same. The reading from thermistor NTC 1  can be compared to either or both of the readings from the other thermistors NTC 2  and NTC 3 . 
         [0033]    Turning now to  FIG. 7 , the switch case base  300  is shown in greater detail. The switch case base  300  provides a receptacle into which the circuit board and connector assembly  201  can be installed. The switch case base  300  generally comprises a polygonal portion  301 , a threaded portion  304  and an O-ring ( FIG. 2 ). It should be noted that a suitable material for the switch case is Chevron-Phillips&#39; Ryton R-4-220 BLK or equivalent. 
         [0034]    Turning now to  FIG. 8 , the liquid level sensor cap  306  is shown. It should be noted that a suitable material for the cap is Chevron-Phillips&#39; Ryton R-4-220 BLK or equivalent. 
         [0035]    Turning next to  FIGS. 9-11 , the normalized resistances of the thermistors can be observed to change over time as a tank in which the liquid level sensor is installed is filled with liquid. With particular reference to  FIG. 9 , with the liquid temperature GTEMP 20  at approximately 20° C., the normalized resistances of the thermistors are equal when the tank is dry and the heaters have been energized long enough for the substrate to reach a steady state temperature. As the tank is filled with liquid, the normalized resistance GNTC 1 T 20  measured with the reference thermistor, the normalized resistance GNTC 2 T 20  measured with the lower threshold thermistor, and the normalized resistance GNTC 3 T 20  measured with the upper threshold thermistor all begin to rise as those thermistors become immersed. Since the reference thermistor is installed at a lowest point in the tank, it becomes immersed first, and its normalized resistance GNTC 1 T 20  begins to rise first. Then, the lower threshold thermistor becomes immersed, followed by the upper threshold thermistor. It should be noted that the upper and lower threshold thermistors are heated by the heaters to a temperature above the temperature of the liquid, so their normalized resistances do not rise to match that of the reference thermistor when the tank is full. 
         [0036]    Referring now generally to  FIGS. 10 and 11 , the normalized resistances fall instead of rising when the temperature of the liquid is significantly higher than 20° C. For example, with the liquid temperature GTEMP 50  at approximately 50° C., the normalized resistances GNTC 1 T 50 , GNTC 2 T 50 , and GNTC 3 T 50  decrease as the tank fills. As another example, with the liquid temperature GTEMP 75  at approximately 75° C., the normalized resistances GNTC 1 T 75 , GNTC 2 T 75 , and GNTC 3 T 75  decrease as the liquid fills the tank. In each of these examples, since the heat of the liquid is high and complements the heat provided by the heaters, the normalized resistances measured by the threshold thermistors fall to match or even fall below the normalized resistance measured by the reference thermistor. 
         [0037]    With knowledge of sensor probe behavior under operation conditions, an end user of the liquid level sensor probe can design their system in which the sensor probe is installed according the needs of the application in question. In particular, a control module  54  ( FIG. 1 ) for use with the probe  52  can determine whether liquid level is too high or too low by comparing the sensed readings from the thermistors. In particular, the reading from thermistor NTC 1  ( FIG. 3 ) can be compared to either or both of the readings from the other thermistors NTC 2  and NTC 3 . For example, if the liquid level is too high or too low, then the readings from the thermistors are all the same. Therefore, the output from the control module  54  ( FIG. 1 ) can be binary in nature, indicating whether the liquid in the container is: (1) either too high or too low; versus (2) neither too high nor too low. This output can be provided to another module that controls filling and emptying of liquid in the container. This other module can then combine this information with its own a priori knowledge regarding whether the container is in the process of being filled or in the process of being emptied to determine whether to begin filling, stop filling, begin emptying, or stop emptying the tank. Alternatively or additionally, control module  54  can receive the knowledge about the current container filling mode, perform the combination, and output a trinary signal indicating whether container is empty, full, or in between. Alternatively or additionally, module  54  can output a quaternary signal indicating whether the container is full, empty, partially filled and still filling, or partially empty and still emptying. Further, control module  54  can alternatively or additionally be capable of deciding whether to fill or empty the container by receiving input from a user and/or executing an automated tank filling and/or emptying process. In this case, control module  54  can output a signal controlling one or more valves or other machinery for filling the liquid container in which the probe  52  is installed. Moreover, control module  54  can take temperature of the liquid into account when interpreting the signals from the thermistors. With test data on hand, the temperature of the liquid can be estimated by the signal from the reference thermistor when the container is full. Alternatively or additionally, a priori knowledge of the temperature can be applied. 
         [0038]    Turning finally to  FIG. 12 , and referring generally to  FIGS. 12A ,  12 B, and  12 C, a method of operation for a liquid level sensor begins by heating heaters of the probe at step  1200  and receiving signals from thermistors of the probe at step  1202 . Then, at decision step  1204 , the signal from the reference thermistor is compared to each of the signals from the threshold thermistors, and the results (e.g., absolute values) compared to a threshold. It should be readily understood that different thresholds can be employed for different liquid temperatures. It should also be readily understood that approximate liquid temperature can be known in some cases (e.g., washing machines) based on whether a cold cycle or hot cycle is being administered. In any event, decision step  1204  makes a determination whether the container is either full or empty at  1206 , versus neither full nor empty at  1208 . This determination occurs continuously, resulting in a binary signal and/or output indicating whether the container is partially filled, or else one of full or empty. 
         [0039]    In the case of a washing machine, this binary signal can be acted upon at decision step  1210  to determine whether to begin filling the container at step  1212 , or else detect/signal an error at step  1258  and cease operation. Ceasing operation can occur on the basis that the container is detected to be partially filled with liquid at the beginning of the washing operation, and thus avoids overflow. Once the container begins to be filled with liquid at step  1212 , then a signal can be output at step  1214  to indicate that the container is empty. Then, at decision step  1216 , the container is detected to be neither full nor empty when the signals from the thermistors have diverged significantly. Accordingly, the container continues to be filled with liquid at step  1218  until this significant divergence has taken place. 
         [0040]    Once the container is detected to be partially filled, a signal can be output to indicate this state at step  1242 . Then, the container continues to be filled at step  1222  until the container is detected to be either full or empty at decision step  1224 . At this point, the container is full, and a signal can be output to indicate this state at step  1228 . Next, at step  1230 , a wash cycle can be performed that involves operation of an agitator. Thereafter, an emptying process begins by once more attempting to detect that the container is full or empty at decision step  1232 . If so, then emptying of the container is begun at step  1234 . Otherwise, an error is detected and/or signaled at step  1258 , and the process ends. 
         [0041]    Once the container begins to be emptied, the container continues to be emptied at step  1240  until it is determined at decision step  1238  that the container is neither full nor empty. A signal can then be output to indicate that the container is partially empty, and this signal can be different from the signal output at step  1242  to distinguish whether the container is filling or emptying. 
         [0042]    Once the container is partially empty, the container continues to be emptied at step  1244  until a determination is made at decision step  1246  that the container is once again either full or empty. At this point, emptying of the container ceases, and a signal can be output to indicate that the container is empty at step  1250 . Thereafter, the aforementioned steps can be repeated to refill the container at step  1252 , perform a rinse cycle at step  1254 , and empty the container at step  1256 . 
         [0043]    While the process described above particularly references a washing machine application, it should be readily understood that this same process can be applied in various ways, such as filling and/or emptying a gas tank of a vehicle, filling and or emptying a swimming pool or hot tub, or any other filling or emptying process. Moreover, portions of the process described above can be used to monitor but not control liquid level in any container of liquid, whether man made or natural. Thus, while it will be appreciated that the preferred embodiments of the invention disclosed are well calculated to provide the advantages and features above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.