Abstract:
A fluid flow rate sensor includes a detection circuit which generates a first signal corresponding to an output voltage of a bridge circuit, and a second signal corresponding to a fluid temperature. A control module can more accurately and quickly determine the fluid flow rate based on the first signal, the second signal and a look-up table. The look-up table includes a plurality of curves plotted according to data, indicating relationship among the fluid temperature, the output voltage and the fluid flow rate. The fluid flow rate sensor is inherently temperature compensated and has a shorter response time.

Description:
FIELD 
   The present disclosure generally relates to fluid flow rate sensors, and more particularly to thermo-anemometers and their methods of operation. 
   BACKGROUND 
   The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
   Fluid delivery systems may include fluid flow meters to measure the flow rate of the fluid in the fluid delivery systems, and thus to determine the volume of the fluid to be dispensed based on the flow rate. Thermo-anemometers have been commonly used for measuring the fluid flow rate over turbine and/or paddlewheel sensors for their lack of moving parts, which are sensitive to contamination of the fluid. A thermo-anemometer operates based on the principles of heat transfer and typically includes a bridge circuit having a resistive heating element subjected to the stream of fluid flow. As the fluid flow passes over the resistive heating element, the fluid carries away heat, resulting in a temperature drop in the resistive heating element. The flow rate of the fluid can be determined by measuring the heat loss from the heating element to the fluid. 
   In one method, the power to the heating element is increased after a temperature drop occurs in the bridge circuit to bring the heating element back to its starting temperature (constant-temperature type). The increased power gives an indication of the fluid flow rate. Another method involves correlating the voltage drop across the bridge circuit to determine the flow rate (constant-current type). 
   In either method, the conventional thermo-anemometers have limitations in response time. The flow rate can not be accurately measured until the fluid flow, and hence the signals indicative of the fluid flow rate, reach a steady state. Some thermo-anemometers may require a relatively long time period to measure the flow rate of the fluid. 
   Another issue with the thermo-anemometers is the need for temperature compensation and part-to-part calibration. A lower temperature fluid has a greater capacity to remove heat from the resistive heating element than a higher temperature fluid at the same flow rate. Therefore, temperature compensation is generally required. Part-to-part calibration may require expensive software and may be difficult to implement in a manufacturing environment. 
   SUMMARY 
   Several embodiments of the present disclosure provide for fluid flow rate sensors which can more accurately and quickly determine the fluid flow rate, and which are self-calibrated and temperature compensated. In one form, a fluid flow rate sensor for detecting a flow rate of a fluid includes a probe module, a look-up table and a control module. The probe module generates a first signal corresponding to an output voltage and a second signal corresponding to a temperature of the fluid. The look-up table includes empirical data of a relationship among the output voltage of the detection circuit, the temperature of the fluid, and the flow rate of the fluid. The control module determines the flow rate of the fluid based on the first signal, the second signal, and the look-up table. 
   In another form, a fluid flow rate sensor for detecting a flow rate of a fluid includes a bridge circuit for generating an output voltage, a temperature sensing circuit for detecting a temperature of the fluid, a heating circuit for heating at least one thermistor of the bridge circuit and a control module. The bridge circuit, the temperature sensing circuit and the heating circuit are connected in parallel. The output voltage is a function of the temperature and the flow rate of the fluid. The control module determines the flow rate of the fluid based on a look-up table which indicates a relationship among the flow rate of the fluid, the temperature of the fluid and the output voltage of the bridge circuit based on empirical data. 
   In yet another form, a method of operating a fluid flow rate sensor comprising a detection circuit is provided and includes measuring a temperature of the fluid, measuring a voltage output of the detection circuit, and determining the fluid flow rate based on a look-up table. The look-up table indicates a relationship among the output voltage of the detection circuit, the temperature of the fluid, and a flow rate of the fluid. 
   Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 

   
     DRAWINGS 
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
       FIG. 1  is a schematic block diagram of a fluid flow rate sensor according to the present disclosure; 
       FIG. 2  is a perspective view of an exemplary probe module of a fluid flow rate sensor according to the present disclosure; 
       FIG. 3  is a front view of an exemplary probe module of a fluid flow rate sensor according to the present disclosure; 
       FIG. 4  is a top view of an exemplary probe module of a fluid flow rate sensor according to the present disclosure; 
       FIG. 5  is a front view of a printed circuit board including an exemplary detection circuit of a fluid flow rate sensor according to the present disclosure; 
       FIG. 6  is a rear view of a printed circuit board including an exemplary detection circuit of a fluid flow rate sensor according to the present disclosure; 
       FIG. 7  is a schematic circuit diagram for an exemplary detection circuit for a fluid flow rate sensor according to the present disclosure; 
       FIG. 8  is a flow chart describing the operation of a fluid flow rate sensor according to the present disclosure; 
       FIG. 9  is an exemplary graph plotting the temperature response over time for a fluid flow rate sensor according to the present disclosure; 
       FIG. 10  is a graph showing the relationship among a flow rate, an output voltage, and the fluid temperature and illustrating the content of a two-dimensional look-up table for use in one embodiment of a fluid flow rate sensor according to the present disclosure; 
       FIG. 11  is a schematic circuit diagram of another exemplary detection circuit for a fluid flow rate sensor with temperature compensation circuitry, according to the present disclosure; and 
       FIG. 12  is a graph showing the relationship between a flow rate and an output voltage, and the fluid temperature and illustrating the content of a one-dimensional look-up table for use in an embodiment of a fluid flow rate sensor with temperature compensation according to the present disclosure. 
   

   Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
   DETAILED DESCRIPTION 
   The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     FIG. 1  generally depicts the major components of a fluid flow rate sensor  10  according to the present disclosure. The fluid flow rate sensor  10  may be employed in a household appliance such as, for example, a refrigerator, washing machine, dishwasher, water dispenser, or automatic ice maker, to monitor water flow therein. 
   The sensor  10  generally includes a probe module  12 , a control module  14 , and, optionally, an I/O module  16 . The probe module  12  is coupled to the control module  14 . As used in this 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 combinational logic circuit, and/or other suitable components that provide the described functionality. 
   When the probe module  12  is subjected to the flow of a fluid  11 , the probe module  12  undergoes a changed condition and generates electrical signals  13  corresponding to such changes. The signals  13  are continuously sent to the control module  14  for processing, which, in turn, generates an output  15  indicative of a fluid flow rate. The I/O module  16  provides a means by which the fluid flow rate sensor  10  can communicate its output  15  to other device(s) or a user. 
   Referring to  FIGS. 2 through 6 , the probe module  12  includes a probe portion  17  and a housing portion  18  for mounting the probe portion  17  to a fluid source. The housing portion  18  includes a body  19 , a pair of tubes  20  formed at opposing ends of the body  19 , a cover  21  secured to the body  19  by screws  22 . The body  19  defines a chamber  23  (shown in  FIG. 4 ) and the tubes  20  define a passageway  24  extending along a longitudinal axis of the tubes  20 . The chamber  23  is in fluid communication with the passageway  24 . The tubes  20  may be provided with annular flanges  25  at their free ends to facilitate connection of the tubes  20  to the fluid source, such as the flexible supply hose of a water dispenser, for example. 
   The probe portion  17  is inserted into the chamber  23  of the body  19  through the cover  21 . The probe portion  17  is a thermo-anemometer and includes a substrate  26  (shown, for example, in  FIGS. 5 and 6 ) on which is disposed a detection circuit  32 . The substrate  26  lies in the chamber  23  and a cable portion  28  is connected to the substrate  26 . The cable portion  28  extends to outside of the chamber  23  and is electrically connected to the control module  14 . 
   The substrate  26  may be a printed circuit board on which a detection circuit  32  (such as the example shown in  FIGS. 5-7 ) is deposited. The substrate  26  is subjected to the flow of fluid  11  that is intended to be monitored. A thin vacuum-deposited Parylene coating may be deposited on the surfaces of the substrate  26  to reduce the thermal mass of the substrate  26 . 
   The probe module  12  may have varied electrical characteristics depending on the arrangement of the detection circuit  32  on the substrate  26 . By unscrewing the screws  22  and removing the cover  21  from the body  19 , the probe portion  17  may be interchanged with another probe portion  17  having different electrical characteristics so that the probe module  12  may be more suitable for other fluids and/or other operating conditions. Moreover, when the probe module  12  ceases to provide the desired function, only the probe portion  17  needs to be replaced. Therefore, it is expected that the probe module  12  of the present disclosure can result in a reduction in replacement and/or component costs. 
   One exemplary detection circuit  32  that can be employed in the fluid flow rate sensor according to the present disclosure is shown in  FIGS. 5 ,  6  and  7 . In  FIG. 7 , a schematic circuit diagram for the detection circuit  32  is shown.  FIGS. 5 and 6  illustrate a physical embodiment of the detection circuit  32  on the substrate  26 . 
   The substrate  26  contains the detection circuit  32  on its two sides. The substrate  26  has pin connectors P 1 , P 2 , P 3 , P 4  and P 5  to which the cable portion  28  is connected at end  30 . The detection circuit  32  ( FIG. 7 ) includes a bridge circuit  34 , a heating circuit  36 , and a temperature sensing circuit  38 . The bridge circuit  34  includes four thermistors NTC 1 , NTC 2 , NTC 3  and NTC 4  screen-printed on a front side  40  of the substrate  26 . The heating circuit  36  includes a heating resistor  1 R 1  screen-printed on a back side  42  of the substrate  26 . The heating resistor  1 R 1  on the back side  42  is disposed in close proximity to thermistors NTC 1  and NTC 2  on the front side  40  to heat thermistors NTC 1  and NTC 2  during operation. NTC 3  and NTC 4  are physically located further away and upstream from heating resistor R 1  such that they are not heated by the heating resistor R 1  during operation. The temperature sensing circuit  38  includes a fixed resistor R 2  and a thermistor NTC 5  screen-printed on the back side  42  of the substrate  26  and adjacent to thermistors NTC 3  and NTC 4  on the front side  40 . Thermistors NTC 1 , NTC 2 , NTC 3 , NTC 4  and NTC 5  have negative temperature coefficient. The thermistor NTC 5  and the fixed resistor R 2 , are also disposed upstream from the heating resistor R 1  and the heated thermistors NTC 1  and NTC 2 . 
   As shown in  FIG. 7 , the bridge circuit  34 , the heating circuit  36  and the temperature sensing circuit  38  are connected in parallel. The bridge circuit  34  is a four-wire bridge circuit having a first leg and a second leg connected in parallel. The thermistor NTC 1  is coupled in series with the thermistor NTC 3  to form the first leg. The thermistor NTC 2  is coupled in series with the thermistor NTC 4  to form the second leg. The two thermistors NTC 2  and NTC 4  may be properly chosen to have a temperature coefficient β and room temperature resistance Ro smaller than that of thermistor NTC 1  and NTC 3  so that the bridge circuit  34 , the function of which is to measure fluid flow rate, becomes much less sensitive to the fluid temperature. 
   The substrate  26  may be formed from a highly thermally conductive ceramic upon which is screen printed a ceramic-filled paste material that forms the thermistors NTC  1  through NTC 5 . Such material is available from Heraeus Incorporated, Circuit Materials Division under the R100 Series designation. Such a configuration completely eliminates discrete thermistor components and helps to reduce the thermal mass of the probe module  12 . 
   The heating circuit  36  is connected in parallel to the two legs of the bridge circuit  34 . The heating resistor R 1  may have a rating of as high as 4 to 6 watts, and as low as 0.5 to 1.5 watts. It should be understood that more than one heating resistor in parallel or in series may be provided in the heating circuit  34  without departing from the spirit of the present disclosure. 
   The temperature sensing circuit  38  includes thermistor NTC 5  and the fixed resistor R 2  formed as a voltage divider. The temperature sensing circuit  38  is connected in parallel to the bridge circuit  34  for measuring the temperature of the fluid upstream from the heating resistor R 1 . 
   The values for the various components in the detection circuit  32  shown in  FIG. 7  are as follows: 
   
     
       
             
             
             
             
           
         
             
                 
                 
             
             
                 
                 
                 
               Coefficient of 
             
             
                 
               Component 
               Resistance 
               Resistance (β) 
             
             
                 
                 
             
           
           
             
                 
               NTC1 
               100 kΩ 
               4,700 K 
             
             
                 
               NTC2 
               100 kΩ 
               4,700 K 
             
             
                 
               NTC3 
               100 kΩ 
               4,700 K 
             
             
                 
               NTC4 
               100 kΩ 
               4,700 K 
             
             
                 
               NTC5 
               100 kΩ 
               4,700 K 
             
             
                 
               R1 
                96 kΩ 
             
             
                 
               R2 
               100 kΩ 
             
             
                 
                 
             
           
        
       
     
   
   The detection circuit  32  includes conductive traces  56 ,  58 ,  60 ,  62 ,  64  that lead to a plurality of pin connectors P 1 , P 2 , P 3 , P 4 , and P 5 , respectively. The pin connectors P 1 , P 2 , P 3 , P 4  and P 5  are provided at the end  30  ( FIGS. 5 and 6 ) of the probe module  12 . 
   The conductive trace  56  is disposed adjacent to NTC 1  and NTC 4  of the bridge circuit  34  and is grounded at pin connector P 1 . The conductive trace  58  is coupled to the second leg of the bridge circuit  34  at terminal  66  between thermistors NTC 2  and NTC 4 . The conductive trace  60  is connected to the temperature sensing circuit  38  at terminal  68  between the thermistor NTC 5  and the fixed resistor R 2 . The conductive trace  62  is connected to the bridge circuit  34  at terminal  70  adjacent to thermistors NTC 2  and NTC 3 . The conductive trace  64  is coupled to the first leg of the bridge circuit  34  between thermistor NTC 1  and thermistor NTC 3 . 
   An input voltage V in  (for example, 12 V DC ) may be applied through pin connector P 4  at terminal  70  to energize the detection circuit  32 . An output voltage V out  is measured across terminals  66  and  72  and can be read at pin connectors P 2  and P 5 . A reference voltage V T  representative of temperature of the fluid can be measured at pin connector P 3 . 
   Referring to  FIGS. 8 through 10 , before the initiation of the fluid flow, an input voltage V in  is applied to the detection circuit  32  so that the heating circuit  36  can preheat thermistors NTC 1  and NTC 2  to a predetermined elevated temperature. The thermistors NTC 3  and NTC 4  are not heated due to physical separation between them and heating resistor R 1 . As shown in  FIG. 9 , as soon as the detection circuit  32  is energized, the detection circuit  32  generates an output voltage of ΔV out  across the bridge circuit  36  read at pin connectors P 2  and P 5 . ΔV out  is an offset voltage indicating an imbalance in the four-wire bridge circuit  34 . To use a bridge circuit to measure the fluid flow rate, a balanced bridge circuit  34  (i.e., zero output voltage) is generally required before the initiation of the fluid flow. This offset voltage is a result of manufacturing deviations from the designed specifications for one or more components of the detection circuit  32 . Instead of calibrating the individual components of the detection circuit  32 , this offset voltage ΔV out  can be stored in the control module  14  and added to or subtracted from a look-up table, which will be described later. Therefore, factory calibration of the individual components may not be necessary. 
   During this preheating period, as the thermistors NTC 1  and NTC 2  are heated, the resistance of the thermistors NTC 1  and NTC 2  is decreased and the output voltage V out  is increased. The rate of the temperature rise of the thermistors NTC 1  and NTC 2  is monitored and recorded so that a signal corresponding to the rate of temperature rise is transmitted to the control module  14 . This pre-heating time is about 250 ms. Because the substrate  26  has a low-thermal-mass Parylene coating, it does not take long to heat the thermistors NTC 1  and NTC 2  to the predetermined elevated temperature. 
   When the thermistors NTC 1  and NTC 2  reach the predetermined elevated temperature, a valve which controls the fluid is opened and the fluid flow is initiated at time=zero seconds as shown in  FIG. 9 . As soon as the fluid is initiated, the probe module  12  is subjected to a stream of fluid, which absorbs the heat of the thermistors NTC 1  and NTC 2 , resulting in a rapid temperature drop in the thermistors NTC 1  and NTC 2 . The output voltage V out  also drops rapidly. The fluid flow is required to reach a steady-state condition before the control module  14  begins to estimate the volume of water that has been dispensed. The time required to reach steady-state depends on the flow rate of the fluid. The faster the fluid flow, the less time required to reach a steady state. During the period that the fluid flow has not reached a steady state condition, the control module  14  may assume a constant fluid flow rate (e.g., a flow rate corresponding to the flow rate first measured after reaching a steady state). 
   Due to the low thermal mass Parylene coating on the substrate  26 , it does not take long to reach a steady state (typically, about 500 msec). At the steady state, the temperature of the fluid is determined from the output voltage V T  read at power connector P 3  in a well-known manner. The output voltage V out  is a function of the fluid flow rate and the fluid temperature and is read at P 2  and P 5 . The output voltages V out  and V T  of the detection circuit  32  may be sampled by the control module  14  at discrete time intervals (e.g., 10 ms). 
   After the data regarding the fluid temperature and the output voltage V out  are recorded, the control module  14  determines the fluid flow rate according to a look-up table. 
     FIG. 10  graphically illustrates an exemplary two-dimensional look-up table used by the control module  14  to determine the flow rate of water. This graph shows a plurality of curves, each indicating a relationship between the output voltage of the bridge circuit  34  and the fluid flow rate at a given fluid temperature. Each of the curves were generated from data points (x=flow rate, y=V out ) that were obtained in a laboratory under controlled conditions (e.g., flow rates and fluid temperatures). A curve was then fit to the data points in a well-known manner. Using the look-up table, the fluid flow rate can readily be determined when the output voltage V out  and fluid temperature V T  are measured or read by the control module  14 . 
     FIG. 11  illustrates another embodiment of a detection circuit  100  for use with a fluid flow rate sensor of the present disclosure, this one incorporating temperature compensation circuitry. The detection circuit  100  in this embodiment is generally similar to the detection circuit  32  of  FIG. 7 , except that it includes a temperature compensation circuit and the temperature sensing circuit may be optional, as desired. More specifically, the detection circuit  100  including a first bridge circuit  102 , a second bridge circuit  104 , a heating circuit  106  connected in parallel. The first bridge circuit  102  includes four thermistors NTC 1 , NTC 2 , NTC 3  and NTC 4  for the purpose of measuring the fluid flow rate. The first bridge circuit  102  has two legs, with each leg including two thermistors. The arrangement of the first bridge circuit  102  and the heating circuit  106  of this embodiment is similar to that of the bridge circuit  34  and the heating circuit  36  in  FIG. 7 . The second bridge circuit  104  includes compensating resistors Rcomp 1 , Rcomp 2 , Rcomp 3  and Rcomp 4  which are connected in parallel with respective bridge thermistors NTC 1 , NTC 2 , NTC 3  and NTC 4 , respectively. This arrangement functions as a temperature compensation circuit. The second bridge circuit  104  includes two legs, with each leg having two compensating resistors. The compensating resistors Rcomp 1  through Rcomp 4  can be deposited (e.g., by screen-printing) directly on the substrate  26 , or externally connected to the four wire bridge circuit (e.g., as part of the control module  14 ). 
   Optionally, the detection circuit  100  may include a temperature sensing circuit  108  including a thermistor NTC 5  and a fixed resistor R 2 . As in the detection circuit  32  of  FIG. 7 , the temperature sensing circuit  108  may be provided upstream of the heating circuit  106 . In this embodiment, however, the measurement of the fluid temperature by the temperature sensing circuit  108  is not necessary for determining the fluid flow rate as discussed above. The temperature sensing circuit  108  is optional and provides an output V T  corresponding to the fluid temperature, if desired. 
   In a fluid flow rate sensor having temperature compensation, only a one-dimensional look-up table is required to determine fluid flow rate, because fluid temperature is removed as a variable under consideration. For the detection circuit  100 , having values for the thermistors NTC 1 , NTC 2 , NTC 3  and NTC 4  and compensating resistors Rcomp 1 , Rcomp 2 , Rcomp 3  and Rcomp 4  as set forth in the table below,  FIG. 12  illustrates the value for V out  at a given fluid flow rate within the range of 0.1 to 1.3 GPM for fluid temperatures of 0.5° C., 25° C. and 50° C. 
   
     
       
             
             
             
             
           
         
             
                 
                 
             
             
                 
                 
                 
               Coefficient of 
             
             
                 
               Component 
               Resistance 
               Resistance (β) 
             
             
                 
                 
             
           
           
             
                 
               NTC1 
               100 kΩ 
               4,700 K 
             
             
                 
               NTC2 
                10 kΩ 
               3,375 K 
             
             
                 
               NTC3 
               100 kΩ 
               4,700 K 
             
             
                 
               NTC4 
                10 kΩ 
               3,375 K 
             
             
                 
               Rcomp1 
                9.8 kΩ 
             
             
                 
               Rcomp2 
               101 kΩ 
             
             
                 
               Rcomp3 
               10.8 kΩ  
             
             
                 
               Rcomp4 
               104 kΩ 
             
             
                 
                 
             
           
        
       
     
   
   As shown in  FIG. 12 , the output voltage V out  is independent of the temperature of the fluid. As a result, the control module  12  requires less memory storage space for the one-dimensional look-up table than is required in a control module for a fluid flow rate sensor not having temperature compensation. The fluid flow rate sensor having temperature compensation also eliminates the need for software that provides temperature compensation, it improves measurement speed, and reduces overall system costs. 
   While the fluid flow sensor response shown in  FIGS. 10 and 12  are for water, the flow rate of other fluids may, of course, be measured by the device according to the present disclosure. Such other fluids would each have corresponding look-up tables of data for use in the device. 
   As previously set forth, if the fluid flow rate sensor  10  has an offset voltage ΔV out  immediately after the detection circuit  32  is energized, the detection circuit  32  may need to be calibrated. The calibration may be achieved by adding/subtracting the offset voltage ΔV out  to/from the output voltage of the look-up table. 
   In addition to determining the fluid flow rate, the sensor of the present disclosure can be used to determine a fault condition of the sensor. During the preheating period and upon initiation of the fluid flow, the rate of the temperature rise of the sensor is monitored and recorded. When the rate of the temperature rise is excessive (e.g., above a pre-determined threshold value), the control module  14  may determine that the fluid flow rate sensor  10  is in a dry condition (i.e., not subjected to the fluid flow). The control module  14  may generate a fault condition and may de-energize the fluid flow rate sensor  10 . Alternatively, or in addition, other appliances and/or components may be shut down due to the dry condition. 
   It should be appreciated that the number of thermistors and heating resistors may vary depending on the application for the fluid flow rate sensor. Also, the heating circuit may include more than one heating resistor connected in series or in parallel. Furthermore, heating resistor(s) and the(ir) corresponding voltage source may be omitted altogether in applications where the thermistor(s) can be internally self-heated. 
   This description is merely exemplary in nature and, thus, variations that do not depart from the gist of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the invention.