Patent Application
20060080050
The present invention generally relates to a fluid flow rate sensor and method for its operation.
Fluid delivery systems have a wide variety of applications and are commonplace in many household appliances, such as metered water dispensers, icemakers, dishwashers, and clothes washers. In such systems, the volume of the fluid to be dispensed is typically controlled by a timing mechanism operable to open and then close a valve after predetermined period of time has elapsed.
These fluid delivery systems operate under the presumed condition that the fluid line pressure and, correspondingly, the fluid flow rate are known and static. All too often, however, the fluid delivery systems are susceptible to fluctuations in the fluid line pressure, and resultantly, the fluid flow rate varies. In such instances, when the fluid line pressure is lower than expected, an under-fill condition typically occurs. Conversely, when the fluid line pressure is greater than expected, an over-fill condition may result.
Consequently, it is desirable for fluid delivery systems to include an in-line sensor that monitors the fluid flow rate and enables the system to accordingly compensate the fluid delivery time.
Another desirable feature for a fluid flow sensor is the absence of any moving parts, such as a paddle-wheel or a turbine, for example. This is because there exists the probability that contaminants, debris or other small particle matter may be present in the fluid. Its function independent of moving parts, the fluid flow sensor is thereby more reliable under such operating conditions.
Thermo-anemometers are a type of flow rate sensor that does not require any moving parts to operate. Thermo-anemometers function based on the principles of heat transfer. Such flow rate sensors, though well-known, have traditionally lacked the necessary response times to make them suitable for many common applications, like water delivery systems for household appliances.
Traditional thermo-anemometers typically include two temperature sensors: one temperature sensor disposed at a downstream location in the fluid path for measuring the fluid temperature downstream; and another temperature sensor disposed at an upstream location in the fluid path for measuring the fluid temperature upstream. The sensor measuring the upstream temperature compensates for fluctuations in the water temperature that might bias the reading of the sensor measuring the downstream temperature. The thermo-anemometer subtracts the upstream temperature from the downstream temperature. By employing various known equations and thermal sensing principles, such as the Seebeck Effect, the temperature difference may be correlated to a fluid flow rate.
Other techniques for determining a fluid flow rate that are fundamentally based on thermodynamic principles are also known. For example, one method is to measure the heat loss, over time, of a known heat source that exposed in the flow of the fluid. The heat loss, expressed as a temperature drop, can be correlated to a fluid flow rate. This method, however, can take a relatively long time period to provide usable results. The reason is primarily because it can take several seconds for the temperature of the source to reach steady-state after being exposed to the fluid flow. In some applications, however, such as in an icemaker, for example, this response time is much too slow.
The need remains, therefore, for a fluid flow sensor that does not involve any moving parts, has a faster response time, and is easily integrated into a variety of fluid delivery systems.
In accordance with the present invention, an improved fluid flow rate sensor and method of operation is provided. The invention comprises a thermo-anemometer-type fluid flow rate sensor design and a method for its operation that overcome the response time limitations of prior known fluid flow rate sensors.
In one aspect of the invention the fluid flow rate sensor comprises a probe having a detection module adapted to change condition in response to the presence of the flow of the fluid, a control module that is electrically connected to the probe that monitors the condition of the detection module over time (e.g., a temperature), determines a rate of change of that condition over time, and generates an output that is indicative of the rate of flow of the fluid, and a I/O module connected to the control module and communicating the output of the control module to another device or a user.
In another aspect of the invention, a method for operating the fluid flow rate sensor is provided.
As a thermo-anemometer-type sensor, the fluid flow rate sensor of the invention does not incorporate any moving parts. The design of the sensor of the invention and its method for operation results in the sensor having a faster response time than prior known fluid flow rate sensors making it suitable for many sensing applications not before available to thermo-anemometer-type fluid flow rate sensors. The fluid flow rate sensor of the invention may be easily integrated into a variety of fluid delivery systems.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
The present invention provides an improved fluid flow rate sensor and associated method for operation improving the valid detection response time of the sensor and being capable of easy integration into a variety of fluid delivery or monitoring systems.
The probe 12 is of the thermo-anemometer-type and has no moving parts. The probe 12 is typically disposed in a fluid environment 22 for detecting the flow rate of a fluid 24. When it is subjected to flow of the fluid 24, the probe 12 experiences changes in its condition represented by a signal 18 (e.g., a voltage).
The signal 18 from the probe 12 is continuously monitored by the control module 14. The control module 12 processes the signal 18 according to the method of the invention, and generates an output 20 that is indicative of a fluid flow rate. The I/O module 16 provides a means by which the sensor 10 can communicate its output 20 to other device(s) or a user.
It is presently contemplated that the fluid flow rate sensor 10 may be employed in a household appliance, such as a refrigerator, to monitor water flow in a water dispenser or automatic ice maker.
One embodiment of a probe 100 for use in the sensor 10 of the invention is shown in
The body 102, as shown, is a generally a cylindrically-shaped tubular member having a passageway 106 extending through its entire length along a longitudinal axis 107. Fluid 24 is able flow through the passageway 106 of the body 102 in a direction along the longitudinal axis 107. Annular flanges 108, 110 may be located at opposite ends 112, 114 of the body 102 to facilitate connection of the probe 100 to a fluid source, such as the flexible supply hose of a water dispenser, for example.
Located intermediate the ends 112, 114 of the body 102 is a housing 116. The housing 116 extends through the body 102 in a direction generally perpendicular to the longitudinal axis 107. The housing 116 is disposed within the passageway 106. The shape of the housing 116 is designed to promote laminar flow of the fluid flow moving through the passageway 106 and across the surface of the housing 116. The detection module 104 is received within the housing 116 such that the housing 116 encapsulates a portion of the detection module 104 to protect it from physical contact with the fluid environment. The housing 116, however, is capable of conducting thermal energy from the fluid environment to the detection module 104.
Both the body 102 and the housing 116 are preferably manufactured from thermally conductive polymers, such as, for example, polypropelene, polyvinylchloride, polyacetylene, polyparaphenylene, polypyrrole, and polyaniline. Ceramic and/or glass fillers mixed in with these base polymers have been shown to greatly enhance the material's thermal conductivity. One such material is known under the trade designation Konduit MT-210-14 and is available from GE/LNP.
The detection module 104 is shown in
An exemplary detection circuit 118 comprises a pair of negative temperature coefficient (NTC) thermistors T1, T2 and a pair of resistors R3, R4 that together form a four-wire bridge circuit. Referring to
As shown, the detection circuit 118 includes traces 126, 128, 130, 132 that lead to a plurality of pin connectors P1, P2, P4, P5 located on the PCB substrate 124. Trace 130 terminates at pin P4, where a reference voltage Vref is applied to the detection circuit 118. Traces 126, 128 are respectively coupled to opposite legs of the detection circuit 118 and terminate at pins P1, P2. An output voltage Vout, which can be calibrated to represent the temperature (T) of the thermistors T1, T2 as is well-known in the art, can be read at pins P1, P2. Trace 132 terminates at pin P5 which is connected to ground.
It can be appreciated that having two thermistors in the detection circuit 118 of the invention accordingly increases the sensitivity of the circuit, thereby eliminating the need for additional signal conditioning to obtain the voltage output Vout.
The heating circuit 120 of the detection module 104 comprises a pair of heating resistors R1, R2. Trace 134 of the heating circuit 120 terminates at pin P3. A voltage VH to power the heating circuit 120 is applied at pin P3. The heating circuit 120 is electrically insulated from the detection circuit 118, but not thermally insulated. The heating resistors R1, R2 are located on the PCB substrate proximate to thermistors T1, T2, respectively, such that heat energy from the heating resistors R1, R2 is conducted to the thermistors T1, T2. The heating resistors are contemplated to provide as much as 4 to 6 watts.
It should be appreciated that the number of thermistors and heating resistors may vary depending on the application for the fluid flow rate sensor. For example, resistors R3 and R4 of the detection circuit 118 could be replaced with thermistors. Also, the heating resistors may be electrically in series or in parallel, or only a single heating resistor need be provided. 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.
The detection module 104 is generally received within the housing 116 such that it is perpendicular to the direction of fluid flow F through the passageway 106. Referring to
The detection circuit 118 and heating circuit 120 are arranged such that the thermistors T1, T2 and heating resistors R1, R2 lie on the portion of the PCB substrate 124 that is within the passageway 106 of the body 102. Resistors R3, R4 of the detection circuit 118 and all of the pin connectors P1, P2, P3, P4, P5 lie on a portion of the PCB substrate 124 that extends outside of the housing 116, as best seen in
Operation of the sensor 10 will now be described. The probe 100 for a fluid flow rate sensor 10 is coupled to the control module 14. Specifically, the control module 14 measures and analyzes Vout from the probe 100 and correlates it to the flow rate of the fluid being measured by the sensor 10 to produce an output 20. An I/O module 16 may be coupled to the control module 14 to provide an interface for communicating the output 20 to other device(s) or a user.
Next, the thermistors T1 and T2 are preheated at 142. In this step, VH is applied to the heating circuit 120 by the control module 14, thereby energizing the heating resistors R1a, R2a and causing the temperature of the thermistors T1 and T2 to increase from their initial starting temperature (T0). The reference voltage Vref is applied to the detection circuit 118.
The output voltage Vout of the detection circuit 118 is sampled at discrete time intervals (e.g., 25 msec). The temperature (Ti) of the thermistors T1 and T2 is determined from the output voltage Vout in a well-known manner and as described below. This process is repeated until a predetermined temperature change threshold for the thermistors T1 and T2 has been reached (e.g., Ti−T0>25° C.). The value of Vout is then stored and operation continues. VH may be held constant, reduced (as described further herein), or removed, depending on the sensor application.
Fluid flow is initiated, e.g., a fluid valve is opened and the fluid flows through the passageway 106 of the probe 100 and passes over and around the housing 116 and, consequently, the portion of the detection module 104 that is enclosed within the housing 116. As the fluid flows past the housing 116, heat is transferred from the thermistors T1 and T2 through the housing 116 and to the fluid. Correspondingly, both the temperature (T) of the thermistors T1 and T2 and the output voltage Vout change over time (t).
At 144, the temperature (T) of the thermistors T1 and T2 is monitored. The output voltage Vout from the detection circuit 118 is again sampled by the control module 14 at discrete time intervals (e.g., 100 msec). Again the temperature (Tj) of the thermistors T1 and T2 is determined from the output voltage Vout in a well-known manner.
At 146, the control module 14 determines the rate of change of temperature (Tj) over time (i.e., it calculates dTj/dt) as is well-known in the art. This process is repeated for a predetermined number of iterations (e.g., 10). At 148, the smallest value of dTj/dt is then correlated to a fluid flow rate. The control module 14 generates a corresponding output 20 at 150.
The I/O module 16 then is able to make the output 20 of the control module 14 available to, for example, a fluid flow controller (not shown) that determines how long a valve should be left open based upon the fluid flow rate that has been sensed by the sensor 10.
It should be understood that the sensor 10 can continue to operate even while fluid flow continues, to sequentially update the output 20. This is particularly useful in applications where the fluid flow rate does not remain constant. When operating in this manner, it is contemplated that power to the resistive heater(s) would be reduced after the initial pre-heat cycle 142. A power reduction of, for example, 50 percent is believed to be sufficient, though other power reductions may be satisfactory depending on the sensor's 10 application. The power reduction can be accomplished by modulating input to the heating resistor(s) in a well-known manner.
The sensor 10 would then continue to operate as previously described. After a first output 20 is generated at 150, the process is then repeated at 152. The heating resistor(s) are, therefore, again pre-heated at 142. The process continues and a second output 20 is generated at 150. Each successive loop of operation, therefore, provides a sequential output 20 that is indicative of the fluid flow rate, which may be changing over time.
It is to be understood that the number and duration of the discrete time intervals referenced above may vary from the examples provided and may be dependent on the processing capabilities of the control module 14, including, for example, the control module's 14 processing speed and resolution.
As stated in the foregoing discussion, the manner in which the temperature of the thermistors' T1a, T2a can be determined from the output voltage (Vout) of the detection circuit 118 is well-known.
First, the output voltage (Vout) can be correlated to the thermistors' T1a, T2a resistance values (Rntc) according to the following equation:
Rntc=Rfixed*(Vref−Vout)/(Vref+Vout)
It is presumed for purposes of the invention that both thermistors T1, T2 are at the same temperature throughout the process, i.e., T=T1=T2, and that R3=R4=Rfixed. The temperature value (T) in degrees Kelvin is calculated, then, according to the following equation:
T=(1/To+ln(Rntc/Ro)/Ro)/β)−1
where temperature To=298.15° K, Ro is the thermistors' T1a, T2a resistance at temperature To, and β is an intrinsic parameter indicative of the temperature sensitivity of the thermistor.
The temperature measurements (T) are differentiated by the control module 14 to generate dT/dt, as is well-known. In the foregoing example, dT/dt is calculated as follows:
(Tj−Tj-1)/(tj−tj-1)
Although the differentiating operation has been discussed in the context of a first order derivative, it should further be understood that higher order derivatives of the temperature versus time curve may be utilized. It is believed that the use of higher order derivatives would further improve the response time of the sensor.
It should be understood that the validity of the operation of the detection circuit 118 can be evaluated theoretically using various equations and principles well known in the art. More specifically, heat transfer theory predicts that as soon as the fluid flow starts, the thermistor temperature (T) begins to drop rapidly. It can be shown that the temperature drop is exponential, namely it can be expressed as:
T−Tfinal=(Tstart−Tfinal)exp(−Kt)
where Tstart is the temperature of the thermistor at the beginning of the fluid flow, Tfinal is the steady state temperature of the thermistor when subject fluid flow, and t is the time after the flow starts. K, a constant, represents the cooling rate by the fluid and is considered to be a measure of the sensor's sensitivity. Since the sensor is cooled convectively by the fluid flow, K is proportional to the square-root of the flow rate.
In terms of the rate of temperature change over time (dT/dt), the minimum value of dT/dt occurs soon after the initiation of fluid flow. From the preceding equation, the maximum value is shown to be proportional to K (Tstart−Tfinal). The temperature drop (Tstart−Tfinal) slightly increases with the fluid flow rate. If the difference between (Tstart−Tfinal) is neglected, the sensitivity of the sensor increases with K, which means that the sensitivity changes roughly as the square-root of the flow rate.
Referring now to
The minimum dT/dt determined as part of the method of the invention can be normalized to calibrate the fluid flow rate sensor to an actual fluid flow rate in any particular sensor application. One example is shown in
Further improvements to optimize the thermal mass of the detection module 104 comprise utilizing a highly thermally conductive ceramic substrate upon which is screen printed a ceramic-filled carbon paste material that forms the thermistors T1 and T2. Such material is available from Heraeus Incorporated, Circuit Materials Division under the R100 Series designation. Such a configuration completely eliminates the discrete thermistor components in the detection circuits 118 and helps to reduce the thermal mass of the detection module 104. A fluid flow rate sensor 10 of the invention constructed according to this configuration and operating as described above has demonstrated a response time in an icemaker application on the order of 0.5 to 1 second. This response time is on the order of a 50 percent or greater decrease in response time over the prior art.
Also, to improve the heat transfer characteristics and durability of the probe, it is contemplated that the housing 116 can be eliminated and a thin layer of a thermally conductive dielectric polymer or a glass material be applied directly to the detection module 104 as a glaze to encapsulate the detection module 104 and protect it from moisture and/or abrasion.
Another embodiment of a probe 200 for use in the sensor 10 of the invention is shown in
Like the detection module 104 the detection module 204 comprises a detection circuit 218 and a heating circuit 220. The detection module 204, however, has a generally more rectangular configuration (as viewed in
The detection circuit 218 comprises a plurality of NTC thermistors T1a, T2a, T3a, T4a deposited on a ceramic substrate 224. The thermistors T1a, T2a, T3a, T4a together form a 4-wire bridge circuit. Referring to
Like the detection circuit 118, the detection circuit 218 includes traces 226, 228, 230, 232 that lead to a plurality of pin connectors P1a, P2a, P4a, P5a located on the ceramic substrate 224. Trace 230 terminates at pin P4a, where a reference voltage Vref2 is applied to the detection circuit 218. Traces 226, 228 are respectively coupled to opposite legs of the detection circuit 218 and terminate at pins P1a, P2a. An output voltage Vout2, which can be calibrated to represent a temperature difference (ΔT) across the bridge and between thermistors T1a T3a and thermistors T2a, T4a as is well-known in the art, can be read at pins P1a, P2a. Trace 232 terminates at pin P5a which is connected to ground.
The heating circuit 220 of the detection module 204 comprises a heating resistor R1a. Trace 234 of the heating circuit 220 terminates at pin P3a. A voltage VH2 to power the heating circuit 220 is applied at pin P3a. The heating circuit 220 is electrically insulated from the detection circuit 218, but not thermally insulated. The heating resistor R1a is located on the ceramic substrate proximate to thermistors T1a, T2a such that heat energy from the heating resistor R1a is conducted to the thermistors T1a, T2a. Heat energy from the heating resistor R1a is not, however, conducted to thermistors T3a, T4a. The heating resistor may have a rating of as much as 4 to 6 watts.
The detection module 204 is generally received within the housing 216 such that it is perpendicular to the direction of fluid flow F through the passageway 206. Referring to
The use of four thermistors T1a, T2a, T3a, T4a in the detection circuit 218 and the thermistors' T1a, T2a, T3a, T4a physical arrangement in the passageway 206 of the body 202 provide further advantages. One significant advantage is that the output voltage Vout2 is automatically compensated for ambient temperature changes, i.e., changes in the temperature of the fluid. This is important because if significant and/or rapid changes in the fluid temperature occur, they could distort the output of the sensor 10 causing the sensor 10 to give inaccurate results.
For example, first assume that the temperature of the static fluid (i.e., before flow is initiated) is at 25° C. In this environment then, the sensor 10 begins its operation as described above with reference to
The output voltage Vout2 of the detection circuit 208, however, represents a temperature difference (ΔT) across the bridge and not an absolute temperature (T). This is because the unheated thermistors T3a, T4a on opposite sides of the bridge of the detection circuit 208 counter-act the impact on the output voltage Vout2 caused by temperature changes in the fluid. Consequently, the compensated sensor measures the change in relative temperature and calculates the rate of change of the relative temperature.
Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and the following claims.
