Patent Grant
7473868
The present invention relates generally to sensors and more particularly to sensor systems and algorithms that operate in a fail-safe manner to detect the temperature of a component or a medium and/or to detect the presence thereof within a heating, ventilating, or air-conditioning (HVAC) system.
Heating systems employ various methods to control the temperature of components with the system. The temperatures of these components are usually regulated within a particular range in order to maintain safe operation. Two such components that require regulation are heat exchangers of furnaces and the water inside a pressurized hot water boiler. Redundant sensors are often used in safety-related components such as these, which provide greater confidence that the sensors are operating properly. Two or more such sensors may reduce the probability of the heating control system recognizing an incorrect temperature, however, the proper functionality of the additional sensors are not known with any greater confidence than the initial sensor.
Temperature measurement is important in many such processes. A common method of temperature measurement uses thermocouple transducers that output an EMF in response to a temperature gradient across two dissimilar materials, typically metals. It is well known, however, that thermocouples degrade over time due to chemical and metallurgical changes in the composition of the materials. Various thermal sensors and detectors such as thermistors, platinum resistance elements, and other types of temperature sensors are also utilized in many heating, ventilation, and air-conditioning (HVAC) applications.
Most temperature sensors used in these HVAC applications, whether used in industrial, commercial, or residential markets, eventually suffer from some form of serious degradation and/or failure of the sensor. Such degradation or failure modes of temperature detectors include thermal degradation, metal fatigue, corrosion, chemical and mechanical changes, which may render the sensor inoperable or otherwise induce a system failure.
During the use of thermocouples, for example, several forms of degradation take place in the thermocouple circuit including chemical, metallurgical, and mechanical changes in the materials and elements or devices of the circuit. Such changes may be accompanied by a shift in the resistivity of the thermoelement, thereby indicating a false temperature measurement.
Heating applications likely produce the greatest potential for sensor failures, where the sensor is particularly susceptible to extremes of thermal degradation and chemical changes. These sensors may include temperature, pressure, flow, and medium presence sensors, and others such as may be used in furnaces and boilers. The exposed portion of the sensor is often the hottest portion of the measurement circuit and may therefore be exposed to the harshest conditions. The temperature sensor and other related sensors are also exposed to processes that may increase the likelihood of changes in the electrical properties of the sensor or cause a complete system failure.
In boiler applications, for example, temperature, pressure, flow, and medium presence detection may be used, wherein the failure of a temperature sensor or an associated low-water level cutoff detector may cause a boiler malfunction or failure. Thus, the failure of such boiler sensors poses a problem. In furnace applications, the temperature sensors and/or limit detectors used in a heat exchanger of a furnace may also reach very high temperatures, and cause overheating conditions that could cause the system to fail. Accordingly, a fail-safe temperature sensor and/or a fail-safe low-water level cut-off detector would be desirable to avoid such problems.
For design, manufacturing, and applications reasons, the HVAC sensors discussed above are generally individually fabricated, packaged and mounted. However, the use of these numerous individual sensors also requires more system mounting difficulties and added complexity in support of the remaining portion of the control system. Such additional support components and circuitry may include related relays, power supplies, and microprocessors that increase the overall cost and complexity of the system.
In many applications, however, several specific sensors are commonly used together. For example, in the case of boiler heating systems, a boiler water temperature sensor is usually accompanied by a low-water cutoff detector, which senses the presence of the water (or another such medium) when strategically placed at the low water level of the boiler. If the water falls below this level, the system is typically shut-down until more water is added, thereby immersing the sensor again.
Accordingly, for fail-safe temperature readings, cost, mounting and system simplicity reasons, there is a need for a fail-safe sensor of a temperature monitoring system that incorporates both temperature and medium detection functions in a single housing.
The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to a fail-safe sensor system and method for detecting a temperature and/or the presence of a component or a medium within an HVAC system in a fail-safe manner. The fail-safe sensor of the present invention comprises a positive temperature coefficient (PTC) resistance element or PTC heater that regulates itself at a known temperature when supplied power. The sensor further comprises a temperature detector (e.g., PTC or NTC thermistor, thermocouple, IC temperature detector) in close thermal proximity to the PTC heater provided within a single sensor housing.
In one method aspect of the present invention, when heated to the self-regulating temperature, the temperature signal of the temperature detector is compared with the known regulated temperature of the PTC heater to confirm whether the sensor is presenting an accurate signal to an analyzer or control system. The device is then allowed to cool to the temperature of the surrounding medium in the component it is designed to sense. The temperature of the component is then measured with greater confidence than would otherwise be provided with a single sensing device or multiple sensing devices.
In another aspect of the present invention, by calculating the time constant of the temperature decay rate of the sensor, a determination is made whether a component or a medium surrounding the sensor is present or absent, for example, whether the sensor is immersed in water or air. In one implementation, for example, a slower decay time constant indicates the sensor is in air, while a faster decay time constant indicates the sensor is immersed in water. Knowledge of the presence of water is important, because boilers may become damaged when fired without water. Thus, the sensor of the present invention eliminates the need for separate and relatively costly medium presence detection (e.g., low-water cutoff) devices and controls (e.g., related relays, power supplies, and microprocessors) currently used in conventional HVAC systems.
In another implementation of the present invention, the sensor is used to measure the temperature of a heat exchanger, an outlet plenum, an air stream, a chamber wall, a stack, or other component, for example, in a furnace or another HVAC system. In such a case, the time constant of the temperature decay rate is used to indicate whether the sensor has adequate thermal contact with the furnace component or has become loose or separated from the furnace component.
In yet another aspect of the invention, the HVAC system may be a furnace, a boiler, a ventilation system, a refrigeration system, or an air-conditioning system.
In still another aspect of the invention, the PTC heater and the temperature detector each have first and second electrical terminals, and are electrically joined together at the first electrical terminals to form a three terminal device.
In another aspect, the PTC heater and the temperature detector are prefabricated on a single integrated circuit die, a single ceramic substrate, or another such common thermal platform.
In yet another aspect of the invention, the sensor housing also has a thermally conductive and electrically insulative material formed about the PTC heater and the temperature detector to provide a close thermal union between the elements of the sensor.
Detecting the temperature or presence of other solids or liquids surrounding the sensor is also anticipated in the context of the systems and methods of the present invention.
A detection system of the present invention monitors the resistance of a temperature detector while alternately heating and cooling a PTC heater to identify the regulation temperature and calculate the thermal time constant of a component or a medium surrounding a sensor in an HVAC system, thereby providing a determination of the health of the sensor and/or the presence or absence of the medium.
In one aspect of the present invention, the PTC heater also serves as the temperature detector when the heater element is not being heated to provide both heater and detector functions within a single element of the sensor.
The present invention further provides an algorithm for HVAC systems to identify a temperature, a low medium alarm, and a failed sensor alarm in a sensor measurement circuit. For example, the algorithm, according to one aspect of the invention, utilizes one or more values supplied by the manufacturer of the sensor and one or more predetermined thermal time constant (TC) levels for comparison to the calculated TC levels, whereby the presence or absence of the medium is determined based on the comparison results.
For example, a first predetermined (cool-down) TC level is initially input into the analyzer for use by the algorithm corresponding to a medium (e.g., water) present at a low water level cut-off location of the sensor. If a determination is made upon comparison that the computed TC level has exceeded the first predetermined TC level, the medium is present at the sensor, however, if the first predetermined TC level is not exceeded, the medium is absent from the sensor, and a low-water cut-off alarm is generated. If the computed TC has not exceeded a second predetermined (cool-down) TC level, or if a third predetermined (warm-up) TC level is not exceeded, a sensor maintenance alarm may be generated.
Thus, by applying parameters specific to the temperature detector and PTC heater of a sensor used in a monitoring system, added accuracy is obtained in determining the TC level for the applicable medium used in the HVAC system using the algorithms of the present invention. Further, it is anticipated that the algorithms used in the methods and temperature monitoring system of the present invention may be used to identify degradation of the sensor in order to predict a future potential sensor system failure therein.
The temperature monitoring system of the present invention comprises a temperature sensor, a storage component, and an analyzer comprising an algorithm for identifying a temperature, a low medium alarm, a sensor alarm, and optionally for predicting certain types of impending failures of the temperature sensor or the HVAC system. The analyzer of the monitoring system is operable to receive sensor parametric input values available from the sensor, monitor one or more sensors (e.g., thermistor, thermocouple) inputs, monitor the temperature detector resistance of the sensor, supply or remove a voltage (e.g., from a power supply) to the PTC heater of the sensor for heating or cooling the sensor, and calculate and store the parameters and predetermined TC levels in the storage component. In response, the analyzer may then provide one or more of a temperature detection, a low medium alarm, a sensor alarm, and a failure prediction based on an analysis of the sensor (temperature detector) (e.g., resistance) measurement results from the algorithm.
For example, the detection system may, according to one aspect of the invention, monitor the resistance of a sensor for changes that are analyzed and determined to be due to a level of sensor degradation greater than a predetermined acceptable level. Although only the sensor resistance need be monitored, an accurate determination may be made using the algorithm and several parameters of the temperature detector from the manufacturer.
In accordance with another aspect of the invention, by creating a time-series history of periodic sensor TC level calculations, a prediction of an imminent sensor or HVAC system failure, or a prediction of a next expected value may be provided by the monitoring system.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
The present invention will now be described with reference to the attached drawings, wherein like reference numerals are used to refer to like elements throughout. The invention relates to a fail-safe sensor system and method for detecting a temperature and/or the presence of a component or a medium within a heating, ventilating, and air-conditioning or HVAC system in a fail-safe manner. The fail-safe sensor of the present invention incorporates the functions of a heater and a temperature detector within a single sensor housing. In one aspect of the invention, the fail-safe sensor of the present invention comprises a positive temperature coefficient (PTC) resistance element or PTC heater that regulates itself at a known or self-regulating temperature when supplied power. In one implementation, the sensor further comprises a temperature detector (e.g., PTC or NTC thermistor, thermocouple, IC temperature detector) in close thermal proximity to the PTC heater provided within a single sensor housing. Alternately, the PTC heater may also serve as the temperature detector when the heater element is not being heated.
When used in a hot water boiler application, a goal of the fail-safe sensor of the present invention is to combine the functions of a temperature detector and a low-water cut-off device within a single sensor. Conventionally, these functions typically require the use of separate devices, which add system complexity as well as cost for the added supporting components (e.g., relays, power supplies, microprocessors, housings, wiring) and for the individual device mounting costs.
Fail-safe operation is obtained by providing the sensor the ability to confirm that the temperature detector is working properly. To accomplish this, in one aspect of the present invention, an algorithm is provided which is used to monitor the health of the sensor and to detect a component or medium in contact with the sensor. When heated to the self-regulating temperature, the temperature signal of the temperature detector is compared with the known regulated temperature of the PTC heater to confirm whether the sensor is presenting an accurate signal to an analyzer or a control system. The sensor is then allowed to cool to the temperature of the surrounding medium in the component it is designed to sense. The temperature of the component is then measured with greater confidence than that which may be provided with a single sensing device or multiple sensing devices.
Initial parameters of the specific thermoelements used in the sensor may be supplied by the manufacturer or otherwise ascertained in another manner. These parameters may be useful for increasing the accuracy of the temperature measurements. In addition, inputting one or more predetermined acceptable levels of thermal decay rate time constants may be useful for identification of specific medium densities or for sensor degradation levels and failure predictions. In order to better appreciate one or more features of the invention, several exemplary implementations of the temperature and presence detection system, the temperature and presence detection method, and several types of system outputs is hereinafter illustrated and described with respect to the following figures.
The conventional boiler 100 of
The low-water cut-off controller 150 of
In operation, transformer 154 supplies voltage through the coil of the relay 156 to the live conductive probe 160, which is mounted into the boiler 100 and insulated from equipment ground 166. If there is water 110 in the boiler 100, current will flow through the coil of relay 156 and the live probe 134 through the water 110 to ground 166, pulling in the relay 156 and passing line voltage power (e.g., 120VAC) to the burner 106.
Thus, in the conventional boiler system configuration 100, separate water temperature sensing and water presence detection may be required for operation in a safe manner. Accordingly, added device, and related equipment costs, including added mounting costs are typically needed in a prior art system.
The particular arrangement of the sensor 400 of the present invention permits the temperature detector 410 to sense the surrounding temperature, while the PTC heater 420 provides heating 435 to the sensor 400 and self-regulation at a known temperature. Measurement using the temperature detector 410 at the known temperature set by the PTC heater 420 then provides a level of confidence that the operation of the temperature detector 410 is providing an accurate temperature measurement. In addition, as indicated supra, when power is removed from the heater 420, the time constant (TC) of the thermal decay rate may be computed (e.g, by an analyzer) from two or more temperature measurements, to indicate whether a component or medium (e.g., a heat sink, heat exchanger, water) is present surrounding the sensor, or is absent. For example, a high TC temperature decay rate may indicate the sensor is immersed in water (medium present), while a low TC rate may indicate the sensor is in air (medium absent).
Further, the sensor housing 430 of
A thermally conductive paste may be applied to the inside of the boiler well so that when the sensor is inserted, there is a good thermal connection. In one preferred implementation, however, the temperature detector 410 and the PTC heater 420 are cast together in a silicon rubber housing 430 that may be inserted into the boiler well with no such thermal paste. Then when a cap (not shown), for example, is screwed down at the opening of the well, it compresses the sensor slightly to cause the sensor to widen and fill the gap between it and the well, creating a good thermal connection. Alternately, in another preferred implementation, a thermal contact side or wet side of the sensor is mounted thru an opening in the boiler wall to directly contact the boiler water, thereby inherently providing intimate thermal contact with the medium.
In another implementation of the present invention, the temperature detector R1410 and the heater R2420 may be fabricated together on a single integrated circuit chip or another such common substrate such as silicon or ceramic for both temperature detection and heating/cooling of the elements of the sensor 400. It is a goal in one aspect of the present invention to minimize the distance and maximize the thermal union between the temperature detector 410 and the heater 420. It is another goal in one aspect of the present invention to minimize the mass of the detector 410 and the heater 420. In these ways, the responsiveness of the sensor to the surrounding medium, and to each other of the elements therein may be maximized.
In this implementation of sensor 460, the PTC heater 420 provides the heat 435 within the sensor 460 when power is applied to the heater 420. Then, when power is removed from the heater 420 of sensor 460, the PTC resistive element of the heater 420 is also used as a temperature detector similar to that of temperature detector 410 of
The difference between the two exemplary sensor implementations 400 and 460 is in the method of temperature detection. In sensor 460, the temperature detector confidence check at the known regulation temperature of PTC heater 420, for example, may be made immediately after removing the heater power supply, and before the sensor has had a chance to cool significantly. However, the time constant of sensor 460 may be too quick (short) to make an accurate measurement practical after power removal. Alternately, therefore, the current and voltage going into sensor 460 may both be monitored and the resistance calculated during the heating phase to provide continuous temperature monitoring from the resistance calculation. Thus, using either sensor 400 or 460, the known regulation temperature may be maintained at a stable temperature level while monitoring the temperature measurement is being obtained.
Although a single temperature detector and heater is discussed in association with the sensor of the present invention, the use of one or more temperature detectors and/or heaters may be used within the sensor, and is anticipated in accordance with the invention.
The exemplary boiler 500 of
The fail-safe sensor 400 of the temperature and presence sensing control device 520 also has a PTC heater 420 that is used to cyclically heat and cool the sensor 400. As the sensor 400 cools in each thermal cycle, the change in temperature is monitored by the analyzer using the change in resistance of the temperature detector 410. From the temperature measurements, the analyzer then computes the thermal decay rate time constant (TC) of the sensor 400, to determine whether water 510 is present surrounding the sensor 400. If water 510 is not present at the sensor 400 (indicating a low water condition), the burner 506 is shut-down until additional water 510 is added, thereby maintaining fail-safe operation of the boiler system 500. Further, the health of the sensor 400 may also be ascertained by using the temperature detector 410 to monitor the PTC heater 420 within the sensor 400, after thermal equilibrium is established at the known self-regulation temperature. Thus, in accordance with several aspects of the present invention, the fail-safe sensor 400 may be used to detect the temperature and presence of a medium in an HVAC system in a fail-safe manner.
In another implementation of the present invention, the temperature and presence of a heat exchanger (not shown) may be detected using the sensor 400 and 460 of the present invention. As a heat exchanger (e.g., comprising a high thermal conductivity metal with fins) is likely to produce a higher thermal decay rate than that of water or another such medium, the temperature swing produced by the PTC heater 420 of the sensor 400/460, is also likely to be low. Thus, the known self-regulation temperature of the PTC heater 420 may be shifted to a significantly lower temperature level when used in the determination of health of the temperature detector 410. Further, the presence detection algorithm as it may be applied to a heat exchanger application, may be somewhat limited to determining whether there is adequate thermal union between the sensor 400/460 and the heat exchanger. For example, if the sensor 400/460 has slipped out of the heat exchanger, the thermal TC would be greatly reduced and a presence determination therefore would indicate that the medium (e.g., heat exchanger) is not present.
Lead wires L1441, L2442, and L3443 may transition at field terminals 670 to field wiring 677, which connects to local terminals 675 of an analyzer 680 for monitoring the fail-safe sensor 400/460. As discussed in association with
For example, system 700 of
Analyzer 730 further comprises an algorithm 735 (e.g., a program, a computer readable media, a hardware state machine) that is applied to the system to calculate and analyze the temperature monitoring, presence detection, and/or sensor degradation and failure prediction. Upon completion of such calculations and/or analysis, the algorithm 735 provides several possible output results from the analyzer 730 that may include a current sensor temperature 750 (e.g., 180° F.), and if a predetermined limit has been achieved, a low medium alarm 760 (e.g., low-water cut-off level, medium absent), and/or a sensor alarm 770 (e.g., sensor or system failure imminent, sensor maintenance required) may be issued.
Similar to system 605 of
Alternately, and as indicated previously, the current and voltage going into sensor 460 may both be monitored and the resistance calculated during the heating phase to provide continuous temperature monitoring based on the resistance calculation.
In another implementation of the present invention, the sensor may comprise an integrated circuit heater and/or detector further operable, for example, to digitally communicate to the analyzer a temperature signal, a sensor parametric input, a sensor model, a sensor serial number, a manufacturing date, and a calibration temperature. Further, the Integrated circuit based sensor, may be operable to provide one or more of the output determination results that are discussed above in association with the analyzer.
The present invention provides one such method and system for monitoring one or more sensors and detecting current or impending sensor or HVAC system failures automatically and without disrupting service. A component or medium detection portion of the algorithm of the present invention utilizes a change in the cool-down time constant that exceeds a predetermined level based on the sensor temperature measurements to detect the presence or absence of a component or medium surrounding the sensor. A failure detection portion of the algorithm of the present invention, for example, utilizes a change over time in the warm-up and/or cool-down time constants of the sensor temperature measurements to detect an impending sensor or HVAC system failure. In addition, no change or an extreme change in the warm-up and/or cool-down TC of the sensor temperature measurements may indicate a present sensor or HVAC system failure.
For example,
The analyzer 830 of
In another aspect of the present invention, an event timing macro 860 is further added to control how often the sensor thermal TC measurement is made via a sensor thermal TC monitoring macro 835b. For example, timings ranging from continuous thermal TC measurements to once per day, or once per thermal process cycle may be enabled with the event timing macro 860.
Another aspect of the invention provides a methodology for monitoring, analyzing, and detecting the temperature and presence of a component or medium in a sensor monitoring system as illustrated and described herein, as well as other types of temperature monitoring systems.
The method relies on a change that exceeds a predetermined level in the cool-down thermal TC as an indicator of the presence or absence of a component or medium surrounding the sensor and of the sensor health. For example, after measurements and calculations, a high slope thermal TC indicates the presence of a medium at the sensor, while a low slope thermal TC indicates the absence of the same medium. However, if no slope or an extremely high slope is detected, a sensor or system failure is likely to be indicated. Optionally, a slope that increases or decreases over time is an indicator of, for example, a sensor or system degradation or an impending failure. The method of the present invention utilizes an algorithm to detect sensor temperature measurements, medium presence, and sensor or system degradations to enable failure predictions as described above.
Referring now to
The exemplary fail-safe temperature and presence detection method 900 of
After waiting for a period of time, such as the delay time tdh, at 920, the sensor will have heated to about the predetermined self-regulation temperature Thf of the PTC heater 420. At 925, for example, after the delay time tdh, the temperature detector 410 is then measured at an initial self-heated temperature Tmi. Accordingly, after an appropriate warm-up period, the measured temperature Tmi indicated by the temperature detector 410 of a healthy sensor will approximate the self regulation temperature Thf, or Tmi˜Thf. Power supply voltage 734 is then removed from the PTC heater 420 at 930. As the sensor cools down toward the temperature of the surrounding medium (e.g., water, Ammonia, Freon) at 935, the sensor temperature detector 410 is monitored and measurements are taken. Optionally, the initial temperature Tmi may be updated again or continuously updated just prior to the thermal cool-down slope measurements, to obtain a fully stabilized measurement Tmi of the self-heating temperature Thf.
When the temperature stabilizes, at 940, the temperature detector 410 is measured at a final temperature Tmf, corresponding to the temperature of the surrounding medium (e.g., water, Freon). A thermal cool-down TC slope (slope 1) is then computed and stored at 945 based on the initial temperature Tmi, the final temperature Tmf, and elapsed time period tdc between the temperature readings.
The computed TC slope level, slope 1 is then compared to the TC 1st level associated with the presence/absence of a medium at 950. If it is determined at 950 that the measured TC level, slope 1 is greater than the TC 1st level, indicating that the medium is present at the sensor (e.g., the sensor is immersed in water), then the medium is present at 955 and the algorithm and thermal cycling continues to 915, wherein the PTC heater is again heated for another temperature and presence detection. If, however, at 950 the measured TC level, slope 1 is not greater than the TC 1st level, then it is determined that the medium is absent from the sensor, and a low-media alarm is output at 960 (e.g., the sensor is in air, alarm for low-water cut-off), and the algorithm continues to 965.
At 965, the computed TC slope level, slope 1 is then compared to the TC 2nd level associated with a sensor low level alarm for maintenance. If it is determined at 965 that the measured TC level, slope 1 is less than the TC 2nd level, then an unacceptable sensor TC slope minimum level is indicated and the algorithm outputs a sensor alarm to maintenance at 970. If, however, the measured TC level, slope 1 is not less than the TC 2nd level, then the sensor is checked further at 975. For example, if a crack or bubble forms in the sensor potting material between the heater and detector elements, if the sensor has been dislodged, or if the sensor otherwise fails, then the calculated slope may become lower than the acceptable minimum slope level.
At 975, a comparison is made to determine if the sensor (as indicated by the initial temperature measurement Tmi) was able to heat to within a predetermined percentage of the self regulation temperature Thf within the delay time tdh. This comparison indicates the ability of the heater 420 to heat properly to the known temperature, as well as the ability of the temperature detector 410 to accurately report the known temperature of the PTC heater 420. If the predetermined percentage of the self regulation temperature Thf is not achieved within the time delay limit tdh, then the algorithm outputs a sensor alarm to maintenance at 970. Otherwise, if the predetermined percentage of the self regulation temperature Thf is successfully achieved by the initial temperature measurement Tmi within the time delay limit tdh, then the algorithm of method 900 ends at 980, and another heating and cooling thermal cycle of the method may begin again, for example, at 915.
Alternately, at steps 935 and 940 of method 900, as the sensor cools down toward the temperature of the surrounding medium, the sensor temperature detector 410 is monitored and measurements are taken after the initial temperature Tmi and before the final temperature Tmf, wherein such intermediate temperature measurements may be used to compute a thermal cool-down TC slope (slope 1) at 945.
Similarly, the method 982 of
In another aspect of the present invention of methods 900 and 982, a time-series history of the initial and final temperatures and/or the calculated thermal TC slopes may be recorded in the storage component 720, 820 for later use. The recorded values may then be used in a trend analysis to anticipate future values based on an acceptable level of sensor or system degradation over time in order to make a failure prediction, or to signal that a failure is imminent.
Plot 1000a and timing diagrams 1030 and 1050 of
The sensor temperature detector 410 may be verified 1065 at or after time period 1, by comparing the detector 410 measurement Tmi 1065 to that of the known self-regulation temperature Thf of the PTC heater 420. In addition, if a predetermined delay time (tdh 1024) is exceeded (1001 to 1001a) during the sensor warm-up before Tmi achieves a predetermined percentage of the self-regulation temperature Thf, a sensor failure may be indicated. Alternately, a warm-up thermal TC slope may be computed to determine such a possible sensor failure. As power remains on the heater 420, after time period 1, the sensor continues to heat but stays at the self-regulation temperature. At time period 2 the medium presence portion of the method 900 (steps 930 to 960) ensues, wherein a thermal cool-down slope is identified. At time period 2, the heater 420 is turned off 1035 and a last self-regulated temperature Tmi measurement 1065 is recorded for future reference at temperature node 1002.
Between time periods 2 and 3, as the sensor cools down toward the temperature of the surrounding medium, the temperature detector 410 is again measured 1070 to determine the thermal decay rate time constant (TC) or slope (slope 1). At time period 3, a final temperature measurement Tmf for calculation of the slope 1 (1070) may be taken. The temperature difference between the self-regulation temperature Tmi and the final temperature measurement Tmf divided by the elapsed time (tdc, 1026) between these temperatures may be used for computation of slope 1. Alternately, two or more temperature measurements, such as 1002a and 1002b, and the elapsed time between the two measurements may be used for computation of slope 1. If the time constant of slope 1 is low as illustrated between time periods 2 and 3, the medium may be absent from contact with the sensor. Between time periods 3 and 4, heater power remains off 1035 and the temperature of the surrounding medium may be measured 1055 with temperature detector 410. This completes one full thermal cycle of the sensor wherein the temperature and presence of the medium is detected.
For example, when a low-water cut-off condition is encountered in a boiler, the medium (water) loses contact with the sensor and the computed slope is lower than a first predetermined TC limit. In such a case, water may be added to the boiler system.
Another thermal cycle of the sensor is illustrated starting at time period 4, wherein heater power is again applied 1040 to heat the sensor to the self-regulation temperature Tmi at time period 5, which is about 105° C. (about 221° F.) in the present example. The method continues between time periods 4-8 as described before between time periods 0-4, wherein a sensor verification temperature is taken between time periods 5 and 6, the allowable sensor warm-up time delay is verified (tdh 1024), and another TC slope, slope 2 is determined over elapsed time (tdc, 1028) between two or more temperature measurements, such as 1006a and 1006b used for computation of slope 2 for indicating medium presence. In this example, slope 2 illustrates a higher slope rate that is an indication of the presence of the medium at the sensor. For example, if water is now present at the sensor of the boiler example, the TC slope level, slope 2 is higher than the first predetermined TC limit. If however, slope 2 is less than a second predetermined TC slope level, this may be an indication of another possible sensor or system failure condition.
Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
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| Number | Date | Country | |
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