WO1983000765A1

WO1983000765A1 - Fluid level and temperature sensor

Info

Publication number: WO1983000765A1
Application number: PCT/US1981/001155
Authority: WO
Inventors: Tractor Co. Caterpillar
Original assignee: Caterpillar Tractor Co
Current assignee: Caterpillar Inc
Priority date: 1981-08-27
Filing date: 1981-08-27
Publication date: 1983-03-03
Anticipated expiration: 1984-02-27
Also published as: BE894100A; JPS58501393A; BR8109034A

Abstract

Un détecteur (10) permet d'indiquer la présence ou l'absence d'un liquide adjacent à ce détecteur (10). Le détecteur (10) possède un premier et un second thermistors espacés l'un de l'autre (26, 28) et un dispositif (38) permettant de chauffer de manière commandée le second thermistor (28). Une tension est appliquée sur un circuit comprenant chacun des thermistors (26, 28) et les tensions à l'entrée (30, 30') de chacun des deux thermistors (26, 28) sont comparées par un comparateur de tension (42). En réponse à une différence entre ces tensions supérieure à une valeur prédéterminée, le comparateur de tension (42) produit un signal destiné à activer soit un système d'alarme soit un système d'arrêt du moteur.A detector (10) makes it possible to indicate the presence or absence of a liquid adjacent to this detector (10). The detector (10) has first and second thermistors spaced from each other (26, 28) and a device (38) for controlled heating of the second thermistor (28). A voltage is applied to a circuit comprising each of the thermistors (26, 28) and the input voltages (30, 30 ') of each of the two thermistors (26, 28) are compared by a voltage comparator (42). In response to a difference between these voltages greater than a predetermined value, the voltage comparator (42) produces a signal for activating either an alarm system or an engine shutdown system.

Description

Fluid Level And Temperature Sensor

Technical Field

This invention relates generally to fluid level sensors and more particularly to electronic liquid level and temperature sensors.

Background Art

Engines and drive trains can be damaged by operation with insufficient coolant, lubricant, transmission fluid and the like. Consequently, it is advantageous to provide the various fluid reservoirs of an engine and vehicle with fluid level sensors to detect the condition of low fluid level and signal this condition to the operator. It is further advantageous to utilize the fluid level sensor as an automatic safety device serving to disable further operation of the engine should the level of a critical fluid fall dangerously low.

Typical of existing safety sensors of this type is that disclosed in U.S. Patent 2,700,153, issued January 18, 1955 to Huckabee. This sensor provides a warning system with an engine shutdown feature for a vehicle radiator. A pair of electrical contacts are placed at different levels in the radiator for initially signalling the loss of a predetermined amount of coolant and, if further coolant loss occurs, the second contact effects cut-off of the electrical supply to the ignition coil of the engine, causing the engine to shut down.

Other types of liquid level sensors utilize thermistors. It is well known that less heat will be transferred from a thermistor thermally adjacent a gaseous heat sink than will be transferred from a ther istor thermally adjacent a liquid heat sink. This has been the basis of such liquid level detectors as that disclosed by Waiwood in U.S. Patent 3,955,416 issued on May 11, 1976. In this device a resistor is utilized as a heat supply adjacent a thermistor. The thermistor is thermally coupled to the fluid of interest such that the thermistor has an increasing apparent heat capacity with increasing fluid level. A measurement of the resistance of the thermistor is utilized as a measurement of fluid level. Waiwood¹s sensor is, unfortunately, sensitive to sloshing of the fluid, the level of which is being measured, and is consequently of limited value for use in environments with significant vibration, as in vehicles and engines. It would be beneficial to develop a liquid level sensor which provides both a measure of fluid temperature and a measure of when fluid is in contact with the probe portion of the sensor. It would also be beneficial if this sensor was as rugged, dependable and simple as possible. It would be further beneficial if such a sensor could be developed which did not rely on contact of the fluid with a metallic contact or depend upon a precisely controlled heat supply.

The present invention is intended to overcome one or more of the problems as set forth above.

Disclosure Of The Invention

In one aspect of the present invention, a sensor has first and second spaced apart current impeding elements, these elements having a temperature dependent impedance. Means is provided for the heating of one of these elements. Further means is provided for generating a signal related to the difference in impedance between the two elements.

θ..-r? It is desired to have a sensor capable of indicating the fact of fluid in a fluid reservoir falling below a certain level. The present development includes a pair of thermistors in thermal contact with the fluid in the reservoir, one of these thermistors being heated. In response to the fluid level falling below the sensor, the apparent heat capacity of the heated thermistor decreases, serving to increase the temperature difference between the two thermistors. This increase in the temperature difference is measured by the sensor as a resistivity difference. The sensor provides an output in response to this resistivity difference exceeding a preselected valve.

Brief Description Of The Drawings

For a better understanding of the present invention, reference may be had to the accompanying drawings in which:

Fig. 1 shows a diagrammatic view of an embodiment of the present invention situated in a fluid reservoir; and

Fig. 2* shows in greater detail a diagrammatic representation of electronics suited for use with the present invention. it is to be understood that the drawings are not intended as a definition of the invention, but are provided for the purpose of illustration only.

Best Mode For Carrying Out The Invention Referring to the drawings, a fluid level and temperature sensor embodying certain of the principles of the present invention is generally indicated by the reference numeral 10.. It should be understood that the following detailed description of the fluid level and temperature sensor 10 relates to the best presently known embodiment of this advance. The fluid level and temperature sensor 10 can assume numerous other embodiments, as will become apparent to those skilled in the art, without departing from the claims.

The sensor 10 has a housing 12 having a periphery 13 and projections 14. These projections 14 define an intermediate spacing or gap 16. This gap 16 can be a physical absence of material intermediate projections 14 or can be a region of relatively high insulative characteristics. In short, this gap 16 is a ° thermal gap. Preferably the housing 12 has a threaded portion 18 of a construction sufficient for being received into a threaded aperture 20 in a wall 22 of a fluid containing chamber 24. The housing 12 is preferably made of a relatively inert, strong, 5 dielectric material capable of withstanding the anticipated temperature range to which it will be exposed. It is further preferable that the material from which the housing 12 is made have a low thermal capacity. 0 Current impeding elements, such as a first and a second thermistor 26,28, are positioned, respectively interior a corresponding one of the two projections 14. Preferably, these thermistors 26,28 are matched to have substantially identical voltage-temperature 5 relationships. Each thermistor 26,28 has an input 30,30' and an output 32,32'. The inputs 30,30' are connected to means 34 for applying an input voltage to each of the thermistors 26,28. This voltage applying means 34 includes a conductor 35 connecting the ⁰ thermistor input 30,30' to a voltage supply 36 which provides stable D.C. voltage. Each of the conductors 35 includes a resistor 37. Preferably these resistors 37 are matched.

Controllable heating means 38 is positioned in ⁵ thermal a jacence to the second thermistor 28.

Preferably, the controllable heating means 38 is also

iRE. C;M?I positioned in thermal adjacence to the periphery 13 of the sensor 10. The controllable heating means 38 preferably includes a heating resistor 39 and a power supply. In the preferred embodiment, the heating resistor 39 is connected to the output of the voltage supply 36. The resistance of the heating resistor 39 is preferably less than that of the thermistors* 26,28^' over the expected range of operating temperatures. As a consequence, the heat dissipated by the heating resistor 39 is significantly greater than that dissipated by the thermistors 26,28. It is preferable that the thermistors 26,28 not generate enough heat to raise their temperature by more than a few degrees above the ambient. Means 40 for monitoring the impedance of the first and second impeding elements 26,28, for comparing the impedances, and for controllably establishing an output signal in response to the measured impedances differing one from the other by an amount within a predetermined range, is electrically connected to the inputs 30,30' of the thermistors 26,28. Preferably this receiving means 40 includes a voltage comparator 42. The voltage comparator 42, in the preferred embodiment, has an output terminal 43 providing an output in response to the voltage inputs to the comparator 42, that is the voltage at the inputs 30,30' of the thermistors 26,28, differing by more than a preselected amount. This output may be utilized to control, for example, a switch 44, which can be a transistor. Preferably, the output terminal 43 of the voltage comparator 42 is connected to a control terminal 46 of the switch 44 which also has an input and an output terminal 48,50. The output provided by the comparator 42 in response to the input voltages differing by more than a preselected amount or set point can be utilized to activate a warning device, a fuel shutoff device for a vehicle, or the like. This can be accomplished by placing the switch 44 in series with the power circuit controlling the warning device or fuel shutoff device. The preselected voltage difference activation point should be set at a level substantially above the small differences in output voltage of the thermistors 26,28 occurring when the sensor 10 is immersed in liquid, yet should not be so great as to prevent the comparator output voltage difference from exceeding it within a few seconds of the fluid being removed from the sensor 10.

The output from the first thermistor 26 is also connected to means 41 for displaying the temperature of the first thermistor 26. The method for converting the resistance of a known thermistor into a temperature reading is well known to those skilled in the art. This provides a true measure of the fluid temperature since the first thermistor 26 is unheated.

Industrial Applicability The present invention is best suited for use in applications in which it is necessary to determine when the level of a fluid has fallen below a certain point. This sensor is especially well suited for detecting low coolant level in radiators. The fluid level sensor may also be utilized for detecting fluid temperature and low fluid level in lubricant systems, brake systems, hydraulic systems, transmission fluid systems, fuel tanks, and the like. It is important to realize that the performance of the fluid level and temperature sensor is not significantly affected by the temperature of the fluid or even by relatively rapid changes in the temperature of the fluid. Consequently, the fluid level and temperature sensor can be utilized, substantially or entirely unaltered, in a variety of applications.

In the operation of the present invention, a liquid normally surrounds the sensor 10. As liquids

O. FI typically have relatively high coefficients of heat transfer, the heat generated by the heating resistor 39 is largely dissipated into the liquid. Consequently, when the sensor 10 is immersed in fluid the two thermistors 26,28 are at substantially the same temperature. As these thermistors are matched and receive substantially the same input voltage, the voltages at their inputs will have a small, relatively fixed difference. This fixed difference is due in part, of course, to the heating of the second thermistor 28. This fixed difference will remain relatively constant as the temperature of the fluid varies. Consequently, when the probe portion of the sensor 10 is mmersed in fluid, the voltage comparator 42.will not detect a significant voltage mismatch situation and, hence, will not provide a switch activating output. The bias point of the comparator 42 is, of course, selected to be greater than the previously mentioned fixed difference in thermistor output for liquid contact situations.

It will be recognized by those skilled in the art that the comparison of the thermistor resistances is obtained through use of voltage dividers. As stated, each of the thermistors 26,28 is in a circuit in which a known voltage source 36 is grounded through a serial combination of a resistor 37 and the corresponding one of the thermistors. That is, each thermistor 26,28 has associated with it a resistor 37. These resistors 37 are preferably matched such that the voltages intermediate each of the thermistors 26,28 and the corresponding one of the resistors 37 are substantially equal when the thermistors 26,-28 are at equal temperatures. In the present embodiment the compared voltages are taken at the thermistor inputs 30,30'; however were the resistors 37 positioned intermediate the thermistors 26,28 and ground the voltage comparison could be of the thermistor output voltages. In either case, this usage of voltage dividers permits comparisons of the relative resistances of the thermistors 26,28 to be made through use of the comparator 42. When the liquid does not surround the sensor

10, however, heat from the second resistor 39 dissipates relatively slowly. This causes the temperature of the second thermistor 28 to rise significantly above that of the first thermistor 26. The resistances of these thermistors 26,28 will then differ significantly, resulting in the voltages at their input terminals 30,30' differing significantly. This voltage differential is initially small and increases with increasing time from the instant thac the liquid is no longer in contact with the sensor 10. After a period of time the voltage differential will stabilize at a relatively high level. The voltage comparator bias point is selected to be smaller than this voltage differential after a short period of time, preferably in the range of about 1-10 seconds, following removal of the liquid from the sensor 10. The voltage comparator 42 detects this condition and provides an output to activate the switch 44 in response thereto. The switch 44 may be placed in a warning circuit or vehicle shutdown circuit. This signal is utilized to activate either a warning system or a vehicle shutdown system.

Preferably, as shown in Fig. 1, the sensor 10 is mounted vertically. This serves to ensure that both projections 14 of the sensor experience loss of liquid contact at substantially the same point in time.

It should be noted that the fluid level and temperature sensor 10 can assume many other configurations without departing from the claims. Other aspects, objects, advantages and uses of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.

Claims

Clai s

1. A fluid level sensor (10), comprising: first and second spaced apart electrical current impeding elements (26,28), said impeding elements (26,28) having a temperature dependent impedance; means (38) for controllably heating one of said first and second impeding elements (26,28); _Q means (40) for monitoring the impedance of said first and second impeding elements (26,28) , and for controllably establishing an output signal in response to said impedances differing one from the other by an amount within a preselected range. 5

2. The fluid level sensor (10), as set forth in claim 1, wherein said monitoring and establishing means (40) delivers a signal in response to the impedances of said first and second impeding elements o (26,28) differing by greater than a preselected amount.

3. The fluid level sensor (10), as set forth in claim 1, wherein said first and second impeding elements (26,28) are thermistors. 5

4. The fluid level sensor (10), as set forth in claim 1, wherein said controllable heating means (38) includes a resistor.

5. The fluid level sensor (10) , as set forth in claim 1, wherein each of said current impeding elements (26,28) is connected between ground and a voltage supply (36) , the two current impeding elements (26,28) being in parallel one with respect to the other, each of said current impeding elements (26,28) having series connected therewith a corresponding resistor (37) , said monitoring and establishing means (40) including a voltage comparator (42) having two inputs, one of said inputs being connected to the junction of said first current impeding element (26) and the corresponding resistor (37) and the other of said inputs being connected to the junction of the second current impeding element (26) and the corresponding resistor (37) .

6. The fluid level sensor (10) , as set forth in claim 1, further including a housing (12) containing said first and second impeding elements (26,28) , said housing defining a gap (16) intermediate said first and second impeding elements (26,28) .

7. The fluid level sensor (10) , as set forth in claim 1, wherein said monitoring and establishing means (40) includes a voltage comparator (42) having an output (43) and a switch (44) in electrical connection with and controlled by said comparator output (43) , said switch (44) having an open and a closed condition, said switch (44 ) being adapted for changing from one of said open and closed conditions to the other of said open and closed conditions in response to said voltage comparator (42) delivering an output signal of a preselected magnitude.

8. The fluid level sensor (10) , as set forth in claim 1, wherein said controllable heating means (38) is adjacent said second impeding element (28) , and said sensor (10) further includes means (41) for receiving a signal from one of an input and an output (30,32) of said first and second impeding elements (26,28) and delivering a signal that varies directly with changes in the impedence of said one of said first and second impeding elements (26,28) .

9. The fluid level sensor (10) , as set forth in claim 8, wherein each of said current impeding elements (26,28) is connected between ground and a voltage supply (36) , the two current impeding elements (26,28) being in parallel each with respect to the other, each of said current impeding elements (26,28) having series connected therewith a corresponding resistor (37) .

10. A liquid level sensor (10) , comprising: a housing (12) ; first and second electrical resisting elements (26,28) in said housing (12) , the resistance of said resisting elements (26,28) being temperature dependent, each of said resisting elements (26,28) having an input (30,30') and and output (32,32'); means (38) for controllably heating one of said first and second resisting elements (26,28) ; a voltage supply (36); a resistor (37) in series with each of said resisting elements (26,28) , the two resistor-resisting element combinations (37,26) , (37,28) each being in parallel one with respect to the other, and each being connected between said voltage supply (36) and ground; and means (40) for comparing the voltages intermediate each of said electrical resisting elements (26,28) and the corresponding one of the resistors (37) and for providing a preselected output signal in response to the difference between said voltages being within a preselected range.

11. The liquid level sensor (10) , as set forth in claim 10, further including a switch (44) having first and second states, said switch ( 44 ) being connected to an output -(43) of said comparing and

C PI providing means (40) and being adapted to switch from one of said states to the other of said states in response to receiving said preselected output signal.