Proximity sensor for level sensing

A field effect sensor is adapted for use in sensing level of fluids and powders. The sensor uses elongated, parallel electrodes disposed on or within the side wall of a tank. The longitudinal axes of the electrodes are parallel to the surface of fluid or powder contained in the tank.

BACKGROUND OF THE INVENTION

1. The Technical Field

The present invention is directed generally to level sensing. More particularly, the present invention is directed to proximity sensors having electrodes adapted for sensing level of fluids and other substances.

2. The Prior Art

It often is convenient or necessary to know the level of fluid in a tank or other container. Known means for doing so include sight glasses, measuring sticks, floats with mechanical linkages which indicate level and floats connected to electrical sending devices. Though widely used, these types of level sensing equipment are not without shortcomings. Whereas sight glasses can provide highly accurate, visual indication of fluid level, they generally must be located at or near the tank whose fluid level is to be measured, and they generally cannot be used to provide remote level indication. Further, the top and bottom of a sight glass generally must be plumbed into the side wall of the tank whose fluid level is to be measured, increasing the potential for fluid spills. Measuring sticks, such as dip sticks, also require presence at the tank whose fluid level is to be measured, and they cannot readily be used remotely. Measuring sticks have the further disadvantage that they must be physically inserted into the fluid whose level they are measuring. As such, their use increases the chance of contaminating the fluid being measured.

Floats with mechanical linkages for level indication are often used in small power equipment, such as lawn mowers, garden tractors, and the like. Such devices can provide reasonably accurate indication at relatively low cost. However, they generally provide only local indication and are not readily adapted for providing remote indication. Further, they are prone to failure due to vibration, exposure to the elements, and other harsh environmental conditions during ordinary use.

Floats with mechanical linkages connected to electrical senders have long been used to detect and provide remote indication of fluid level in tanks, such as automobile gas tanks. Such devices typically are mounted inside a tank and require sufficient space inside the tank to allow movement of the float and linkage as the fluid level rises and falls. As such, devices of this nature place constraints on tank design and packaging efficiency. Further, such units operate on the assumption that the tank cross section from top to bottom is substantially uniform, such that fluid volume within the tank is simply a function of the height of fluid in the tank. Such units typically would not give accurate data when used in tanks with irregular cross sections. Although multiple units could be used to mitigate this concern, such use would add cost, complexity, and might not be feasible in all situations due to space constraints.

Field effect sensors can detect proximity of some fluids, such as water. However, conventional field effect sensors are not sensitive to certain other fluid types, for example, hydrocarbons such as gasoline.

SUMMARY OF THE INVENTION

The present invention senses level of a fluid or powder uses a proximity sensor having elongated, generally parallel electrodes, each having a longitudinal axis generally parallel to the surface of the fluid or powder the level of which is to be measured.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1illustrates a field effect sensor10located on the side wall22of a tank20containing a fluid, such as the gas tank of an automobile. In other embodiments, tank20could contain multiple fluids or a powder. Preferably, sensor10is located on the outside of tank20, but also could be located on the inside of tank20. Alternatively, sensor10could be embedded within the side wall22of tank20.

Sensor10includes first and second, substantially parallel, electrodes12,14coupled to a control circuit16. Preferably, control circuit16is embodied as the control circuit provided with the TS100 sensor available from TouchSensor Technologies, LLC of Wheaton, Ill. Many of the design and operating principles of the TS100 sensor are described in U.S. Pat. Nos. 6,230,282 and 6,713,897 and related U.S. patent application Ser. Nos. 10/272,377 10/725,908, the disclosures of which are incorporated herein by reference.

Electrodes12,14differ from conventional sensor electrodes in that they are generally elongated and parallel. Preferably, electrodes12,14are disposed on tank20such that their longitudinal axes are substantially parallel with the surface of the fluid contained within tank20. Generally, the greater the ratio of electrode length to width, the more quickly sensor10responds to stimuli proximate to an electrode, as discussed further below. Also, closely spaced pairs of electrodes provide greater resolution. That is, a sensor10using a closely spaced pair of electrodes generally is more sensitive to small changes in level a sensor10using a widely spaced pair of electrodes. However, a sensor using a closely spaced pair of electrodes may be more prone to providing erratic indication resulting from, for example, sloshing of fluid within tank20.

Electrodes12,14can be embodied in many different forms. For example, they can comprise thin, parallel, equal length planar traces, as illustrated inFIG. 1. They can comprise cylindrical rods of unequal length, as illustrated inFIG. 2. In other embodiments, they can resemble unequal length planar traces or equal length cylindrical rods. They also can comprise rods of dissimilar diameter. Their overall shapes and cross-sections can vary, as well. In general, similar electrodes respond to similar stimuli substantially equally. An electrode that is longer, wider, or of greater cross-sectional area than another electrode generally is more sensitive to a given stimulus. This principle can be used to tailor a sensor's sensitivity and ability to reject common mode interference as needed or desired in connection with a given application. Generally, improved sensitivity comes with decreased ability to reject common mode interference. WhereasFIGS. 1 and 2show electrodes12,14as generally linear, electrodes12,14can be configured to wrap around or otherwise conform to the side wall of tank20.

Sensor10preferably is disposed on a flexible or rigid substrate (not shown) which is bonded to or otherwise integrated with tank20. For example, the substrate bearing sensor10can be embedded within the side wall of tank20. Alternatively, sensor10can be disposed directly onto or embedded within tank20, omitting the substrate.

When both electrodes12,14sense the same medium, for example, air/vapor above the surface of gasoline in an automobile's gas tank, both electrodes12,14have similar capacitance-to-ground. Put another way, when both electrodes12,14sense the same medium, the electric field coupling of each electrode to ground is substantially the same, resulting in negligible electric field potential between the two electrodes. In this condition, sensor10is in the “off” state. As the liquid level rises, covering lower electrode14, the electric field potential between lower electrode14and upper electrode12increases until it is great enough to switch sensor10to the “on” state, as would be known to one skilled in the art. As the liquid level continues to rise, covering upper electrode12, the electric field potential between upper electrode12and lower electrode14returns to a negligible level. In this condition, sensor10returns to the “off” state. (The foregoing discussion assumes that both electrodes are similarly configured. The capacitance-to-ground of the two electrodes could differ in the condition where both electrodes sense the sam medium if one electrode is longer, larger, or otherwise configured substantially differently than the other, as would be understood by one skilled in the art. Thus, a sensor's response to level changes in tank20can be adjusted by adjusting the structure of electrode12relative to the structure of electrode14.)

FIG. 3illustrates how a plurality of sensors10A–10C disposed on or embedded within the side wall of a tank20can be used to provide substantially continuous indication of the fluid level within the tank. When the fluid level is lower than the lower electrode14A of lowermost sensor10A, each of sensors10A–10C is in the “off” state. When the fluid covers only lower electrode14A of lowermost sensor10A, sensor10A is in the “on” state and sensors10B,10C are in the “off” state. When the fluid also covers lower electrode14B of intermediate sensor10B, sensors10A,10B are in the “on” state and sensor10C is in the “off” state. When the fluid also covers upper electrode12A of sensor10A, sensor10A is in the “off” state, sensor10B is in the “on” state, and sensor10C is in the “off” state. When the fluid also covers lower electrode14C of uppermost sensor10C, sensor10A is in the “off” state and sensors10B,10C are in the “on” state. When the fluid also covers upper electrode12B of sensor10B, sensors10A,10B are in the “off” state and sensor10C is in the “on” state.

In the preferred embodiment, the outputs VoutA–VoutCof sensors10A–10C are coupled to a microcomputer (not shown) which converts the sensor outputs to level indication. For example, with sensor10A in the “off” state and sensors10B and10C in the “on” state (corresponding to the condition where electrodes12A,14A,14B, and14C are covered), the microcomputer (not shown) would provide an output indicating that tank20is about half full, assuming that tank20has a substantially uniform cross section. If tank20has non-uniform cross-section, the tank geometry can be taken into account in the microcomputer's analysis so as to yield an accurate level indication.

Though described above in terms of measuring the level of a single fluid, the present invention also can be used to measure the level of a powder in a container, or to measure the level of an interface between different liquid layers in a container. One skilled in the art would know how to modify the teachings of this disclosure without departing from the scope of the claims which define the invention.