This project, a capacitive liquid level sensor from 1989, uses an ingenious technique where the senor is located outside the liquid container.
This is a very useful technique and has been used in later designs to create a portable liquid level sensor for locating the exact height of liquid in a plumbing system.
Normal capacitive liquid level sensors work by having the sensor within the liquid and sensing the capacitance change when as they are exposed when the liquid level falls. This has the disadvantage of plate electrolysis when an ac signal is developed across the capacitive sensor. This design completely avoids that problem.
A non-intrusive fluid level detector including a single point
capacitive sensor mounted on the outside surface of a receptacle such
that capacitive principles can be utilized to sense the level of a
liquid contained within the receptacle. The sensor assembly is disposed
in a substantially fixed position on the exterior wall of the
receptacle wherein the dielectric effect of the liquid changes the
effective capacitance of the sensing capacitor as the liquid rises and
falls within the receptacle.
This change in effective capacitance is detected by electronic circuitry included in the detector device. In one embodiment, the fluid level detector is directly mounted to a completely non-conductive receptacle. In another embodiment, the fluid level detector is mounted to a non-conductive window which is an integral part of a receptacle fabricated out of a conductive material.
Fluid level detectors which sense the level of a liquid
contained within a receptacle are well known for use in conjunction
with automotive engines. Historically, such detectors have been made in
the form of float operated switches involving moving parts which are
subject to friction and wear.
Other devices utilize an electrical probe to detect fluid levels by measuring the conductivity of the coolant. However, these devices require complicated current amplifying systems because there is often an insufficient amount of current passing through the electrodes to power an indicator lamp.
In either of these systems, the measuring sensor is located in the fluid where contaminants are likely to collect on the sensor and disturb the fluid level measurement. Furthermore, the devices in the prior art, as described, are categorized as "intrusive" in that they require an opening into the receptacle containing the fluid.
This creates an additional potential for fluid leaks as well as potential deterioration of the sensing devices.
Due to the problems discussed above, it has become desirable to employ non-intrusive means to sense fluid levels contained in receptacles. These non-intrusive systems typically involve wave propagation techniques which are implemented through a transmitter/receiver system. Typically, these systems involve the transmission of an ultrasonic signal from a transmitting transducer through a fluid to a receiving transducer.
Such ultrasonic transmission systems require a liquid transmission medium in order to carry the ultrasonic signal from the transmitting transducer to the receiving transducer. Lack of signal at the receiving transducer relates to a lack of liquid transmission medium, activating a no fluid present indication.
However, a failure in the transducer pair or in their respective electronic connections results in a lack of signal from the receiving transducer which, in turn, activates the normal failure mode thereby falsely indicating a dry or no fluid present state.
The increasing importance of monitoring fluid levels in automotive and other applications is creating a need for more reliable non-intrusive fluid level sensors. It is, therefore, important that a fluid level detection system be developed which can provide reliable data and which does not require contact with the fluid being measured.
In accordance with the present design a non-intrusive fluid
level detector is provided for mounting on the exterior wall of a
receptacle. The present design provides single point sensing of
predetermined low levels of fluids, as in an automobile cooling system
or a windshield washer solvent reservoir, without contact with the
liquid being sensed.
The design also provides an electrical signal which can energize an alarm light or other indicator. The fluid level detector of the present design incorporates capacitive sensor means established relative to a fluid receptacle in a substantially fixed relation on the outside wall of the receptacle.
The capacitive sensor can be made to be an integral part of a printed circuit containing electronic detection circuitry, thereby making the device self-contained. This sensor assembly is mounted on the exterior wall of the fluid receptacle at a position to sense the lower limit of acceptable fluid level.
The present design utilizes capacitive principles to sense the level of a liquid contained within a non-metallic receptacle. As a liquid rises and falls in the container, the dielectric effect of the liquid changes the effective capacitance of the sensing capacitor which is detected by electronic circuitry coupled to the sensor. The device remains activated whenever power is applied and provides an indication to the user only when the low liquid level is detected.
The present design is a passive device in that the device monitors the level of the liquid within the receptacle at all times and requires no interaction or other monitoring by the user.
FIG. 1 (A) illustrates the circuit diagram of one embodiment
of the fluid level detector of the present design. The supply voltage
for this circuit is typically provided by a 12-volt automobile battery
which is reduced to a 5-volt DC source voltage by means of a voltage
regulator comprising the combination of resistor R1 and zener diode Z1.
Capacitor C1 serves as a filter for this voltage supply regulator.
The detector of FIG. 1A incorporates an amplifier A1 in conjunction with a resistor/capacitor network R4 and C2 and resistors R5 and R6 to form a square wave oscillator. A reference voltage is supplied from the voltage regulator where the reference voltage value is determined by the voltage divider circuit R2/R3.
This voltage serves as a reference voltage for the square wave oscillator and voltage comparators A2, A4 located in the circuit, as will be described.
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