Inductive sensor for monitoring fluid level and displacement

Details Inductive sensor for monitoring fluid level and displacement Inductive sensor for monitoring fluid level and displacement

1998-03-19

2001-02-27

Kwok, Helen C. (Department: 2856)

Measuring and testing

Liquid level or depth gauge

Float

C073SDIG005, C073S29000R

Reexamination Certificate

active

06192753

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a sensor system for monitoring fluid level and displacement and, more particularly, to a sensor system for monitoring the level of crude oil in storage containers.
2. Background Art
There are many types of sensors known in the art for monitoring fluid level, and especially for measuring the level of crude oil in storage containers. Many of these sensors utilize a float designed to interact with the sensor. These sensors can be expensive in order to obtain the accuracy necessary and are often affected by the fluids and other contaminates associated with storage containers in which such fluids are stored. The following is a listing of known sensors for monitoring or measuring fluid level and disadvantages of such sensors.
Linearly Variable Differential Transformers (VDT) require high precision manufacturing of the coils and a sensor length of more than two times the useful length of the sensor. They also have resolution that is limited by the resolution of their data acquisition system and by the electrical noise of the whole system.
Ultrasonic transducers are affected by changes in pressure, temperature and other variations in the composition of the media in which they operate due to their sensitivity to the density of the media. This limitation thereby increases the probability of errors.
Reed switch arrays, used in the oil industry, provide an incremental readout with limited resolution. However, they are sensitive to shock and vibrations and can be damaged by electrical storms. Furthermore, they are labor intensive to manufacture, which makes them expensive, and are unreliable due to the hundreds of switch contacts and internal connections. The accuracy of such arrays is typically +/− 6.4 mm and clearance required between a float used with such reed switch arrays and the sensor elements must be between 0-3 mm. The arrays also require yearly cleaning and float replacement due to contaminant buildup.
Optical encoders are sensitive to contamination and are expensive. They also require high precision during manufacturing and implementation.
Magnetostrictive wave guide transducers are expensive and require high precision electronics. Also, the clearance between the float and the sensing element is limited.
Radar is expensive and has limited accuracy.
Capacitive probes are expensive and very sensitive to contamination. They also require high precision electronics and have a limited range.
Pressure transducers can be affected by contamination and have a resolution limited by the acquisition system employed.
It is an object of the present invention to provide a fluid sensor system that is inexpensive and easy to manufacture, has high reliability and accuracy, is easy to implement, and has a low sensitivity to contamination, shock, electrical storms and the media in which it operates.
It is an object of the present invention to provide a fluid sensor system having a float that includes an activatable resonator circuit.
It is an object of the present invention to provide a fluid sensor system having two or more floats each responsive to a different frequency excitation signal.
It is an object of the present invention to provide a fluid sensor system having two or more floats each responsive to a common excitation frequency and each having a resonating circuit that is activatable independent of resonating circuits of other floats.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention a sensor system is provided for measuring the level of a fluid. The sensor system includes a tube having a first end, a second end and a longitudinal axis extending therebetween. The sensor system further includes a float adapted to move adjacent the tube between the ends thereof and a control system. A primary coil is wound around the longitudinal axis between the first end and second end of said tube. The primary coil produces a time varying electromagnetic field adjacent to the tube in response to a time varying signal output by the control system. A first secondary coil is wound around the longitudinal axis at a first periodically varying winding density distribution between the ends of the tube. A second secondary coil is wound at the first periodically varying winding density distribution around the longitudinal axis between the ends of the tube. The winding density distribution of the second secondary coil is shifted relative to the winding density distribution of the first secondary coil. In response to excitation by the time varying electromagnetic field the first and second secondary coils produce first and second signals having amplitudes that vary periodically in response to movement of the float between the ends of the tube.
The fluid level sensor can include a third secondary coil wound at a monotonically varying winding density distribution around the longitudinal axis between the ends of the tube. In response to excitation by the time varying electromagnetic field the third secondary coil produces signals having amplitudes that vary monotonically in response to movement of the float between the ends of the tube. Alternatively, the fluid level sensor can include third and fourth secondary coils wound around the longitudinal axis at a second periodically varying winding density distribution between the ends of the primary coil. In this alternative, in response to excitation by the time varying electromagnetic field, the third and fourth secondary coils produce third and fourth signals having amplitudes that vary periodically in response to the movement of the float between the ends of the tube. The winding density distributions of the first and second secondary coils are repeated N cycles between the ends of the tube, wherein N is greater than 1. The winding density distribution of the third and fourth secondary coils are repeated M cycles between the ends of the tube, wherein M equals one of (i) N+1 and (ii) a number that does not have a denominator in common with N other than the number one.
The float of the fluid level sensor includes a combination resonator coil and capacitor or a ferromagnetic core. Alternatively, the float of the fluid level sensor includes a resonator coil connected in parallel with a series connected capacitor and switch. A switch control circuit is connected to the resonator coil and is connected to control the operation of the switch. In response to the primary coil of the fluid level sensor producing a time varying electromagnetic field at a first frequency, the resonator coil generates a signal at the first frequency. The signal is provided to the switch control circuit which controls the switch to be opened during a first interval determined by the first frequency and to be closed during a second interval determined by the first frequency. The first interval and the second interval occur at different intervals of time. Closing the switch connects the capacitor and the resonator coil in parallel to form a resonating circuit. The resonating circuit is active when the switch is closed and the capacitor and resonator coil are in parallel. Similarly, the resonating circuit is inactive when the switch is open.
The switch control circuit can include a reset control responsive to the resonator coil generating a signal at a second frequency. In response to detecting the second frequency, the reset control generates a reset signal which resets a counter of the switch control circuit. The counter is utilized to detect a first predetermined number of cycles of the first frequency corresponding to the first interval, and to detect a second predetermined number of cycles of the first frequency corresponding to the second interval. In response to detecting the cycles of the first frequency corresponding to the first interval and second interval, the counter causes the switch to be open and closed, respectively.
When the primary coil is excited at the first frequency, the control system samples during the first interval a compensation signal from each secondary coil

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