Flow sensor having high impedance circuit with capacitive sensing electrode

• US 5,392,657 A
• Filed: 02/22/1993
• Issued: 02/28/1995
• Est. Priority Date: 02/13/1991
• Status: Expired due to Fees

• Current Assignee: ONICON, Inc. (TASI Holdings, Inc.)
• Original Assignee: ONICON, Inc. (TASI Holdings, Inc.)
• Inventors: Feller, Murray F.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): MAH, RAYMOND
• Application Number:
  US08/020,908
• Time in Patent Office: 736 Days
• Field of Search: 73/861.77, 73/861.78, 361/285, 331/665, 340/870.37, 340/870.31, 324/160, 324/163, 324/671, 324/663, 324/686
• US Class Current: 73/861.77
• CPC Class Codes: G01F 1/106 with electrostatic coupling to the indicating device

Abstract

Apparatus is provided in which flow to be monitored actuates a device to move past a sensing electrode, causing changes of a capacitive sensing impedance in series with an impedance transformation device. The sensing electrode is at the high-impedance level of the impedance transformation device. Only a small fraction of the stray capacitance of circuit wiring, etc., which is connected at the low-impedance level of the impedance transformation device is reflected as a dulling shunt across the sensing capacitance.

Claims

1. Flow sensing apparatus including a series circuit having opposite end terminals and having a sensing capacitor and an inductor in series with each other interposed between said opposite end terminals, said sensing capacitor having a capacitive sensing electrode and a companion electrode which, when energized in operation, have a distributed field between them in a passage for fluid whose flow is to be monitored and said sensing capacitor having a flow-actuated device operated by the fluid through a range of positions in relation to the sensing electrode and to said field for developing flow-representing changes in said sensing capacitor, said inductor having first and second inductor terminals, said capacitive sensing electrode being in close proximity to and having a direct connection to said first inductor terminal, said direct connection being connected to the remainder or the apparatus only via said sensing capacitor and said inductor, said companion electrode and said second inductor terminal being connected to said opposite end terminals, respectively, and circuit means connected to and interposed between said opposite end terminals of the series circuit, for deriving flow-representing signals in response to said flow-activated device, said apparatus having excitation means for providing said series circuit with constant-frequency excitation at or close to the series-resonant frequency of the series circuit.

2. Flow sensing apparatus as in claim 1 wherein said excitation means includes an oscillator for generating said constant-frequency excitation and wherein said oscillator includes an amplifier connected to the first and second end connections of the series circuit.

3. Flow sensing apparatus as in claim 1 including an amplifier having a variable gain-control circuit that includes said series circuit, and means acting through said amplifier to impress constant-frequency excitation on said series circuit, the excitation frequency being at or close to a frequency at which the series-circuit is series-resonant when the flow-actuated device has any particular relationship to said sensing electrode.

4. Flow sensing apparatus as in claim 1 wherein said excitation means includes a source of constant-frequency excitation and means for impressing the excitation on said series circuit, the excitation frequency being at or close to a frequency at which the series circuit is series-resonant.

5. Flow sensing apparatus as in claim 4 wherein the flow-actuated device has a dielectric constant that contrasts significantly with the dielectric constant of the fluid whose flow is to be sensed, whereby the flow-actuated device causes the reactance of the capacitive impedance at the excitation frequency to vary when the flow-actuated device is flow-driven.

6. Flow sensing apparatus as in claim 4 wherein the flow-activated device is prominently lossy, whereby the flow-actuated device causes variations of effective resistance in the capacitive impedance when the sensing electrode is excited and the flow-actuated device is driven by fluid whose dielectric constant approximates the dielectric constant of said flow-actuated device.

7. Flow sensing apparatus as in claim 1 wherein said companion electrode constitutes one of said end connections of the series circuit.

8. Flow sensing apparatus as in claim 7 wherein said flow-actuated device is a rotor having flow-actuated vanes movable successively into proximity to the sensing electrode.

9. Flow sensing apparatus as in claim 1 wherein said circuit means includes an amplifier having an impedance-responsive gain-control circuit, said series circuit being connected in said gain-control circuit and being an impedance rendered variable by operation of said flow-actuated device.

10. Flow sensing apparatus as in claim 9 wherein said gain-control circuit is arranged to impress said excitation on said series circuit.

11. Flow sensing apparatus as in claim 1 wherein said sensing capacitor and said inductor constitute an assembly and wherein said apparatus includes an elongated support.

12. Flow responsive apparatus as in claim 11, further including an elongated support having an assembly at an end thereof, said assembly comprising said inductor, said junction and said sensing capacitor, and said elongated support being adapted to extend through the wall of a pipe and to support said assembly within the pipe.

13. Flow sensing apparatus for monitoring the flow of fluid, said apparatus comprising a circuit which constitutes an amplifier, said amplifier having at least one amplifying device, and said amplifier having a pair of terminals and gain control impedance connected between said terminals for providing substantial control of the gain of the amplifier, said gain control impedance including variable impedance that changes in value in dependence on the frequency of the excitation signal applied to it and whose impedance varies in dependence on the flow of fluid to be monitored, said apparatus having means for providing a constant-frequency excitation signal and for applying said excitation signal to said variable impedance, and said apparatus having means for deriving from said variable impedance a signal representing the flow to be monitored.

14. Flow sensing apparatus as in claim 13, wherein said excitation signal applying means includes an oscillator that comprises said amplifier.

15. Flow sensing apparatus as in claim 13 wherein said gain control impedance comprises a capacitive impedance and an inductive impedance connected to each other as a series circuit, one of said impedances comprising said flow-responsive variable impedance means.

16. Flow sensing apparatus as in claim 13 wherein said flow-responsive variable impedance means comprises a sensing electrode which, in operation, has a distributed field that extends to a first one of said pair of gain-control terminals and said flow-responsive variable impedance means further including a fluid-actuated rotor having vanes of plastic which, in operation, sweep successively close to the sensing electrode, said gain control impedance means including an inductor having a first end connection of minimal length directly connected only to said sensing electrode and said inductor having a second end connection connected to the second one of said pair of gain-control terminals.

17. Flow sensing apparatus as in claim 16 wherein said inductive impedance comprises a resonant crystal device whose resonant frequency is so related to the excitation frequency that said resonant crystal device simulates an inductive impedance.

18. Flow sensing apparatus as in claim 15, wherein said series circuit is a series-resonant circuit having a connection of minimal length joining said impedances to each other and said series circuit having separate connections at its opposite ends constituting said pair of terminals.

19. Flow sensing apparatus including a probe for insertion within an opening of a wall of a conduit having an elongated support bearing at an end thereof an assembly comprising (a) a capacitive electrode, (b) a flow activated vaned rotor supported close to said electrode so that, in operation, vanes of the rotor move in succession past said electrode, (c) an inductive impedance disposed in close proximity to said electrode, said inductive impedance having first and second terminals, and (d) a junction that forms a direct connection from said capacitive electrode only to said first terminal of said inductive impedance.

20. Flow sensing apparatus as in claim 19 further including circuit means remote from said assembly for deriving a flow-representing signal representing the operation of said vaned rotor, said circuit means including an amplifier having an impedance-responsive variable gain-control circuit that comprises said inductive impedance and said capacitive sensing electrode, and an oscillator comprising said amplifier for developing an excitation signal which is applied to the gain-control circuit by the amplifier.

21. Apparatus for sensing the flow of fluid in a passage, including a sensing capacitor comprising a sensing electrode and a companion electrode, said electrodes when in operation developing a distributed field between them in said passage, a flow-actuated rotor having vanes operable successively past said sensing electrode for interacting variably with said field and thereby developing flow-representing variations of impedance of said sensing capacitor, circuit means for both applying excitation to and deriving flow-representing output signals from said sensing capacitor, and an impedance transformation device interposed between said circuit means and said sensing electrode for transforming the impedance from a relatively high level prevailing at said sensing electrode to a relatively low level prevailing at said circuit means, said impedance transformation device having a high impedance terminal disposed in close proximity to said sensing electrode and connected thereto, and said impedance transformation device having a low-impedance signal output terminal, said circuit means having respective connections to said companion electrode of the sensing capacitor and to said low-impedance signal output terminal of the impedance transformation device, said connections of said circuit means to said low-impedance signal output terminal and to said companion electrode having distributed capacitance that is largely isolated by said impedance transformation device from said capacitive sensing electrode.

22. Apparatus for sensing the flow of fluid as in claim 21, wherein said impedance transformation device is an inductor which, with said capacitive sensing impedance, is at or close to series resonance at the frequency of said excitation when vanes of the rotor have any particular relationship to said sensing electrode.

23. Apparatus for sensing the flow of fluid as in claim 21, wherein said impedance transformation device is a semi-conductor-follower device.

24. Apparatus for sensing the flow of fluid as in claim 21, including an elongated support and an assembly carried at one end thereof, said assembly including said sensing electrode and said rotor and said impedance transformation device, said support and said assembly forming a probe proportioned for insertion into a pipe via a hole in the wall of the pipe.

25. Flow-sensing apparatus including:

a flow-responsive capacitor comprising a capacitive sensing electrode and a companion electrode which, when energized in operation, have a distributed field between them, and a device operable by flowing fluid into varying physical relationships to said sensing electrode and to said field for causing variations in the impedance of said flow-responsive capacitor,

an inductor having multiple terminals including first and second terminals, said first terminal being in close proximity to said sensing electrode,

means of minimal length for connecting said sensing electrode only to said first terminal of the inductor, said flow-responsive capacitor and said connecting means and said inductor forming a series circuit, said companion electrode of the sensing capacitor and said second terminal of the inductor being opposite end terminals of the series circuit, and

circuit means connected between said companion electrode and said second terminal of the inductor for applying constant-frequency excitation to and deriving flow-representing signals from said series circuit.

26. Flow-sensing apparatus as in claim 25, wherein said flow-activated device is a vaned turbine whose vanes have edges that sweep past the sensing electrode in response to fluid flow.

27. Flow-sensing apparatus as in claim 25, wherein said flow-activated device, in operation of the apparatus, is immersed in the fluid whose flow is to be monitored, and wherein said sensing electrode is covered by protective insulation.

28. Apparatus for sensing the flow of fluid in a passage, including a capacitive sensing impedance comprising a sensing electrode and a companion electrode, said electrodes when energizes in operation, developing a distributed field between them in said passage, a flow-actuated rotor having vanes operable successively past said sensing electrode for interacting variably with said field and thereby developing flow-representing variations of said capacitive sensing impedance, an inductive impedance having first and second terminals, said first terminal being in close proximity to said sensing electrode and connected thereto, said capacitive impedance and said inductive impedance forming a series-resonant circuit, said companion electrode and said second terminal of said inductive impedance, constituting opposite terminals of said series-resonant circuit, and circuit means interposed between and connected to the opposite terminals of said series-resonant circuit for both applying excitation to and deriving flow-representing signals from said series-resonant circuit.

29. Apparatus for sensing the flow of fluid in a passage, said apparatus including

a probe including an elongated support and a sensing assembly at an end of said elongated support, said assembly including a capacitive sensing electrode and an inductive impedance, the latter having first and second terminals, said first terminal of said inductive impedance and said capacitive sensing electrode being connected to each other at a junction, said apparatus having means acting as a companion capacitive electrode, and said sensing assembly having a flow-activated rotor including vanes which, as the rotor turns, sweep along a path close to said capacitive sensing electrode so that said sensing capacitive electrode and said companion capacitive electrode are terminals of a capacitive impedance which is rendered variable by said rotor, said capacitive impedance and said inductive impedance thus being connected in series in a series-resonant circuit, said junction being at a first impedance level of the series-resonant circuit and both said companion electrode and said second terminal of the inductive impedance being at a second impedance level of the series-resonant circuit, said first impedance level and said second impedance level being relatively high and low, respectively, in relation to each other,

circuit means for both providing excitation to and deriving flow-representing variations from said series-resonant circuit,

and a line extending at least along said elongated support for interconnecting said circuit means and the second terminal of said inductive impedance, said apparatus including connection means from said companion electrode to said circuit means, said line having distributed capacitance to said connection means at said second impedance level.

30. Flow responsive apparatus including a flow sensing capacitor that includes a sensing electrode and a companion electrode, said sensing capacitor including flow-activated means for varying the impedance between said electrodes, an inductor having first and second terminals, said first terminal of the inductor and the sensing electrode being in close proximity to each other, a junction constituting a direct connection between said first terminal of the inductor and said sensing electrode so that said inductor and said sensing capacitor are connected to each other as a series circuit, said companion electrode and said second terminal of the inductor constituting the opposite end terminals of the series circuit, and a signal utilization circuit having respective connections to said opposite end terminals of said series circuit for applying excitation thereto and for deriving flow representing signals therefrom, said junction of the series circuit being connected to said signal utilization circuit only via said indicator and said sensing capacitor.

31. A probe for flow sensing apparatus, said probe including an elongated metal tubular support having a longitudinal bore, and said elongated support bearing at a first end thereof an assembly comprising (a) a capacitive electrode, (b) a flow activated vaned rotor supported close to said electrode so that, in operation, vanes of the rotor move in succession past said electrode, (c) an inductive impedance having a terminal disposed in close proximity to said capacitive electrode, and (d) a junction that forms a direct connection from said capacitive electrode only to said terminal of said inductive impedance, said probe including a conductor extending from a second terminal of said inductive impedance via said longitudinal bore to and beyond a second end of said elongated tubular support remote from said first end thereof.