Admittance measuring system for monitoring the condition of materials

• US 4,146,834 A
• Filed: 11/22/1976
• Issued: 03/27/1979
• Est. Priority Date: 09/19/1974
• Status: Expired due to Term

• Current Assignee: Drexelbrook Controls, Inc.
• Original Assignee: Drexelbrook Controls, Inc.
• Inventors: Maltby, Frederick L., Loewenstern, Kenneth M., Kramer, Jonathan
• Primary Examiner(s): Chatmon, Jr., Saxfield
• Application Number:
  US05/743,618
• Time in Patent Office: 855 Days
• Field of Search: 324/60 R, 324/61 R, 324/57 R, 324/58 R, 324/58 A, 324/58.5, 324/58.5 A, 328/160, 328/158
• US Class Current: 324/610
• CPC Class Codes: G01N 27/028 Circuits therefor measuring impedance per se G01R27/02
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00

Abstract

A two-wire transmitter includes an admittance sensing probe adapted to sense the conditions and corresponding admittance of materials. The probe is coupled into an admittance responsive network which generates an admittance signal representing the condition of materials. The output current from the transmitter is varied in response to the admittance signal. In one embodiment of the invention, the admittance responsive network comprises a variable frequency oscillator whose frequency varies with the admittance of the materials. In another embodiment, the admittance responsive network comprises a ramp generator with a frequency which varies with the admittance of the materials. In another embodiment, the admittance responsive network comprises a bridge whose balance changes in response to the admittance of the materials.

Claims

1. In a two-wire transmitter system comprising a power supply and a load at one location and a two-wire transmitter at another location interconnected by a pair of transmission lines carrying a variable signaling current, the improvement comprising:

an admittance sensing probe including a probe electrode adapted to sense the condition and corresponding admittance of materials;

an admittance responsive network coupled to said probe representing the condition of materials; and

output means coupled to said admittance responsive network for varying the signaling current in response to the condition of materials.

2. The transmitter of claim 1 wherein said admittance responsive network comprises:

first oscillator means comprising a first frequency determinative circuit including said admittance sensing probe, said first oscillator means generating a first signal changing frequency in response to changes in the admittance of said materials;

second oscillator means comprising a second frequency determinative circuit generating a second signal having a substantially constant reference frequency; and

frequency difference detector means coupled to said first oscillator means and said second oscillator means, said frequency difference detector means being coupled to said output means.

3. The transmitter of claim 2 wherein said admittance responsive network is supplied by said power supply through said pair of transmission lines.

4. The transmitter of claim 3 further comprising voltage regulating means coupled to said admittance responsive network for supplying a regulated voltage from said power supply to said admittance responsive network.

5. The transmitter of claim 4 further comprising spark protection means coupled to said pair of transmission lines.

6. The transmitter of claim 5 comprising a full wave rectifying bridge coupled to said pair of transmission lines so as to permit polarity reversals of said transmission lines when connecting the transmitter thereto.

7. The transmitter of claim 2 wherein said frequency difference detector means comprises means for digitizing the output from said frequency detector means.

8. The transmitter of claim 7 wherein said frequency difference detector means comprises comparator means for generating a difference signal representing the frequency difference between said first signal and said second signal.

9. The transmitter of claim 2 wherein said first oscillator means and said second oscillator means comprise a bridge-like circuit, one-half of said bridge-like circuit comprising said first frequency determinative circuit of said first oscillator means and the other half of said bridge-like circuit comprising said second frequency determinative circuitry of said second oscillator means, said second frequency determinative circuit including a reference admittance.

10. The transmitter of claim 9 comprising a probe cable coupled to said probe and a compensate terminal associated with said second oscillator means, and a cable matched to a probe cable connected to said compensate terminal.

11. The transmitter of claim 2 wherein said output means comprises means for generating a feedback signal substantially proportional to the signal current.

12. The transmitter of claim 2 wherein the admittance of said admittance sensing probe is DC isolated from said first oscillator means.

13. The transmitter of claim 1 wherein said admittance responsive network comprises:

first admittance means coupled to said sensing probe so as to include the admittance of said materials;

second admittance means comprising a reference admittance;

charge current means coupled to said first admittance means and said second admittance means for charging thereof;

discharge means coupled to said first admittance means and said second admittance means for discharging thereof; and

charge rate detection means for detecting the difference in charging rates between said first admittance means and said second admittance means.

14. The transmitter of claim 13 wherein said charge current means comprises a first zero current source and a second span current source.

15. The transmitter of claim 13 wherein said first admittance means and said second admittance means comprises an admittance bridge.

16. The transmitter of claim 15 wherein said first admittance means forms a first side of said bridge and said second admittance means forms a second side of said bridge, said charge rate detection means detecting a difference in time to charge said first side as compared with said second side.

17. The transmitter of claim 16 wherein said bridge and said charge current means are DC isolated from said transmission lines.

18. The transmitter of claim 17 wherein charge current means comprises means for adjusting the DC charging current by adjusting DC current flow.

19. The transmitter of claim 16 further comprising feedback means coupling said charge rate detection means to said current source means for rebalancing said bridge.

20. The transmitter of claim 13 including guard means and guard amplifier means having an input coupled to said sensing probe and an output coupled to said guard means for driving said guard means at substantially the same potential as said sensing probe.

21. The transmitter of claim 20 comprising parallel reverse poled diodes coupled between said sensing probe and said guard means and a pair of series reverse poled Zener diodes coupled between said shield means and ground.

22. The transmitter of claim 13 wherein said admittance responsive network is supplied by said power supply through said pair of transmission lines.

23. The transmitter of claim 22 wherein said output means comprises means for generating a feedback signal substantially proportional to the signal current.

24. The transmitter of claim 23 wherein said output means comprises modulator means coupled to said charge rate detection means for generating an AC signal representing the feedback signal, demodulator means for demodulating the modulated AC signal, DC isolation means for coupling demodulator means to said modulator means, and output amplifier means coupled to said demodulator means, said output amplifier means being coupled to said pair of transmission lines so as to control the current drawn by said two-wire transmitter.

25. The transmitter of claim 13 including spark protection means coupled to said admittance sensing probe.

26. The transmitter of claim 25 including guard means associated with said probe, said spark protection means coupled between said probe and said guard and said guard means and ground such that said guard means functions to break up the stray path comprised of the protecting devices.

27. The transmitter of claim 13 including spark protection means coupled to said transmission lines.

28. The transmitter of claim 13 further comprising:

oscillator means coupled to said pair of transmission means;

Dc power supply means including rectifying means coupled to said admittance responsive network; and

transformer means coupling said oscillator means to said DC power supply means.

29. The transmitter of claim 1 wherein said output means comprises an output amplifier including a voltage feedback network including a resistor through which DC current drawn by the two-wire transmitter flows so as to stabilize the flow of the current at all current levels.

30. The two-wire transmitter of claim 1 further comprising a full wave rectifying bridge coupled to said pair of transmission lines for permitting current to flow through one pair of diodes when the terminals are connected to the transmission wires with one polarity and current to flow through the other pair of diodes when the terminals of the bridge are connected to the transmission wires with the opposite polarity.

31. The transmitter of claim 1 wherein said admittance responsive network comprises a bridge having an unknown admittance representing the condition of materials on one side of said bridge and a reference admittance on the other side of the bridge.

32. The transmitter of claim 31 including span adjustment means associated with said bridge.

33. The transmitter of claim 31 including zero adjustment means associated with said bridge.

34. The transmitter of claim 1 further comprising a regulated power supply coupled to and supplied by said transmission lines.

35. The transmitter of claim 1 wherein the admittance of said admittance sensing probe is DC isolated from said transmission lines.