Method and apparatus for circuit fault detection with boiler water level detection system

• US 6,650,128 B2
• Filed: 02/19/2002
• Issued: 11/18/2003
• Est. Priority Date: 02/19/2002
• Status: Expired due to Term

• Current Assignee: Tyco Valves & Controls Incorporated (Emerson Electric Company)
• Original Assignee: Tyco Flow Control
• Inventors: Sanders, Gary G.
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): Dole, Timothy J.
• Application Number:
  US10/078,599
• Publication Number:
  US 20030155938A1
• Time in Patent Office: 637 Days
• Field of Search: 324/715, 324/693, 324/697, 324/662, 340/450, 340/618, 340/620, 732/90.R, 733/04.R, 733/04.L
• US Class Current: 324/715
• CPC Class Codes: G01F 23/244 comprising oscillating circuits
• CPC Class Codes: G01F 25/24 Testing proper functioning of electronic circuits

Abstract

A fault detection circuit in a boiler-water level system includes a dual-frequency signal generator which develops two AC components with no associated DC component. The two frequencies are mixed and sent through an impedance matching circuit to match the impedance of the signal generating portion of the system with the impedance of the boiler water under measurement. The impedance-matched signal is then directed to two legs, one leg directed through one of a plurality of electrode probes and then to a first filter circuit, and the other leg is directed a second set of filters. In combination, the filters pass either the higher or the lower of the two frequencies to determine an open or short condition in the level sensing circuitry, as well as a steam vs. water condition.

Claims

1. A sensing circuit for a boiler level indicator having a level probe, the circuit comprising:

a. a dual frequency signal generator;

b. a first high pass filter coupled to the signal generator through the probe;

c. a second high pass filter coupled to the signal generator;

d. a positive going rectifier coupled to the first high pass filter to develop a positive sense level;

e. a negative going rectifier coupled to the second high pass filter to develop a negative sense level; and

f. a summing junction to receive the positive and negative sense level and to develop a summed output, wherein equal positive and negative sense levels at the summing junction indicates that no open circuit exists in the sensing circuit.

2. The sensing circuit of claim 1, further comprising a comparator, biased by a predetermined short voltage and coupled to the negative going rectifier, to develop a short indication on a predetermined difference between the negative sense level and the short voltage.

3. The sensing circuit of claim 1, further comprising a boiler chemistry comparator, biased by a predetermined boiler water impedance voltage and coupled to the negative going rectifier, to develop an out of specification boiler water chemistry indication on a predetermined difference between the negative sense level and the boiler water impedance voltage.

4. The sensing circuit of claim 1, further comprising a low pass filter coupled to the signal generator to develop a signal indicative of a steam vs. water condition at the probe.

5. The sensing circuit of claim 4, further comprising a frequency mixer to couple the signal generator to the low pass filter.

6. The sensing circuit of claim 4, further comprising

a. a low pass filter rectifier to receive the signal indicative of a steam vs. water condition and to develop a rectified signal indicative of a steam vs. water condition; and

b. a steam over water comparator, biased by a steam vs. water voltage reference, to develop a steam vs. water indication on a predetermined difference between the rectified signal indicative of a steam vs. water condition and the steam over water voltage reference.

7. The sensing circuit of claim 1, further comprising a frequency mixer to couple the signal generator to the first and second high pass filters.

8. The sensing circuit of claim 1, wherein the signal generator develops two sine waves.

9. The sensing circuit of claim 8, further comprising a frequency mixer to impress the two sine waves onto a conductor.

10. The sensing circuit of claim 1, further comprising a window comparator coupled to the summing junction to receive the summed output and to compare the summed output to a window reference voltage, thereby developing an open signal indication on a predetermined difference between the summed output and the window reference voltages.

11. The sensing circuit of claim 1, wherein the signal generator develops a dual frequency signal defining a net sum integral zero signal level.

12. A method of analyzing a boiler level circuit of a boiler, comprising the steps of:

a. developing a dual frequency signal having a higher frequency element and a lower frequency element;

b . passing the dual frequency signal through a boiler level probe to develop a sense signal;

c. filtering the lower frequency element from the sense signal;

d. filtering the lower frequency element from the dual frequency signal; and

e. analyzing the filtered sense signal and the dual frequency signal to determine if an open condition exists in the boiler level circuit.

13. The method of claim 12, further comprising the step of comparing the filtered dual frequency signal to a short reference signal to determine if a short condition exists in the boiler level circuit.

14. The method of claim 12, further comprising the steps of:

a. filtering the higher frequency element from the dual frequency signal; and

b. comparing the filtered dual frequency signal to a reference steam over water reference to determine if a steam over water condition exists in the boiler.

15. The method of claim 12, wherein higher frequency element and a lower frequency element define a net sum integral zero signal level.