Circuit for measuring source resistance of a sensor

{
  "publication_number": "US5469070A",
  "country": "US",
  "kind": "A",
  "title": "Circuit for measuring source resistance of a sensor",
  "abstract": "A method and apparatus are disclosed for measuring a resistance of a sensor. The apparatus injects a first test current into the sensor and then measures a substantially stable first voltage level across the sensor when the first test current is present. The apparatus then injects a second test current into the sensor, the second test current being substantially equal to but opposite in polarity to the first test current. A substantially stable second voltage level across the sensor is measured when the second test signal is present. Using the first measured voltage level and the second measured voltage level, the apparatus calculates the resistance of the sensor.",
  "claims": [
    "1. A method for measuring a resistance of a sensor, the method comprising the steps of: injecting a first test current into the sensor; measuring a substantially stable first voltage level across the sensor when the first test current is present; injecting a second test current into the sensor, the second test current being substantially equal to but opposite in polarity to the first test current; measuring a substantially stable second voltage level across the sensor when the second test current is present; calculating the resistance of the sensor as a function of the measured first voltage level and the measured second voltage level.",
    "2. The method of claim 1 and further comprising the step of calculating a source voltage of the sensor as a function of the measured first voltage level and the measured second voltage level.",
    "3. The method of claim 2 wherein the step of calculating the source voltage comprises averaging the measured first voltage level and the measured second voltage level.",
    "4. The method of claim 2 wherein the step of calculating the source voltage comprises filtering the source voltage from the measured first voltage level and the measured second voltage level.",
    "5. The method of claim 1 wherein the steps of injecting comprise generating the first and second test currents with a differential circuit having a first input and a second input for receiving selected reference control signals.",
    "6. The method of claim 5 wherein the selected reference control signals are provided by oscillation means.",
    "7. The method of claim 1 wherein the step of calculating comprises obtaining the difference between the first measured voltage level and the second measured voltage level.",
    "8. The method of claim 7 wherein the difference between the first measured voltage level and the second measured voltage level is obtained by a differential circuit.",
    "9. The method of claim 7 wherein the difference between the first measured voltage level and the second measured voltage level is obtained by a demodulating circuit.",
    "10. An apparatus for measuring a resistance of a monitoring system comprising a sensor, the apparatus comprising: means for injecting a first test current and a second test current into the sensor, the second test current being substantially equal to but opposite in polarity to the first test current; means for measuring a substantially stable first and second voltage level across the sensor when the corresponding first test current and second test current are present; and means for calculating the resistance of the sensor as a function of the measured first voltage level and the measured second voltage level.",
    "11. The apparatus of claim 10 and further comprising means for calculating a source voltage of the sensor as a function of the measured first voltage level and the measured second voltage level.",
    "12. The apparatus of claim 11 wherein the means for calculating the source voltage comprises an averaging circuit for averaging the measured first voltage level and the second voltage level.",
    "13. The apparatus of claim 11 wherein the means for calculating the source voltage comprises a filtering circuit for filtering the source voltage from the measured first voltage level and the second voltage level.",
    "14. The apparatus of claim 10 wherein the means for injecting comprise generating the first and second test currents with a differential circuit having a first input and a second input for receiving selected reference control signals.",
    "15. The apparatus of claim 14 wherein the selected reference control signals are provided by oscillation means.",
    "16. The apparatus of claim 10 wherein the means for calculating comprises difference means for obtaining the difference between the first measured voltage level and the second measured voltage level.",
    "17. The apparatus of claim 16 wherein the difference means comprises a differential circuit.",
    "18. The apparatus of claim 16 wherein the difference means comprises a demodulating circuit.",
    "19. The apparatus of claim 10 wherein the monitoring system comprises a second sensor, the apparatus further measuring a resistance of the second sensor wherein the means for injecting further injects a third test current and a fourth test current into the second sensor, the fourth test current being substantially equal to but opposite in polarity to the third test current; wherein the means for measuring further measures a substantially stable third and fourth voltage level across the second sensor when the corresponding third test current and fourth test current are present; and wherein the means for calculating further calculates the resistance of the second sensor as a function of the measured third voltage level and the measured fourth voltage level.",
    "20. The apparatus of claim 19 wherein the monitoring system comprises a selective ion monitoring system for monitoring a test solution and the first mentioned sensor comprises a selective ion electrode and the second sensor comprises a reference electrode.",
    "21. The apparatus of claim 20 wherein the reference electrode comprises a second selective ion electrode.",
    "22. The apparatus of claim 20 wherein the first mentioned sensor is a pH electrode.",
    "23. The apparatus of claim 20 wherein the reference electrode is a pH electrode immersed in a reservoir filled with a pH buffer solution having a porous junction in contact with the test solution.",
    "24. The apparatus of claim 20 and further comprising a ground probe forming a common ground path for the selective ion electrode and the reference electrode."
  ],
  "description_excerpt": "This invention relates to a circuit for measuring a source resistance of a sensor. In particular, this invention provides for measurement of the source resistance of the sensor by applying a known test current to the sensor to obtain substantially stable voltages, and measuring the resulting voltages to calculate the source resistance.\n\nContinuous measurement of electrodes in a solution is known throughout the art. For example, Blackmer in U.S. Pat. No. 3,661,748 discloses a device in which an ac signal is applied via an electrode to the conductive fluid in which the electrode system is disposed. An ac signal detector is connected to the dc circuitry to measure the ac current flow. A threshold circuit responsive to the output of the ac signal detector indicates fault in the electrochemical sensor system when the output is of a predetermined magnitude. The system measures a change in resistance of the electrode membrane by a phase detector using the ac signal source as a phase reference. A resistance threshold is provided such that an alarm will sound when the threshold is exceeded.\n\nConnery et al., U.S. Pat. No. 4,189,367 disclose another system for testing electrodes. In Connery et al., the electrodes comprise a glass membrane pH electrode and a reference electrode. The membrane is tested for damage during periodic testing periods by applying a test current through the electrodes and measuring the corresponding changing voltages produced between the electrodes. A reverse current of the same magnitude and duration is then applied through the electrode system to discharge capacitance.",
  "cpc": [
    "G01R 27/14"
  ],
  "ipc": [
    "G01R 27/14"
  ],
  "assignees": [
    "Rosemount Analytical Inc"
  ],
  "inventors": [
    "Roland H. Koluvek"
  ],
  "filing_date": "1992-10-16",
  "publication_date": "1995-11-21",
  "grant_date": "1995-11-21",
  "priority_date": "1992-10-16",
  "application_number": "US-96213392-A",
  "family_id": "25505465",
  "cited_by_count": 101,
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}