Apparatus and method for detecting electrical resistance change in connectors to a remote mounted sensor

• US 6,198,295 B1
• Filed: 01/06/1999
• Issued: 03/06/2001
• Est. Priority Date: 01/06/1999
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Hill, Bruch Lyman
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): KERVEROS, DEMETRIOS C
• Application Number:
  US09/225,462
• Time in Patent Office: 790 Days
• Field of Search: 324/711, 324/133, 324/677, 324/441, 236/20.R
• US Class Current: 324/711
• CPC Class Codes: G01D 3/08 with provision for safeguarding the apparatus, e.g. against abnormal operation, against breakdown
• CPC Class Codes: G01R 27/205 Measuring contact resistance of connections, e.g. of earth connections
• CPC Class Codes: G01R 31/2829 Testing of circuits in sensor or actuator systems testing of apparatus for measuring electric or magnetic variables G01R35/00; testing of indicating or recording apparatus G01D; in airbag systems B60R21/0173; checking gas analysers G01N33/007; monitoring or fail-safe circuits for electromagnets H01F7/1844

Abstract

A control system detects an electrical resistance change in a connector coupling a remote mounted temperature sensor to a control system. A control system determines the resistance of the sensor or the connector by selectively charging or discharging a capacitor through a unidirectional circuit element or the sensor. The control system is disclosed as controlling a water heater system, although such control systems may be used to control other systems using remote mounted sensors. A calibrating circuit is used to measure a transient response of an RC circuit, from which resistance can be derived.

Claims

1. A method for detecting a measured connector resistance in a connector coupling a remote mounted sensor to a microcontroller and a capacitor, the connector having a predetermined resistance, the method comprising:

coupling a unidirectional current element in parallel with the remote mounted sensor to form a remote mounted sensor net;

coupling the remote mounted sensor net to the microcontroller and the capacitor via the connector to form an RC circuit including at least the remote mounted sensor net, the connector, and the capacitor;

determining a first transient response associated with the RC circuit and being characterized by current substantially flowing through the unidirectional current element and the connector and substantially bypassing the remote mounted sensor; and

determining the measured connector resistance in the connector from the first transient response.

2. The method of claim 1 further comprising:

detecting a resistance change from a difference between the predetermined resistance and the measured connector resistance.

3. The method of claim 2 further comprising:

indicating an error if the difference exceeds a predetermined threshold.

4. The method of claim 3 further comprising:

determining a second transient response associated with the RC circuit and being characterized by current substantially flowing through the remote mounted sensor and the connector and substantially bypassing the unidirectional current element; and

determining a sensor resistance in the remote mounted sensor from a difference between the measured connector resistance and a measured sensor resistance determined from the second transient response.

5. A device for detecting a resistance change of a connector as seen by a control circuit looking into the connector connecting a remote circuit to the control circuit, the device comprising:

a unidirectional current element electrically connected in parallel with the remote circuit, the unidirectional current element and the remote circuit being coupled to the control circuit via the connector;

a non-linear passive circuit element in the control circuit coupled to the remote circuit and the unidirectional current element via the connector, wherein the non-linear passive circuit element, the unidirectional current element, the remote circuit, and the connector form at least a portion of an RC circuit; and

a microcontroller coupled to the connector and to the non-linear passive circuit element and being further coupled to the unidirectional current element via the connector, the microcontroller being configured to;

detect a first response time of a first transient signal in the RC circuit and resulting from transient current flowing substantially through the connector and the unidirectional current element and bypassing substantially the remote circuit,

determine a first resistance of the connector and the unidirectional current element based on the first response time, and

compare the first resistance to a predetermined resistance for the connector and the unidirectional current element to detect a derogation in the connector.

6. The device of claim 5 wherein the microcontroller is further configured to:

detect a second response time of a second transient signal in the RC circuit resulting from transient current flowing substantially through the connector and the remote circuit and bypassing substantially the unidirectional current element,

determine from the second response time a second resistance corresponding to the resistance of the connector and the remote circuit, and

subtract the first resistance from the second resistance to determine a resistance associated with the remote circuit.

7. The device of claim 5 wherein the remote circuit is a sensor changing resistance based on changes in a condition sensed by the sensor.

8. The device of claim 5 wherein the non-linear passive circuit element is a capacitor.

9. The device of claim 5 wherein the remote circuit is a thermistor changing resistance with a change in temperature of the thermistor.

10. The device of claim 5 wherein the unidirectional current element is a diode and the diode is forward-biased during determination of the first resistance and reverse-biased during determination of the second resistance.

11. A water heater control system comprising:

a unidirectional current element electrically connected in parallel with a remote circuit, the unidirectional current element and the remote circuit being coupled to the control circuit via a connector;

a non-linear passive circuit element in the control circuit coupled to the remote circuit and the unidirectional current element via the connector, wherein the non-linear passive circuit element, the unidirectional current element, the remote circuit, and the connector form at least a portion of an RC circuit; and

a microcontroller coupled to the connector and to the non-linear passive circuit element and being further coupled to the unidirectional current element via the connector, the microcontroller being configured to;

detect a first response time of a first transient signal in the RC circuit and resulting from transient current flowing substantially through the connector and the unidirectional current element and bypassing substantially the remote circuit,

determine a first resistance of the connector and the unidirectional current element based on the first response time, and

compare the first resistance to a predetermined resistance for the connector and the unidirectional current element to detect a derogation in the connector.

12. The water heater control system of claim 11 wherein the microcontroller is further configured to:

detect a second response time of a second transient signal in the RC circuit resulting from transient current flowing substantially through the connector and the remote circuit and bypassing substantially the unidirectional current element,

determine from the second response time a second resistance corresponding to the resistance of the connector and the remote circuit, and

subtract the first resistance from the second resistance to determine a resistance associated with the remote circuit.

13. The water heater control system of claim 11 wherein the non-linear passive circuit element is a capacitor.

14. The water heater control system of claim 11 wherein the remote circuit is a thermistor changing resistance with a change in temperature of the thermistor.

15. The water heater control system of claim 11 wherein the unidirectional current element is a diode and the diode is forward-biased during determination of the first resistance and reverse-biased during determination of the second resistance.

16. Apparatus for detecting a measured connector resistance in a connector coupling a remote mounted sensor to a microcontroller and a capacitor, the connector having a predetermined resistance, the apparatus comprising:

means for coupling a unidirectional current element in parallel with the remote mounted sensor to form a remote mounted sensor net;

means for coupling the remote mounted sensor net to the microcontroller and the capacitor via the connector to form an RC circuit including at least the remote mounted sensor net, the connector, and the capacitor;

means for determining a first transient response associated with the RC circuit and being characterized by current substantially flowing through the unidirectional current element and the connector and substantially bypassing the remote mounted sensor; and

means for determining the measured connector resistance in the connector from the first transient response.

17. The apparatus of claim 16 further comprising:

means for detecting a resistance change from a difference between the predetermined resistance and the measured connector resistance.

18. The apparatus of claim 16 further comprising:

means for determining a second transient response associated with the RC circuit and being characterized by current substantially flowing through the remote mounted sensor and the connector and substantially bypassing the unidirectional current element; and

means for determining a sensor resistance from a difference between the measured connector resistance and a measured sensor resistance determined from the second transient response.

19. A method for detecting a resistance change in a connector coupling a remote mounted sensor to a microcontroller and a capacitor, the method comprising:

coupling a unidirectional current element in parallel with the remote mounted sensor to form a remote mounted sensor net;

coupling the remote mounted sensor net to the microcontroller and the capacitor via the connector to form an RC circuit including at least the remote mounted sensor net, the connector, and the capacitor;

determining a first transient response associated with the RC circuit and being characterized by current substantially flowing through the unidirectional current element and the connector and substantially bypassing the remote mounted sensor;

determining a second transient response associated with the capacitor and being characterized by current substantially flowing through a calibration resistor; and

detecting the resistance change in the connector based on the first transient response and the second transient response.

20. The method of claim 19 further comprising:

indicating an error if the ratio of the first and second transient responses exceeds a predetermined threshold.

21. A device for detecting a resistance change of a connector as seen by a control circuit looking into the connector connecting a remote circuit to the control circuit, the device comprising:

a unidirectional current element electrically connected in parallel with the remote circuit, the unidirectional current element and the remote circuit being coupled to the control circuit via the connector;

a non-linear passive circuit element in the control circuit coupled to the remote circuit and the unidirectional current element via the connector to form an RC circuit including at least the remote mounted sensor net, the connector, and the capacitor;

a calibration element coupled to the non-linear passive circuit element;

a microcontroller coupled to the connector, the calibration element and the non-linear passive circuit element and being further coupled to the unidirectional current element and the remote sensor via the connector, the microcontroller being configured to;

detect a first response time of a first transient signal in the RC circuit and resulting from transient current flowing substantially through the connector and the unidirectional current element,

detect a second response time of a second transient signal across the non-linear passive circuit element and resulting from transient current flowing through the calibration element,

determine a ratio of the first response time to the second response time, and compare the ratio to a predetermined threshold to detect a derogation in the connector.

22. The device of claim 21 wherein the microcontroller is further configured to provide a signal if the ratio exceeds the predetermined threshold.