Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-01-06
2001-03-06
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C236S02000A
Reexamination Certificate
active
06198295
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electrical systems and devices, and more particularly to apparatus and methods for detecting resistance changes in connectors to a remote mounted sensor.
2. Description of Related Art
In a typical water heater control system, a remote mounted sensor is used to detect water temperature inside a water heater tank. In some systems, the remote mounted sensor is a Positive Temperature Coefficient (PTC) or Negative Temperature Coefficient (NTC) thermistor. Generally, a thermistor is a temperature sensitive passive semiconductor that exhibits a large change in electrical resistance when subjected to a relatively small change in temperature. The resistance of a PTC thermistor goes up as the temperature increases; whereas the resistance of an NTC thermistor goes down as the temperature increases. Such a sensor can be electrically connected to a microcontroller that monitors the resistance of the thermistor, converts the resistance to a corresponding water temperature value in accordance with the positive or negative relationship of resistance to temperature, and adjusts the water heater operation to maintain the water temperature within a desired range.
In some systems, the remote sensor is electrically connected to the microcontroller by way of a quick connection means. Over time, such connection means can corrode, thereby increasing in resistance. The increased resistance of the connector translates to an increase in the overall resistance detected by the microcontroller, ultimately corrupting the accuracy of the temperature detection function. For example, in a sensor comprising a PTC thermistor, an increase in the resistance of the connector would cause the microcontroller to detect a temperature that is higher than that actually sensed by the thermistor. In contrast, an increase in the resistance of the connector in an NTC thermistor circuit would cause the microcontroller to detect a temperature that is lower than that actually sensed by the thermistor. As a result, the accurate temperature detection and control of the water temperature are compromised by the increased resistance of the connector. In particular, in the case of an NTC thermistor circuit, such an increased resistance would tend to cause the microcontroller to unnecessarily increase the water temperature. For safety reasons, it is generally preferable to avoid unintended increases in temperature.
One approach for checking a temperature sensing circuit is addressed in U.S. Pat. No. 5,768,429 to Peterson. The object of the invention in Peterson is to provide a temperature sensing circuit having a safe failure mode and employing resistive temperature sensors suitable for high limit heating plant temperature detection. In Peterson, a temperature sensing circuit is described as including two switches and parallel NTC and PTC thermistors, where the NTC thermistor is also coupled in series with a diode. Failure of one of the resistive temperature sensors may be detected by monitoring charging and discharging time constants associated with the resistive temperature sensors. A failure is evidenced by a dramatic change in the ratios of the time constants. During discharge, if the diode is reverse biased, current flows through one thermistor, and if the diode is forward biased, current flows through both thermistors. Peterson does not, however, disclose or suggest determining or detecting a change in, a connector resistance of a connector coupling a remote mounted thermistor to a microcontroller.
Therefore, need exists for a means of detecting changes in resistance of the connectors to a remote mounted sensor. Furthermore, need exists for a means of determining the accurate resistance of the a remote mounted sensor, notwithstanding changes in connector resistance.
SUMMARY OF THE INVENTION
In accordance with this invention the above problems have been solved by a method for detecting a measured connector resistance in a connector coupling a remote mounted sensor to a microcontroller and a capacitor. The connector has an initial predetermined resistance, and the method begins by coupling a unidirectional current element to the microcontroller and the capacitor via the connector and in parallel with the remote mounted sensor. A first transient response associated with the capacitor is determined when current flows substantially through the unidirectional current element and the connector and substantially bypasses the remote mounted sensor. The measured connector resistance is determined from the first transient response.
In another feature of the invention a resistance change is detected by detecting a difference between the predetermined resistance and the measured connector resistance. If the difference exceeds a predetermined threshold, an error indicating derogation of the connector is flagged.
In another feature of the invention, a second transient response is determined. This transient response is also associated with the capacitor but is characterized by current substantially flowing through the remote mounted sensor and the connector and substantially bypassing the unidirectional current element. A sensor resistance is determined from a difference between the measured connector resistance and a measured sensor resistance determined from the second transient response.
The invention can also be implemented by an apparatus or device for detecting electrical resistance as seen by a control circuit looking into connectors connecting a remote circuit to the control circuit. The device includes a unidirectional current element, a non-linear passive element, and a microcontroller. The unidirectional current element is electrically connected in parallel with the remote circuit and across the connectors. The non-linear passive circuit element in the control circuit is coupled to the remote circuit and the unidirectional current element by one of the connectors. The microcontroller is coupled to both of the connectors. Through the one connector the microcontroller is also coupled to the non-linear passive circuit element.
The microcontroller is configured to perform a plurality of operations. First, it detects a response time of a transient signal across the non-linear passive circuit element. This response time results from a transient current flowing substantially through the connectors and only the unidirectional current element. Second, it determines a first resistance of the connectors and the unidirectional current element from the response time. Third, it compares the first resistance to a predetermined resistance for the connectors and the unidirectional current element to indicate a derogation in connectors.
As another feature of the invention the microcontroller is further configured to perform the following operations. It detects a second response time of a transient signal across the non-linear passive circuit element. The second response time results from a transient current flowing substantially through the connectors and only the remote circuit. The microcontroller determines from the second response time a second resistance corresponding to the resistance of the connectors and the remote circuit element. Last, the microcontroller subtracts the first resistance from the second resistance to determine a resistance for substantially only the remote circuit.
REFERENCES:
patent: 3711850 (1973-01-01), Kelly
patent: 3750146 (1973-07-01), Lucas
patent: 3836847 (1974-09-01), Lucas
patent: 4217542 (1980-08-01), Abbe et al.
patent: 4242631 (1980-12-01), Hall
patent: 4841458 (1989-06-01), Levine et al.
patent: 5189376 (1993-02-01), Roberge et al.
patent: 5657238 (1997-08-01), Lindeboom
patent: 5748429 (1998-05-01), Peterson
patent: 5874825 (1999-02-01), Daniele
Honeywell Inc.
Kerveros James C
Merchant & Gould P.C.
Metjahic Safet
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