An improved self-validating thermocouple (SVT) instrumentation system not only acquires readings from a thermocouple but is also capable of detecting deterioration and a variety of discrete faults in the thermocouple and its lead wires. Prime examples of detectable discrete faults and deterioration include open- and short-circuit conditions and debonding of the thermocouple junction from the object, the temperature of which one seeks to measure. Debonding is the most common cause of errors in thermocouple measurements, but most prior SVT instrumentation systems have not been capable of detecting debonding.
The improved SVT instrumentation system includes power circuitry, a cold-junction compensator, signal-conditioning circuitry, pulse-width- modulation (PWM) thermocouple-excitation circuitry, an analog-to-digital converter (ADC), a digital data processor, and a universal serial bus (USB) interface. The system can operate in any of the following three modes:
In this mode, the ADC samples the output voltages of the thermocouple and the cold-junction compensator. Because the output voltage of the thermocouple is very small (typically of the order of microvolts or millivolts), it is necessary to utilize the gain of the ADC. The processor uses the cold-junction-compensator reading to obtain a compensated thermocouple output voltage, Vout, then calculates the temperature at the thermocouple tip by use of the equation of the form
where the Aks are calibration parameters, Vout is the compensated thermocouple output voltage, and k and n are integers.
For the purpose of determining whether there is a short or open circuit, the two thermocouple leads are subjected to a common-mode DC excitation or, via capacitors, to a differential-mode PWM excitation. From the response to the DC excitation, the processor can determine whether or not there is a short circuit. From response to the PWM excitation, the processor can determine whether there is an open circuit.
The processor commands the application of a PWM excitation, via a capacitor, to the thermocouple for a certain amount of time to heat the thermocouple. (Inductors in the thermocouple leads prevent the PWM excitation from reaching the thermocouple cold junction.) The characteristic time or rate of increase in temperature during the excitation is analyzed by the processor as an indication of the integrity of the thermocouple. The characteristic time or rate of decay of the temperature after the excitation is analyzed by the processor as an indication of the thermal resistance (and, hence, of bonding or debonding) between the thermocouple and the object, the temperature of which one seeks to measure.
The software running in the processor includes components that implement statistical algorithms to evaluate the state of the thermocouple and the instrumentation system. When power is first turned on, the user can elect to start a diagnosis/ monitoring sequence, in which the PWM is used to estimate the characteristic times corresponding to the correct configuration. The user also has the option of using previous diagnostic values, which are stored in an electrically erasable, programmable read-only memory so that they are available every time the power is turned on.
This work was done by Jose Perotti and Josephine Santiago of Kennedy Space Center and Carlos Mata, Peter Vokrot, Carlos Zavala, and Bradley Burns of ASRC Aerospace Corp.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Kennedy Innovative Partnerships Office
at (321) 861-7158.
Refer to KSC-12875.