Data processing: measuring – calibrating – or testing – Calibration or correction system – Sensor or transducer
Reexamination Certificate
1998-12-29
2001-06-19
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Sensor or transducer
C702S087000, C702S107000, C340S501000, C341S155000
Reexamination Certificate
active
06249753
ABSTRACT:
TECHNICAL FIELD
The invention relates to a sensors, such as pressure transducers, and more particularly to a sensor having improved calibration.
BACKGROUND OF THE INVENTION
Modem sensing systems typically include a sensor, a microprocessor, and an A/D converter coupled there between. Sensors typically provide a low-level signal which needs to be amplified before being of use. Major error sources in such a system include offset error and gain error.
Offset error calibration of the sensor can be done by applying a zero input to the sensor and reading the resulting value output from an A/D converter. If the offset error is small, the processor may simply add or subtract that value from the signal to arrive at a compensated value. Doing this with a large offset error, however, can result in the loss of almost half of the A/D converter's resolution. Thus it is desired to output a signal from the processor, pass it through a D/A converter and counter-act the offset error within the signal conditioning circuit. This allows the full resolution of the A/D converter to be used for signal values. This also compensates for any offset errors occurring in the signal conditioning circuit itself, another point of concern.
To get best resolution, a full scale input into the sensor should result in a full scale output reading from the A/D converter. Since the sensor's full scale output can vary from unit to unit, some means for adjusting the circuit's gain is needed.
Gain calibration can be done by applying a full scale input to the sensor after offset calibration is complete. The gain of the circuit is adjusted until the out put of the A/D converterjust reaches a full scale reading. At this point the whole signal path is calibrated for gain and offset.
The offset null and gain control signals can compensate for errors occurring anywhere in the entire signal path from the sensor to the output of the A/D converter. Once the system is calibrated, the values of the necessary adjustments are stored in the microprocessor's non-volatile memory for recall after every power-up sequence. If the system is such that the output of the sensor is known to be zero immediately after power up, offset calibration may be performed automatically at every power up time and the offset null adjustment value would not need to be stored in non-volatile memory.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a signal conditioner for a sensor system.
The sensor system includes a sensor for sensing an input between a first, minimum value and a second, maximum value. The sensor generates a sensor signal relative to the sensor input. The sensor is subject to offset and gain errors. The sensor system further includes a microprocessor having first and second microprocessor output signals. The signal conditioner calibrates the sensor output signal.
In accordance with the invention, the signal conditioner comprises means responsive to the sensor output signal and the first microprocessor signal for generating a null signal. The null signal is substantially equal to zero when the sensor input is at the minimum sensor value. The signal conditioner further comprises means responsive to the null signal and the second microprocessor signal for generating a gain signal substantially equal to the maximum signal conditioner value when the sensor input is at the maximum sensor value.
It is contemplated that the null signal generating means includes a summing circuit for summing the sensor output signal and the first microprocessor signal.
It is further contemplated that the gain signal generating means includes a frequency controlled amplifier.
It is a further object of the invention to provide a method of providing a calibrated sensor signal substantially free of offset and null errors.
The sensor senses an input between a first, minimum value and a second, maximum value. The sensor generates a sensor output signal relative to the sensor input value.
In accordance with the invention, the method includes providing a microprocessor having first and second output signals, and providing a signal conditioner including means responsive to the sensor output signal and the first microprocessor signal for generating a null signal, and means responsive to the null signal and the second microprocessor signal for generating a gain signal. The sensor input value is set to zero and the first microprocessor signal is adjusted until the null signal equals zero. In addition, the sensor input value is set to the maximum value and the second microprocessor signal is adjusted until the signal conditioner signal equals the maximum value.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.
REFERENCES:
patent: 4800513 (1989-01-01), Deutsch
patent: 5479096 (1995-12-01), Szczyrbak et al.
patent: 5798692 (1998-08-01), Crispie et al.
patent: 5995033 (1999-11-01), Roeckner et al.
patent: 6013108 (2000-01-01), Karolys et al.
Information from MAXIM—1%-Accurate, Digitally Trimmed Sensor Signal Conditioner.
Mason Robert C.
Washington Rodney B.
Bui Bryan
Golden Larry I.
Shah Kamini
Shrout Larry T.
Square D Company
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