THERMISTOR CIRCUIT CALIBRATION

• US 20090110022A1
• Filed: 10/29/2007
• Published: 04/30/2009
• Est. Priority Date: 10/29/2007
• Status: Active Grant

• Current Assignee: Smiths Medical ASD, Inc. (Smiths Group International Holdings Ltd.)
• Original Assignee: Smiths Medical ASD, Inc. (Smiths Group International Holdings Ltd.)
• Inventors: Wheatley, Maurice, Snyder, Robert L.
• Granted Patent: US 8,197,123 B2
• Time in Patent Office: Days
• US Class Current: 374/1
• CPC Class Codes: A61M 16/026 specially adapted for predicting, e.g. for determining an information representative of a flow limitation during a ventilation cycle by using a root square technique or a regression analysis
• CPC Class Codes: A61M 16/109 the humidifying liquid or the beneficial agent
• CPC Class Codes: A61M 16/1095 in the connecting tubes
• CPC Class Codes: A61M 16/16 Devices to humidify the respiration air A61M16/1045 takes precedence
• CPC Class Codes: A61M 16/162 Water-reservoir filling system, e.g. automatic
• CPC Class Codes: A61M 2205/3368 Temperature
• CPC Class Codes: A61M 2205/3653 by Joule effect, i.e. electric resistance
• CPC Class Codes: A61M 2205/702 automatically during use
• CPC Class Codes: G01K 15/00 Testing or calibrating of thermometers
• CPC Class Codes: G01K 7/16 using resistive elements resistive elements per se H01C, H01L

Abstract

Temperature detection circuitry is selectively coupled to a thermistor and one of two sources representing the impedance at respective ends of the expected range of temperature to which the thermistor is to be exposed. The offset of an amplifier and a scale factor to account for gain set of the amplifier are determined in an automatic calibration process while coupled to the source(s), and thereafter temperature readings are taken from the thermistor. During the calibration process, if the gain or scale factor are outside of expected ranges, a failure is determined and an alarm given and/or a heater is disabled.

Claims

1. An automatically calibrating thermistor circuit comprising:

a port adapted to be coupled to a thermistor exposed to a heated location over which a range of temperatures is expected, and configured to obtain a temperature reading representative of a temperature of said heated location;

a first source adapted to provide a temperature reading associated with one end of said expected range of temperatures; and

detection circuitry having an input adapted to be selectively coupled to the port and the first source and having an output, the detection circuitry including an amplifier coupled to the input and having a generally preset gain and a variable offset and an A/D converter coupled to the amplifier and the output by which to provide a digital temperature signal correlated to a temperature reading at the input, the amplifier adapted to output amplified temperature reading signals across an expected operating range in relation to said expected temperature range of said heated location, the first source adapted to cause the amplifier to output a minimum value; and

a processor coupled to the detection circuitry output and adapted to determine and to output an adjustable offset signal operatively coupled to set the variable offset of the amplifier based on digital temperature signals obtained from the first source.

2. The automatically calibrating thermistor circuit of claim 1, the first source being a precision resistor having an impedance associated with a high end of said expected temperature range.

3. The automatically calibrating thermistor circuit of claim 1 further comprising a multiplexer responsive to the processor to selectively couple the port or the first source to the detection circuitry.

4. The automatically calibrating thermistor circuit of claim 1 further comprising a D/A converter coupling the offset signal from the processor to the amplifier.

5. The automatically calibrating thermistor circuit of claim 1, the processor further adapted to iteratively adjust the offset signal until the digital temperature signal from the detection circuitry correlates to an expected digital signal.

6. The automatically calibrating thermistor circuit of claim 5, the processor adapted to iteratively output as an offset signal an average of upper and lower boundary values and to vary the boundary values in relation to the digital temperature signal from the detection circuitry until the boundary values satisfy a predetermined criterion.

7. The automatically calibrating thermistor circuit of claim 1, the processor adapted to indicate failure in response to determining an offset signal that is outside of an acceptable range.

8. The automatically calibrating thermistor circuit of claim 1 further comprising a second source adapted to provide a temperature reading related to a second temperature associated with another end of said expected range of temperatures to cause the amplifier to output a maximum value, the detection circuitry input further adapted to be selectively coupled to the second source, the processor further adapted to determine a scale factor based on a relationship between the digital temperature signal obtained from the second source and a desired maximum output of the amplifier, the processor adapted to adjust the digital temperature signals obtained from the port in relation to the scale factor.

9. The automatically calibrating thermistor circuit of claim 8, the second source being a precision resistor having an impedance associated with a low end of said expected temperature range.

10. The automatically calibrating thermistor circuit of claim 8 further comprising a multiplexer responsive to the processor to selectively couple the port, the first source, or the second source to the detection circuitry.

11. The automatically calibrating thermistor circuit of claim 8, gain of the amplifier being preset to provide a maximum output of the amplifier below the upper end of the expected operating range of the amplifier.

12. The automatically calibrating thermistor circuit of claim 8, the processor adapted to indicate failure in response to determining the scale factor is outside a permissible range.

13. An automatically calibrating thermistor circuit comprising:

a port adapted to be coupled to a thermistor exposed to a heated location over which a range of temperatures is expected, and configured to obtain a temperature reading representative of a temperature of said heated location;

a first source adapted to provide a temperature reading associated with one end of said expected range of temperatures; and

detection circuitry having an input adapted to be selectively coupled to the port and the first source and having an output, the detection circuitry including an amplifier coupled to the input and having a generally preset gain and a variable offset and an A/D converter coupled to the amplifier and the output by which to provide a digital temperature signal correlated to a temperature reading at the input, the amplifier adapted to output amplified temperature reading signals across an expected operating range in relation to said expected temperature range of said heated location, the first source adapted to cause the amplifier to output a minimum value; and

a processor coupled to the detection circuitry output and adapted to determine and to output an adjustable offset signal and to indicate failure in response to determining an offset signal that is outside of an acceptable range.

14. The automatically calibrating thermistor circuit of claim 13 further comprising a second source adapted to provide a temperature reading related to a second temperature associated with another end of said expected range of temperatures to cause the amplifier to output a maximum value, the detection circuitry input further adapted to be selectively coupled to the second source, the processor further adapted to determine a scale factor based on a relationship between the digital temperature signal obtained from the second source and a desired maximum output of the amplifier and to indicate a failure in response to determining the scale factor exceeds a permissible value.

15. A method of automatically calibrating a thermistor circuit in which a thermistor is to be exposed to heat from heated location, the temperature of which is expected to vary over a range of temperatures comprising:

coupling temperature readings from a first source adapted to provide a temperature reading associated with one end of said expected range of temperatures, through an amplifier having a generally preset gain and a variable offset and an A/D convener to generate digital temperature signals corresponding to the temperature readings, the amplifier adapted to output amplified temperature reading signals across an expected operating range in relation to the expected temperature range, the temperature reading provided by the first source corresponding to a minimum value output of the amplifier;

determining with a processor an adjustable offset signal based on digital temperature signals obtained from the first source;

setting the offset of the amplifier in relation to the offset signal determined by the processor;

thereafter coupling temperature readings from the thermistor through the amplifier and the A/D convener to generate digital temperature signals corresponding the temperature readings from the thermistor; and

determining with the processor a temperature of said heated location.

16. The method of claim 15 further comprising determining the adjustable offset signal by iteratively adjusting the offset signal until the digital temperature signal obtained from the first source correlates to an expected digital signal.

17. The method of claim 16 further comprising iteratively determining the offset signal as an average of upper and lower boundary values and varying the boundary values in relation to the digital temperature signal from the A/D converter until the boundary values satisfy a predetermined criterion.

18. The method of claim 17 further comprising initially setting the boundary values to respective minimum and maximum values.

19. The method of claim 15 further comprising indicating failure if the offset signal determined is outside of an acceptable range.

20. The method of claim 15 further comprising:

coupling a temperature reading from a second source adapted to provide a temperature reading associated with another end of said expected range of temperatures through the amplifier and the A/D converter to generate a digital temperature signal corresponding to the temperature reading from the second source, the temperature reading provided by the second source corresponding to a maximum value output of the amplifier;

determining a scale factor based on a relationship between the digital temperature signal corresponding to the temperature reading from the second source and a desired maximum output of the amplifier; and

adjusting the digital temperature signals corresponding to the temperature readings from the thermistor in relation to the scale factor.

21. The method of claim 20 further comprising setting gain of the amplifier to provide a maximum value output below the desired maximum output.

22. The method of claim 20 further comprising indicating failure if the scale factor is outside a permissible range.

23. A method of automatically calibrating a thermistor circuit in which a thermistor is to be exposed to heat from heated location, the temperature of which is expected to vary over a range of temperatures comprising:

coupling temperature readings from a first source adapted to provide a temperature reading associated with one end of said expected range of temperatures, through an amplifier having a generally preset gain and a variable offset and an A/D converter to generate digital temperature signals corresponding to the temperature readings, the amplifier adapted to output amplified temperature reading signals across an expected operating range in relation to the expected temperature range, the temperature reading provided by the first source corresponding to a minimum value output of the amplifier;

determining with a processor an adjustable offset signal based on digital temperature signals obtained from the first source;

setting the offset of the amplifier in relation to the offset signal determined by the processor; and

indicating failure if the offset signal is outside of an acceptable range.

24. The method of claim 23, indicating failure including one of terminating power to a heater, providing an alarm, and a combination thereof.

25. The method of claim 23 further comprising:

coupling a temperature reading from a second source adapted to provide a temperature reading associated with another end of said expected range of temperatures through the amplifier and the A/D converter to generate a digital temperature signal corresponding to the temperature reading from the second source, the temperature reading provided by the second source corresponding to a maximum value output of the amplifier;

determining a scale factor based on a relationship between the digital temperature signal corresponding to the temperature reading from the second source and a desired maximum output of the amplifier; and

indicating failure if the scale factor is outside a permissible range.