Low cost calibration system for frequency varying temperature sensing means for a thermostat

• US 5,255,975 A
• Filed: 11/26/1991
• Issued: 10/26/1993
• Est. Priority Date: 11/26/1991
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Adams, John T.
• Primary Examiner(s): YASICH, DANIEL
• Application Number:
  US07/798,453
• Time in Patent Office: 700 Days
• Field of Search: 374/1, 73/1 R, 364/571.03, 236/DIG. 8
• US Class Current: 374/1
• CPC Class Codes: G01K 15/005 Calibration
• CPC Class Codes: G01K 7/32 using change of resonant frequency of a crystal
• CPC Class Codes: G05D 23/1917 using digital means
• CPC Class Codes: G05D 23/24 the sensing element having a resistance varying with temperature, e.g. a thermistor
• CPC Class Codes: Y10S 236/08 Computer

Abstract

Low cost temperature sensor for thermostats utilizing a calibration resistor and a microprocessor with switchable ports wherein said microprocessor selects between high impedance input or an output for a temperature sensing element and a calibration resistor where by selecting the input, the temperature sensing element or the calibration resistor are effectively removed from the circuit thus allowing the low cost temperature sensor to self calibrate itself.

Claims

1. A self-calibrating thermostat for controlling the application of electrical power to a temperature modifying load, said thermostat utilizing a variable frequency signal having a characteristic dependent upon the ambient temperature to calculate the ambient temperature, comprising:

a microprocessor having first and second selectable ports, said ports capable of being selected as input or output, said input being a high impedance; and

means for generating a variable frequency signal having a characteristic which varies as a function of the ambient temperature, said means comprising an oscillator having a temperature dependent element, said temperature dependent element having a variable resistance dependent upon the ambient temperature wherein the frequency of the oscillator varies with respect to the variable resistance, and said oscillator further having a resistive calibration element, a first end of said temperature dependent element electrically connected to said first port and a first end of said calibration element electrically connected to said second port, a second end of said temperature dependent element electrically connected to a second end of said calibration element, said microprocessor interchangeably selecting said calibration element or said temperature dependent element by interchangeable selecting said first port and said second port as an input or an output port.

2. The thermostat of claim 1 wherein said variable frequency source further comprises a pair of inverters and a tank capacitor, wherein said tank capacitor is charged and discharged through either said temperature sensitive element or said calibration element as selected by said microprocessor, said capacitor providing a second signal to said pair of inverters representative of said charge on said capacitor, whereby said pair of inverters provide said variable frequency to said microprocessor.

3. The thermostat of claim 2 wherein said input port of said microprocessor comprise an open collector port.

4. The thermostat of claim 1 wherein said variable frequency source further comprises a comparator and a tank capacitor, whereby said tank capacitor is charged and discharged utilizing said temperature sensitive element or said calibration element as selected by said microprocessor, said charging and discharging of said tank capacitor providing an oscillating signal to said comparator, said comparator providing a variable frequency to said microprocessor.

5. The thermostat of claim 4 wherein said input port of said microprocessor comprise an open collector port.

6. A self-calibrating temperature sensor, said sensor calculating ambient temperature by providing a variable frequency signal wherein the frequency is dependent upon the ambient temperature, said sensor comprising:

a microprocessor having first and second selectable ports, said ports capable of being selected as input or output, said input being a high impedance; and

means for generating a variable frequency signal having a characteristic which varies as a function of the ambient temperature, said means comprising an oscillator having a temperature dependent element, said temperature dependent element having a variable resistance dependent upon the ambient temperature wherein the frequency of the oscillator varies with respect to the variable resistance, and said oscillator further having a resistive calibration element, a first end of said temperature dependent element electrically connected to said first port and a first end of said calibration element electrically connected to said second port, a second end of said temperature dependent element electrically connected to a second end of said calibration element, said microprocessor interchangeably selecting said calibration element of said temperature dependent element by interchangeable selecting said first port and said second port as an input or an output port.