Selective temperature control system

Abstract

A method and apparatus is disclosed for determining the temperature to which a thermistor is exposed. A capacitor is charged through a reference resistor and the length of time required to charge the capacitor to a predetermined level is measured to obtain a reference charge time. The capacitor is discharged, and then charged through the thermistor and the length of time required to charge the capacitor to the predetermined level through the thermistor is measured to obtain a temperature charge time. The temperature to which the thermistor is exposed is determined by computing the difference between the reference charge time and the temperature charge time divided by the sum of the times.

I claim: 
     
       1. A method of determining the temperature to which a thermistor is exposed utilizing a capacitor, a reference resistor, means coupled to the capacitor and actuable to selectively charge the capacitor through either of the reference resistor and the thermistor, means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time;   (c) actuating the discharging means to discharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time; and   (f) computing the difference between the reference charge time and the temperature charge time and dividing the difference by the sum of said reference charge time and said temperature charge time to determine the temperature to which said thermistor is exposed.   
     
     
       2. The method of claim 1 in which a second thermistor is exposed to a different temperature, the second thermistor being coupled to the charging means, the charging means being actuable to selectively charge the capacitor through the second thermistor comprising the further steps of actuating the charging means to charge said capacitor through said second thermistor, operating the timing means to measure said length of time required to charge the capacitor to the predetermined level to derive a second temperature charge time, and computing the difference between the reference charge time and the second charge time with the difference being divided by the sum of said reference charge time and said second charge time to determine said different temperature. 
     
     
       3. A method of determining the temperature to which a thermistor is exposed in a temperature control system for a space cooling device having an evaporator, an evaporator fan, a compressor, control means including a microcomputer, a capacitor and a reference resistor, the microcomputer including charging means selectively actuable to charge the capacitor through either of the reference resistor and the thermistor, discharging means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time input for said microcomputer;   (c) actuating the discharging means to discharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time input for said microcomputer; and   (f) computing in said microcomputer the difference between the reference charge time and the temperature charge time and dividing the difference by the sum of said reference charge time and said temperature charge time to determine the temperature to which said thermistor is exposed.   
     
     
       4. An apparatus for determining the temperature to which a thermistor is exposed, comprising: a capacitor;   a reference resistor coupled to the capacitor;   first charging means coupled to the reference resistor and to the capacitor for charging said capacitor from a first predetermined charge level to a second predetermined charge level through said reference resistor;   timing means coupled to the capacitor for measuring the length of time required to charge said capacitor to said second predetermined level to obtain a reference charge time;   discharge means coupled to the capacitor for discharging said capacitor to said first predetermined charge level;   second charging means coupled to the thermistor and to the capacitor for charging said capacitor to said second predetermined charge level through said thermistor, said timing means measuring the length of time required to charge said capacitor to said second predetermined level to obtain a thermistor charge time; and   computing means for computing the difference between the reference charge time and the thermistor charge time, the difference being divided by the sum of the reference charge time and the thermistor charge time to determine the temperature to which the thermistor is exposed.   
     
     
       5. An apparatus for determining the temperature to which a thermistor is exposed, comprising: a capacitor having one end coupled to a voltage source, and another end coupled to said thermistor;   a reference resistor coupled to the other end of the capacitor;   sequential charging means coupled to the reference resistor, the thermistor and the capacitor for sequentially charging the capacitor to a predetermined value by the voltage source through the resistor and the thermistor;   timing means coupled to the capacitor for measuring the lengths of time required to sequentially charge the capacitor through the reference resistor and the thermistor to obtain a reference charge time and a thermistor charge time, respectively; and   computing means for computing the difference between the reference charge time and the thermistor charge time with the difference being divided by the sum of the reference charge time and the thermistor charge time to obtain a value proportional to the temperature to which the thermistor is exposed, such value being substantially independent of leakage of the capacitor. .Iadd.   
     
     
       6.  A method of determining the temperature to which a temperature sensor is exposed, comprising the steps of: providing a reference resistor;   providing a capacitor;   providing means for charging the capacitor a predetermined amount sequentially through the reference resistor and the temperature sensor;   the capacitor by the predetermined amount a first time through the reference resistor to obtain a first indication of the time required to charge the capacitor by the predetermined amount;   charging the capacitor by the predetermined amount a second time through the temperature sensor to obtain a second indication of the time required to charge the capacitor by the predetermined amount; and   comparing the first indication to the second indication to determine the temperature to which the sensor is exposed. .Iaddend. .Iadd.   
     
     
       7.  The method of claim 6, wherein the comparing step includes the step of computing the temperature from the first and second indications in a microcomputer. .Iaddend. .Iadd. 
     
     
       8.  The method of claim 6, wherein the step of charging the capacitor by the predetermined amount a first time and the step of charging the capacitor by the predetermined amount a second time each includes the step of increasing the voltage on the capacitor from a first level to a second level. .Iaddend. .Iadd. 
     
     
       9.  The method of claim 6, including the further step of discharging the capacitor after the first indication has been obtained and before the capacitor is charged through the temperature sensor. .Iaddend. .Iadd.10. The method of claim 6, in which a second temperature sensor is exposed to a different temperature, including the further steps of charging the capacitor through the second temperature sensor to obtain a third indication of the time required to charge the capacitor by the certain amount and using the first and third indications to obtain a measure of 
     
     
        the different temperature. .Iaddend. .Iadd.11.  A method of determining the temperature to which a thermistor is exposed utilizing a capacitor, a reference resistor, means coupled to the capacitor and actuable to selectively charge the capacitor through either of the reference resistor and the thermistor, means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps of: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time;   (c) actuating the discharging means to discharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time; and   (f) computing a value of the temperature to which the thermistor is exposed using the reference charge time and the temperature charge time whereby such value is independent of the capacitance value of the capacitor.   
     
     
        .Iaddend. .Iadd.12.  A method of determining the temperature to which a thermistor is exposed utilizing a capacitor, a reference resistor, means coupled to the capacitor and actuable to selectively charge the capacitor through either of the reference resistor and a thermistor, means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps of: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time;   (c) actuating the discharging means to discharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time; and   (f) comparing the reference charge time to the temperature charge time to determine the temperature to which the thermistor is exposed. .Iaddend.   
     
     
        .Iadd.13.  A method of determining the temperature to which a thermistor is exposed in a temperature control system for a space cooling device having an evaporator, an evaporator fan, a compressor, control means including a microcomputer, a capacitor and a reference resistor, the microcomputer including charging means selectively actuable to charge the capacitor through either of the reference resistor and the thermistor, discharging means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps of: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time input for said microcomputer;   (c) actuating the discharging means to discharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time input for said microcomputer; and   (f) computing in said microcomputer a value of the temperature to which said thermistor is exposed as a function of the reference charge time and the temperature charge time using an algorithm that is independent of the   
     
     
        capacitance value of said capacitor. .Iaddend. .Iadd.14.  A method of determining the temperature to which a thermistor is exposed in a temperature control system for a space cooling device having an evaporator, an evaporator fan, a compressor, control means including a microcomputer, a capacitor and a reference resistor, the microcomputer including charging means selectively actuable to charge the capacitor through either of the reference resistor and the thermistor, discharging means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps of: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time input for said microcomputer;   (c) actuating the discharging means to dishcharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time input for said microcomputer; and   (f) comparing in said microcomputer the reference charge time to the temperature charge time to determine the temperature to which the   
     
     
        thermistor is exposed. .Iaddend. .Iadd.15.  A method of determining the temperature to which a thermistor is exposed in a temperature control system for a space cooling device having an evaporator, an evaporator fan, a compressor, control means including a microcomputer, a capacitor and a reference resistor, the microcomputer including charging means selectively actuable to charge the capacitor through either of the reference resistor and the thermistor, discharging means actuable to discharge the capacitor and timing means coupled to the capacitor, comprising the steps of: (a) actuating the charging means to charge the capacitor through the reference resistor;   (b) operating the timing means to measure the length of time required to charge said capacitor to a predetermined level to obtain a reference charge time input for said microcomputer;   (c) actuating the discharging means to discharge the capacitor;   (d) actuating the charging means to charge said capacitor through said thermistor;   (e) operating the timing means to measure the length of time required to charge said capacitor to said predetermined level through said thermistor to obtain a temperature charge time input for said microcomputer; and   (f) computing in said microcomputer a value of the temperature to which said thermistor is exposed as a function of the reference charge time and the temperature charge time using an algorithm that is independent of the capacitance value of said capacitor and the voltage of said charging   
     
     
        means. .Iaddend. .Iadd.16.  An apparatus for determining the temperature to which a thermistor is exposed, comprising: a capacitor;   a reference resistor;   sequential charging means coupled to the capacitor, the thermistor and the reference resistor for sequentially charging the capacitor by a predetermined charge amount through the reference resistor and the thermistor;   timing means coupled to the capacitor for measuring the length of time required to charge the capacitor by the predetermined charge amount through the reference resistor and the thermistor to obtain a reference resistor charge time and a thermistor charge time, respectively; and   computing means for computing the temperature to which the thermistor is exposed responsive to the reference charge time and the thermistor charge   
     
     
        time. .Iaddend. .Iadd.17.  The apparatus of claim 16, wherein the computing means comprises a microcomputer. .Iaddend. .Iadd.18. The apparatus of claim 16, wherein the sequential charging means includes a first series circuit comprising the reference resistor, the capacitor and a voltage source and a second series circuit comprising the thermistor, 
     
     
        the capacitor and the voltage source. .Iaddend. .Iadd.19.  The apparatus of claim 17, wherein the capacitor is coupled to an input of the microcomputer and wherein the timing means senses the voltage developed at the microprocessor input. .Iaddend.