Analog to digital conversion by measuring the ratio of RC time constants

• US 4,841,458 A
• Filed: 07/07/1987
• Issued: 06/20/1989
• Est. Priority Date: 07/07/1987
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Levine, Michael R., Russo, James, Skogler, Nicholas, Rigotti, Victor
• Primary Examiner(s): Lall, Parshotam S.
• Assistant Examiner(s): Cosimano, Edward R.
• Application Number:
  US07/070,727
• Time in Patent Office: 714 Days
• Field of Search: 307/117, 340/584, 364/557, 374/170, 374/184, 374/185, 324/62
• US Class Current: 702/133
• CPC Class Codes: G01K 1/028 arrangements for numerical indication
• CPC Class Codes: G01K 7/245 in an oscillator circuit
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00

Abstract

The present invention is a technique for forming a digital signal representing an environmental condition. The technique consists of measuring the variable time constant of an analog sensing device having a variable resistance dependent upon the environmental condition and a capacitor as well as measuring the reference time constant of a precision fixed resistor and the same capacitor. The time constants are measured by discharging the capacitor, charging the capacitor through the appropriate element and measuring the time for the voltage across the capacitor to reach a predetermined level. Lastly, the digital ratio between the two time constants is formed. This digital ratio is the digital representation of the environmental condition sensed by said analog sensing device.

Claims

1. An analog to digital conversion apparatus comprising:

an analog sensing device having a variable resistance dependent upon an environmental condition, having a first terminal and a second terminal;

a precision fixed resistor having a resistance value near the nominal value of resistance of said analog sensing device, having a first terminal and a second terminal;

a capacitor having a first terminal connected to said second terminal of said analog sensing device and said second terminal of said precision fixed resistor and a second terminal connected to a reference voltage;

a discharge means connected to said capacitor for discharging said capacitor;

a first time constant measuring means connected to said analog sensing means, said capacitor and said discharge means for measuring the variable time constant of said analog sensing device by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said analog sensing device and counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage;

a second time constant measuring means connected to said precision fixed resistor, said capacitor and said discharge means for measuring the reference time constant of said precision fixed resistor and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor and counting the number of predetermined time intervals required for the voltage across said capacitor to reach said predetermined threshold voltage;

a measurement sequence means connected to said first time constant measurement means and said second time constant measurement means for producing a sequence of measurements by causing said second time constant measurement means to measure said reference time constant a first time thereby producing a first count, then causing said first time constant measurement means to measure said variable time constant, then causing said second time constant measurement means to measure said reference time constant a second time thereby producing a second count, and for comparing said first count and said second count and repeating said sequence of measurements if said first count and said second count differ by more than a predetermined amount; and

a ratio means connected to said first time constant measurement means, said second time constant measurement means and said measurement sequence means for computing the digital ratio of the count of predetermined time intervals of said variable time constant and the count of predetermined tim intervals of said reference time constant if said first count and said second count do not differ by more than a predetermined amount, said ratio being the digital representation of the environmental condition sensed by said analog sensing device.

2. An analog to digital conversion apparatus for use with a microprocessor device having at least two tristate output lines and an interrupt input comprising:

a analog sensing device having a variable reference resistance dependent upon an environmental condition, having a first terminal connected to a first of said tristate output lines and a second terminal connected to said interrupt input;

a precision fixed resistor having a resistance value near the nominal resistance value of said analog sensing device, having a first terminal connected to a second of said tristate output lines and a second terminal connected to said interrupt input;

a capacitor having a first terminal connected to said interrupt input, and a second terminal connected to a reference voltage;

a discharge means connected to said capacitor and controlled by said microprocessor device for discharging said capacitor; and

the microprocessor device being programmed for

measuring the reference time constant of said precision fixed resistor and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor by applying a digital "1" output to said second tristate output line and applying a high impedance output to the other of said at least two tristate output lines and counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage at said interrupt input, thereby producing a first count,

measuring the variable time constant of said analog sensing device and said capacitor by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said analog sensing device by applying a digital "1" output to said first tristate output line and applying a high impedance output to the other of said at least two tristate output lines and counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage at said interrupt input,

measuring the reference time constant of said precision fixed resistor and said capacitor a second time by discharging said capacitor via said discharge means, charging said capacitor through said first terminal of said precision fixed resistor by applying a digital "1" output to said second tristate output line and applying a high impedance output to the other of said at least two tristate output lines and counting the number of predetermined time intervals required for the voltage across said capacitor to reach said predetermined threshold voltage at said interrupted input, thereby producing a second count,

comparing said first count and said second count,

if said first count and said second count differ by more than a predetermined amount, again measuring the reference time constant of said precision fixed resistor and said capacitor thereby producing another first count, again measuring the variable time constant of said analog sensing device and said capacitor, again measuring the reference time constant of said precision fixed resistor and said capacitor thereby producing another second count, and again comparing said first count and said second count, and

if said first count and said second count do not differ by more than said predetermined amount computing the digital ratio of said count of predetermined time intervals of said variable time constant and said count of predetermined time intervals of said reference time constant, said digital ratio being the digital representation of the environmental condition sensed by said analog sensing device.

3. The analog to digital conversion apparatus claimed in claim 2, wherein:

said microprocessor device is further programmed for

counting the number of predetermined time intervals required for the voltage across said capacitor to reach a predetermined threshold voltage at said interrupt input by incrementing a memory register at the rate of operation of said microprocessor under closed loop program operation until an interrupt input is received.

4. The analog to digital conversion apparatus claimed in claim 2, wherein:

said microprocessor device is further programmed for discharging said capacitor by applying a digital "O" output at least one of said tristate output lines for a predetermined period of time, whereby said discharge means consists of said analog sensing device, said precision fixed resistor and said at least two tristate output lines of said microprocessor device.

5. A method of forming a digital signal representing an environmental condition comprising the steps of:

measuring the reference time constant of a precision fixed resistor and a capacitor, by discharging said capacitor, charging said capacitor through said precision fixed resistor and counting the number of predetermined time intervals for the voltage across said capacitor to reach a predetermined threshold voltage, thereby producing a first count;

measuring the variable time constant of an analog sensing device having a variable resistance dependent upon the environmental condition and said capacitor, by discharging said capacitor, charging said capacitor through said analog sensing device and counting the number of predetermined time intervals for the voltage across said capacitor to reach said predetermined threshold voltage;

again measuring the reference time constant of a precision fixed resistor and said capacitor, by discharging said capacitor, charging said capacitor through said precision fixed resistor and counting the number of predetermined time intervals for the voltage across said capacitor to reach said predetermined threshold voltage thereby producing a second count;

comparing said first count and said second count,

if said first count differs from said second count by more than a predetermined amount then again measuring the reference time constant of said precision fixed resistor and said capacitor, thereby producing a new first count, again measuring the variable time constant of said analog sensing device and said capacitor, and again measuring the reference time constant of said precision fixed resistor and said capacitor, thereby producing a new second count, and then again comparing said first count and said second count, and

if said first count does not differ from said second count by more than said predetermined amount, computing the digital rates of said count of the number of predetermined tim intervals of said variable time constant and said count of the number of predetermined time intervals of said reference time constant, said digital ratio being the digital representation of the environmental condition sensed by said analog sensing device.