CONDITIONING CIRCUIT AND METHOD FOR VARIABLE FREQUENCY SENSOR

• US 3,790,910 A
• Filed: 04/21/1972
• Issued: 02/05/1974
• Est. Priority Date: 04/21/1972
• Status: Expired due to Term

• Current Assignee: William H. Mccormack
• Original Assignee: William H. Mccormack
• Inventors: McCormack, William H.
• Primary Examiner(s): Morrison, Malcolm A.
• Assistant Examiner(s): Smith, Jerry
• Application Number:
  US05/246,471
• Time in Patent Office: 655 Days
• Field of Search: 235/92MT,92FQ,150.3,150.53,151.3,151.34,197 73/393 324/130 331/44,65,66
• US Class Current: 702/138
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

A conditioning circuit includes a computer connected to a pressure transducer having a variable frequency oscillator the output signal of which varies with pressure, a temperature sensor, a read only memory storing constants which are dependent upon the particular transducer and application, and a counter connected to count the variable frequency output signal when gated. In accordance with the disclosed method, the computer obtains a digital count signal indicative of oscillator frequency by gating the output signal to the counter for a predetermined interval, converts an analog signal from the temperature sensor to a digital signal, and then computes a linearized, temperature compensated digital pressure signal in accordance with a predetermined function which utilizes constants stored in the read only memory to initially scale and translate the count signal for maximum computer accuracy, corrects for nonlinearities in the count signal, and corrects for variations in the count signal with temperature. The computed signal then appears at the output as a binary digital word accurately representing sensed pressure over a wide range.

Claims

1. Apparatus for indicating the status of a physical condition comprising a transducer interchangeably connected to a computer, the transducer sensing the physical condition and generating a first signal varying with the physical condition in response thereto, a transducer mounted memory storing scale and offset constants related to the particular characteristics of the transducer for rationalizing the first signal to a predetermined scale and zero point wherein the resulting rationalized function varies between predetermined values as the physical condition varies over a selected range regardless of the particular characteristics of the transducer, and a computer connected to rationalize the first signal by computing a rationalized second signal dependent upon the first signal, the scale constant and the offset constant in accordance with a predetermined mathematical function.

2. The invention as set forth in claim 1 above, wherein the sensed physical condition is pressure, the first signal is a digital signal, the transducer mounted memory is a digital read only memory and the computer is a digital computer.

3. The invention as set forth in claim 2 above, wherein the transducer includes a variable frequency oscillator connected to generate an output signal having a frequency varying with the magnitude of the sensed pressure and a counter connected to count successive cycles of the oscillator output signal, the first signal being the count stored in the counter at the end of a predetermined period of time.

4. The invention as set forth in claim 3 above, wherein the second signal varies monotonically with the sensed physical condition and wherein the counter may overflow during the predetermined period of time without loss of accuracy.

5. The invention as set forth in claim 3 above, wherein the second signal varies monotonically with the sensed physical condition and wherein the predetermined period of Time is sufficiently long that the counter overflows at least once.

6. The invention as set forth in claim 1 above, further comprising transducer mounted temperature sensing means connected to provide the computer with a third signal indicative of transducer temperature and wherein the transducer mounted memory stores at least one constant in addition to the scale and offset constants for linearizing and temperature compensating the second signal in accordance with a second predetermined mathematical function dependent upon the second signal, the additional at least one constant and the third signal.

7. The invention as set forth in claim 6 above, wherein the second mathematical function has the form of a generalized finite power series

8. The invention as set forth in claim 1 above wherein the rationalized second signal is obtained by adding/subtracting an offset constant stored in the transducer mounted memory to the first signal and dividing/multiplying the result by a scaling constant stored in the transducer mounted memory.

9. A pressure sensor conditioning circuit comprising:

an interchangeable, computer connectable pressure transducer sensing a manifolded pressure and including;

a variable frequency oscillator connected to provide an output signal having a frequency varying with the manifolded pressure being sensed, a resettable, gated counter connected to count cycles of the oscillator output signal when gated by a computer generated gating signal, a register connected to receive a count CP,T'"'"''"'"' stored in the counter in response to a computer generated transfer signal, a read only memory storing a plurality of constants having values determined by the particular characteristics of the transducer, said constants being outputed in response to computer generated address signals, and means for sensing transducer temperature and providing an analog signal indicative thereof; and

a digital computer operatively connected to reset the counter, generate a gating signal to gate the counter for a predetermined period of time to obtain a count signal CP,T'"'"''"'"', generate a transfer signal to transfer the count signal from the counter to the register, utilize the count signal stored in the register in making computations, provide address signals to the read only memory and receive output signals indicative of constants stored at addressed locations in response thereto, and receive the analog temperature indicative signal, the digital computer including an analog to digital converter receiving the analog temperature indicative signal, converting said signal to a digital signal KT, the digital computer further including an output register providing a digital output signal P accurately indicating the manifolded pressure, and computing P in accordance with a predetermined mathematical function dependent upon CP,T'"'"''"'"', KT and the constants stored in the read only memory.

10. The pressure conditioning circuit as set forth in claim 9 above, wherein the mathematical function for computing P has the form P fO(KT) + f1(KT) f(CP,T) + f2(KT) f(CP,T)2 where CP,T (CP,T'"'"''"'"'+CC)/Z, CC and Z being initial offset and scale constants stored in the read only memory for the purpose of making CP,T a rationalized parameter varying monotonically with pressure and having a predetermined zero point and slope independent of the particular characteristics of the transducer.

11. A pressure sensor conditioning circuit comprising:

a pressure transducer sensing a manifolded pressure and including;

a variable frequency oscillator connected to provide an output signal having a freqUency varying with the manifolded pressure being sensed, a resettable, gated counter connected to count cycles of the oscillator output signal when gated by a gating signal, a register connected to receive a count CP,T'"'"''"'"' stored in the counter in response to a transfer signal, a read only memory storing a plurality of constants having values determined by the particular characteristics of the transducer, said constants being output in response to address signals, and means for sensing transducer temperature and providing an analog signal indicative thereof; and

a digital computer operatively connected to reset the counter, gate the counter for a predetermined period of time to obtain a count signal CP,T'"'"''"'"', transfer the count signal from the counter to the register, utilize the count stored in the register for making computations, provide address signals to the read only memory and receive output signals in response thereto, and receive the analog temperature indicative signal, the digital computer including an analog to digital converter receiving the analog temperature indicative signal and converting said signal to a digital signal KT, the digital computer further including an output register providing a digital output signal P accurately indicating the manifolded pressure, and computing P in accordance with the predetermined mathematical function depending upon CP,T'"'"''"'"', KT and the constants stored in the read only memory wherein the mathematical function for computing P has the form P fO(KT) + f1(KT) f(CP,T) + f2(KT) f(CP,T)2 where CP,T (CP,T'"'"''"'"'+CC)/Z, CC and Z being initial offset and scale constants stored in the read only memory for the purpose of making CP,T a rationalized parameter varying monotonically with pressure and having a predetermined zero point and slope independent of the particular characteristics of the transducer, and wherein

12. An interchangeably connectable pressure transducer for use in a conditioning system having a digital computer operatively connected to the transducer comprising:

a variable frequency oscillator connected to provide an output having a frequency varying with a manifold pressure being sensed;

a gated counter connected to count cycles of the oscillator output in response to gating signals from the computer;

a temperature sensor providing a temperature signal varying with transducer temperature; and

a read only memory storing a plurality of constants having values determined by the particular characteristics of the transducer, the constants and the temperature signal being usable by the computer without change of programming to account for particular characteristics of the transducer to convert a count stored in the counter at the end of a predetermined gating period to a temperature compensated signal varying linearly with the sensed pressure.

13. The method of operating a conditioning circuit including an interchangeable, computer connectable transducer generating a first signal varying over a given range in response to a given range of a sensed physical condition, a transducer mounted memory storing slope and offset constants having predetermined values determined in accordance with particular characteristics of the transducer and a computer connected to communicate with the first signal and transducer mounted memory generating an output signal varying linearly with the sensed physical condition comprising the steps of:

rationalizing the first signal to a predetermined zero point and scale independent of the range of the first signal generated by the particular transducer by using the slope and offset constants storeD in the transducer mounted memory to compute a rationalized second signal; and

performing computations on the rationalized second signal within the computer in accordance with a predetermined mathematical function, the result of the computations being a linearized output signal.

14. The method as set forth in claim 13 above, wherein the transducer is a pressure sensor, the computer is a digital computer and the first signal is rationalized by adding the offset constant thereto and dividing the resulting sum by the slope constant.

15. The method of conditioning the output of an oscillator mounted on a transducer and generating a signal varying in frequency with a sensed condition comprising the steps of:

incrementing a counter by a count of one for each cycle of the generated signal for a predetermined period of time, there being an accumulated count stored in the counter at the end of the predetermined period of time; and

rationalizing the accumulated count by adding a predetermined offset constant and dividing the resulting sum by a predetermined slope constant, the result being a rationalized count which varies linearly and monotonically with the frequency of the generated signal.

16. The method as set forth in claim 15 above, further comprising the steps of obtaining a digital signal indicating the temperature of the transducer and computing a temperature compensated, linearized signal indicating the sensed condition in accordance with a predetermined mathematical function dependent upon the rationalized count, the temperature indicating signal and a plurality of constants having predetermined values dependent upon the particular characteristics of the transducer.

17. The method of generating a temperature compensated output signal P varying linearly with a sensed pressure with an interchangeably connectable pressure transducer having a gated counter, an oscillator having an output varying in frequency with sensed pressure connected to the gated counter, a temperature sensor providing an analog temperature signal varying with transducer temperature and a transducer mounted memory storing constants determined in accordance with particular characteristics of the transducer, and a digital computer connected to communicate with the counter, the memory and the temperature signal comprising the steps of gating the oscillator output to the counter for a predetermined period of time to obtain an accumulated count indicative of sensed pressure, rationalizing the accumulated count in accordance with a first predetermined mathematical function containing constants stored in the transducer mounted memory to obtain a rationalized count CP,T varying monotonically with the sensed pressure and having a predetermined zero point and slope with respect to the frequency of the oscillator output, converting the analog temperature signal to a digital temperature signal KT, computing a temperature compensated linearized output signal P in accordance with a second predetermined mathematical function utilizing constants stored in the transducer mounted memory, each of these steps being performed by the digital computer.

18. The method of generating a temperature compensated output signal P varying linearly with a sensed pressure with a pressure transducer having a gated counter, an oscillator having an output varying in frequency with sensed pressure connected to the gated counter, a temperature sensor providing an analog temperature signal varying with transducer temperature and a transducer mounted memory storing constants determined in accordance with particular characteristics of the transducer, and a digital computer connected to communicate with the counter, the memory and the temperature signal comprising the steps of gating the oscillator output to the counter for a predetermined period of time to obtain an accumulated count indicative of sensed pressure, rationalizing the accumulated count in accordance with a first predetermiNed mathematical function containing constants stored in the transducer mounted memory to obtain a rationalized count CP,T varying monotonically with the sensed pressure and having a predetermined zero point and slope, converting the analog temperature signal to a digital temperature signal KT, computing a temperature compensated linearized output signal P in accordance with a second predetermined mathematical function having the form

19. The method of utilizing a transducer having an oscillator generating a signal varying in frequency over a predetermined range with a sensed condition and a memory storing predetermined constants indicative of the relationship between the frequency of the particular oscillator and the sensed condition mounted thereon, a counter connected to count cycles of the variable frequency signal on command, and a digital computer having a two'"'"''"'"'s complement fractional notation input word operatively connected to the counter and transducer to provide a rationalized digital signal indicative of the sensed condition comprising the steps of:

incrementing the counter by a count of one for each cycle of the variable frequency signal for a predetermined period of time, the capacity of the counter being sufficiently large that no two frequencies within the predetermined range can cause the counter to store the same count at the end of the predetermined period of time and the capacity of the counter being sufficiently small that the counter overflows at least twice during the predetermined period of time;

communicating a count signal stored in the counter at the end of the predetermined period of time to the digital computer with the most significant bit of the count signal aligned with the sign bit of the input word of the digital computer, the numbering of bits in the count signal being no greater than the number of bits in the input word (including the sign bit) of the computer;

adding a first constant stored in the memory to the count signal to obtain an offset compensated signal, the first constant having a predetermined value selected to make the offset compensated signal equal to zero when the variable frequency oscillator signal is at a frequency at the center of the predetermined range, the offset compensated signal varying monotonically with oscillator frequency over the predetermined range; and

dividing the offset compensated signal by a second constant stored in the memory to obtain a rationalized signal, the second constant having a predetermined value selected to cause the rationalized signal to vary substantially over the full scale capability of the computer as the oscillator frequency varies over the predetermined range.