Method of digital process variable transmitter calibration and a process variable transmitter system utilizing the same

• US 4,734,873 A
• Filed: 07/18/1986
• Issued: 03/29/1988
• Est. Priority Date: 02/02/1984
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Malloy, deceased, John R., Olsen, Arthur M.
• Primary Examiner(s): Krass, Errol A.
• Assistant Examiner(s): Dixon, Joseph L.
• Application Number:
  US06/886,999
• Time in Patent Office: 620 Days
• Field of Search: 364/178, 364/179, 364/571, 364/573, 73/1 R, 73/4 R, 324/130
• US Class Current: 702/98
• CPC Class Codes: G01D 18/008 with calibration coefficients stored in memory
• CPC Class Codes: G01L 27/002 Calibrating, i.e. establishing true relation between transducer output value and value to be measured, zeroing, linearising or span error determination
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

A method of digital process variable transmitter calibration includes the steps of sensing the process variable (PV) to produce an analog output which is converted to a digital signal representation. The digital signal is corrected by a microprocessor using a characterization equation previously individually developed during a manufacturing testing of the process variable transmitter over a predetermined range of environmental stimuli to ascrtain the coefficients for a polynomial in the form of PV=A_o =A₁ P+A₂ +. . . A_i Pⁱ. Thus, the digital computer produces a process variable representation as a modified digital output. Subsequently, the digital output from the computer is converted to a conventional 4-20 mA analog signal for use as a process control signal. The process variable transmitter apparatus utilizing the aforesaid method includes means for receiving an output from a process variable sensor, an analog-to-digital converter for converting the received output from the sensor to a digital representation, a digital computer, a non-volatile memory for storing an individualized correction equation determined during the manufacturing testing of the transmitter and a digital-to-analog converter for converting the computer output to an analog control signal.

Claims

1. A method for digital calibration of a process variable sensor comprising the steps of

storing a characterization equation in the form of PV=A₀ A₁ P+A₂ P² + . . . +A_i Pⁱ where A_i =f (S,T) and PV is the process variable, P is the output of a process variable differential value measuring sensor, S is the output of a process variable static value measuring sensor and T is the output of a sensor measuring an environmental temperature affecting the outputs of the process variable measuring sensors and where ##EQU2## measuring a process variable by the process variable sensor to produce an analog output signal representative of the process variable,

converting the output from the process variable sensor to a digital representation thereof and

supplying the digital representation to a digital computer to be used in computing a digital output in accordance with the stored characterization equation to produce a corrected version of the output of the process variable sensor independent of the effects of the environmental conditions.

2. A method as set forth in claim 1 which includes the further step of converting the digital output from the digital computer into an analog control signal.

3. A method as set forth in claim 2 wherein the analog control signal is a 4-20 mA signal.

4. A method as set forth in claim 1 wherein i≦

5, n≦

7 and m≦

2.

5. A method as set forth in claim 1 wherein i≦

5.

6. A method as set forth in claim 1 wherein the process variable is pressure (PP) and the environmental conditions are differential pressure (P) and static pressure (S) whereby PP=f (P,S,T) as the basis of the characterization equation.

7. A process variable transmitter apparatus comprising

analog-to-digital converter means for converting an analog output from a process variable sensor into a digital representation thereof,

a non-volatile memory means for storing a transmitter characterization equation in the form of

PV=A₀ A₁ P+A₂ P² + . . . +A_i Pⁱ where

A_i =f (S,T) and PV is the process variable, P is the output of a process variable differential value measuring sensor, S is the output of a process variable static value measuring sensor and T is the output of a sensor measuring an environmental temperature affecting the outputs of ther process variable measuring sensors and where ##EQU3## a digital computer means connected to said converter means and said memory means for computing a digital output in accordance with the stored transmitter characterization equation to produce a corrected version of the digital representation from said converter means independent of the effects of the environment conditions.

8. A pressure transmitter as set forth in claim 7 and further including a digital-to-analog converter means for converting the digital output from said computer means into an analog signal.

9. A pressure transmitter as set forth in claim 8 wherein said analog signal is a 4-20 mA signal.

10. A pressure transmitter as set forth in claim 7 wherein

i≦

5, n≦

7 and m≦

2.

11. A process variable transmitter as set forth in claim 7 where i≦

5.

12. A transmitter as set forth in claim 7 wherein the process variable is pressure (PP) and the environmental conditions are differential pressure (P) and static pressure (S) whereby PP=f (P,S,T) as the basis of the characterization equation.

13. A transmitter as set forth in claim 7 where i<

5.