Digital converter apparatus for improving the output of a two-wire transmitter

• US 5,187,474 A
• Filed: 04/28/1988
• Issued: 02/16/1993
• Est. Priority Date: 10/02/1986
• Status: Expired due to Fees

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Nelson, Richard L., Kielb, John A., Pederson, David L.
• Primary Examiner(s): Oberley, Alvin E.
• Assistant Examiner(s): Farnandez, Kara
• Application Number:
  US07/188,110
• Time in Patent Office: 1,755 Days
• Field of Search: 340/870.34, 340/870.37, 340/310 A, 340/870.19, 340/310 R, 340/646, 340/870.18, 364/571, 364/573, 364/571.01, 324/60 R, 324/61 R
• US Class Current: 340/870.18
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

An existing analog two-wire transmitter has a sensor module, an analog excitation circuit, and an analog detector circuit which provides an output representative of a sensed process variable, such as pressure, to a two-wire current loop. The analog detection circuit is removed and replaced with apparatus including a digital converter that digitally calculates the transmitter'"'"'s output. The output is improved by calculating correction for linearity. The analog excitation circuit may also be replaced with a replacement excitation circuit. The replacement excitation circuit and the digital converter can be energized in series to control energization current. A charge pump can be coupled to the loop to balance the current in the series circuit. The digital converter can also provide electrical span and zero adjustments.

Claims

1. A method for adding a capability to a parameter value transmitter for receiving input information where such a transmitter initially has, through a set of transmitting circuits therein, a capability for transmitting, along a two-wire current carrying loop adapted for electrical connection to first and second terminals of the transmitter, output information formed by those values measured by a sensing means in the transmitter of a parameter the values of which depend on conditions in a structure to which the transmitter is affixed, the method comprising:

disconnecting and removing at least a portion of the set of transmitting circuits which are initially electrically connected between the sensing means and the first and second terminals in the transmitter affixed to the structure; and

providing in the transmitter affixed to the structure a set of transmitting and receiving circuits electrically connected between the sensing means and the first and second terminals, the set of transmitting and receiving circuits being capable of enabling the then-modified transmitter to simultaneously transmit the output information and to receive the input information.

2. The method of claim 1 wherein the transmission of the output information by the modified transmitter occurs through use of transmitted signals having a frequency content in a first frequency range, and wherein the reception of the input information by the modified transmitter occurs through receiving signals having a frequency content in a second frequency range separated from the first frequency range.

3. The method of claim 1 wherein the modified transmitter receives the input information and stores it therein at least temporarily.

4. The method of claim 1 wherein the sensing means comprises a sensor and sensor output signal correction circuitry, the sensor and the sensor output correction circuitry being sealed from that space in the transmitter in which the set of transmitting circuits were located before removal and from that space in the modified transmitter in which the set of transmitting and receiving circuits are provided.

5. The method of claim 1 wherein the modified transmitter provides a current between the first and second terminals on the two-wire current carrying loop of a value which has been adjusted based on the input information received at the first and second terminals.

6. The method of claim 2 wherein the transmitting and receiving circuits of the modified transmitter can also transmit information in the second frequency range.

7. The method of claim 2 wherein the modified transmitter provides a current between the first and second terminals on the two-wire current carrying loop of a value which has been adjusted based on the input information received at the first and second terminals.

8. The method of claim 3 wherein the input information stored is representative of span adjustment.

9. The method of claim 3 wherein the input information stored is representative of zero adjustment.

10. The method of claim 1 wherein the sensing means comprises a sensor which is sealed from that space in the transmitter in which the set of transmitting circuits were located before removal and from that space in the modified transmitter in which the set of transmitting and receiving circuits are provided, and wherein said structure contains a fluid therein both during said disconnecting and removing of at least a portion of the set of transmitting circuits and during the providing in the transmitter affixed to the structure of a set of transmitting and receiving circuits.

11. The method of claim 10 wherein said fluid is pressurized.

12. A method for adding a capability to a parameter value transmitter for transmitting a selected second kind of information where such a transmitter initially has, through a first set of transmitting circuits therein, a capability for a transmitting, along a two-wire current carrying loop adapted for electrical connection to first and second terminals of the transmitter, a first kind of information formed by those values measured by a sensing means in the transmitter of a parameter the values of which depend on conditions in a structure to which the transmitter is affixed, the method comprising:

disconnecting and removing at least a portion of the first set of transmitter circuits which are initially electrically connected between the sensing means and the first and second terminals in the transmitter affixed to the structure; and

providing in the transmitter affixed to the structure a second set of transmitting circuits electrically connected between the sensing means and the first and second terminals, the second transmitting circuits being capable of enabling the then-modified transmitter to transmit both the first kind of information and the second kind of information simultaneously along the two-wire loop.

13. The method of claim 12 wherein the transmission of the first kind of information by the modified transmitter occurs through use of transmitted signals having a frequency content in a first frequency range, and wherein the transmission of the second kind of information by the modified transmitter occurs through use of transmitted signals having a frequency content in a second frequency range separated from the first frequency range.

14. The method of claim 12 wherein the sensing means comprises a sensor and sensor output signal correction circuitry, the sensor and the sensor output correction circuitry being sealed from that space in the transmitter in which the first set of transmitting circuits were located before removal and from that space in the modified transmitter in which the second set of transmitting circuits are provided.

15. The method of claim 12 wherein the sensing means comprises a sensor which is sealed from that space in the transmitter in which the first set of transmitting circuits were located before removal and from that space in the modified transmitter in which the second set of transmitting circuits are provided, and wherein said structure contains a fluid therein during said disconnecting and removing of at least a portion of the first set of transmitting circuits and during the providing in the transmitter affixed to the structure of a second set of transmitting circuits.

16. The method of claim 15 wherein said fluid is pressurized.

17. A parameter value transmitter having transmitting and receiving circuits therein for receiving input information signals and for transmitting, along a two-wire loop adapted for electrical connection to first and second terminals in the transmitter, values measured by a sensing means in the transmitter of a parameter the values of which depend on conditions in the structure to which the transmitter is affixed, the transmitter comprising:

three terminal means provided therein, including first and second terminal means electrically connected to the first and second terminals respectively; and

the transmitting and receiving circuits provided therein comprising;

a power supply means capable of maintaining a differing voltage value on each of the three terminal means including first and second voltage values on the first and second terminal means, and a third voltage value on that third terminal means remaining which is intermediate the first and second voltage values, there being portions of the transmitting and receiving circuits electrically connected between the second and third terminal means capable of passing a greater total current therethrough than portions of the transmitting and receiving circuits electrically connected between the first and third terminal means; and

a charge storage means having a pair of terminals and capable of being alternately electrically connected between the first and third terminal means with a selected one of the charge storage means terminals being connected to the third terminal means and then electrically connected between the second and third terminal means with the opposite charge storage means terminal being connected to the third terminal means.

18. The apparatus of claim 17 wherein the first terminal means is electrically connected to the first terminal through a voltage regulation circuit.

19. The apparatus of claim 17 wherein the second terminal means is electrically connected to the second terminal through a current sensing resistor.

20. The apparatus of claim 17 wherein the charge storage means is a capacitive means.

21. The apparatus of claim 17 wherein the transmitting and receiving circuits receive information signals at the first and second terminals.

22. The apparatus of claim 17 wherein the power supply means is capable of being operated from current supplied at the first and second terminals.

23. A method for adding a capability to a parameter value transmitter for receiving input information where such a transmitter initially has, through a set of transmitting circuits therein, a capability for transmitting, along a two-wire current carrying loop adapted for electrical connection to first and second terminals of the transmitter, output information formed by those values measured by a sensing means in the transmitter of a parameter the values of which depend on conditions in a structure to which the transmitter is affixed, the method comprising:

disconnecting and removing at least a portion of the set of transmitting circuits which are initially electrically connected between the sensing means and the first and second terminals in the transmitter affixed to the structure;

providing in the transmitter affixed to the structure a set of transmitting and receiving circuits electrically connected between the sensing means and the first and second terminals, the set of transmitting and receiving circuits being capable of enabling the then-modified transmitter to transmit the output information and to receive the input information;

providing electrical current to the transmitter through the two-wire current carrying loop which is substantially prevented in any part from flowing through at least one selected circuit component in the receiving circuits by a switch means in the absence of an information signal on the two-wire current carrying loop containing the input information;

detecting in the transmitter receipt of an information signal containing input information on the two-wire current carrying loop;

switching the switch means upon detection of the information signal to provide a flow of electrical current through the selected circuit components in the receiving circuits;

and switching the switch means again on termination of the detection of the information signal to thereby prevent flow of electrical current through the selected circuit components.

24. A method for adding a capability to a parameter value transmitter for transmitting a selected second kind of information where such a transmitter initially has, through a first set of transmitting circuits therein, a capability for transmitting, along a two-wire current carrying loop adapted for electrical connection to first and second terminals of the transmitter, a first kind of information formed by those values measured by a sensing means in the transmitter of a parameter the values of which depend on conditions in a structure to which the transmitter is affixed, the method comprising:

disconnecting and removing at least a portion of the first set of transmitter circuits which are initially electrically connected between the sensing means and the first and second terminals in the transmitter affixed to the structure;

providing in the transmitter affixed to the structure a second set of transmitting circuits electrically connected between the sensing means and the first and second terminals, the second transmitting circuits being capable of enabling the then modified transmitter to transmit both the first and second kinds of information along the two-wire loop;

providing electrical current to the transmitter through the two-wire current carrying loop which is substantially prevented in any part from flowing through at least one selected circuit component in the transmitting circuits by a switch means in the absence of a received signal on the two-wire current carrying loop;

selecting the second kind of information through a received signal selectively provided from an external source;

detecting in the transmitter receipt of a received signal on the two-wire current carrying loop;

switching the switch means upon detection of the received signal to provide a flow of electrical current through the selected circuit components in the transmitting circuit; and

switching the switch means again on termination of the detection of the information signal to thereby prevent flow of electrical current through the selected circuit components.