Conversion circuit for process control system

• US 6,307,483 B1
• Filed: 04/07/1999
• Issued: 10/23/2001
• Est. Priority Date: 06/07/1995
• Status: Expired due to Fees

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Brown, Gregory C., Anderson, Stephen D., Louwagie, Bennett L., Piechowski, Todd A., Westfield, Brian L.
• Primary Examiner(s): EDWARDS JR, TIMOTHY
• Application Number:
  US09/287,775
• Time in Patent Office: 930 Days
• Field of Search: 340/870.11, 340/870.16, 340/870.15, 340/310.06, 340/310.01
• US Class Current: 340/870.11
• CPC Class Codes: G05B 19/0423 Input/output
• CPC Class Codes: G05B 19/4185 characterised by the network communication
• CPC Class Codes: G05B 2219/25189 Current mode sensor I-O, current loop, 40-mA loop instead of voltage
• CPC Class Codes: G05B 2219/25191 Current loop
• CPC Class Codes: G05B 2219/31121 Fielddevice, field controller, interface connected to fieldbus
• CPC Class Codes: G05B 2219/31126 Transmitter coupled to fieldbus and to sensor, a-d conversion
• CPC Class Codes: G05B 2219/31135 Fieldbus
• CPC Class Codes: G05B 2219/33197 Current loop 4-20-mA milliampere
• CPC Class Codes: Y02P 90/02 Total factory control, e.g. smart factories, flexible manufacturing systems [FMS] or integrated manufacturing systems [IMS]

Abstract

Conversion circuitry for use in a process control system is adapted for coupling to a primary process control loop. Digital receiver circuitry in the conversion circuitry receives a digital signal transmitted over the primary process control loop from a field transmitter and responsively provides a digital output. A microprocessor receives the digital output and responsively provides a secondary loop control output. Secondary loop control circuitry for coupling to a secondary process control loop receives the secondary loop control output from the microprocessor and responsively controls current flowing through the secondary process control loop. The current flowing through the secondary process control loop is related to the digital signal transmitted by the field transmitter.

Claims

1. Conversion apparatus adapted to couple to a field transmitter through a primary process control loop, comprising:

a digital receiver adapted to couple to the primary process control loop and having a process variable signal input and a digital output related to the process variable signal input;

a controller having a digital signal input coupled to the digital output and having a secondary loop control output related to the process variable signal input; and

a secondary loop controller coupled to the secondary loop control output and comprising a current source having an analog current output adapted to couple to a secondary process control loop and being responsive to the secondary loop control output.

2. The apparatus of claim 1 and further comprising a power supply coupled to the secondary loop adapted to receive power from the secondary loop to power the digital receiver, controller and secondary loop controller.

3. The apparatus of claim 1 wherein current in the analog current output is between 4 mA and 20 mA.

4. The apparatus of claim 1 including an opto-isolator optically coupled between the controller and the secondary loop controller and providing isolation between the controller and the secondary loop controller.

5. The apparatus of claim 1 wherein the secondary loop controller is powered by current from the secondary process control loop.

6. The apparatus of claim 1 wherein the process variable signal input from the field transmitter is in accordance with the HART®

communication standard.

7. The apparatus of claim 1 wherein the process variable signal input from the field transmitter is in accordance with a Fieldbus communication standard.

8. The apparatus of claim 1 and including a digital transmitter controlled by the controller, coupled to the primary loop, and adapted to responsively transmit a digital signal on the primary loop to the field transmitter.

9. The apparatus of claim 1 wherein the secondary loop controller provides an alarm signal on the secondary loop in response to an alarm condition.

10. The apparatus of claim 1 including a plurality of secondary loop controllers coupled to a plurality of secondary process control loops, wherein the controller provides a plurality of secondary loop control outputs to the plurality of secondary loop controllers to responsively control current through the secondary process control loops.

11. The apparatus of claim 10 wherein the field transmitter provides a digital signal representative of a plurality of process variables and current on each of the plurality of secondary loops is related to one of the plurality of process variables.

12. The apparatus of claim 10 wherein multiple field transmitters are coupled to the primary loop, the multiple field transmitters transmitting digital signals representative of process variables, and wherein current on each of the plurality of secondary loops corresponds to one of the process variables.

13. Conversion circuitry for coupling to a field transmitter over a primary process control loop, comprising:

means for receiving a digital process variable signal from the primary process control loop and providing a digital output related to the digital process variable signal;

means for receiving the digital output and responsively providing a secondary loop control output as a function of the digital process variable signal; and

means for controlling current flowing through the secondary loop in response to the secondary loop control output whereby an analog current in the secondary loop is a function of the digital process variable signal provided by the primary process control loop.

14. A method of controlling current in a secondary process control loop, comprising:

receiving a digital process variable signal from a primary process control loop;

providing a digital output related to the digital process variable signal;

providing a secondary loop control output responsive to the digital output; and

controlling analog current in the secondary control loop in response to the secondary loop control output as a function of the digital process variable signal.

15. The method of claim 14 and further comprising controlling the current in the secondary control loop between 4 mA and 20 mA.

16. The method of claim 14 and further comprising sending a digital signal on the primary process control loop.

17. The method of claim 14 and further comprising providing an alarm signal based upon an alarm condition on the primary loop.