Communication bus suitable for use in a hazardous area of a process plant

• US 7,684,167 B2
• Filed: 09/30/2003
• Issued: 03/23/2010
• Est. Priority Date: 09/30/2003
• Status: Active Grant

• Current Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Original Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Inventors: Aneweer, Tom, Burr, Kent A., Law, Gary K.
• Primary Examiner(s): Jackson; Stephen W
• Assistant Examiner(s): HOANG, ANN THI
• Application Number:
  US10/675,446
• Publication Number:
  US 20050068710A1
• Time in Patent Office: 2,366 Days
• Field of Search: 361/119
• US Class Current: 361/119
• CPC Class Codes: H02H 9/008 Intrinsically safe circuits

Abstract

A communication bus suitable for use in a hazardous area of a process plant includes a first transmission path adapted to communicate electrical signals in a first direction, and a second transmission path adapted to communicate electrical signals in a second direction. A safety device is coupled to each of the first and second transmission paths and includes a control unit adapted to detect a fault condition associated with the communication bus. The safety device further includes a switch unit adapted to interrupt the flow of electrical signals along each of the first and second transmission paths in response to the detected fault condition. In this manner, the detection circuitry of the communication bus automatically reduces or prevents the occurrence of sparks or arcing in the event of a fault condition associated with the communication bus.

Claims

1. A communication bus suitable for use in a hazardous area of a process plant to transmit electrical signals from one process device to a second and different process device disposed within the process plant and to interrupt the transmission of electrical signals in response to the detection of a fault condition in the communication bus, the communication bus comprising:

a first end to connect to the one process device;

a second end to connect to the second and different process device;

a first transmission path between the first end and the second end that communicates electrical signals in a first direction between the first end and the second end;

a second transmission path between the first end and the second end that communicates electrical signals in a second direction between the first end and the second end; and

a safety device coupled to each of the first and second transmission paths between the first and second ends, wherein the safety device includes a first control unit to detect a fault condition associated with the communication bus, and wherein the safety device further includes a first switch unit connected to the first and second transmission paths and having a closed position allowing a flow of electrical signals along the first and second transmission paths and an open position preventing the flow of electrical signals along the first and second transmission paths, and wherein the first control unit causes the first switch unit to move to the open position to interrupt the flow of electrical signals between the first and second ends along each of the first and second transmission paths in response to detecting a fault condition in the communication bus at the first control unit, wherein the first control unit does not use the first and second transmission paths to detect the fault condition in the communication bus.

2. The communication bus of claim 1, wherein the detected fault condition associated with the communication bus includes at least one of an open circuit, an electrical discontinuity, a cut in the communication bus, a severed communication bus, and a disconnected end of the communication bus.

3. The communication bus of claim 1, wherein the safety device includes an intrinsically safe housing and the first control unit and the first switch unit are disposed in the intrinsically safe housing.

4. The communication bus of claim 1, wherein the safety device includes an explosion proof housing and the first control unit and the first switch unit are disposed in the explosion proof housing.

5. A communication bus suitable for use in a hazardous area of a process plant to transmit electrical signals from one process device to a second and different process device disposed within the process plant and to interrupt the transmission of electrical signals in response to the detection of a fault condition in the communication bus, the communication bus comprising:

a first end to connect to the one process device;

a second end to connect to the second and different process device;

a first transmission path between the first end and the second end that communicates electrical signals in a first direction between the first end and the second end;

a second transmission path between the first end and the second end that communicates electrical signals in a second direction between the first end and the second end;

a safety device coupled to each of the first and second transmission paths between the first and second ends, wherein the safety device includes a first control unit to detect a fault condition associated with the communication bus, and wherein the safety device further includes a first switch unit connected to the first and second transmission paths and having a closed position allowing a flow of electrical signals along the first and second transmission paths and an open position preventing the flow of electrical signals along the first and second transmission paths, and wherein the first control unit causes the first switch unit to move to the open position to interrupt the flow of electrical signals between the first and second ends along each of the first and second transmission paths in response to detecting a fault condition in the communication bus at the first control unit; and

a third transmission path between the first and second ends and coupled to the safety device, wherein the first control unit is configured to detect the fault condition on the third transmission path and to cause the first switch unit to move to the open position to interrupt the flow of electrical signals between the first and second ends along each of the first and second transmission paths in response to detecting the fault condition on the third transmission path at the first control unit.

6. The communication bus of claim 5, wherein each of the first, second, and third transmission paths includes twisted pair cable or coaxial cable.

7. The communication bus of claim 5, wherein the first control unit includes a first control device coupled to the third transmission path, and wherein the first control device includes a first signal source that generates an electrical signal that is communicated in the first direction along the third transmission path.

8. The communication bus of claim 7, wherein the first control device includes a first sensor that measures an electrical characteristic associated with the third transmission path.

9. The communication bus of claim 8, wherein the measured electrical characteristic associated with the third transmission path is one of current, voltage, and resistance.

10. The communication bus of claim 8, wherein the first control device includes a first comparator that compares the measured electrical characteristic associated with the third transmission path to a normal operational value.

11. The communication bus of claim 10, wherein the first switch unit includes a first switch coupled to the first control device.

12. The communication bus of claim 11, wherein the first switch includes a first relay and a second relay, and wherein each of the first and second relays is coupled to the first control device.

13. The communication bus of claim 12, wherein the first control device energizes and de-energizes coils associated with each of the first and second relays to open and close the first and second relays.

14. The communication bus of claim 12, wherein each of the first and second relays includes contacts that are closed during normal operation.

15. The communication bus of claim 14, wherein the first control device opens the contacts of the first and second relays in response to a change in the measured electrical characteristic associated with the third transmission path from the normal operational value.

16. The communication bus of claim 5, further including a fourth transmission path between the first end and the second end and coupled to the safety device, wherein the safety device comprises:

a first switch assembly coupled to each of the first, second, third and fourth transmission paths proximate the first end of the communication bus and including the first control unit and the first switch unit; and

a second switch assembly coupled to each of the first, second, third and fourth transmission paths proximate the second end of the communication bus, wherein the second switch assembly includes a second control unit configured to detect a fault condition associated with the communication bus on the fourth transmission path, and a second switch unit connected to the first and second transmission paths and having a closed position allowing a flow of electrical signals along the first and second transmission paths and an open position preventing the flow of electrical signals along the first and second transmission paths, wherein the second control unit is configured to cause the second switch unit to move to the open position to interrupt the flow of electrical signals between the first and second ends along each of the first and second transmission paths in response to detecting the fault condition on the fourth transmission path at the second control unit.

17. The communication bus of claim 16, wherein at least one of the first switch assembly and the second switch assembly is housed in a protective enclosure.

18. A safety device adapted for use in a hazardous area of a process plant, the safety device comprising:

a communication bus including a first end to connect to one process device and a second end to connect to a second process device, first, second and third transmission paths disposed between and communicatively connecting the first end and the second end, wherein transmission paths are disposed between the one process device and the second process device disposed at different locations within the process plant and the first and second transmission paths are coupled to communicate electrical signals between the one process device and the second process device;

a first control unit coupled to the third transmission path to detect a fault condition on the third transmission path associated with the communication bus; and

a first switch unit coupled to the first and second transmission paths between the first end and the second end and to the first control unit and having a closed position allowing a flow of electrical signals along the first and second transmission paths and an open position preventing the flow of electrical signals along the first and second transmission paths, wherein the first control unit causes the first switch unit to move to the open position to interrupt the flow of electrical signals along the first and second transmission paths between the first end and the second end in response to detecting the fault condition associated with the communication bus on the third transmission path at the first control unit.

19. The safety device of claim 18, wherein the first control unit includes a sensor to measure an electrical characteristic associated with the third transmission path.

20. The safety device of claim 19, wherein the measured electrical characteristic associated with the third transmission path includes one of current, voltage, and resistance.

21. The safety device of claim 19, wherein the first control unit includes a comparator to compare the measured electrical characteristic associated with the third transmission path to a normal operational value.

22. The safety device of claim 21, wherein the first transmission path communicates electrical signals in a first direction, and the second transmission path communicates electrical signals in a second direction.

23. The safety device of claim 22, wherein the third transmission path communicates electrical signals in the first direction.

24. The safety device of claim 23, wherein each of the first, second, and third transmission paths includes one wire or two wires.

25. The safety device of claim 23, wherein the first control unit includes a first control device coupled to the third transmission path.

26. The safety device of claim 23, comprising:

a fourth transmission path communicating electrical signals in the second direction, wherein the first control unit is coupled to the third transmission path proximate the first end of the communication bus and the first switch unit is coupled to the first and second transmission lines proximate the first end of the communication bus;

a second control unit coupled to the fourth transmission path proximate the second end of the communication bus to detect a fault condition on the fourth transmission path associated with the communication bus; and

a second switch unit coupled to the second control unit and to the first and second transmission paths between the first end and the second end proximate the second end, and having a closed position allowing a flow of electrical signals along the first and second transmission paths and an open position preventing the flow of electrical signals along the first and second transmission paths, wherein the second control unit causes the second switch unit to move to the open position to interrupt the flow of electrical signals along the first and second transmission paths between the first end and the second end in response to detecting a fault condition associated with the communication bus on the fourth transmission path at the second control unit.

27. The safety device of claim 25, wherein the first switch unit includes a first switch and a second switch each coupled to the first control device, wherein the first switch is coupled to the first transmission path, and wherein the second switch is coupled to the second transmission path.

28. The safety device of claim 27, wherein each of the first and second switches includes contacts that are closed during normal operation.

29. The safety device of claim 28, wherein the first control device operates to open the contacts of the first and second switches in response to a change in the measured electrical characteristic associated with the third transmission path from the normal operational value.

30. The safety device of claim 18, wherein each of the transmission paths includes a twisted pair cable or a coaxial cable.

31. The safety device of claim 18, wherein the first and second transmission paths communicate electrical signals using a communication protocol based on Ethernet, Fieldbus, HART, PROFIBUS, WORLDFIP, Device-Net, As-Interface, or CAN.

32. The safety device of claim 18, wherein the first control unit includes a signal source that operates to generate an electrical signal that is communicated along the third transmission path.

33. A method for providing a communication bus suitable for use in a hazardous area of a process plant, the method comprising:

communicating electrical signals from a first process device to a second process device by communicating the electrical signals from a first end of the communication bus to a second end of the communication bus along a first transmission path and a second transmission path disposed between and communicatively connecting the first end and the second end of the communication bus;

communicating electrical signals along a third transmission path within the communication bus;

measuring an electrical characteristic associated with the third transmission path;

detecting a fault condition associated with the communication bus in response to the measured electrical characteristic associated with the third transmission path; and

interrupting the flow of electrical signals along the first and second transmission paths at a point between the first end and the second end of the communication bus in response to detecting a fault condition associated with the communication bus on the third transmission path.

34. The method of claim 33, wherein detecting the fault condition associated with the communication bus includes detecting at least one of an open circuit, an electrical discontinuity, a cut in the communication bus, a severed communication bus, and a disconnected end of the communication bus.

35. The method of claim 33, wherein communicating electrical signals along the first and second transmission paths includes communicating electrical signals in a first direction along a first pair of transmission wires of the first transmission path and communicating electrical signals in a second direction along a second pair of transmission wires of the second transmission path, and wherein communicating electrical signals along the third transmission path includes communicating electrical signals in the first direction along a third pair of transmission wires of the third transmission path, wherein the fault condition is detected on the third pair of transmission wires.

36. The method of claim 33, wherein communicating electrical signals along the first and second transmission paths includes communicating electrical signals in a first direction along a first transmission wire of the first transmission path and communicating electrical signals in a second direction along a second transmission wire of the second transmission path, and wherein communicating electrical signals along the third transmission path includes communicating electrical signals in the first direction along a third transmission wire of the third transmission path, wherein the fault condition is detected on the third transmission wire.

37. The method of claim 33, wherein measuring the electrical characteristic associated with the third transmission path includes measuring current, voltage, or resistance.

38. The method of claim 33, further including comparing the measured electrical characteristic associated with the third transmission path to a normal operational value.

39. The method of claim 38, wherein interrupting the flow of electrical signals along the first and second transmission paths includes opening switch contacts coupled to the first and second transmission paths in response to a change in the measured electrical characteristic associated with the third transmission path from the normal operational value.

40. A communication bus suitable for use in a hazardous area of a process plant to transmit electrical signals from one process device to a second and different process device disposed within the process plant and to interrupt the transmission of electrical signals in response to the detection of a fault condition in the communication bus, the communication bus comprising:

a first end to connect to the one process device;

a second end to connect to the second and different process device;

a first transmission path between the first end and the second end that communicates electrical signals in a first direction between the first end and the second end;

a second transmission path between the first end and the second end that communicates electrical signals in a second direction between the first end and the second end;

a safety device coupled to each of the first and second transmission paths between the first and second ends, wherein the safety device includes a first control unit to detect a fault condition associated with the communication bus, and wherein the safety device further includes a first switch unit connected to the first and second transmission paths and having a closed position allowing a flow of electrical signals along the first and second transmission paths and an open position preventing the flow of electrical signals along the first and second transmission paths, and wherein the first control unit causes the first switch unit to move to the open position to interrupt the flow of electrical signals between the first and second ends along each of the first and second transmission paths in response to detecting a fault condition in the communication bus at the first control unit; and

a third transmission path and a fourth transmission path connected in a loop within the communication bus, wherein the safety device is coupled to each of the third and fourth transmission paths and wherein the control unit includes a signal source to send a generated signal through the third transmission path and receives a received signal on the fourth transmission path and detects a fault condition based on the received signal.