Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-04-28
2001-10-23
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06307652
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to fiber optic communication systems and particularly to fiber optic communication systems for use in industrial control applications. More particularly, the invention relates to an optical communication system which continues uninterrupted communication between various elements of an industrial control system even in the event of a failure in the fiber optic lines.
Industrial control systems are used in industrial processes to control pressures, temperatures, mass transfers, and other process parameters. Control systems may included many input/output devices such as temperature sensors, pressure sensors, pressure regulators, and other similar devices positioned at various locations in the process equipment being controlled. Modern control systems also include one or more programmable logic controllers (“PLCs”) for controlling various input/output devices in the system, based upon data collected from the input/output devices and based upon instructions programmed on the controller.
These input/output devices and PLCs must communicate with each other in order for the control system to provide the desired control for the process. Both data and instructions must be communicated between devices in the control system. Thus, an industrial control system requires a communication arrangement to enable the various control system devices to communicate with each other.
Some industrial processes may require very precise control. Failure of communications between the various elements of an industrial control system may upset the entire process and have dire results. For example, failure of process control communications in a chemical manufacturing process may result in the production of an entirely different chemical than what was intended. Thus, a communications failure in an industrial control system may require that the entire process being controlled be aborted or at least suspended until communication is reestablished. Also, failure of communication and control in some processes may pose very serious safety risks. Therefore, the communication systems employed in industrial control systems must be very robust.
Yet the communication lines required between the various elements of an industrial control system must commonly traverse harsh environments or areas of high activity. In these areas, there is a constant danger of damage to the communication lines resulting in a loss of communication in the control system. Also, conditions in some areas of an industrial process may interfere with certain types of transmissions. For example, electrical noise in certain areas of a plant may interfere with electrical transmissions. In these situations and others, data and instructions may be converted to optical signals for transmission between the various control system devices through optical fibers.
The various elements of an industrial control system are advantageously connected for communications in a ring configuration allowing communications in both directions around the ring. In this closed ring configuration, a single break in the ring does not cause a loss of communications since communication is still possible in the opposite direction from the location of the break. Closed communications rings, however, raise the problem of signal oscillation which interferes with communications.
One way to solve the problem of ensuring communications in the event of a failure along a communication path in an industrial process control system is to provide multiple communication paths between the various elements of the system. In the event of a failure on one communication path, communications are still maintained along a secondary or tertiary path. However, systems having multiple communication paths require duplicate communication lines and associated material and are therefore more costly. Furthermore, multiple communication paths complicate communication protocols.
Detecting a failure of communication in an industrial control system process is complicated by the nature of communications in the system. While some communications of data and instructions may be relatively continuous in some process control applications, communications may be infrequent in other control systems. In situations in which long gaps appear between data or instructions, a communication failure may not be readily apparent. A monitoring system which monitors for communication failures may mistake a communication fault for a gap in data, or mistake a gap in data for a communication fault.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome the above-described problems and others associated with optical communication systems used in industrial control applications. It is also an object of the invention to provide a fault tolerant optical communication method for industrial control applications.
In order to accomplish these objects, the various components of the control system which require communication are connected in a ring configuration. Each component which requires communication has associated with it a node on the ring. The ring configuration includes a first optical communication path along which signals may travel in one direction, and a second optical communication path connected in parallel with the first path. Signals on the second optical communication path travel in the opposite direction to the signals traveling on the first optical communication path. Fiber optic communication lines or segments extend between each node.
Each node has associated with it a first receiver, first transmitter, second receiver, and second transmitter. Each receiver is adapted to receive optical signals from the optical fiber line or segment to which it is connected and convert the received signals to electrical signals. Each transmitter is adapted to receive electrical signals, including signals which are transferred from the respective receiver, convert the electrical signals to optical signals, and transmit the optical signals along the fiber optic communication line or segment to which the transmitter is connected. The first receiver is connected to transfer signals to the first transmitter for retransmission along the next segment of the ring, and the second receiver and second transmitter are similarly connected.
As used herein unless otherwise indicated, the term “data” when referring to transmissions in the communication system shall include any type of information that may be communicated in the system. For example, “data” may include either input/output data or operational instructions.
According to the invention, one node comprises a control unit node having a control unit associated therewith (alternatively referred to as a “master unit”). Each other node in the communication system is associated with a data unit (alternatively referred to as a “slave unit”). Each data unit relays data from each receiver to its respective transmitter for retransmission along the optical fiber lines or segments to the adjacent nodes. Also, each data unit is connected to a control system device such as an input/output device or a PLC associated with the particular control system. The data unit transfers data from each of its receivers to the control system device and transfers data from the control system device to the transmitters. Each particular industrial control system has its own communication protocol and addressing arrangement so that the signals transmitted around the ring are useful to the desired control system device.
Unlike the data units, the control unit according to the invention operates in either an open mode or a closed mode and is not associated with an input/output device or a PLC. In the open mode, data is not relayed from each receiver of the control unit to its respective transmitter. Although a transmission path is present between each receiver and its respective transmitter in the control unit, transmissions between the receiver and its respective transmitter are blocked when the control unit is operating in the open mode. In the closed mode of opera
Cisneros Ernest A.
Cram Milton E.
Fassih-Nia Mohammed
Taylor Donald Ray
Culbertson Russell D.
Pascal Leslie
Shaffer & Culbertson LL
Singh Dalzid
Weed Instrument Company, Incorporated
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