Data processing: generic control systems or specific application – Generic control system – apparatus or process – Having protection or reliability feature
Reexamination Certificate
2001-01-17
2004-07-06
Patel, Ramesh (Department: 2121)
Data processing: generic control systems or specific application
Generic control system, apparatus or process
Having protection or reliability feature
C700S002000, C700S019000, C700S020000, C700S014000, C714S011000, C714S012000
Reexamination Certificate
active
06760634
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the art of industrial controllers, and more particularly to a system and method for resumption of periodic tasks following a redundant control system switchover.
BACKGROUND OF THE INVENTION
Industrial controllers are special purpose computers used for controlling industrial processes, manufacturing equipment, and other factory automation applications. In accordance with a control program, an industrial controller may measure one or more process variables or inputs reflecting the status of a controlled process, and change outputs effecting control of the process. The inputs and outputs may be binary, (e.g., on or off), as well as analog inputs and outputs assuming a continuous range of values. The control program may be executed in a series of execution cycles with batch processing capabilities.
The measured inputs received from a controlled process and the outputs transmitted to the process generally pass through one or more input/output (I/O) modules. These I/O modules serve as an electrical interface between the controller and the controlled process, and may be located proximate or remote from the controller. The inputs and outputs are recorded in an I/O table in processor memory. Input values may be asynchronously read from the controlled process by one or more input modules and output values are written directly to the I/O table by the processor for subsequent communication to the process by specialized communications circuitry. An output module may interface directly with a controlled process, by providing an output from an I/O table to an actuator such as a motor, valve, solenoid, and the like.
During execution of the control program, values of the inputs and outputs exchanged with the controlled process pass through the I/O table. The values of inputs in the I/O table are asynchronously updated from the controlled process by dedicated scanning circuitry. This scanning circuitry may communicate with input and/or output modules over a bus on a backplane or network communications. The scanning circuitry also asynchronously writes values of the outputs in the I/O table to the controlled process. The output values from the I/O table are then communicated to one or more output modules for interfacing with the process. Thus, the processor may simply access the I/O table rather than needing to communicate directly with the controlled process.
An industrial controller may be customized to a particular process by writing control software that may be stored in the controller's memory and/or by changing the hardware configuration of the controller to match the control task. In distributed control systems, controller hardware configuration is facilitated by separating the industrial controller into a number of control modules, each of which performs a different function. Particular control modules needed for the control task may then be connected together on a common backplane within a rack and/or through a network or other communications medium. The control modules may include processors, power supplies, network communication modules, and I/O modules exchanging input and output signals directly with the controlled process. Data may be exchanged between modules using a backplane communications bus, which may be serial or parallel, or via a network. In addition to performing I/O operations based solely on network communications, smart modules exist which may execute autonomous logical or other programs.
Various control modules of a distributed industrial control system may be spatially distributed along a common communication link in several racks. Certain I/O modules may thus be located in close proximity to a portion of the control equipment, and away from the remainder of the controller. Data is communicated with these remote modules over a common communication link, or network, wherein all modules on the network communicate using a standard communications protocol.
In a typical distributed control system, one or more I/O modules are provided for interfacing with a process. The outputs derive their control or output values in the form of a message from a master or peer device over a network or a backplane. For example, an output module may receive an output value from a processor, such as a programmable logic controller (PLC), via a communications network or a backplane communications bus. The desired output value is generally sent to the output module in a message, such as an I/O message. The output module receiving such a message will provide a corresponding output (analog or digital) to the controlled process. Input modules measure a value of a process variable and report the input values to a master or peer device over a network or backplane. The input values may be used by a processor (e.g., a PLC) for performing control computations.
Conventional control devices typically provide a run mode wherein a module executes a control program and a configure mode wherein the control program execution is suspended. As control systems become more widely distributed, the logic or control program associated with a particular process or system may be executed on a large number of modules or devices. In this way, individual processors in the devices execute a program autonomously from the rest of the system components. Smart devices, such as I/O modules, transducers, sensors, valves, and the like may thus be programmed to execute certain logical or other programs or operations independently from other such devices.
In many control systems, redundant control devices are provided in order to further ensure proper control of a process or machine in the event of a device failure. Such redundant control systems may be employed, for example, where the operation of the controlled process or machine is in some manner critical. Thus, primary and secondary controllers may be provided in a control system, wherein the primary controller runs the process and the secondary controller is adapted to assume control if the primary controller fails. Such controllers typically execute or run various tasks, some of which may be periodic in nature. In conventional redundant control systems, however, it is difficult or impossible to guarantee the periodicity of such periodic tasks upon switchover from the primary controller to the secondary controller. Thus, there is a need for improved methods and apparatus by which timely execution of periodic tasks may be improved in redundant control systems following a switchover event.
SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
A method and apparatus are disclosed for performing timely execution of periodic tasks in a redundant control system. A secondary controller in the redundant system receives a wakeup time array having one or more wakeup time elements corresponding to periodic tasks, as well as a current time value from a primary controller. For example, the array may include entries for each periodic task and a corresponding element representing an estimated wakeup or execution time for the task. The array may be provided from the redundant primary controller to the secondary controller, for example, across a system redundancy module bridge. The secondary controller then schedules a run time for the periodic tasks at switchover based on the wakeup time elements and the current time value from the primary controller.
In the situation where the primary and secondary controllers determine task execution times according to internal timers (e.g., 1 &mgr;s timers), the primary may provide the secondary with its internal timer count value when sending the wakeup time array in
Cook William
Flood Mark
Amin & Turocy LLP
Barnes Crystal J
Patel Ramesh
Rockwell Automation Technologies Inc.
Speroff R. Scott
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