Data processing: generic control systems or specific application – Generic control system – apparatus or process – Plural processors
Utility Patent
1998-06-30
2001-01-02
Heckler, Thomas M. (Department: 2787)
Data processing: generic control systems or specific application
Generic control system, apparatus or process
Plural processors
Utility Patent
active
06169928
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to control devices and, more particularly, to the sharing of data among a plurality of control devices such as programmable logic controllers.
Programmable logic controllers (PLC's) are a relatively recent development in industrial and process control technology. As a part of this control technology, a PLC is used to monitor input signals from a variety of sources which report events and conditions occurring in a controlled process. For example, a PLC can monitor such input conditions as motor speed, temperature, pressure, volumetric flow and the like. A PLC can also include input circuits which enable it to monitor whether input devices are on or off. A control program is stored in a memory within the PLC to instruct the PLC what actions to take upon encountering particular input signals or conditions which are provided to input points of the PLC. In response to these input signals provided by input sensors, the PLC derives and generates output signals which are transmitted via PLC output points to various output devices to control the process. For example, the PLC issues output signals to speed up or slow down a conveyer, rotate the arm of a robot, open or close a relay, raise or lower temperature as well as many other possible control functions too numerous to list. As will be apparent, a PLC is a specific type of control device and performs functions similar to other types of control devices such as microcomputer based drive controllers for motor drive applications, e.g., multiple motor drive systems for paper mills and steel mills. For purpose of this description, reference to “controller” or to “PLC” in each instance is intended to include both the specific PLC and the general class of microprocessor based controllers.
It is often desirable for control systems to share data among a plurality of individual controllers such as the programmable logic controllers. However, in many cases, a given set of data is needed by some but not all controllers in a networked system. Several different approaches have been developed to address this situation. For example, in one approach, all data is sent to all devices and any unneeded data is discarded by protocol software or applications software. Another approach is to send an explicit copy of each set of data to each device needing that data. Unfortunately, this results in multiple copies of the shared data being sent over the connecting network. Yet another approach is to build a highly specialized network using specialized hardware which can be configured to accept only data of interest.
In conventional data sharing arrangements, an “acknowledgment” or “ack-back” approach is used to take care of the situation where data is lost in transmission over the network. The sending device transmits data to a receiving device at a specific address on the network. When the receiving device receives the data, it transmits an “acknowledgment” back to the sending device which resides at another particular address. If the sending device fails to receive an “acknowledgment” within a predetermined time period, the sending device retransmits the data to the receiving device. Unfortunately, this approach requires the transmission of overhead data in addition to the control data. The overhead data undesirably consumes bandwidth on the network as well as in the sending and receiving devices even when no retransmission of the data is required.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide an apparatus for efficiently sharing data among multiple control devices without the requirement of specialized hardware.
Another object of the present invention is to provide an apparatus for efficiently sharing data among multiple control devices without the requirement of extensive protocol processing within the devices sharing the data.
Yet another object of the invention is to provide an apparatus for sharing data among control devices which reduces error correction overhead by configuring each control device to rely on its knowledge of when data should arrive and to take corrective action when such data fails to arrive.
In accordance with one embodiment of the present invention, a system of programmable logic controllers is provided wherein the controllers communicate with each other and share data with each other under a predetermined communications protocol. More specifically, the plurality of programmable logic controllers includes one or more producer controller and one or more consumer controllers. The system further includes a communications network connecting the producer controller to the consumer controller. A given producer controller takes a data sample and transmits the data sample to the consumer controllers on the network. The consumer controller checks for loss of data samples transmitted from the producer controller to the consumer controller.
It is noted that the disclosed system provides high reliability by placing the responsibility of checking for lost data on the consumer device as opposed to the conventional technique of having each data producer check for a data acknowledgment back from a consumer device.
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Cinalli Anthony Joseph
Olson Ronald Wayne
Spencer Andrew Edward
GE Fanuc Automation North America Inc.
Heckler Thomas M.
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