Excavating
Patent
1995-02-23
1998-04-21
Chung, Phung
Excavating
G06F 1100, G08C 2500, H03M 1300, H04L 100
Patent
active
057426244
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a fault detecting apparatus and method. More particularly, the invention relates to an apparatus and method for detecting a location at which a failure is possible in one or both of a detector and actuator in a controlled system by investigating the internal states of a programmable logic controller (hereinafter referred to simply as a "PLC").
BACKGROUND ART
PLCs are used widely in the sequence control of a large variety of controlled systems.
The controlled system is provided with many detectors (sensors and switches such as proximity switches, limit switches and photoelectric detectors) for extracting the operating state of the system, as well as a plurality of actuators (air cylinders, hydraulic cylinders, motors, etc.) for driving each component of the controlled system.
The signals from the detectors are applied to a PLC as input signals. The PLC, in which a user program for controlling the controlled system has been installed in advance, generates outputs signals (relay signals) for controlling the actuators in accordance with the user program in response to the input signals.
In general, the function of the PLC, namely the user program that has been installed in the PLC, is expressed by a ladder circuit (diagram). A ladder circuit comprises a plurality of individual ladder circuits.
FIG. 27 shows an example of a ladder circuit represented by a ladder diagram. This ladder circuit has individual ladder circuits L01, L02 and L03.
The individual ladder circuit L01 is composed of contacts (actual input contacts) 001 and 002 representing the status of input signals provided by the detectors of a controlled system, and an output relay 003. The individual ladder circuit L02 is composed of actual input contacts 004, 005, 006 and 007, a contact (internal auxiliary contact) 003 controlled by the output relay 003, and an output relay 008. The individual ladder circuit L03 is composed of actual input contacts 009, 010, an internal auxiliary contact 008 controlled by the output relay 008, a self-holding contact 011 of an output relay 011, and the output relay 011.
The contacts 001, 003, 005, etc., are a contacts (make contacts), and the contacts 002, 004 and 006, etc., are b contacts (break contacts).
If all of the serially connected contacts are 0N (closed), then the output relay connected to these contacts also is ON (closed). The relay outputs are sent to actuators to actuate them. For example, if the contacts 001 and 002 in the individual ladder circuit L01 are ON, then the output relay 003 also is ON. Further, if the contacts 004, 005, 003 and 006 are ON or the contacts 007 and 006 are ON in the individual ladder circuit L02, then the output relay 008 also is ON.
In a case where a prescribed operation is not performed in the PLC constituted by such a ladder circuit owing to a failure in the controlled system or a defect in a contact, the operator ascertains the abnormal location by referring to the states of the input/output signals and the ladder diagram of the PLC or the program list of the ladder program.
More specifically, the operator specifies the actuator that is not operating normally and specifies the output relay that is applying the output signal to the specified actuator. Then, while referring to the ladder diagram or program list, the operator scans and checks, one by one, the plurality of contacts connected to the specified output relay.
For example, consider a case in which the specified output relay is the output relay 011, which is in the OFF state regardless of the fact that it should be ON.
First, the contacts 010, 008, 009 and 011 connected to the output relay 011 are investigated to see if they are ON. If the contact 008 is OFF, then the output relay 008 controlling the contact 008 and the contacts 006, 003, 005, 004 and 007 connected to the output relay 008 are investigated to see if they are ON. If, by way of example, the contact 003 is OFF, then the output relay 003 and the contacts 001 and 002 connected to the output relay 003 are investi
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"Industrial Applications Socciety 36th Annual Petroleum and Chemical Industry Conference," The Institute of Electrical and Electronics Engineers Inc., San Diego, CA, Sep. 11-13, 1989 p.1 & Emergency shutdown system, pp. 23-27.
Irie Atsushi
Ishiguro Susumu
Nishidai Hajime
Soma Hiroshi
Tabata Naohiro
Chung Phung
Omron Corporation
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