The 2011 International Conference of Youth Communication ReliabilityReliability ofof AutomaticAutomatic ControlControl SystemSystem PLCPLC Abstract:Abstract: Programmable controller as anti-interference ability, high reliability, simple programming, high cost performance, in the field of industrial control to be more widely applied.Keywords:Keywords: reliability; automatic control system; PLC1.1. IntroductionIntroduction Date of machine industry as the central control unit, equipped with configuration software, use large-screen real-time monitoring interface, the achievement of the control points of the dynamic display, data modification, fault diagnosis, automatic alarm, but also show the query record of historical events, the system of major parts total running time, the device process flow diagram, the device structure and so on. Central control unit and slave serial communications between the PLC data exchange, usually selected from the 485 in less than 1000m twisted pair communication, fiber can choose from more frequent communication, more long-distance wireless communication can also be used. PLC control of the lower computer use, according to the number of control objects, control the scope of the object, the choice of one or more control PLC, PLC data exchange between the registers is to use internal links, data exchange and sharing. The PLC real-time monitoring of the site into a high reliability, and programming is simple, flexible, and therefore more and more people attention. 2.2. TheThe mainmain reasonreason ofof controlcontrol systemsystem reliabilityreliability reducereduce Although the industrial control and programmable controller itself has a high reliability, but if the input switch signal to the PLCs error, the analog signal to a larger deviation, PLC output port to control the implementation of the institutions were not required action, These mistakes can make the control process, resulting in irreparable economic losses. Live input signals to the PLC of the main reasons for errors include: 1) The signal transmission line caused a short circuit or open circuit (due to mechanical pull, line their own aging, especially rodents), when the transmission signal when the fault line, the scene can not send signals to the PLC, resulting control error; 2) mechanical contact bounce, live close contact though only once, PLC has closed a number that, although the hardware plus a filter circuit, the software to increase the differential instruction, but because of PLC scan cycle is too short, may still be in the count, accumulate, displacements instruction error, error control results; 3) on-site transmitter, mechanical switches itself out of trouble, such as bad contact, the transmitter reflects the larger field of non-power deviation, or does not work, etc. These failures also make the control system does not work.Error of the main reasons for the implementing agencies are: 1) can not reliably control the movement of contact load, although the PLC to issue a motion command, but the agency did not perform the required action;2) control the drive started, the drive itself fails, the drive motor has not brought the required work; 3) all kinds of electric valve, solenoid valve in the open could not open and shut the place could not relevant, as the executing agency did not follow the requirements of PLC, motion control, the system does not work, reducing system reliability. To improve the reliability of the whole control system, the input signal must increase the reliability and accuracy of the implementation of agency action, otherwise the PLC should be able to find problems with the sound and light and other warning methods prompted to the operator as soon as possible troubleshooting so system security , reliable and correctly. 3.3. FailureFailure alarmalarm systemsystem designeddesigned The automatic control system design, we designed the three fault alarm system, a setting in the control field of the control cabinet panel, with the light indicates normal operation and fault conditions, corresponding to normal operation when the indicator light, when the equipment failure indicator light running frequency of 1Hz flash. Light bulb to prevent damage to equipment not properly reflect the work, set up a special fault reset / lamp test button, the system is running continuously press the but-ton any time 3s, all lights should be fully lit, if not lit when there is indication that such The light is bad, should be replaced immediately after the light to change button to reset the status of display devices based upon the work of the original work status. 2 failures in the central control room display settings on the large screen monitor, when the device fails, the fault type with text display, process map is a device flashes, history will record the fault event table. 3 Fault display settings in the central control room signal bo