Sunday, March 4, 2012

Automatic control system reliability PLC





Programmable controller because of high reliability, strong anti-interference ability, high ratio of performance, and simple programming in industrial control areas to be more and more extensively applied.

Machine of industry as years central control unit, with the software configuration, select a big display real-time interface of monitoring, recognize the control points dynamic display, error diagnosis, and modification of data, automatic alarm, still can demonstrate questions the historical events record, system main parts accumulative total time of running, each machine, diagram of process flow, every structure of device and so on. The PLC and a central control unit under method between the serial communications replace data, generally in the 1000 m distance inside select 485 cable communication of twisted-pair, more frequently distance can prefer communication of fiber optic, more long distance also can select wireless communication. A JiXuan under PLC control, along with the amount control object, the controlled objects scope, can select one or many PLC control sets, PLC is using data exchange between realize data exchange, sharing, and internal links registers. Because of the scene into synchronized monitoring PLC has programming simple and flexible, and high reliability, consequently more and more notice by people.

The input signal to influence PLC fault is the major reasons:
1) Generate signal short circuit or open circuit transmission, when the error signal transmission, site, the signal can not transmit to the PLC, cause wrong control;
2) Contacts jitter of mechanical, on-site links though only closed one time, but a lot of times that closed PLC, though additional hardware of filter circuit, software enlarge differential instructions, but as the scanning cycle of PLC is too short, it can still be in the calculate, collect, and shift and directive fault, results of error control;
3) The transmitter, a switch of mechanical their own error, for example touch bad contacts, reproduce the deflection of transmitter non-electric quantity or can’t work generally, these error also can create the system of control can’t work in general.

Fault control actuators are the major reasons:
1) Contact can’t consistent control load action,
Even though PLC issued the commands of motion, but agencies enforcement as necessary and no action;

2) Control start of frequency converter
Since their individual error frequency converter, inverter with motor and did not depress for work;

3) Various valves of electric
Valve of solenoid this open couldn’t open the, the April didn’t shut in place, as the actuators can’t depress PLC control need action, create the system doesn’t work correctly, and reduce the reliability of system. To develop the whole control reliability system and the requirement to improve the input signal reliability and the executive agency action accuracy, or PLC should be capable to find out in time problems, with light and sound alarm measures for instance offered to operating personnel, out error as soon as possible, and let the system reliable, right, and safe to work.

In the automatic control system design, they designed a grade 3 in error display system of alarming, level 1 set in the panel of control every control cabinet, using lights the normal instructions and operation equipment fault, when the equipment normal operation communicating indicator, when the equipment failure operation indicator to blinking of 1 Hz frequency.

Level 2 error display settings in the big screen monitors of control room, when the tools malfunction, have text specify that the error type, the diagram of process flow on the corresponding shine of device, historical event list will record the error.

Level 3 error display settings in the signal of control room, when the equipment malfunction, signal box will utilize sound and light alarm means hint staff, deal with failure. In the treatment of the fault, and error classification, some faults are required to stop operation system, but some fault on the system work the impact is not big, the system can run with errors, error can be in operation of excluded, so significantly decreases the entire system to stop operating time, and develop the operation level system reliability.

4) The research of input signal reliability.
To develop the input signal reliability to PLC, the first to select transmitter high reliability and all types of switch, avoid a variety of causes transmission line open circuit, short circuit or contact unwanted. Secondly in design of program raise when digital filter program, raise the input signal credibility.

In the input contacts with a timer after, timing time according to contacts jitter and the response speed of the system to determine, generally in dozens of ms, so can ensure that contacts really stable after closing, just have other response. Three data sample are saved in the data registers of DT10, DT11, DT12, when the last one time after data comparison sampling, the data replace instructions, data, more instructions period from the maximum and minimum, maintain value as the results of sampling data saved in registers of DT0.

Improve the site of reliability the signal read PLC can control characteristics of system, the connection between the signal use to judge the reliable degree signal. For example control liquid level, because of the storage size tank is recognized, into the liquid or out of the valve opening and pressure of fluid is acknowledged, in several time adjusts in highly liquid tank regarding what the range is distinguish, if this gauge level to PLC data and approximation the height of liquid level is large, judge may be level gauge error, throughout the system of fault alarm notify the operating personnel verify the level gauge.

And as a liquid level limit protection and the storage tanks, when change to signal to the PLC action, this signal is correct, consistent, if the position of level limit also in reading that the signal is correct; If the position of level limit is not in reading, indicator may be level limit switch error or transmit signal line error, throughout the same system of alarm update the operating personnel handling of the error.

5) Actuators reliability research
The results of PLC program execution will through the actuators on the field devices on the adjustment when the signal precisely input to PLC. How to make sure those implementation institutions along with control for operating, when actuators did not depress for work, how to find error? It takes the following measurements: when control load by contactor, begin or stop this type of load to make contact with device control to coil, contactor start-up is dependable suction close, ends contactor is consistent release.

Y0 for control output coil, X0 for contactor action condition, X1 for lead back to input terminal of PLC the contactor supplementary frequently open contacts, the timer is longer than the time contact action device.

When close or open the valve electric, consistent with close and open the valve diverse time, delay time setting, after delay recognition in position or off in position a signal, if these signals cannot accurately on time back to the PLC, clarify valve may have error, perform the error alarm processing. Y1 for control action output valves, X2 for the opens conditions valve, the timing longer than the valve opens in position time, R1 for valve an error, X3 for valve in place to return to signal.



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