PLC系统的软件故障和硬件故障(Software failure and hardware failure of PLC system)

  plc故障分为软件故障和硬件故障,电工学习网小编结合PLC系统现场故障处理实例,分享PLC故障维修经验,本文是PLC高手速成秘籍!!

  PLC主要由中央处理单元、输入接口、输出接口、通信接口等部分组成,其中CPU是PLC的核心,I/0部件是连接现场设备与CPU之间的接口电路,通信接口用于与编程器和上位机连接。对于整体式PLC,所有部件都装在同一机壳内;对于模块式PLC,各功能部件独立封装,称为模块或模板,各模块通过总线连接,安装在机架或导轨上。(初学者可以结合plc视频教程来学习)

  PLC系统故障分析

  PLC控制系统故障分为软件故障和硬件故障两部分。PLC系统包括中央处理器、主机箱、扩展机箱、I/O模块及相关的网络和外部设备。现场生产控制设备包括I/0端口和现场控制检测设备,如继电器、接触器、阀门、电动机等。

  1、PLC软件故障

  PLC具有自诊断能力,发生模块功能错误时往往能报警并按预先程序作出反应,通过故障指示灯就可判断。当电源正常,各指示灯也指示正常,特别是输入信号正常,但系统功能不正常(输出无或乱)时,本着先易后难、先软后硬的检修原则首先检查用户程序是否出现问题。

  用户程序储存在PLC的RAM中,是掉电易失性的,当后备电池故障系统电源发生闪失时,程序丢失或紊乱的可能性就很大,强烈的电磁干扰也会引起程序出错。

  2、PLC硬件故障

  ①PLC主机系统故障

  A、电源系统故障。电源在连续工作、散热中,电压和电流的波动冲击是不可避免的。

  B、通讯网络系统故障。通讯及网络受外部干扰的可能性大,外部环境是造成通讯外部设备故障的最大因素之一。系统总线的损坏主要由于PLC多为插件结构,长期使用插拔模块会造成局部印刷板或底板、接插件接口等处的总线损坏,在空气温度变化、湿度变化的影响下,总线的塑料老化、印刷线路的老化、接触点的氧化等都是系统总线损耗的原因。

  ②PLC的I/O端口故障。

  I/O模块的故障主要是外部各种干扰的影响,首先要按照其使用的要求进行使用,不可随意减少其外部保护设备,其次分析主要的干扰因素,对主要干扰源要进行隔离或处理。

  ③现场控制设备故障

  A、继电器、接触器。减少此类故障应尽量选用高性能继电器,改善元器件使用环境,减少更换的频率。现场环境如果恶劣,接触器触点易打火或氧化,然后发热变形直至不能使用。

  B、阀门或闸板等类设备。长期使用缺乏维护,机械、电气失灵是故障产生的主要原因,因这类设备的关键执行部位,相对的位移一般较大,或者要经过电气转换等几个步骤才能完成阀门或闸板的位置转换,或者利用电动执行机构推拉阀门或闸板的位置转换,机械、电气、液压等各环节稍有不到位就会产生误差或故障。

  C、开关、极限位置、安全保护和现场操作上的一些元件或设备故障,其原因可能是因为长期磨损,或长期不用而锈蚀老化。对于这类设备故障的处理主要体现在定期维护,使设备时刻处于完好状态。对于限位开关尤其是重型设备上的限位开关除了定期检修外,还要在设计的过程中加入多重的保护措施。

  D、PLC系统中的子设备,如接线盒、线端子、螺栓螺母等处故障。这类故障产生的原因主要是设备本身的制作工艺、安装工艺及长期的打火、锈蚀等造成。根据工程经验,这类故障一般是很难发现和维修的。所以在设备的安装和维修中一定要按照安装要求的安装工艺进行,不留设备隐患。

  E、传感器和仪表故障。这类故障在控制系统中一般反映在信号的不正常。这类设备安装时信号线的屏蔽层应单端可靠接地,并尽量与动力电缆分开敷设,特别是高干扰的变频器输出电缆,而且要在PLC内部进行软件滤波。

  F、电源、地线和信号线的噪声(干扰)故障。

  PLC系统故障实例分析

  1、PLC软故障实例

  一台停机一段时间的PLC控制系统上电后无法启动

  故障检查、处理:检修人员在检查后认为程序出错,很自然地将EPROM卡插人PLC中,总清后拷贝程序,完成后重启,故障依旧,由于程序不大,逐条把EPROM上的程序读出,与手册上的指令核对后发现完全一样,重复拷贝无效后认为是PLC硬件故障。用PG将备份程序调出,与EPROM上的程序进行比对,结果语句指令表相同,但程序存放地址发生了变化,把备份程序发送到PLC后设备运行正常。可见EPROM上的程序也出现了错误,擦除后重新写入问题解决。

  2、PLC硬件故障实例

  ①某石化装置西门子PLC(S7-300,CPU315-2DP)在使用时,突然停止运行

  故障检查、分析:检查报警灯、程序、供电电源,在检查报警时,发现CPU上BAT灯亮起。检査程序时,发现没有对电池失效进行故障处理。

  故障处理:更换CPU电池,对电池失效故障在程序中进行相应处理。

  ②某日晚,压缩机PLC与主控PLC通讯突然中断,主控dcs上显示压缩机PLC与主控PLC通讯中断报警,压缩机控制室里的电机信号在主控合成DCS上均显示红色(停止状态),压缩机控制室里的一些流量、压力、温度等信号,在主控合成DCS上均显示高低报警。由于通讯中断使压缩机控制室里一些重要联锁不能送到主控,从而使全厂停车

  故障检查、分析:从理论上讲,引起压缩机PLC和主控PLC通讯中断的原因主要是两个:一个是软件不同步;另一个是由于硬件如CP525卡、CPU卡故障。

  首先从软件方面进行处理。在主控PLC进行了同步操作,强制通讯数据字DW13的第14位,结果通讯仍然没有建立起来,看来不是主控PLC不同步引起的。接着在压缩机PLC对其进行了同步操作,强制通讯数据字MW10的第14位,结果通讯建立。从而确认这次压缩机的PLC与主控PLC通讯中断的原因是由于压缩机程序不同步引起的,造成程序不同步的原因是外界的电磁干扰。

  故障处理:为了避免此类故障的再次发生,应加强控制室的屏蔽,禁止在控制室使用移动电话等通讯工具。

  ③西门子PLC(S7-300)的SF灯报警

  故障检查、分析:SF灯报警说明输入点有故障。

  故障处理:检查各个输入点工作状态,在检查时发现现场一台温度变送器没有输入信号,经处理后故障消失。

  ④PLC某个输入点外部没有被接通(即使拆开该输人端子上的连接线效果也相同),但该输入点实际已经被接通而且相应输入指示灯常亮

  故障分析:判断该端子的相邻端子已经被接通,而PLC的输入端子之间存在铁屑,导致了该输入点被接通,或该输入点已经被损坏。

  故障处理:拆开PLC的所有输入端子的连线,发现输入端子排上存在很多铁屑,将端子上的铁屑吹干净,然后恢复接线,故障被排除。

  ⑤控制系统PLC数字输入卡SF灯变红色

  故障检查、分析:将卡件电源重新送电后,故障现象依然存在;重新启动PLC主机后,故障指示灯仍旧是红色。于是对卡件所接收的现场信号一一进行检查后发现一回讯开关有异常。用万用表测量后发现,回路电阻无穷大,这说明回讯开关坏而被数字输入卡检测到。

  故障处理:更换备件后故障指示灯灭。

  ⑥造粒机PLC控制系统模拟输入卡接收的现场信号在DCS上指示无穷大

  故障检查、分析:分析可能是现场压力变送器和接线箱之间相互连接的通讯电缆出现故障,于是更换通讯电缆,但现象依然如故。仔细检查分析整个回路后发现,在回路中容易出现的地方有三个,压力变送器本身、通讯电缆、卡件,压力变送器、通讯电缆都已排除。将卡件拆开来看后发现里面的一个小的集成块已经被烧毁。

  故障处理:更换卡件。

  ⑦两个PLC互为热备的控制器中只有一个能够运行,另一个始终处于停止

  故障检查、分析:将整个控制柜断电、送电后同时启动两个PLC主机还是只有一个PLC主机运行。查询相关资料后发现OB70,OB72两个系统功能块负责冗余故障,如果没有插入这两个功能块则系统冗余丢失,即只有一个CPU能够运行。

  故障处理:插入这两个系统功能块后,控制系统恢复正常。

  本文来源:https://www.jcpeixun.com

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PLC faults are divided into software faults and hardware faults. The small editor of electrician learning network combines the on-site fault handling examples of PLC system to share PLC fault maintenance experience. This paper is a quick secret script of PLC experts!!

PLC is mainly composed of central processing unit, input interface, output interface and communication interface. CPU is the core of PLC, I / 0 component is the interface circuit between field equipment and CPU, and the communication interface is used to connect with programmer and upper computer. For integral PLC, all components are installed in the same cabinet; For modular PLC, each functional component is encapsulated independently, which is called module or template. Each module is connected by bus and installed on the rack or guide rail. (beginners can learn with PLC video tutorial)

< strong > PLC system fault analysis < / strong >

PLC control system fault is divided into software fault and hardware fault. PLC system includes central processing unit, host box, expansion box, I / O module and related network and external equipment. Field production control equipment includes I / 0 port and field control and detection equipment, such as relay, contactor, valve, motor, etc.

< strong > 1. PLC software failure < / strong >

PLC has the ability of self diagnosis. In case of module function error, it can often give an alarm and respond according to the pre program, which can be judged by the fault indicator. When the power supply is normal and all indicators also indicate normal, especially when the input signal is normal but the system function is abnormal (no or disordered output), first check whether there is a problem in the user’s program based on the maintenance principle of easy before difficult and soft before hard.

The user program is stored in the ram of PLC, which is volatile after power failure. When the backup battery fails and the system power supply fails, the possibility of program loss or disorder is great, and strong electromagnetic interference will also cause program error.

< strong > 2. PLC hardware failure < / strong >

< strong > ① PLC host system failure < / strong >

A. power system failure. In the continuous operation and heat dissipation of power supply, the fluctuation and impact of voltage and current are inevitable.

B. communication network system failure. Communication and network are likely to be disturbed by external interference. External environment is one of the biggest factors causing communication external equipment failure. The damage of system bus is mainly due to the plug-in structure of PLC. The long-term use of plug-in modules will cause bus damage at local printed boards or backplanes, connector interfaces, etc. under the influence of air temperature and humidity changes, the aging of bus plastic, printed circuit and oxidation of contact points are all the causes of system bus loss.

< strong > ② I / O port failure of PLC

The failure of I / O module is mainly affected by various external interference. Firstly, it shall be used according to its use requirements, and its external protection equipment shall not be reduced at will. Secondly, the main interference factors shall be analyzed, and the main interference sources shall be isolated or treated.

< strong > ③ field control equipment failure < / strong >

A. relay and contactor. To reduce such faults, high-performance relays shall be selected as far as possible to improve the service environment of components and reduce the replacement frequency. If the site environment is bad, the contactor contact is easy to ignite or oxidize, and then heat and deform until it can not be used.

B. valves or rams and other equipment. Lack of maintenance in long-term use and mechanical and electrical failure are the main reasons for the failure. Because the relative displacement of the key executive parts of such equipment is generally large, or the position conversion of the valve or RAM can be completed through several steps such as electrical conversion, or the position conversion of the valve or ram can be pushed and pulled by the electric actuator, mechanical, electrical If the hydraulic and other links are not in place, errors or faults will occur.

C. some components or equipment failures in switches, limit positions, safety protection and field operation may be due to long-term wear or corrosion and aging due to long-term use. The treatment of such equipment faults is mainly reflected in regular maintenance to keep the equipment in good condition at all times. For limit switches, especially those on heavy equipment, in addition to regular maintenance, multiple protection measures should be added in the design process.

D. faults of sub equipment in PLC system, such as junction box, wire terminal, bolt and nut, etc. This kind of failure is mainly caused by the manufacturing process and installation process of the equipment itself, as well as long-term ignition and corrosion. According to engineering experience, such faults are generally difficult to find and repair. Therefore, the installation and maintenance of the equipment must be carried out according to the installation process required by the installation, leaving no hidden dangers of the equipment.

E. sensor and instrument failure. This kind of fault is generally reflected in the abnormal signal in the control system. During the installation of such equipment, the shielding layer of the signal line shall be reliably grounded at one end and laid separately from the power cable as far as possible, especially the output cable of the frequency converter with high interference, and software filtering shall be carried out inside the PLC.

F. noise (interference) fault of power supply, ground wire and signal wire.

< strong > PLC system fault case analysis < / strong >

< strong > 1. PLC soft fault example < / strong >

A PLC control system that has been shut down for a period of time cannot be started after it is powered on

Fault inspection and handling: after inspection, the maintenance personnel think that the program is wrong, so they naturally insert the EPROM card into the PLC, copy the program after general cleaning, and restart after completion. The fault is still the same. Because the program is small, read out the programs on the EPROM one by one, which is exactly the same as the instructions in the manual. If the repeated copy is invalid, it is considered to be a PLC hardware fault. Call out the backup program with PG and compare it with the program on EPROM. The result is that the statement instruction table is the same, but the program storage address has changed. After sending the backup program to PLC, the equipment runs normally. It can be seen that the program on EPROM also has errors. The problem of rewriting after erasing is solved.

< strong > 2. PLC hardware failure example < / strong >

① Siemens PLC (S7-300, CPU315-2DP) of a petrochemical unit suddenly stopped running when it was in use

Fault inspection and analysis: check the alarm light, program and power supply. When checking the alarm, it is found that the bat light on the CPU is on. When checking the procedure, it is found that no fault treatment is carried out for battery failure.

Fault handling: replace the CPU battery and handle the battery failure in the program.

② one night, the communication between the compressor PLC and the main control PLC was suddenly interrupted. The main control DCS displayed the alarm of communication interruption between the compressor PLC and the main control PLC. The motor signals in the compressor control room displayed red (stop state) on the main control synthetic DCS. Some flow, pressure, temperature and other signals in the compressor control room displayed high and low alarms on the main control synthetic DCS. Due to the interruption of communication, some important interlocks in the compressor control room cannot be sent to the main control, resulting in the shutdown of the whole plant

Fault inspection and analysis: theoretically, there are two main reasons for the interruption of communication between compressor PLC and main control PLC: one is that the software is not synchronized; The other is the failure of hardware such as cp525 card and CPU card.

Firstly, it is processed from the aspect of software. The synchronous operation was carried out in the main control PLC and the 14th bit of the forced communication data word dw13. As a result, the communication was still not established. It seems that it was not caused by the non synchronization of the main control PLC. Then, the synchronous operation is carried out in the compressor PLC to force the 14th bit of the communication data word mw10. As a result, the communication is established. Thus, it is confirmed that the communication interruption between the PLC of the compressor and the main control PLC is caused by the asynchronous program of the compressor, and the asynchronous program is caused by external electromagnetic interference.

Fault handling: in order to avoid the recurrence of such faults, the shielding of the control room shall be strengthened, and the use of mobile phones and other communication tools in the control room is prohibited.

< strong > ③ SF light alarm of Siemens PLC (S7-300) < / strong >

Fault inspection and analysis: SF light alarm indicates that there is a fault at the input point.

Fault handling: check the working status of each input point. During the inspection, it is found that there is no input signal for a temperature transmitter on site, and the fault disappears after processing.

< strong > ④ the outside of an input point of PLC is not connected (even if the connecting line on the input terminal is disconnected, the effect is the same), but the input point has actually been connected and the corresponding input indicator is always on < / strong >

Fault analysis: judge that the adjacent terminals of the terminal have been connected, and there are iron chips between the input terminals of PLC, resulting in the connection of the input point or the input point has been damaged.

Troubleshooting: disassemble the wiring of all input terminals of PLC and find that there are many iron filings on the input terminal strip. Blow the iron filings on the terminal, and then restore the wiring. The fault is eliminated.

< strong > ⑤ SF light of PLC digital input card of control system turns red < / strong >

Fault inspection and analysis: the fault still exists after the card power supply is re powered; After restarting the PLC host, the fault indicator is still red. Therefore, after checking the field signals received by the card one by one, it is found that the first communication switch is abnormal. After measuring with a multimeter, it is found that the loop resistance is infinite, which indicates that the return switch is broken and detected by the digital input card.

Fault handling: the fault indicator light is off after replacing the spare parts.

< strong > ⑥ the field signal received by the analog input card of the PLC control system of the granulator indicates infinity on the DCS < / strong >

Fault inspection and analysis: it is possible that the communication cable connected between the field pressure transmitter and the junction box has failed, so the communication cable is replaced, but the phenomenon remains the same. After careful inspection and analysis of the whole circuit, it is found that there are three easy places in the circuit. The pressure transmitter itself, communication cable, clamp, pressure transmitter and communication cable have been eliminated. After taking apart the card, I found that a small integrated block inside had been burned.

Troubleshooting: replace the card.

< strong > ⑦ only one of the two PLCs is hot standby for each other, and the other is always stopped < / strong >

Fault inspection and analysis: after the whole control cabinet is powered off and powered on, start two PLC hosts at the same time or only one PLC host runs. After querying relevant data, it is found that two system function blocks OB70 and OB72 are responsible for redundancy faults. If these two function blocks are not inserted, the system redundancy will be lost, that is, only one CPU can run.

Troubleshooting: after inserting these two system function blocks, the control system returns to normal.

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