Siemens PLC common problems

Siemens PLC series products

PLC system problem analysis

PLC is mainly composed of central processing unit, input interface, output interface and communication interface. In the meantime, the CPU is the center of the PLC. The I/O component is the interface circuit between the field device and the CPU. The communication interface is used to connect with the programmer and the host computer. For the integral PLC, all components are installed in the same chassis; for modular PLCs, each functional component is individually packaged, called a module or a template, each module is connected by a bus, and the device is on a rack or a rail.

The PLC control system is divided into two parts: software faults and hardware faults. The PLC system consists of a central processing unit, mainframe, expansion chassis, I/O modules, and related network and external devices. On-site production control equipment includes I/O ports and on-site control and inspection equipment such as relays, contactors, valves, motors, etc.

First, PLC software problems

PLC has self-diagnosis ability, when the module function is faulty, it can often alarm and respond according to the pre-program, and can be judged by the fault indicator. When the power supply is normal, the indicators are also normal, especially if the input signal is normal, but the system function is not normal (output is not or chaotic), first check the user program to present whether it is easy to use, first soft and hard. problem.

The user program is stored in the RAM of the PLC, which is power-off and volatile. When the power supply of the backup battery system is lost, the possibility of program loss or disorder is great, and the electromagnetic interference is also strong. Will cause the program to make mistakes.

Second, PLC hardware problems

1, PLC host system problems

(1), power system problems. In the continuous operation and heat dissipation of the power supply, the shaking of voltage and current is not prevented.

(2), communication network system problems. Communication and network are highly likely to be disturbed by the outside world, and the external environment is one of the major factors that form a problem with communication external devices. The damage of the system bus is mainly because the PLC is mostly plug-in structure. The long-term use of the plug-in module will form the bus damage of the local printed board or the backplane, the connector interface, etc., under the influence of air temperature change and humidity change, the bus plastic aging The aging of printed circuits and the oxidation of contact points are the causes of system bus losses.

2. The I/O port of the PLC is faulty.

The fault of the I/O module is mainly the influence of various external disturbances. First, it should be applied according to the requirements of its application. It is not necessary to reduce its external maintenance equipment at will, and then analyze the primary disturbance elements, and the primary disturbance. The source is to be blocked or treated.

3, on-site control equipment problems

(1), relays, contactors. To reduce such problems, high-performance relays should be used as much as possible to improve the operating environment of components and reduce the frequency of replacement. If the site environment is bad, the contactor contacts are easy to be ignited or oxidized, and then deformed by heat until they cannot be used.

(2), valves or gates and other equipment. Long-term use lacks maintenance, mechanical and electrical failures are the primary cause of faults. Because of the critical parts of such equipment, the relative displacement is generally large. It may be necessary to complete the valve or gate orientation through several steps such as electrical conversion. Conversion, perhaps using the electric performance mechanism to push the valve or the azimuth transformation of the gate, mechanical, electrical, hydraulic and other links are not in place, there will be errors or problems.

(3) Some of the components or equipment on the switch, limit orientation, safety maintenance, and on-site operation may be caused by prolonged wear or aging due to prolonged periods of time. The handling of such equipment problems is mainly reflected in the regular maintenance, so that the equipment time is in good condition. For limit switches, especially for limit switches on heavy equipment, in addition to regular maintenance, multiple maintenance measures must be added during the design process.

(4), sub-devices in the PLC system, such as junction boxes, wire terminals, bolts and nuts, etc. The cause of such defects is the formation of the equipment itself, the equipment process and the long-term fire, rust and so on. According to engineering experience, such problems are generally difficult to find and repair. Therefore, in the equipment and maintenance of the equipment must be carried out in accordance with the equipment technology required by the equipment, leaving no hidden dangers.

(5), sensor and instrument problems. Such problems are generally reflected in the abnormality of the signal in the control system. In this type of equipment, the shielding layer of the signal line should be grounded firmly at one end, and should be laid separately from the power cable as much as possible, especially the high-interference inverter output cable, and software filtering should be performed inside the PLC.

(6) Noise, noise, and interference of power, ground, and signal lines.