Problems needing attention in PLC application - Database & Sql Blog Articles

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Plc is a device for industrial automation control that can be used directly in industrial environments without any measures. However, although the reliability is higher as described above and the anti-interference ability is strong, when the production environment is too harsh, the electromagnetic interference is particularly strong, or the installation is improperly used, it may cause a program error or an operation error, thereby causing a false input and Causes false output, which will cause the device to run out of control and malfunction, thus failing to ensure the normal operation of the PLC. To improve the reliability of the PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of the equipment; on the other hand, it requires high attention in design, installation and maintenance, and multi-party cooperation can solve the problem and effectively enhance the system's resistance. Interference performance. Therefore, the following issues should be noted in use.

Working environment

(1) Temperature: The PLC requires an ambient temperature of 0 to 55 ° C. It should not be placed under the component with large heat output during installation. The space for ventilation and heat dissipation should be large enough.

(2) Humidity: In order to ensure the insulation performance of the PLC, the relative humidity of the air should be less than 85% (no condensation).

(3) Vibration: The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibration of the vibration frequency of 10 to 55 Hz. When the use environment is unavoidable, shock absorption measures must be taken, such as using damping rubber.

(4) Air: Avoid corrosive and flammable gases such as chemical acids and bases. For environments with more dust or corrosive gases in the air, the PLC can be installed in a well-closed control room or control cabinet. For example, the dry slag and dry ash removal of power plants have added closed huts in the late stage of infrastructure construction.

(5) Power supply: PLC has a certain resistance to interference caused by the power line. In environments with high reliability requirements or particularly severe power disturbances, an isolation transformer with a shield can be installed to reduce interference between the equipment and the ground. The general PLC is supplied to the input terminal by the DC 24V output. When the input terminal uses an external DC power supply, the DC stabilized power supply should be selected. Because of the ordinary rectified and filtered power supply, it is easy for the PLC to receive error information due to the influence of ripple. (Exclusive original, please indicate the source: )

2. Interference and its source in the control system

On-site electromagnetic interference is one of the most common and most susceptible factors affecting system reliability in PLC control systems.

(1) Interference sources and general classification

The interference sources that affect the PLC control system are mostly generated in places where the current or voltage changes drastically. The reason is that the current changes to generate a magnetic field, which generates electromagnetic radiation to the device; the magnetic field changes to generate current, and the electromagnetic high-speed generates electromagnetic waves. Generally, electromagnetic interference is classified into common mode interference and differential mode interference according to different interference modes. Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the superposition of the common state (same direction) voltage induced by the power grid, the ground potential difference and the space electromagnetic radiation on the signal line. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which directly affects the measurement and control signals, causing component damage (this is the main reason for the high damage rate of some system I/O modules). This common mode interference can be DC. It can also be an exchange. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling of the space electromagnetic field between the signals and the common mode interference caused by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control. Precision.

(2) Main sources and ways of interference in PLC system

Strong electrical interference: The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induce voltage on the line. In particular, internal changes in the power grid, surges in knife switch operation, start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, and short-circuit transients of power grids are transmitted to the primary side of the power supply through transmission lines.

Interference in the cabinet: high-voltage electrical appliances in the control cabinet, large inductive loads, and chaotic wiring are all likely to cause a certain degree of interference to the PLC.

Interference from signal line introduction: Various types of signal transmission lines connected to the PLC control system, in addition to transmitting various types of information, there will always be external interference signals. There are two main ways of this interference: one is the power grid interference through the power supply of the transmitter or the power supply of the shared signal meter, which is often neglected; the second is that the signal line is interfered by the space electromagnetic radiation, that is, the signal line The external inductive interference, which is very serious. The introduction of interference from the signal will cause the I/O signal to work abnormally and the measurement accuracy will be greatly reduced. In severe cases, the component will be damaged.

Interference from chaotic grounding systems: Grounding is one of the effective means of improving electromagnetic compatibility (EMC) of electronic equipment. The correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the external interference of the equipment; and the wrong grounding will introduce serious interference signals, so that the PLC system will not work normally.

Interference from the PLC system: mainly caused by mutual electromagnetic radiation between internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the interaction between analog ground and logic ground and the mutual interaction between components Match usage, etc.

Inverter interference: First, the harmonics generated during the startup and operation of the inverter cause conduction interference to the power grid, causing grid voltage distortion and affecting the power supply quality of the grid; second, the output of the inverter will generate strong electromagnetic radiation interference, affecting the surrounding The device works normally.

3. Main anti-interference measures

(1) Reasonable handling of the power supply, suppression of interference introduced by the power grid. For the power grid interference introduced by the power supply, an isolation transformer with a shielding ratio of 1:1 can be installed to reduce the interference between the equipment and the ground. The power input terminal is connected in series with the LC filter circuit.

(2) Correct selection of grounding points and perfect grounding system Good grounding is an important condition to ensure reliable operation of PLC, and can avoid accidental voltage shock hazards. The purpose of grounding is usually

Two, one for safety and the other for suppression of interference. The perfect grounding system is one of the important measures for the anti-electromagnetic interference of the PLC control system.

The ground wire of the PLC control system includes system ground, shield ground, AC ground and protection ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven distribution of potential at each grounding point. There is a ground potential difference between different grounding points, causing ground loop current and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and a current flows through the shielding layer. When an abnormal state such as a lightning strike occurs, the ground current will be larger.

In addition, the shielding layer, the grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system ground and other grounding treatments are confusing, the generated ground circulation may generate unequal potential distribution on the ground line, affecting the normal operation of the logic circuit and the analog circuit in the PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution of logic ground potential easily affects the logic operation and data storage of PLC, resulting in data confusion, program running or crash. The distribution of the analog ground potential will result in a decrease in measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.

Grounding safely or power: Ground the power cord ground and cabinet wiring to a safety ground. If the power supply is leaking or the cabinet is charged, it can be introduced into the ground from the safety ground without causing harm to people.

System grounding: The PLC controller is grounded in order to be at the same potential as the controlled devices, and the system is grounded. The grounding resistance value must not exceed 4 Ω. Generally, the PLC equipment system ground and the negative terminal of the switching power supply in the control cabinet should be connected together as the control system ground.

Signal and shield grounding: Generally, the signal line must have a unique reference ground, that is, “single-point grounding”. When the shielded cable encounters a possibility of conductive interference, it must be grounded locally or in the control room to prevent the formation of “ground”. Loop". When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be avoided; When the shielded twisted pair of the point signal is connected with the multi-core twisted pair shielded cable, the shield layers should be connected to each other and insulated, and a single point contact at the appropriate grounding point should be selected.

(3) Suppression of inverter interference

Inverter interference processing generally has the following ways: Adding an isolation transformer, mainly for conducted interference from the power supply, can block most of the conducted interference before the isolation transformer; using filters, the filter has strong resistance The ability to disturb can also prevent the interference of the device itself from being transmitted to the power supply, and some also have the function of peak voltage absorption; using the output reactor to increase the AC reactor between the inverter and the motor is mainly to reduce the output of the inverter in energy transmission. During the process, the circuit generates electromagnetic radiation, which affects the normal operation of other equipment.