The six countermeasures to improve the credibility of the plc automatic control system will enable you to master a series of rectification measures for the Mitsubishi plc automatic control system, such as moderate working temperature, moderate air relative humidity, air pollution, effective installation and wiring, appropriate Grounding device, etc. when making PLC automatic control system, power line, safety maintenance section, necessary software measures, etc. The web editor shares six countermeasures to improve the reliability of the PLC automatic control system for everyone.

1. Suitable office environment

1. Suitable working temperature

Each manufacturer has certain requirements on the working temperature of PLC. Under normal circumstances, the allowable operating temperature of PLC is around 0-55 degrees Celsius, so during installation, do not place components with high calorific value under the PLC, and there must be sufficient ventilation heat pipe cooling space around the PLC, and it is not necessary to install the PLC Directly irradiate or close to heating devices such as heating, electric heaters, and high-power power supplies in places with sufficient sunlight; if the temperature of the control box is too high, install a PLCZUI control box with ventilated venetian blinds. High, centrifugal fan forced natural ventilation should be installed in the cabinet.

2. Suitable air relative humidity

The air humidity of the environmental quality in PLC work is generally lower than 85%, in order to ensure the dielectric strength performance of PLC. Excessive ambient humidity can also compromise the precision of analog input/output devices. Therefore, the plc cannot be installed in dew and rainy places.

3. care about air pollution

It is not suitable for installation in areas with a lot of air pollutants (such as dust, dust, vermicelli, etc.), rotten candles, and flammable gases, especially in areas with a lot of corrosive gases that are likely to cause corrosion of electronic devices and printed circuit boards. PLC. If it is only installed in such a place, the PLC can be turned off if the temperature permits, or the PLC can be installed in a highly airtight operating room and air filtration equipment installed.

4. Avoid sources of vibration and shock

Control boxes with programmable control boards should be kept away from areas with significant vibrations and shocks, especially areas with constant, frequent vibrations. If necessary, appropriate countermeasures can be taken to reduce the risk of vibration and shock, and to prevent loosening of wiring or software.

5. Eliminate strong interference signals

The plc should avoid strong interference signals such as high-power thyristor devices, high-frequency equipment, and high-power equipment. In addition, the PLC should also avoid strong magnetic fields and strong radioactive substances, as well as places that are prone to strong electrostatic induction.

two. Reasonable installation and wiring

1. Pay attention to electrical installation

The power supply is the main way to interfere with PLC access. There are two kinds of power sources in plc system: external power source and internal power source.

The external power supply is used to drive the PLC output device (load) and provide input signals, also known as the user power supply, and the same PLC external power supply may have multiple specifications. The capacity and performance of the external power supply are determined by the input circuit and PLC of the output device. Because the I/O circuit of PLC has filtering and isolation functions, the external power supply has little influence on the performance of PLC. Therefore, the requirements for the external power supply are not high.

The internal power supply is the working power supply of the PLC, that is, the working power supply of the internal circuit of the PLC. Its performance directly affects the reliability of PLC. Therefore, in order to ensure the normal operation of PLC, higher requirements are put forward for the internal power supply. The internal power supply of the plc generally adopts a switching regulated power supply or a regulated power supply with a low-pass filter on the primary sideband.

In the case of large interference or high reliability requirements, an isolation transformer with a shielding layer is used to supply power to the plc system. The LC filter circuit can also be connected in series on the secondary side of the isolation transformer. At the same time, the following issues should be paid attention to during installation:

2) The power line of the system should be thick enough to reduce the line voltage drop caused by the startup of large-capacity equipment.

3) When the PLC input circuit uses an external DC power supply, it is best to use a regulated power supply to ensure that the input signal is correct. Otherwise, the plc may receive wrong signals.

Six Measures to Improve the Reliability of PLC Control System

2. stay away from high voltage

The plc cannot be installed near high-voltage and high-voltage power lines, and it cannot be installed in the same control cabinet as high-voltage electrical appliances. In the cabinet, the PLC should be far away from high-voltage power lines, and the distance between them should be greater than 200mm.

three. Reasonable wiring

1) I/O lines, power lines and other control lines should be separated and not routed in the same slot as much as possible.

2) AC line and DC line, input line and output line are well separated.

3) The switch I/O line ZUI is separated from the analog I/O line ZUI, and the I/O line ZUI for transmitting analog signals uses a shielded wire, and one end of the shielding layer of the shielded wire is grounded.

4) The cable transmission signal between the PLC basic unit and the expansion unit is small, the frequency is high, and it is susceptible to interference, so it cannot be laid in the same slot as other lines.

5) The I/O circuit wiring of the PLC must use crimp terminals or single-pair wires. It is not advisable to use multi-strand wires to directly connect to PLC terminals, otherwise sparks will easily occur.

6) Installed in the same control cabinet as the PLC, although it is not an induction element controlled by the PLC, the RC or diode arc suppression circuit should also be connected in parallel.

three. properly grounded

Good grounding is an important condition for the safe and reliable operation of PLC. In order to suppress interference, PLC is usually grounded separately, using its own grounding device together with other equipment, or using a common ground, but the series grounding method shown in Figure 6-37C is prohibited, because this grounding method will generate programmable logic control devices and equipment.

The grounding wire of the PLC should be as short as possible, so that the grounding point is as close as possible to the PLC. At the same time, the grounding resistance should be less than 100, and the cross section of the grounding wire should be greater than 2mm2.

In addition, the cpu unit of plc must be grounded. When using the I/O expansion unit, the CPU unit should share a grounding body with the CPU unit, and the resistance of any unit’s protective ground terminal to ground should not be greater than 100.

4. Necessary safety protection links

1. Short circuit protection

When the output device of the PLC is short-circuited, in order to avoid damage to the internal output components of the PLC, a fuse should be installed in the external output circuit of the PLC for short-circuit protection. Most fortunately, fuses are installed in the circuit of each load.

2. Interlocks and interlocking measures

In addition to ensuring the interlocking relationship of the circuit in the program, hardware interlocking measures should also be taken in the external wiring of the PLC to ensure the safe and reliable operation of the system, such as the forward and reverse control of the motor, and the normal closing contact. KM1 and KM2 of the contactor are used for interlocking outside the PLC. When there are interlocking requirements between different motors or electrical appliances, Zuo Hao also performs hardware interlocking outside the PLC. It is a common practice in PLC control system to use PLC external hardware for interlocking.

three. Loss of pressure protection and emergency shutdown measures

The power supply line of the external load of the plc should have voltage loss protection measures. When the power supply is restored after a temporary power failure, the external load of the PLC cannot be started by itself without pressing the "Start" button. Another function of this connection method is that when an emergency shutdown is required under special circumstances, pressing the "stop" button can cut off the load power supply, which has nothing to do with the PLC.

Sometimes hardware measures may not be able to completely eliminate the influence of interference, but the use of certain software measures can play a very good role in improving the anti-interference ability and reliability of the PLC control system.

1. Eliminate the jitter of the digital input signal

In practical applications, when the input signal of some switches is turned on, due to external interference, there will be a phenomenon of "jitter" in the switching time. This phenomenon will not affect the relay system due to the electromagnetic inertia of the relay, but in the PLC system, due to the fast scanning speed of the PLC, the scanning cycle is much shorter than the actual relay running time, so the jitter signal can be detected by the PLC , causing system failure. g wrong result. Therefore, in order to ensure the normal operation of the system, some "jitter" signals must be processed.

Influence of Input Signal Jitter and Its Elimination

a) Effect of jitter phenomenon b) Method of eliminating jitter