The importance of grounding to PLC control system

The importance of grounding to PLC control system

Grounding technology has been widely and deeply applied in the field of modern control. The "ground" of PLC control system usually has two meanings: - one is "earth" (safety ground; the other is "system reference ground" (signal ground) .Grounding refers to establishing a low-impedance conductive path between the system and a certain potential reference plane. "Grounding" is based on the potential of the earth, and the ground is used as the zero potential to benchmark the metal shell circuit of the PLC control system equipment The ground is connected to the ground. Because the capacitance of the ground is very large, the potential of the ground is generally considered to be zero. The grounding in the PLC control system generally does not refer to the grounding connected to the earth in the true sense, but the grounding point is the common reference point in the circuit. The voltage of the circuit is OV, and the voltage of other points in the circuit is based on this reference point. Generally, the voltage data of each point marked in the circuit diagram is relative to the size of the ground terminal, which can greatly facilitate testing and Although the voltage measurement in debugging, the connection between the same ground points is called the ground wire. The connection between the ground plane and the earth is often considered to improve the stability of the circuit operation, discharge the static battery, and provide safety guarantee for the staff.

The introduction of grounding technology was initially a protective measure to prevent electrical equipment or control equipment from being struck by lightning. At the same time, grounding is also an effective means to protect personal safety, when the phase line due to some reason (such as poor wire insulation, aging, etc.) and the control equipment shell touch. Dangerous voltage will be generated on the shell of the control equipment, and the fault current generated will flow through the PE line to the ground, thus playing a protective role.

In the grounding system, only considering lightning protection and safety is far from meeting the requirements. For example, in the PLC control system, the interconnection of signals between a large number of control devices requires that each control device must have a reference "ground" as the reference ground for the signal. . And with the complexity of the control equipment, the signal frequency is getting higher and higher, so in the grounding design, special attention must be paid to electromagnetic compatibility issues such as mutual interference between signals, otherwise, improper grounding will "heavily affect the operation of the PLC control system" reliability and stability.

Grounding is the establishment of a conductive path between two points in order to connect a device or component to the "earth" reference point. The ground is the signal to form a voltage reference network (typically, the analog circuit is plus or minus 10mV, and the digital circuit is plus or minus 200mV): the signal is returned, the power extraction is returned to form the reference plane for the antenna; the high-frequency potential near the antenna is maintained to protect people and people Control equipment is not damaged by lightning, power line failure, and discharges static electricity. Therefore, the grounding system must be carefully designed to meet all the requirements while keeping the unwanted coupling between the signals of certain noise problems small.

Connection is also one of the effective and important means to improve PLC control ENC. Correct connection can not only suppress the original sound of external electromagnetic interference, but also prevent the PLC control system from emitting electromagnetic waves to the outside. Wrong grounding often introduces very serious interference, and even makes the PLC control system unable to work normally. The PIC control system has a variety of control devices scattered in many places, so their respective grounding often forms a very complex grounding network. The grounding not only needs to be carefully considered during system design, but also must be carefully checked and properly done during installation and debugging. adjustment. The purpose of grounding is as follows.

(1) Grounding makes all unit circuits in the entire PLC control system have a common reference zero potential to ensure that each unit circuit system can work stably.

(2) Prevent the disturbance of the external electromagnetic field. The grounding of the chassis can make the large amount of charges accumulated on the chassis due to electrostatic induction discharged through the ground. Otherwise, the high voltage formed by these charges may cause spark discharges inside the PLC control system and cause interference. In addition, for the shield of the circuit, if you choose a suitable ground. A good screen quilt effect can also be obtained. Avoid the PLC control system changing the potential of the equipment to the earth under the action of the external electromagnetic environment, resulting in the instability of the PLC control system.

(3) Ensure the personal safety of operators. When the electromagnetic induction of direct lightning occurs, grounding can avoid the destruction of the PLC control cabinet; when the input voltage of the power frequency AC power supply is directly connected to the casing due to poor insulation or other reasons, it can avoid the occurrence of electric shock accidents for operators. The grounding for the purpose of ensuring the safety of personnel and PLC control cabinets is called "protective grounding", and it must be reliably connected to the ground potential. - In general. The grounding of the metal shell chassis frame of the PLC control cabinet belongs to the category of protective grounding.

It can be seen that the connection of the PLC control cabinet to the ground is not only for personnel safety and equipment safety. It is also an important means of suppressing the occurrence of disturbances. Good grounding can protect the normal operation and personal safety of equipment or systems in the PLC control system, and can eliminate various electromagnetic interference and lightning strikes. Therefore, the grounding design is very important and difficult.

There are many types of ground wires, including logic ground and signal ground. Shield ground, protective ground, etc. The grounding method can also be divided into single-point grounding, multi-point grounding, mixed grounding and floating ground. The ideal ground plane should have zero potential and no potential difference between the ground points. But in reality, any "ground" or ground wire has resistance. When there is current passing through, a voltage drop will occur, making the potential on the ground wire unnecessary. A ground voltage exists between the two ground points. When the circuit is grounded at multiple points and there is a signal connection, it will form a ground loop interference voltage.

The transmission and information conversion of circuit currents at all levels in the PLC control cabinet require a reference potential, which also prevents the invasion of external electromagnetic field signals. This potential is often referred to as "logical ground". This "ground" is not necessarily "geographical ground", it may be the metal chassis base of the PLC control cabinet, the ground wire on the printed circuit board or the general ground terminal in the building, the grounding trunk, etc. The logical ground can be in contact with the earth. It can also not be in contact, and the "electrical ground" must be in contact with the earth.

Although from the angle of anti-lightning and electrostatic discharge and safety, the PLC control cabinet needs to be connected to the ground, but from the perspective of electromagnetic interference, the ground may form a ground loop, causing interference to the signal. In the PLC control cabinet, there are cabinet ground terminals that connect the internal circuit to the gold tail cabinet, and there are also signal ground terminals that provide a low-impedance path for the internal circuit ground current. In the application of PLC control system, the cabinet ground and the signal ground are connected internally, and then used as the ground terminal. The two PLCs are insulated from the floor, and the signal terminals and the cabinet are grounded at one point respectively. so. The noise current flowing through the PLC control cabinet will not flow to the signal ground wire. Therefore, there will be no variation in the potential reference plane.

And since each signal terminal is grounded separately, there is no mutual interference of ground currents.