This article summarizes the grounding of various signals and hopes to help you learn.
In addition to proper grounding design and installation, grounding of various signals must be performed correctly. In the control system, there are roughly the following ground lines:
(1) Digital ground: Also called logical ground, it is the zero potential of various switching (digital) signals.
(2) Analog ground: It is the zero potential of various analog signals.
(3) Signal ground: Usually the ground of the sensor.
(4) AC ground: The ground wire of the AC power supply, which is usually the ground where noise is generated.
(5) DC ground: the ground of the DC power supply.
(6) Shielding ground: also called the chassis ground, designed to prevent static induction and magnetic field induction.
These ground wire treatments are an important issue in system design, installation, and commissioning. Here are some thoughts on the grounding problem:
(1) The control system should be grounded at one point. Under normal circumstances, the high-frequency circuit should be grounded at multiple points, and the low-frequency circuit should be grounded at one point. In the low-frequency circuit, the inductance between the wiring and the component is not a big problem. However, the interference of the loop formed by the ground has a great influence. Therefore, the grounding point is often used as a grounding point; but one-point grounding is not suitable for the high frequency because the high frequency At the time, the ground line has an inductance, which increases the impedance of the ground line, and inductive coupling between the lines. Generally speaking, the frequency is below 1MHz, and one point can be grounded; when it is higher than 10MHz, multi-point grounding is used; one point can be grounded between 1~10MHz, and multiple points can be grounded.
(2) The exchange ground and the signal ground cannot be shared. Since there are several mV or even a few V voltages between two points of a power ground, this is a very important interference for low level signal circuits and must be isolated and prevented.
(3) Comparison of floating ground and grounding. The whole system is floating, that is, all parts of the system are floating with the earth. This method is simple, but the insulation resistance of the whole system and the earth cannot be less than 50MΩ. This method has a certain anti-interference ability, but it will cause interference once the insulation is lowered. Another method is to ground the case and leave the rest floating. This method is strong in anti-interference ability, safe and reliable, but it is complicated to implement.
(4) Analog ground. The connection of the analog ground is very important. In order to improve the anti-common mode interference capability, shielded floating technology can be used for analog signals. Grounding of specific analog signals should be designed in strict accordance with the requirements in the operation manual.
(5) Shielding ground. In the control system, in order to reduce the capacitive coupling noise in the signal, accurate detection and control, it is necessary to use shielding measures for the signal. Depending on the purpose of the shielding, the connection of the shielding ground is different. Electric field shielding solves the problem of distributed capacitance, generally connected to the earth; electromagnetic field shielding mainly avoids high-frequency electromagnetic field radiation interference such as radar and radio. Made of high-conductivity of low-resistance metal materials, it can be grounded. The magnetic field shielding is used to prevent magnetic induction of magnets, motors, transformers, coils, etc. The shielding method is to use a high magnetic conductive material to close the magnetic circuit, generally it is better to connect the earth. When the signal circuit is grounded at one point, the shield of the low frequency cable should also be grounded at one point. If there is more than one shield location of the cable, a noise current will be generated, creating a source of noise interference. When a circuit has an ungrounded signal source connected to a grounded amplifier in the system, the shield at the input should be connected to the common terminal of the amplifier; conversely, when the grounded signal source is connected to an ungrounded amplifier in the system, the input to the amplifier The terminal should also be connected to the common end of the signal source.
For the grounding of electrical systems, it should be classified according to the requirements and purpose of grounding. It is not possible to simply and arbitrarily connect different types of grounding. Instead, it should be divided into several independent grounding subsystems, each with its common grounding point. Or ground the trunk, and finally connect together to implement the total grounding.
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