What are the measures to improve electromagnetic compatibility?
Various forms of electromagnetic interference are the main factors affecting the electromagnetic compatibility of electrical equipment. Electromagnetic interference can be divided into internal interference and external interference. Internal interference refers to mutual interference between various components inside an electronic device. For example, the working power supply generates interference caused by leakage of distributed capacitance and insulation resistance of the line, interference of component heating, impedance interference of signal through ground wire, power supply and transmission wire, magnetic field and electric field interference generated by high-power and high-voltage components. External interference refers to interference caused by factors other than electronic equipment or systems. For example: interference caused by space electromagnetic waves, interference generated by power supply network, strong magnetic field interference generated by external high-power equipment, interference caused by external high voltage through insulation leakage, and disturbance caused by internal electronic components due to unstable environmental temperature Wait. In order to ensure the normal operation of medical electronic equipment, the impact of interference must be weakened and prevented. There must be three factors in the electromagnetic compatibility problem between the systems: electromagnetic interference source, coupling path, and sensitive equipment. These three aspects are indispensable. Therefore, we can take corresponding measures from the above three aspects, such as eliminating or suppressing interference source, cut off the interference channel and weaken the sensitivity of the equipment to interference, etc., by adopting various anti-interference technical measures, the equipment instrument works stably and reliably, and effectively solves the electromagnetic compatibility problem. Common techniques for suppressing interference are:
Electromagnetic shielding
The shielding body is used to absorb energy (eddy current loss), reflected energy (electromagnetic wave reflection at the interface of the shielding body) and offset energy for electromagnetic waves from the outside or inside (electromagnetic induction generates a reverse electromagnetic field on the shielding layer to cancel part of the interference). The role of electromagnetic waves, to achieve the function of reducing interference. When the frequency of the interfering electromagnetic field is high, a conductive metal material is used as a shielding layer, and the eddy current generated in the shielding metal by the high-frequency interference electromagnetic field is used to form a canceling effect of the external electromagnetic wave. When the frequency of the interfering electromagnetic field is low, a high magnetic permeability material is used as a shielding layer to limit the magnetic lines of force inside the shielding body to prevent outward diffusion.
In recent years, plastic cases, plastic parts or panels have been widely used in medical electronic devices, so that external electromagnetic waves can easily penetrate the outer casing or the panel, which interferes with the normal operation of the instrument, and the electromagnetic waves generated by the instrument are also very easy to radiate. Go to the surrounding space and affect the normal operation of other electronic instruments. Such instruments can be treated by a plastic metallization process, such as sputter galvanizing, electroless copper plating, pasting metal foil, and coating of a conductive coating. After metallization, the fully insulated plastic surface has a metal. This reflects, absorbs, conducts, and attenuates the characteristics of electromagnetic waves, and acts to shield electromagnetic waves. The Chinese and foreign instruments and devices with display function, because the display screen uses ordinary glass, does not have the function of electromagnetic shielding, and the electromagnetic waves generated by the internal module work radiate outward through here to form interference. In order to improve the electromagnetic compatibility of the instrument, the original display glass can be changed into a screened glass to cut off the electromagnetic coupling path.
Electrostatic shielding
It is shielded by a metal container with good conductivity connected to the ground, so that the internal electric field is not transmitted, and the external electric field does not affect the inside.
Ground
Grounding is the main method to suppress noise and prevent interference, including power system grounding, equipment grounding, circuit signal grounding, shield grounding, etc. The main purpose of grounding is:
(1) Make all the unit circuits in the whole circuit system have a common reference zero potential to ensure stable operation of the circuit system.
(2) Prevent interference from external electromagnetic fields. By grounding the casing, a large amount of electric charge accumulated on the casing due to electrostatic induction is transmitted to the earth, and the high voltage formed by these charges is prevented from interfering with the device.
(3) Ensure safe work. In order to prevent damage caused by lightning strikes and personal safety of the workers, the casing of the electronic device must be connected to the earth. In order to prevent electromagnetic interference between the medical electronic devices sharing the power supply through the power line, a three-phase five-wire and single-phase three-wire power supply mode should be adopted. The grounding network must be standardized in the hospital. All kinds of grounding should be self-contained and cannot share the same grounding pole. There should be an independent AC grounding grid in the medical building, and no multi-point grounding is allowed in each room. It is forbidden to use the heating pipe and the water pipe as the grounding wire. It is strictly forbidden to work without the grounding wire.
Filtering is an important measure to suppress and prevent interference. According to the signal and noise frequency distribution range, the filter of the corresponding frequency band is connected into the signal transmission channel, and the noise is filtered or as much as possible to achieve the purpose of improving the signal to noise ratio. The use of a filtering network is a powerful measure to suppress interference sources and eliminate interference coupling, or to enhance the anti-jamming capability of receiving devices. There are many types of filters, such as low-pass filters, high-pass filters, band-pass filters, band-stop filters, etc. Depending on the characteristics of the signal spectrum and the spectral characteristics of the interference, choosing the appropriate filter can eliminate unwanted coupling. For example, a high-frequency circuit can use a CLCπ-type filter composed of two capacitors and one inductor (high-frequency choke). Decoupling and sensing decoupling networks isolate the circuit from the power supply, eliminate coupling between the circuits, and prevent interfering signals from entering the circuit.
Balance circuit
Also known as a symmetrical circuit, it refers to two wires in a two-wire circuit and all circuits connected to the wires, and have the same impedance to ground or to other wires. The purpose is to make the interference signals detected by the two wires equal. The interference noise at this time is a common state signal, which can disappear on the load itself.
Floating
Also known as floating, floating, refers to the electronic signal instrumentation of the input signal amplifier public line is not connected to the chassis and does not connect to the ground, a measure to suppress interference. The goal is to block the path of the interference current.
Isolation
Isolation is a technical measure to destroy the interference path and cut off the noise coupling channel to achieve the purpose of suppressing interference. Commonly used circuit isolation methods are transformer isolation method and photoelectric coupling method.
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