“This article expounds the causes of signal interference and the anti-interference effect of isolators. Combined with the principle of isolators, the classification and application of signal isolators are popularized to help instrument workers improve their skills in selecting signal isolators.
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This article expounds the causes of signal interference and the anti-interference effect of isolators. Combined with the principle of isolators, the classification and application of signal isolators are popularized to help instrument workers improve their skills in selecting signal isolators.
In the control system, various physical quantities need to be monitored and controlled, such as temperature, humidity, pressure, etc. These different physical quantities are always changing. Devices used to detect these states and changes must faithfully reflect these changes. After these parameters are accurately measured, they must be guaranteed not to be affected during transmission from the field to the control room, despite being affected by external factors such as atmosphere and installation. reduce or interfere. Signal isolators can convert or transmit these signals requiring high-quality Electronic systems that can adapt or resist harsh environments, such as temperature changes, electromagnetic interference, vibration, corrosion or explosions.
1. The reason for the interference
In the process of industrial production, there are various interferences to signal transmission, and absolute isolation is impossible to achieve. Usually interference is mainly caused by the following reasons:
①Ground loop
Various instruments and equipment need to be grounded according to the requirements and purposes. For example, for safety, the chassis needs to be grounded; in order to make the circuit work normally, the system needs to have a common reference point; in order to suppress interference and add a shield, the shield also needs to be grounded, but due to There is a potential difference between the reference point between the meter and the device, creating a “ground loop”. The negative terminal of the DC power supply of the control system is usually grounded. If the input signal is powered by another power supply or the sensor itself is grounded, the transient current will flow through the loop formed by the ground potential difference generated between the △ ground conductors, thereby interfere with normal signals. As shown in Figure 1.
Figure 1 Ground loop
② Radiated noise
Radiated electromagnetic fields from space are mainly generated by transient processes of power networks, electrical equipment, etc., lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., and are often referred to as radiation interference. Usually the sensors send out small signals, so they are easily interfered by capacitive or inductive devices. For example, the time domain waveform of lightning interference is a large spike superimposed on a series of random pulse backgrounds. Cosmic noise is produced by ionizing radiation and changes throughout the day. Solar noise varies dramatically with solar activity. Induction motors, frequency converters or other power switching devices. Radiated noise often interferes with measured values and can damage sensitive I/O cards in control systems. as shown in picture 2.
Figure 2 Radiated noise
③ Line introduction
Under normal circumstances, the power supply of the control system is powered by the grid. Due to the wide coverage of the grid, voltages and circuits will be induced on the line due to electromagnetic interference from all spaces. In particular, changes within the power grid, such as switching operation surges, starting and stopping of large power equipment, harmonics caused by AC and DC rotating devices, and short-circuit transient shocks in the power grid, etc., will all pass through the transmission line to the power source. The power supply of the control system usually adopts an isolated power supply, but its mechanism and manufacturing process factors make its isolation not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.
2. Principle of isolator
The working principle of the signal isolator: It modulates and converts the signal of the transmitter or instrument through the semiconductor device, and then realizes the isolation conversion through the light-sensing or magnetic-sensing device, and then demodulates and converts it back to the original signal before isolation. The power supply of the signal is isolated to ensure absolute independence between the converted signal, power supply and ground. At the same time, the interference signal superimposed on the measured value is filtered, and the signal is matched according to the input and output requirements of the control system. Therefore, isolation, amplification, filtering and matching are the functions of the signal isolator. There are eight main performance indicators for high-quality signal isolators. This article introduces some parameters.
①Maximum input signal
The maximum input signal is the value that will not damage the module and the signal generator. If these values are exceeded, the voltage-limiting diode can short-circuit this input on the basis of overvoltage detected by the system, so that the transmission range of the analog signal is only within the given input range.
②Common mode rejection ratio
The isolation amplifier uses an operational amplifier to transmit the signal. The operational amplifier theoretically has ideal transmission and amplification performance, but it is not the case in practice. When the voltages of the two input segments change in the same direction, that is, the exact same ground voltage is applied to the two input terminals, it will cause an undesired output signal. The output signal should not appear because the difference signal at the input is “0V”. Common mode rejection ratio This parameter indicates how many times the common mode input voltage is less amplified than the differential mode input voltage.
③Load impedance
The load impedance on the output side is a standard for measuring the load capacity of a measuring transmitter or isolation amplifier. The current output end can withstand a load of less than 500 in most cases, and the voltage output end can usually bear a load of at least 10K.
④ Linearity
The process can be evaluated from zero to maximum by the linearity of the signal, usually the linearity error represents a few percent deviation from the ideal transmission characteristic.
⑤ Limit frequency
The isolation amplifier is generally used to transmit DC signals, but the change of the signal requires the module to have dynamic characteristics, which are determined by the limit frequency. Therefore, even a small amount of communication can be transmitted. At the same time, the lower limit frequency will suppress high frequency AC components.
3. Isolator classification and application
Signal isolators are divided into active signal isolators and passive signal isolators.
①Active signal isolator
Active signal isolators are powered by a separate power supply to ensure that the isolators work well, modules require an active signal on the input side, and on the output side they provide a filtered and amplified signal, depending on the application between the input/output and the power supply isolated from each other.
a, three-terminal isolation
Three-terminal isolation only requires one power supply, which is isolated from the measurement circuit. Using this technology to isolate the signal isolator, all components connected to the input terminal, output terminal or power supply will not interfere with each other, and there will be no interference between the three terminals. Correspondingly, they are electrically isolated from each other. As shown in Figure 3.
Figure 3 Three-terminal isolation of signal isolators
Signal isolators with three-terminal isolation can be used directly in the industrial field for decentralized terminal boxes and are electrically isolated next to the control part in the control cabinet. The three-terminal isolation not only acts as an electrical isolation between the detection probe and the control part, but also between the control part and the adjustment part.
b. Input terminal isolation
Modules using this isolation technology should protect the electronic equipment connected on the output side (eg, the input card of the controller) from various disturbances in the field. Therefore, the input terminal and the equipotential output terminal and the power supply section are galvanically isolated. As shown in Figure 4. Such signal isolators require an active signal on the input side (eg from a pressure transmitter) and they provide a filtered and amplified signal on the output side (eg a controller).
Figure 4 Isolation at the input of the isolator
c, output isolation
Modules using this isolation technology should protect the electronic equipment connected on the input side (eg, the output card of the controller) from various disturbances in the field. Therefore, the output terminal and the equipotential input terminal and the power supply part are galvanically isolated. As shown in Figure 5. Such modules require active signals on the input side (eg the output card of the controller), which send out processed and amplified signals on the output side (eg to the regulating valve).
Figure 5 Isolation at the output of the isolator
②Passive signal isolator
The passive signal isolator provides an additional and substantial convenience. It does not require additional power supply. The working power of the module is provided through the input or output loop, and its internal circuit consumes very little current, which does not affect the correctness of the signal. transmission. According to the power supply mode of the signal isolator, it is divided into the following types:
a. Power supply on the input side
Passive signal isolators (such as YR9101A) that use this input-side power supply isolation technology to obtain energy on the input side are obtained from active input loops (such as electromagnetic flowmeters or control system output cards) The required energy is used for 4-20mA signal transmission and electrical isolation, and the output side provides a processed current signal for control or regulation. As shown in Figure 6. This passive isolation is suitable for signal processing (separate from ground return) and filtering. without the need for additional auxiliary power.
Figure 6 Input side isolation of passive signal isolator for energy harvesting on the input side
b. Power supply on the output side
With this isolation technique, the modules draw the required energy for signal transmission and electrical isolation from an active output loop (preferably powered from an auxiliary power supply from the control system input card). As shown in Figure 7.
Figure 7 Output isolation
This passive isolator (such as YR9201A) that obtains energy from the output side can process various active signals (such as electromagnetic flowmeters) on the input side, and on the output side, the loop-powered module uses a standard signal of 4-20mA. When using this isolation technology, it must be noted that the active signal connected to the output side (such as the control system input card) can either power the passive isolator or drive the load (such as the input resistance of the control system input card).
c. Passive feed isolator
With this isolation technology, the modules take the energy needed from the active output circuit for signal transmission and electrical isolation. The passive distributor supplies this energy from the output circuit to a second-wire connected to the input side. Control transmitters (such as pressure transmitters), the two-wire transmitter sends an active signal with the help of the energy provided by the passive distributor, which is electrically isolated through the passive distributor and output from the output side. As shown in Figure 8. Therefore, the signal and energy flow are in principle reversed in the passive distributor mode.
Figure 8 Passive Feed Isolation
In addition to the electrical isolation of the signal isolator, the signal is also amplified in the isolation amplifier in order to achieve long-distance transmission or to connect high-impedance loads. It can be used in all industrial applications requiring electronic measurement and control systems, such as process control in power plants, metallurgical plants, water and sewage treatment plants, oil and gas production plants and chemical production plants.
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