Application of Isolation Transmitters in Industrial Instruments

A variety of automated instruments, computers and corresponding actuators are used to monitor and control the production process. The signals in the process range from small signals as weak as millivolts to large signals of tens of volts, and even signals as high as thousands of volts and hundreds of amperes. In terms of frequency, there are DC low-frequency ranges and high-frequency/pulse spikes. The mutual interference between equipment and instruments has become a problem that must be solved in system debugging. In addition to electromagnetic shielding, solving the "ground" of various equipment and instruments, that is, the potential difference of the signal reference point, will become an important issue. Because the signals of different equipment and instruments need to be transmitted to each other, there is a problem of signal reference point. In other words, in order to transmit the signal completely, the ideal situation is that the signals of all equipment and instruments have a common reference point, that is, the potential difference between the reference points of all equipment and instrument signals is "zero". However, in the actual environment, this is almost impossible. In addition to the voltage drop caused by the connection resistance between the "ground" of each equipment and instrument, there are many factors such as different interferences of various equipment and instruments in different environments, and the quality of the wire contacts is reduced due to wind and rain, which leads to differences between the "grounds".

In industrial production process control systems, industrial field instruments need to monitor the electrical signals transmitted by front-end sensors and transmit the processed signals to the control center host. In order to ensure the reliability and stability of signal transmission, the isolation processing of each signal port must be considered to eliminate the ground potential difference problem after the instrument is connected to the system.

2. The general composition of industrial field instruments.

Industrial field instruments are used in industrial production process control systems. Their function is to sample field sensor signals, calculate the sampled values ​​according to the preset algorithm to obtain the actual value of the monitored physical quantity, and output the actual field physical conditions to the main control center through the communication bus or 4~20mA current signal. The general system structure diagram is shown in the figure below.

Figure 1

As can be seen from the figure above, when the sensor signal is input to the industrial instrument, the first thing is the EMC protection circuit. Due to the complex electrical environment of the industrial site, there are strong electrical equipment (such as high-power motors) and high-frequency switching equipment (such as inverters). These devices will generate large surges and spike pulses when starting and stopping. This type of interference contains relatively large energy. Through EMC protection devices such as varistors, TVS tubes and other devices to absorb this type of interference, the subsequent circuit will not be damaged by this type of interference. After the sensor signal passes through the EMC protection circuit, it is input to the isolation amplifier at the input end for signal isolation and conditioning. Signal isolation can ensure that the internal circuit of the instrument and the external sensor form a ground wire block, eliminate common-mode interference, and thus eliminate the ground wire loop caused by the ground potential difference. On the one hand, it can ensure that the sampling accuracy of the sensor signal is not affected by interference, and on the other hand, it can protect the internal MCU system from damage to the device due to large common-mode voltage; signal conditioning, through signal type conversion and signal amplification, the sensor signal is linearly amplified to meet the signal type and range of ADC sampling, ensuring that the conditioned signal range matches the ADC signal input range, thereby ensuring the ADC sampling accuracy. The MCU performs digital operations on the sensor signal after ADC digital quantization, obtains the actual status of the physical quantity on site after a series of operations such as digital filtering and linearization, and then outputs it to the display panel. At the same time, the obtained physical quantity value on site is output to the DAC circuit in a linear ratio. The DAC circuit outputs a voltage signal. In order to form a 4~20mA current signal, it is also necessary to isolate and condition the voltage signal into a 4~20mA current signal output through an isolation amplifier at the output end. Of course, the signal output end also needs to design an EMC protection circuit. Industrial intelligent instruments generally have communication interfaces. Currently, the common ones are 485 communication interfaces and CAN communication interfaces. The physical quantity values ​​on site can be directly uploaded in digital form through the communication bus. At the same time, the bus interface can also be used to complete tasks such as instrument parameter setting, instrument operation status query, and instrument self-calibration. Both 485 communication and CAN communication use differential signal communication, which has strong anti-interference ability. However, the common-mode input voltage range that the communication interface IC can withstand is limited, especially the 485 interface IC, which is generally between -7V and +12V. In long-distance communication, the common-mode interference formed by the ground potential difference can easily cause damage to the communication interface IC, so the communication interface must also be designed for isolation to improve system reliability.

3. Application of isolation transmitter.

Guangzhou Mornsun Technology Co., Ltd. has launched a special solution for the isolation and conditioning design of the analog signal input and output ports of industrial field instruments.

1. Input type isolation transmitter TE_N series.

The principle block diagram of the input type isolation transmitter is shown in Figure 2. This series of products has the following features:

l With internal isolated power supply, no external DC/DC isolated power supply is required, simplifying the user's power supply configuration design;

l Isolation conversion of various signal types;

l Small size: DIP package (26*9.5*12.5mm), ultra-small board area, reliable welding reliability;

l High accuracy level (0.1%FS);

l Wide operating temperature range: - 40～+85℃;

l Adjustment function: signal full scale adjustment and zero point adjustment.

The input type isolation transmitter is designed for the input port of analog signals. The product has its own isolated power supply and does not require an external DC/DC isolated power supply. The power supply end and the signal output end of the product share a common ground, so it is very convenient to draw power from the MCU system side, simplifying the user's power supply configuration design; TE_N products have high transmission accuracy, with a factory accuracy of 0.1% FS, and are equipped with full-scale adjustment and zero-point adjustment pins. When the product is used in the system, if the signal linearity of other parts of the system needs to be adjusted, it can be easily achieved through the full-scale adjustment and zero-point adjustment pins of the TE_N product; the small size of TE_N products can meet the small size requirements of industrial instruments, and The pin arrangement of dual in-line plug-in has good welding reliability and vibration resistance. The TE_N product also provides a set of isolated power output on the signal input side, which is common to the signal input end. When the isolated transmitter cannot meet the actual conversion and conditioning needs, a simple signal conditioning circuit can be built using the isolated power supply to meet the actual conditioning needs. The TE_N series products can provide mutual isolation conversion of industrial conventional signals (0~20mA, 4~20mA, 0~5V, 0~10V), and can also customize the corresponding input and output range for the actual range of the customer, and can also provide conventional 24V, 15V, 12V, 5V and other power supply range products. It has the characteristics of high precision, small size, low power consumption and low cost.

Figure 2: Input type isolation transmitter block diagram

2. Output type isolation transmitter TF_N series.

The principle block diagram of the output type isolation transmitter is shown in Figure 3. This series of products has the following features:

l With internal isolated power supply, no external DC/DC isolated power supply is required, simplifying the user's power supply configuration design;

l Isolation conversion of various signal types;

l Small size: DIP package (26*9.5*12.5mm), ultra-small board area, reliable welding reliability;

l High accuracy level (0.1%FS);

l Wide operating temperature range: - 40～+85℃;

l Adjustment function: signal full scale adjustment and zero point adjustment.

Industrial instruments generally need to convert the collected physical quantity signals into 0~20mA or 4~20mA signal outputs to achieve the purpose of remote transmission to the main control center. The TF_N series output type isolation transmitter products are specially designed for the application of isolated conversion output of such analog signals. The power supply end and the signal input end of the product share the same ground, and it is very convenient to draw power from the signal input end. The output side of the product is the required isolated 4~20mA signal, and no other additional circuits are required. The product functions and features are similar to those of the TE_N series input type isolation transmitter, and it also has the characteristics of high precision, small size, low power consumption, and low cost.

Figure 3: Principle block diagram of output type isolation transmitter 4. Application of bus isolation transceiver.

Among industrial field buses, the 485 bus has the advantages of simple design, strong anti-interference, and low cost; the CAN bus has the advantages of high transmission rate, strong anti-interference, advanced bus arbitration mechanism, excellent bus fault detection and fault self-locking function; both buses can use low-cost twisted pair media for long-distance communication transmission and are widely used by industrial instruments. For the port isolation design of these two commonly used buses, Guangzhou Jinshengyang Technology Co., Ltd. has also launched an integrated solution.

1. 485 bus isolation transceiver TD501D485H, CAN bus isolation transceiver TD501DCAN

The application wiring diagram of the 485 bus isolation transceiver TD501D485H is shown in Figure 4; the application wiring diagram of the CAN bus isolation transceiver TD501DCAN is shown in Figure 5. This series of products is an integrated product that optimizes the power supply and port electrostatic protection level for the traditional 485 bus and CAN bus port isolation design. This series of products has the following functional features:

l Built-in isolated power supply, no external DC/DC isolated power supply required, simplifying the user's power supply configuration design;

l Integration, small size, low power consumption;

l Higher port electrostatic protection capability;

l Wide operating temperature range: - 40～+85℃;

l Complete EMC protection circuit recommendation.

The TD series bus isolation transceiver has optimized the power consumption of actual communication in terms of power supply design, which can ensure lower power consumption while ensuring communication quality. It has also optimized and strictly designed the circuit for digital isolation of traditional optocouplers to ensure reliable transmission of digital isolation in the entire high and low temperature working range. The design also takes into account the CTR attenuation problem of long-term operation of optocouplers to ensure the long-term reliability of the design. In terms of overall structural design, it ensures a small size while ensuring a smaller board area to meet the small size requirements of industrial instruments. In addition, considering the electrical complexity of the industrial environment, there are complete EMC protection circuit recommendations in the product technical manual to simplify the actual port design of customers.

Figure 4: 485 isolated transceiver application wiring diagram

Figure 5: CAN isolated transceiver application wiring diagram

3. Summary

As industrial instruments become more intelligent and have more functions, there are more and more issues to consider in the overall design of the product, such as power configuration, MCU system design, display and human-machine interface design, software algorithms, communication protocols, etc. The electrical reliability design of the port is often overlooked by designers. The integrated products of several port designs recommended above can greatly simplify the electrical reliability design of the ports of industrial instruments, thereby shortening the development cycle of industrial instruments and improving system reliability.