Eight Questions and Answers about Transmitters (Part 2)

With the rapid development of modern industry, multi-motor AC control systems have been widely applied in industries such as papermaking, chemicals, steel, and food processing, giving rise to unique inverter control systems for each of these fields. In actual operation, such control systems exhibit high reliability and strong resistance to interference, but the corresponding maintenance costs increase over time. For example, a certain type of inverter uses its own unique communication protocol and interface; if a communication failure leads to the destruction of the communication interface board, it will result in the collapse of the system, unless a communication interface of that same model is purchased ； Similarly, if the frequency converter is damaged and burned out, it is also necessary to purchase a frequency converter of the same model ； Furthermore, to upgrade the system, it is necessary to replace all inverters completely. As a result, issues such as high procurement costs and excessive inventory of spare parts will inevitably arise. 　　This paper introduces a multi-motor AC control system based on fieldbuses, which integrates the most widely used fieldbus technologies currently available and takes into account the convenience of practical application. It details the main features of this multi-motor AC control system from three aspects: bus integration, hardware configuration, and software principles. 2. Introduction of the fieldbus Profibus The fieldbus Profibus, also known as Process Fieldbus, began development in 1984. After more than a decade of development, production, and application, Profibus has become one of the leading open fieldbus systems in Europe. Currently, PNO (Profibus User Organization) has over 600 members and produces nearly a thousand products. Profibus products hold a market share of over 40% in the European market, and are widely used in areas such as process automation, building automation, power generation, and power transmission and distribution. 　　Based on ISO7498 and using the Open System Interconnection (OSI) as a reference model, Profibus defines the physical transmission characteristics, bus access protocols, and application functions. Profibus-DP, Profibus-PA, and Profibus-FMS constitute the Profibus family. Among them, Profibus-DP (Decentralized Periphery) is a high-speed and inexpensive communication interface, designed specifically for communication between automatic control systems and device-level distributed I/O. The Profibus-DP module can replace 24-volt or 4–20 mA serial signal transmission. The user interface provided by the Direct Data Link Mapping (DDLM) simplifies and facilitates access to the data link layer, with transmission possible using RS-485 transmission technology or fiber optic media. This article covers the DP section. 　　The hardware of the standard field bus Profibus DP consists of three components: master devices, slave devices, and the network itself. The master device is used to control data transmission on the bus, and it sends information as well as is authorized to access the bus in the absence of any external requests ； A slave device is a relatively simple external device as compared to the master device, and it does not have permission to access the bus ； Networks include transmission media and network connectors; the former includes electrical networks made of shielded twisted pair cables, fiber optic networks constructed from plastic or glass fiber optic cables, or hybrid networks that utilize conversion by OLM between these two types of media, while the latter includes RS485 bus connectors, RS485 bus terminators, RS485 repeaters, optical link modules OLM, and so on. 　　In multi-motor AC control systems, there are motor control points, and the distribution lines of various AC frequency converters are long and dispersed. By using fieldbus technology, distributed I/O modules can be installed in both the control room and the field operator panels, forming a bus network that enables serial transmission of control data in digital form. As a result, the control system gains improved resistance to interference, and the reference values provided to the frequency converters become more accurate. 3. Hardware architecture of multi-motor AC control system Figure 1 (upper) shows the hardware connection diagram of the multi-motor AC control system. As can be seen from the diagram, the master device of the fieldbus Profibus uses a Siemens programmable controller PLC S5, which connects to the Profibus bus via an IM308 interface module. As the master device, the PLC S5 is responsible for reading the inverter status words (including digital and analog values) from all distributed I/O modules connected to the bus, as well as transmitting the inverter control words (including digital and analog values). The device in question is the distributed I/O module ET200; each ET200 slave can be equipped with up to 32 digital or analog I/O cards (such as DI, DO, AI, AO, etc.). The address values of these connected I/O cards can be determined by setting the address DIPs using the IM318 interface card for the ET200 slave. 　　Due to the remote distribution of ET200 slaves, corresponding slaves are installed in areas where the frequency converters are concentrated, and the associated frequency converters are connected in a point-to-point manner; the specific wiring method is shown in Figure 1 (below). All inverters have I/O interfaces, including digital input, digital output, analog input, and analog output. The digital inputs include forward start, reverse start, external faults (such as motor temperature rise), jog, enable, and start at set frequency, while the digital outputs comprise the inverter operation signal, fault signal, operating frequency range signal, alarm signal, etc ； For analog input and output, the U/I mode must be set first to determine whether a 4–20mA signal or a 0–10V signal will be used. Analog input is primarily used to set the reference values for speed or frequency, while analog output provides the actual values of current, speed, frequency, and DC circuit voltage. Figure 1: Hardware connection diagram of the multi-motor AC control system. The hardware structure of the multi-motor AC control system is relatively simple; it is centered around a programmable controller, with the associated ET200 slave units connected in sequence via the Profibus bus, and the corresponding frequency converter units connected to these slave units. The bus uses electrical twisted-pair wiring, with clear cabling ； A shorter control circuit can be used to connect the ET200 slave unit to the associated frequency converter group. This data transmission method relies primarily on serial digital signals, with only some analog signals being used upon reaching the inverter; as a result, it does not affect the reliability and noise resistance of the transmitted data at all. 4. Software Design of Multi-Motor AC Control Systems 4.1 Software Flow of Multi-Motor AC Control Systems The software design for multi-motor AC control systems based on field buses includes the following elements, as shown in Figure 2 (upper): 1) Initialization of the PLC IM308 interface module: Before activating the field bus, it is necessary to configure the Profibus bus first and assign parameters to each bus station (transmission rate on the bus, bus standard – usually DP-Standard, fault mode settings, including diagnostic settings and diagnostic addresses, etc.). 　　2) Reading the status words and actual values of Inverter #1: By using the I/O terminals of the ET200 slave connected to the inverter, and through operations on the remote I/O, the status words (digital values) and actual values (analog values) of the inverter can be read. 　　3) Determine whether the frequency converter is faulty; if so, trigger an alarm and archive the information, otherwise output analog values according to the system’s requirements to the I/O terminals of the ET200 slave connected to terminal 1# of the frequency converter, including control words and set values. 　　4) Repeat steps 2 and 3 to perform read and write operations on inverters 2#, 3#, 4#…N# in sequence. 　　4.2 Logical Control Principles of Multi-motor AC Control Systems Logical control refers to the logical interlocking between the inverter and other external control signals to enable the activation and operation of specific devices. For example, when implementing variable-frequency control on the feed pump in a paper mill, the necessary conditions for starting the feed pump are: (1) The valve is fully open ； (2) The liquid level in the tank is normal ； (3) The seal water of the feed pump is normal. If any one of these 3 conditions is missing, the device cannot be turned on. For the supply pump to continue operating, in addition to the above 3 conditions, it is also necessary that the flow rate and pressure in the pipeline be normal; otherwise, the operation of the supply pump will stop automatically. 　　The purpose of logical control is to protect human safety and ensure the normal operation of equipment as well as consistent production quality; therefore, implementing logical interlock protection in multi-motor AC control is an essential step. 　　4.3 Principle of closed-loop control for frequency converters Generally, frequency converters operate in an open-loop manner, but when higher production standards are required, especially in the control of multi-motor AC drives, frequency converters must be operated under closed-loop control in order to achieve the desired production outcomes. In continuous production lines such as film-making machines, cold rolling mills, and papermaking machines, closed-loop control is particularly important. 　　In Figure 2 (below), the frequency converter operates in closed-loop control. Among them, the current inner-loop torque control is carried out directly within the inverter and is not indicated in this diagram. The diagram shows the speed control loop: starting from the analog input signal (which, according to the instructions for each inverter, can be either a current or voltage signal; the corresponding DIP switches and parameters must be set), the signal is restricted by upper and lower frequency limits; after comparison with the signal fed back from the speed sensor after filtering, it enters PID control (the PID parameters can be adjusted within the inverter’s settings). Following this, through a slope generator (which includes RAMP ON TIME/RAMP DOWN TIME), the signal goes directly to the current loop to control the motor’s output. This thus constitutes the closed-loop control of the inverter. Figure 2: Software flowchart of the multi-motor AC control system. 5. Conclusion Through the development of this system, it can be widely applied in fields such as small pulp and paper mills, powder processing plants, and building ventilation pump control. Since it is structured through an open, IEC-standard-based fieldbus system called Profibus, it features (1) low flexibility in terms of modification and expansion ； (2) Achieve distributed control to improve the system’s response speed and control accuracy ； (3) Reduce system unreliability and enhance maintainable functions.

Sensor transmitters are widely used; analog sensors can be found in various fields such as industry, agriculture, national defense, as well as in daily life, education, and scientific research. However, in the design and use of analog sensors, there is always the issue of how to achieve the highest possible measurement accuracy. Numerous interferences continue to affect the measurement accuracy of sensors; for example, there are many energy-intensive devices in the field, and the start-up and stop-off of high-power inductive loads often cause sharp voltage spikes of several hundred volts or even several thousand volts in the power grid ； The industrial power grid experiences under-voltage or over-voltage conditions (the supply voltage at the Shexian Steel Plant fluctuates between 160V and 310V), often reaching around 35% of the rated voltage. Such poor power supply conditions can last for minutes, hours, or even days ； When various signal wires are tied together or run along the same multi-core cable, the signals can be interfered with; this interference is particularly severe when the signal wires and alternating current power lines share the same long conduit ； Poor performance of the multiplexer or holder can also cause crosstalk in the channel signals ； Various electromagnetic and meteorological conditions in space, as well as changes in lightning activity and even the Earth’s magnetic field, can also interfere with the proper functioning of sensors ； Furthermore, changes in temperature and humidity at the site can cause variations in circuit parameters. Corrosive gases, acids, alkalis, and salts, as well as wind, sand, rain in outdoor environments, and even rodent bites and insect damage, can all affect the reliability of sensors. The outputs of analog sensors are generally low-level signals, and issues related to the amplification, processing, shaping of these signals as well as noise suppression arise. In other words, it is necessary to accurately amplify the weak signals from the sensors to standard signal levels such as 1VDC–5VDC or 4mADC–20mADC, in order to meet the required technical specifications. This requires designers to pay attention to certain issues that are not shown in the circuit diagrams of analog sensors, namely interference resistance issues. Only by identifying the sources of interference and the ways in which they affect analog sensors, and by designing circuits to eliminate such interference or measures to prevent it, can the optimal performance of analog sensors be achieved. II. Interference sources, types of interference, and interference phenomena. Sensors and instruments are subject to a variety of interferences during operation in the field; each case must be analyzed individually, and different measures should be taken for different types of interference as this is the principle of interference resistance. Such a flexible strategy is undoubtedly at odds with universality; the solution is to adopt a modular approach, allowing the instrument to be equipped with various optional components tailored to different operating conditions, in order to effectively counter interference and enhance reliability. Before further discussing the selection of circuit components and the applications of circuits and systems, it is necessary to analyze the sources and types of interference that affect the accuracy of analog sensors. 1. Main sources of interference 　(1) Electrostatic induction: Electrostatic induction occurs due to the presence of parasitic capacitance between two branch circuits or components, which allows charges on one branch to be transferred to the other branch through this parasitic capacitance; hence it is also known as capacitive coupling. 　(2) Electromagnetic induction: When there is mutual inductance between two circuits, a change in current in one circuit is coupled to the other circuit through a magnetic field; this phenomenon is known as electromagnetic induction. Such as the leakage magnetic field of transformers and coils, energized parallel wires, etc. 　(3) Leakage current induction: Due to poor insulation of components such as mounts, terminals, printed circuit boards, the dielectric material inside capacitors, or their casings within electronic circuits, and especially when the humidity in the environment where the sensor is used is high, the insulation resistance of the insulators decreases, which leads to an increase in leakage current and thus interference. The impact is particularly severe, especially when leakage current flows into the input stage of the measurement circuit. 　(4) Radio frequency interference mainly includes interference caused by the startup and shutdown of large power equipment, as well as high-order harmonic interference. Such as interference from thyristor rectifier systems, etc. 　(5) Other interferences: In addition to being susceptible to the interferences mentioned above, on-site safety production monitoring systems are also prone to mechanical interferences, thermal interferences, and chemical interferences due to their poor operating environment. 2. Types of interference (1) Norm interference: Norm interference refers to a situation where the intrusion of interfering signals is consistent on both the incoming and outgoing lines. The source of normality interference is generally a strong alternating magnetic field in the surroundings, which affects the instrument and induces an alternating electromotive force, thereby causing interference; this type of interference is difficult to eliminate. 　(2) Common-mode interference: Common-mode interference refers to a situation where the interference signal flows through each of the two wires to some extent, with the ground serving as the common return path, while the signal current flows only between these two wires. The sources of common-mode interference are generally equipment leakage current to ground, ground potential differences, and interference from the wires themselves with respect to ground. Due to the unbalanced condition of the circuit, common-mode interference is converted into differential-mode interference, making it more difficult to eliminate. 　(3) Persistent interference: Persistent interference refers to interference that exists over a long period of time. Such interference is characterized by a constant voltage level with little variation, and it can be easily detected using measuring instruments. For example, electromagnetic interference from power cables or nearby power lines is continuous AC interference at a frequency of 50 Hz. 　(4) Sudden accidental interference: Sudden accidental interference occurs mainly during the operation of electrical equipment, such as when switching it on or off; it can also arise alongside lightning strikes or at the moment radio equipment is in use. Interference can be roughly divided into 3 aspects: (a) local generation (i.e., unwanted thermocouples) ； (b) Coupling within the subsystem (i.e., the issue of ground path) ； (c) External generation (interference at the Bp power supply frequency). 　　3. Interference phenomena: In practical applications, the following main interference phenomena often occur: (1) The motor rotates irregularly when commands are issued ； (2) When the signal is zero, the value displayed on the digital meter jumps around randomly ； (3) When the sensor is in operation, its output value does not match the signal value corresponding to the actual parameter, and the error value is random and irregular ； (4) When the parameter under measurement remains stable, the difference between the value output by the sensor and the signal value corresponding to that parameter is a constant or periodically varying value ； (5) Devices that share the same power supply as the AC servo system (such as monitors, etc.) are not functioning properly. 　　There are mainly two types of channels through which interference can enter a positioning control system: signal transmission channel interference, which occurs via the signal input and output channels connected to the system ； Power supply system interference. 　　The signal transmission path is the means through which a control system or driver receives feedback signals and sends out control signals. Since pulse waves experience delays, distortions, attenuation, and channel interference along the transmission lines, interference from long distances becomes the main factor during transmission. Any power source and transmission line possesses internal resistance, and it is this internal resistance that causes noise interference from the power source. Without internal resistance, any noise would be absorbed by the short circuit in the power source, and no interference voltage would be generated in the circuit ； Furthermore, the AC servo system drive itself is also a significant source of interference, as it can interfere with other devices through the power supply. III. Interference suppression measures 1. Interference resistance design of the power supply system The most serious threat to the proper operation of sensors and instruments is interference caused by spikes in the electrical grid. Devices that generate such spikes include welding machines, large motors, variable-frequency drives, relay contactors, fluorescent lighting lamps with ballasts, and even soldering irons. Spike interference can be suppressed using a combination of hardware and software. 　(1) Suppressing the impact of spike interference using hardware circuits. There are mainly three common methods: ① Inserting an interference controller designed based on the principle of spectral equalization at the AC power input terminal of the instrument, in order to distribute the energy concentrated in the spike voltages across different frequency bands, thereby reducing their destructive effects ； ②A super isolation transformer is connected to the AC power input of the instrument, utilizing the principle of ferroresonance to suppress spike pulses ； ③A varistor is connected in parallel at the input of the instrument’s AC power supply; when spike pulses arrive, the resistance value of this varistor decreases, which reduces the voltage supplied to the instrument from the power supply and thereby mitigates the impact of interference. 　(2) Suppressing spike interference using software methods: For periodic interference, time filtering can be implemented through programming; in other words, the program can control so that sampling does not take place at the moment when the thyristor is turned on, thereby effectively eliminating the interference. 　(3) A hardware and software combined watchdog technology is used to mitigate the effects of spike pulses. Software-wise: before the timer reaches its set time, the CPU accesses the timer once, causing it to start timing over again; thus, normal program execution continues, and the timer does not generate an overflow pulse, so the watchdog is not activated. Once a spike disturbance triggers a \"flight program,\" the CPU will not access the timer before the scheduled time, resulting in the generation of a timing signal that causes a system reset interrupt, ensuring that the intelligent instrument returns to its normal operating procedure. 　(4) Implement grouped power supply; for example, separate the drive power supply for the actuator motors from the control power supply to prevent interference between devices. 　(5) Using a noise filter can also effectively suppress the interference caused by the AC servo drive on other devices. This measure can effectively suppress the aforementioned interference phenomena. 　(6) Use of isolation transformers: Considering that high-frequency noise propagates through transformers not primarily through the mutual inductance between the primary and secondary coils, but rather through the parasitic capacitance between them, the primary and secondary sides of isolation transformers are separated by shielding layers to reduce their distributed capacitance, thereby enhancing their ability to resist common-mode interference. 　(7) Use a power supply with high anti-interference performance, such as one designed using spectrum equalization techniques. This type of power supply is highly effective at resisting random interference; it converts high-peak disturbance voltage pulses into voltages with lower peak values (peak voltages below TTL levels), while keeping the energy of the interference pulses unchanged, thereby enhancing the resistance to interference of sensors and instruments.

Thank you for your hard work, OP. After reading your article, I have gained a basic understanding of the ET200 module. Thanks.