Abstract This paper analyzes the mechanism of various interferences affecting PLCs and servo drives, and proposes relevant anti-interference measures from both hardware and software perspectives. These measures have practical value for the application of PLC systems in motion control. Keywords PLC; Servo; Anti-interference; Digital filtering I. Overview With the development of industrial control technology, PLC and servo technology have made great strides. PLCs are computer control devices designed specifically for industrial production environments, and are widely used in various industries due to their advantages such as high reliability, complete hardware support, simple and easy-to-learn user programs, and convenient maintenance. AC servo motor control adopts the field-oriented vector control principle, which has the characteristics of fast dynamic response, high steady-state operation accuracy, small torque ripple, and smooth low-speed operation. Moreover, it has a large speed range and is widely used as a feed transmission device. PLCs generally have a pulse output interface, so a simple CNC system composed of PLC and pulse-type servo is the first choice for economical machine tools. PLCs and servos are designed specifically for industrial control environments, so they are inherently reliable. Therefore, in general control systems, anti-interference design is not required or only simple anti-interference design is needed to ensure the safe and reliable operation of the system. However, in particularly harsh application environments, such as strong electric fields, strong magnetic fields, and severe shock and vibration environments, the control system and actuators may not operate reliably. Furthermore, in applications with particularly high reliability requirements, special anti-interference design is necessary for the control system and actuators. To improve system reliability, it is essential to first carefully analyze various potential sources of interference in the corresponding application environment. Based on this analysis, select a reliable PLC and related modules, and conduct anti-interference design from a hardware perspective, including engineering design, construction wiring, and usage and maintenance. Additionally, targeted anti-interference design should be implemented from a software perspective. II. Main Sources of Interference and Suppression Measures in the System Numerous sources of interference disrupt system stability. The main manifestations of system instability are the corruption of internal information, leading to control system chaos, actuator malfunctions, and network errors, affecting the normal operation of the equipment. 2.1 PLC From a formal perspective, interference in PLC control systems can be divided into two categories: internal interference and external interference. Internal interference is a problem inherent to the PLC itself; external interference includes interference introduced through wires (interference introduced by external lines such as power lines, control lines, and signal lines), spatial induction and radiation interference, and interference introduced through ground wires. In actual industrial situations, internal interference is relatively rare. The following is an analysis of external interference. (1) Select a high-performance power supply and take measures to suppress grid interference. In the PLC control system, the power supply occupies an extremely important position and is also one of the main ways for interference to enter the PLC. Various electrical devices are connected to the power grid, such as high-power motors, AC/DC transmission devices, frequency converters, household appliances, etc. The start and stop of these devices will cause current and voltage fluctuations in the power grid, generating large-amplitude surges and high-order harmonics. If the AC power supply of the PLC system is used, in situations where the interference is strong or the reliability requirements are high, an isolation transformer with a shielded layer and a low-pass filter can be added to the AC power input terminal of the PLC. The shielding layer should be reliably grounded; a shielding layer can also be added between the primary and secondary windings and grounded together with the iron core to improve the high-frequency common-mode interference capability. (2) Interference from spatial induction and radiation . Most PLC control systems are located in spaces with various electric and magnetic fields, which all affect the control system. Electromagnetic interference (EMI) is mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc.; poorly shielded PLC control systems also generate electromagnetic fields, which in turn affect the control system itself. These electromagnetic fields are collectively called radiated interference, and their distribution is extremely complex. As long as the PLC control system is within the radiation range, it will be interfered with. The degree of interference to the control system is related to the strength and frequency of the radiation. Radiation affects the PLC control system through the following two pathways: ① Direct radiation to the inside of the PLC, which generates interference through circuit induction; ② Radiation to the PLC communication network, which introduces interference through the induction of the communication lines. Shielding, filtering, and grounding are the three main methods to deal with this kind of interference. (3) Interference introduced by signal lines Crosstalk signals on adjacent signal lines will generate noise on the crosstalked single line or generate coupling signals on the crosstalked line pairs, that is, there are crosstalk signals on the crosstalked lines. Interference introduced by signals will cause abnormal operation of I/O interface signals and a significant reduction in measurement accuracy, and in severe cases, it will cause damage to components. For systems with poor isolation performance, it will also cause mutual interference between signals, cause the common ground system's total ground wire to return current, resulting in changes in logic data, malfunctions and crashes. (4) Interference introduced by ground wire. There are two purposes for grounding: one is for safety; the other is to suppress interference. Improper connection of ground wire will cause ground loop current. Ground loop current generates an electromagnetic field inside the shield wire, which in turn interferes with the shield wire and causes signal distortion. The following figure shows the correct grounding method. Series grounding should be avoided. (5) Unscientific installation and wiring. Different types of PLCs have different installation specifications, such as the installation position of the CPU and power supply, the distance between racks, the installation position of interface modules, the number of I/O modules, the connection resistance between the rack and the mounting part, etc. There are clear requirements. During installation, the installation requirements of the product used must be followed. PLC should have an independent and good grounding device. The grounding resistance should be less than 100Ω, the grounding wire should not exceed 20m, and PLC should not share a grounding body with other equipment. PLC power lines, I/O lines, and power lines should ideally be placed in their respective cable trays or conduits, with a center-to-center distance of at least 300mm. Analog input/output lines should ideally be shielded, and one end of the shield should be grounded. PLCs should be kept away from interference sources. If signal lines cannot avoid interference sources, fiber optic cables should be used. When installed outdoors, lightning protection measures must be taken, such as running the cables in metal conduits grounded at both ends. To reduce electromagnetic interference from power cables, especially those from frequency converter feeder cables, two basic principles are adopted: First, in practical engineering, copper-clad shielded power cables should be used as much as possible to reduce electromagnetic interference generated by power lines. This method has proven very effective in many cases. Second, different types of signals should be transmitted using different cables. Signal cables should be laid in layers according to the type of signal transmitted. It is strictly forbidden for different conductors of the same cable to transmit power and signals simultaneously. Signal lines should not be laid parallel to power cables to reduce electromagnetic interference. In PLC control systems, hardware anti-interference design is fundamental and the basic measure to suppress interference. In addition, in software design, economical and effective methods such as digital filtering and software fault tolerance can be adopted to further improve the reliability of the system. (1) Digital Filtering The analog signals on site are converted into digital signals after A/D conversion and stored in the PLC. Then, the digital filtering program is used to process them, filter out noise signals, and obtain the required useful signals. There are many digital filtering methods in engineering, the commonly used ones are: average value filtering method, median value filtering method, weighted filtering, sliding filtering method, etc. (2) Software Fault Tolerance Although various anti-interference technologies are adopted, interference cannot be completely eliminated. Interference always exists, whether more or less, large or small, and under certain conditions, it may cause large interference to the control system. Therefore, we should also adopt software fault tolerance technology in program compilation. Fault tolerance means that when interference cannot be avoided, if it causes large interference to the control system and causes the system to malfunction, the control system can react to it in time and decide the next remedial measures based on the state of the error. The main fault-tolerance technologies include: ① Program re-execution technology: During program execution, if a field fault or error is detected, the interfered preceding instructions can be re-executed several times under certain circumstances. If the re-execution is successful, it indicates that the cause of the control system fault is interference; otherwise, it is due to reasons other than interference. In this case, a software failure (Fault) should be output, and the system should be stopped and an alarm should be triggered. ② Handling of infinite loops: A timer (WDT) program is designed in the program. When the timer exceeds the original time, it can be determined that the system has entered an infinite loop. When the control system enters an infinite loop, the program's judgment can determine whether to stop the system or enter the relevant subroutine to restore the system. ③ Software delay: To ensure the correctness of the detection of important switch input signals, easily jittered signals, and control loop data acquisition, a software delay of 15ms-20ms can be used. The same signal is read multiple times, and the result is considered consistent before it is confirmed as valid. This can eliminate the influence of occasional interference. 2.2 Servo Servo systems are similar to PLC systems. The external interference sources and anti-interference measures of PLCs are also applicable to servo systems. At the same time, there are also differences between servo systems and PLCs. The anti-interference of servo drives is mainly to prevent the input of interference pulses. (1) The pulse input port of the servo drive is divided into open collector mode and differential input mode. Since the anti-interference capability of the open collector mode is much worse than that of the differential input mode, the servo drive with differential input mode should be selected as much as possible. (2) In order to minimize the input of interference signals by the servo drive when there is no upper positioning instruction, the "pulse input prohibition" signal of the servo drive should be activated in the program design when there is no pulse input. This can effectively reduce the input of interference pulses. (3) The connection between the servo drive and the servo motor should use shielded wire. The part of the cable that is stripped of the shield layer should not be greater than 75mm. The shield layer should be reliably grounded on the servo drive side. (4) If conditions permit, the servo speed control mode and the upper controller should be used to form a closed loop control. Three Case Studies A company produced a simple CNC drilling machine. The control system was a Mitsubishi Fx series PLC. The X and Y axes were positioned using servo motors driving lead screws, while the Z axis used hydraulic feed. The spindle was driven by a frequency converter, which in turn drove a standard three-phase asynchronous motor controlled by a gearbox. During actual debugging, inaccurate positioning was found. Inspection revealed that the machine tool's servo motors were still inputting pulses even without pulse commands, and the number of pulses received by the servo driver was not equal to the number of pulses sent by the PLC, especially at the moment the frequency converter started. Therefore, it was determined that the system suffered from severe interference. Based on the above analysis, it is proposed to add an input filter at the PLC power supply; to use shielded twisted-pair cable for the pulse signal connection between the PLC and the servo drive, and to keep this cable as short as possible; to add an input filter at the servo drive power supply; to add a freewheeling diode at the DC solenoid valve; and to add a surge absorber at the AC contactor; to lay signal lines and power lines in separate cable trays with a 200cm interval; to add an input filter at the inverter input terminal; to replace the connection between the inverter and the motor with shielded cable; and to ensure proper grounding at the inverter side; and to modify the PLC control program so that the "pulse input disable" signal on the servo drive takes effect when there is no pulse output from the upper controller. After these improvements, the machine tool's performance fully meets the requirements. IV. Conclusion To improve equipment reliability, on the one hand, PLC and servo manufacturers need to further improve the anti-interference capabilities of their products; on the other hand, multi-party cooperation is required in engineering design, installation, construction, and maintenance to effectively solve interference problems and enhance the system's anti-interference capabilities. [1] Liao Changchu, PLC Programming and Application, Machinery Industry Press, May 2005, 2nd edition [2] Gao Yongsheng, Anti-interference measures for programmable controllers [J]. Mechanical Management and Development, No. 5 (Total No. 69), December 2002 [3] Tang Yuan, Causes of interference in PLC control system and anti-interference measures [J]. Petrochemical Construction, Vol. 27 No. 3 June. 2005 [4] Xiong Xingming, Research on grounding anti-interference technology of PLC control system [J]. 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