Although PLCs are highly reliable and have strong anti-interference capabilities, harsh environments or improper installation and use can still corrupt internal information, leading to control chaos or even damage to internal components. To improve the reliability of PLC system operation, the following points should be noted during use.
I. Suitable working environment
1. Suitable ambient temperature
Each manufacturer has specific requirements regarding the ambient temperature for PLCs. Generally, the permissible ambient temperature for PLCs is approximately 0~55°C. Therefore, during installation, do not place components that generate a lot of heat under the PLC; ensure sufficient ventilation space around the PLC; do not install the PLC in direct sunlight or near heat-generating devices such as radiators, heaters, or high-power power supplies; the control cabinet where the PLC is installed should ideally have ventilation louvers, and if the control cabinet temperature is too high, a fan should be installed inside for forced ventilation.
2. Suitable ambient humidity
The relative humidity of the air in the PLC operating environment should generally be less than 85% to ensure the PLC's insulation performance. Excessive humidity can also affect the accuracy of analog input/output devices. Therefore, PLCs should not be installed in locations prone to condensation or rain.
3. Pay attention to environmental pollution.
PLCs should not be installed in locations with large amounts of pollutants (such as dust, fumes, iron powder, etc.), corrosive gases, or flammable gases, especially corrosive gases, as these can easily cause corrosion to components and printed circuit boards. If installation in such locations is unavoidable, the PLC can be enclosed if the temperature permits; alternatively, the PLC can be installed in a well-sealed control room with an air purification system.
II. Main Sources of Interference and Preventive Measures in PLC Systems
The main source of interference
Interference from the power supply.
The normal power supply for PLC systems is provided by the power grid. Due to the wide coverage of the power grid, it is susceptible to electromagnetic interference from all sources. Radiated electromagnetic fields (EMI) in space are mainly generated by power networks, transient processes in electrical equipment, lightning, radio broadcasts, television, radar, etc., and are commonly referred to as radiated interference. If the PLC system is placed within this radio frequency field, it will receive radiated interference, inducing voltage on the lines. In particular, changes within the power grid, such as surges from knife switch operations, starting and stopping of large power equipment, harmonics caused by AC/DC drives, and transient impacts from power grid short circuits, are all transmitted to the primary side of the power supply through the transmission lines. This can cause program errors or calculation errors, resulting in incorrect inputs and outputs, which will lead to equipment malfunction and misoperation, thus failing to guarantee the normal operation of the PLC.
Interference introduced by the signal line.
In addition to transmitting valid signals, various signal transmission lines connected to the PLC control system are always subject to external interference signals. This interference mainly occurs through two pathways: first, interference from the power grid introduced through the power supply of transmitters or shared signal instruments; and second, interference induced by electromagnetic radiation from the surrounding environment. This often leads to system malfunctions.
Interference in the grounding system.
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can suppress the effects of electromagnetic interference and prevent the equipment from emitting interference; while incorrect grounding can introduce serious interference signals, making the PLC system unable to work properly.
Inverter interference.
First, the harmonics generated during the startup and operation of the frequency converter cause conducted interference to the power grid, resulting in voltage distortion and affecting the power supply quality. Second, the output of the frequency converter generates strong electromagnetic radiation interference, affecting the normal operation of surrounding equipment.
Anti-interference measures
Suppression of power supply interference.
Protection is generally achieved through shielded cables, partial shielding of the PLC, and high-voltage discharge components. Selecting equipment with good isolation performance, choosing high-quality power supplies, and ensuring more rational routing of power and signal lines are crucial. For key components such as power transformers, central processing units, and programmers, shielding with conductive and magnetically permeable materials is essential to prevent external interference. Power supply adjustment and protection : Power fluctuations causing voltage distortion or glitches will adversely affect the PLC and I/O modules. The +5V power supply required by the microprocessor core components should be multi-stage filtered and adjusted using an integrated voltage regulator to adapt to AC power grid fluctuations and the effects of overvoltage and undervoltage. Power lines should be routed parallel as much as possible, and the power lines should have low impedance to ground to reduce power supply noise interference. Different grounding methods for the shielding layer have different interference suppression effects; generally, the secondary coil should not be grounded. Input and output lines should use twisted-pair cables, and the shielding layer should be reliably grounded to suppress common-mode interference. Additionally, a shielded isolation transformer with a 1:1 turns ratio can be installed to reduce interference between the equipment and ground, and an LC filter circuit can be connected in series at the power input.
Anti-interference measures introduced by the signal line.
Power lines, control lines, and PLC power and I/O lines should be wired separately. Twisted-pair cables should be used to connect the isolation transformer to the PLC and I/O lines. PLC I/O lines and high-power lines should be routed separately. If they must be in the same cable tray, AC and DC lines should be bundled separately. If conditions permit, separate cable trays are best, as this maximizes spatial distance and minimizes interference. Furthermore, using signal isolators is an ideal solution to interference problems. The principle is to first modulate and transform the signal received by the PLC using semiconductor devices, then isolate and convert it using optical or magnetic sensors, and finally demodulate and transform it back to the original signal or a different signal. Simultaneously, the power supply to the isolated signal is isolated, ensuring absolute independence between the transformed signal, power supply, and ground. Simply adding such an isolator between the input and output terminals in areas with interference can effectively solve the problem.
Choose the correct grounding point and improve the grounding system.
Proper grounding is crucial for ensuring the reliable operation of a PLC, preventing damage from accidental voltage surges. Grounding typically serves two purposes: safety and interference suppression. A robust grounding system is one of the important measures for PLC control systems to resist electromagnetic interference. In PLC control systems, there are various forms of grounding, primarily including:
(1) Signal ground. The ground of the input signal element;
(2) AC ground. The neutral (N) line of the AC power supply;
(3) Shielding ground. An outer casing or wire mesh installed to prevent static electricity and magnetic field induction, connected to the ground via a dedicated copper conductor;
(4) Protective ground. Grounding the outer casing of the machine or the casing of independent components inside the equipment is used to protect personal safety and prevent equipment leakage.
To suppress interference from the power supply and input/output terminals, the PLC system must be properly grounded. Generally, the grounding method depends on the signal frequency. For frequencies below 1MHz, single-point grounding is sufficient; for frequencies above 10MHz, multi-point grounding is used; and for frequencies between 1 and 10MHz, the PLC control system typically uses single-point grounding, connecting all ground terminals to the nearest grounding point to achieve the best anti-interference capability. The cross-sectional area of the grounding wire should not be less than 2mm², the grounding resistance should not exceed 100Ω, and a dedicated grounding wire should be used.
Suppression of frequency converter interference.
(1) Adding an isolation transformer is mainly for conducted interference from the power source. It can block most of the conducted interference before the isolation transformer.
(2) Use filters. Filters have strong anti-interference capabilities and can also prevent interference from the device itself from being conducted to the power supply. Some filters also have peak voltage absorption functions.
(3) Use an output reactor. Adding an AC reactor between the frequency converter and the motor is mainly to reduce the electromagnetic radiation generated by the frequency converter output during energy transmission, which may affect the normal operation of other equipment.
III. Reasonable installation and wiring
1. Pay attention to power supply installation.
There are two types of power supplies for PLC systems: external power supplies and internal power supplies.
An external power supply is used to drive the PLC's output devices (loads) and provide input signals; it is also called the user power supply. The same PLC may have multiple external power supply specifications. The capacity and performance of the external power supply are determined by the output devices and the PLC's input circuitry. Because the PLC's I/O circuits have filtering and isolation functions, the external power supply has little impact on PLC performance. Therefore, the requirements for the external power supply are not high.
The internal power supply is the operating power source for the PLC, that is, the power supply for the PLC's internal circuitry. Its performance directly affects the reliability of the PLC. Therefore, to ensure the normal operation of the PLC, high requirements are placed on the internal power supply. Generally, PLCs use switching power supplies or power supplies with primary-side low-pass filters for their internal power supply.
In applications with strong interference or high reliability requirements, a shielded isolation transformer should be used to power the PLC system. An LC filter circuit can also be connected in series on the secondary side of the isolation transformer. Additionally, the following points should be noted during installation:
1) It is best to use twisted-pair cables to connect the isolation transformer to the PLC and I/O power supply in order to control crosstalk interference;
2) The system's power lines should be thick enough to reduce the voltage drop caused by the start-up of large-capacity equipment;
3) When using an external DC power supply for the PLC input circuit, it is best to use a regulated power supply to ensure correct input signals. Otherwise, the PLC may receive incorrect signals.
2. Stay away from high pressure.
PLCs must not be installed near high-voltage electrical appliances or high-voltage power lines, and they must not be installed in the same control cabinet as high-voltage electrical appliances. Inside the cabinet, the PLC should be kept away from high-voltage power lines, with a distance of more than 200mm between them.
3. Reasonable wiring
1) I/O lines, power lines and other control lines should be routed separately and should not be routed in the same cable tray as much as possible.
2) AC lines and DC lines, as well as input lines and output lines, should ideally be routed separately.
3) It is best to route digital and analog I/O lines separately. For I/O lines that transmit analog signals, it is best to use shielded lines, and one end of the shielding layer of the shielding line should be grounded.
4) The signals transmitted by the cables between the basic unit and the expansion unit of the PLC are small and high-frequency, and are easily interfered with. Therefore, they cannot be laid in the same cable tray as other connections.
5) PLC I/O circuit wiring must use crimp terminals or single-strand wires. Multi-strand twisted wires should not be used to directly connect to the PLC terminals, otherwise sparks may easily occur.
6) Even if an inductive component is not controlled by the PLC and is installed in the same control cabinet as the PLC, it should still be connected in parallel with an RC or diode arc suppression circuit.
IV. Essential Safety Protection Measures
1. Short circuit protection
When a PLC output device short-circuites, a fuse should be installed in the PLC's external output circuit for short-circuit protection to prevent damage to the PLC's internal output components. Ideally, a fuse should be installed in the circuit of each load.
2. Interlocking and Linkage Measures
In addition to ensuring interlocking relationships in the program, hardware interlocking measures should also be implemented in the external wiring of the PLC to ensure the safe and reliable operation of the system. For example, for the forward and reverse control of the motor, the normally closed contacts of contactors KM1 and KM2 should be used for interlocking externally to the PLC. When there are interlocking requirements between different motors or electrical appliances, it is best to also implement hardware interlocking externally to the PLC. Using external hardware for interlocking and locking is a common practice in PLC control systems.
3. Pressure loss protection and emergency shutdown measures
The power supply line for the PLC's external load should have undervoltage protection. When a temporary power outage occurs and power is restored, the PLC's external load should not start automatically without pressing the "Start" button. Another advantage of this wiring method is that in special circumstances requiring an emergency shutdown, pressing the "Stop" button will cut off the load power supply, without affecting the PLC.
V. Necessary Software Measures
Sometimes hardware measures cannot completely eliminate the effects of interference. Using certain software measures in conjunction can play a significant role in improving the anti-interference capability and reliability of the PLC control system.
1. Eliminate jitter in digital input signals.
In practical applications, some switch input signals exhibit a "jittering" phenomenon, where the signal is intermittently switched on and off due to external interference. This phenomenon generally has little impact on relay systems due to the electromagnetic inertia of the relays. However, in PLC systems, because the PLC's scanning speed is much faster and the scanning cycle is much shorter than the actual relay's operating time, the jitter signal may be detected by the PLC, leading to erroneous results. Therefore, it is necessary to process certain "jittering" signals to ensure the normal operation of the system.
The impact and elimination of input signal jitter
a) The impact of jitter b) Methods to eliminate jitter
2. Fault Detection and Diagnosis
PLCs are highly reliable and have a comprehensive self-diagnostic function. If a PLC malfunctions, the cause of the malfunction can be easily found with the help of the self-diagnostic program, and normal operation can be restored after troubleshooting.
Extensive engineering practice has shown that the failure rate of PLC external input/output devices is far higher than that of the PLC itself. Furthermore, the PLC generally cannot detect these device malfunctions, potentially allowing the fault to escalate until the high-voltage protection device trips, sometimes even causing equipment damage and personal injury. After shutdown, troubleshooting also takes considerable time. To promptly detect faults, enabling the PLC to automatically shut down and issue alarms before accidents occur, and to facilitate fault finding and improve maintenance efficiency, PLC programs can be used to implement self-diagnosis and self-handling of faults.
Modern PLCs have a wealth of software resources. For example, the FX2N series PLC has thousands of auxiliary relays and hundreds of timers and counters, providing a considerable margin that can be utilized for fault detection.
(1) The time required for the mechanical equipment to operate in each step of the process is generally constant, and even if it changes, it will not be too large. Therefore, these times can be used as a reference. When the PLC sends an output signal and the corresponding external actuator starts to operate, a timer is started. The timer's set value is about 20% longer than the normal duration of the operation. For example, suppose that after a certain actuator (such as a motor) runs for 50 seconds under normal conditions, the component it drives causes the limit switch to operate and sends an operation end signal. If the operation time of the actuator exceeds 60 seconds (i.e., the corresponding timer's set time), and the PLC has not yet received the operation end signal, the normally open contact of the timer, which is delayed, sends a fault signal. This signal stops the normal loop program and starts the alarm and fault display program, so that operators and maintenance personnel can quickly identify the type of fault and take timely measures to eliminate the fault.
(2) Logic Error Detection: During normal system operation, there are definite relationships between the PLC's input and output signals and internal signals (such as the status of auxiliary relays). If an abnormal logic signal appears, it indicates a fault. Therefore, some abnormal logic relationships for common faults can be programmed. Once an abnormal logic relationship is ON, it should be treated as a fault. For example, if two limit switches are activated successively during a mechanical movement, these two signals will not be ON at the same time. If they are ON at the same time, it means that at least one limit switch is stuck, and the machine should be stopped for processing.
3. Eliminate predictable interference
Some interference is predictable. For example, when a PLC outputs commands to activate actuators (such as high-power motors or electromagnets), it often generates interference signals such as sparks and arcs. These interference signals may cause the PLC to receive incorrect information. During periods when such interference is likely to occur, certain PLC input signals can be blocked using software, and the blocking can be lifted after the interference-prone period has passed.
VI. Employ redundant or hot standby systems.
Some control systems (such as those in chemical, papermaking, metallurgical, and nuclear power plants) require extremely high reliability. If the control system malfunctions, the resulting production stoppage or equipment damage will cause significant economic losses. Therefore, simply improving the reliability of the PLC control system itself is insufficient. In such large-scale systems demanding extremely high reliability, redundant systems or hot standby systems are often used to effectively address these issues.
1. Redundant system
A redundant system refers to a system with spare parts. The system can function without these spare parts, but in the event of a failure, the spare parts can immediately replace the faulty parts, allowing the system to continue operating normally. Redundant systems typically consist of two identical sets of hardware for the most critical components of the control system (such as the CPU module). If one set fails, the other immediately takes over control. Whether to use two identical sets of I/O modules depends on the system's reliability requirements.
Two CPU modules operate in parallel using the same program, one as the primary CPU and the other as a backup. During normal system operation, the backup CPU module's output is disabled, and the primary CPU module controls the system's operation. Simultaneously, the primary CPU module continuously refreshes the backup CPU module's I/O image registers and other registers via the Redundancy Processing Unit (RPU). When the primary CPU module issues a fault signal, the RPU switches control to the backup CPU within 1-3 scan cycles. The I/O system switching is also handled by the RPU.
a) Redundant system b) Hot standby system
2. Hot standby system
A hot standby system has a simpler structure than a redundant system. Although it also has two CPU modules running a program simultaneously, it lacks a redundant processing unit (RPU). Switching between the two CPU modules is accomplished by the primary CPU module communicating with the standby CPU module via a communication port. The two sets of CPUs are connected through a communication interface. When a system failure occurs, the primary CPU notifies the standby CPU, and a switchover is initiated; this process is generally slow.
