Introduction
PLCs, or Programmable Logic Controllers, serve as core components in the field of industrial automation.
They play a crucial role in controlling various mechanical equipment, automated processes, and systems.
This article presents 10 practical tips for PLC applications to help you efficiently resolve issues encountered in daily operations.
01
Grounding Issues
PLC systems have stringent grounding requirements. It is best to have a dedicated grounding system, and ensure other PLC-related equipment is also reliably grounded.
Connecting multiple circuit grounding points together can generate unexpected currents, leading to logic errors or circuit damage.
Differential ground potentials typically arise when ground points are physically separated by significant distances. When widely spaced equipment is connected via communication cables or sensors, currents may flow through the entire circuit between the cable and ground. Even over short distances, load currents from large equipment can cause potential shifts relative to ground, or unpredictable currents may be generated directly through electromagnetic effects. Destructive currents capable of damaging equipment may flow between power sources connected to incorrect grounding points.
PLC systems generally employ single-point grounding. To enhance common-mode interference immunity, shielded floating ground techniques can be applied to analog signals. This involves grounding the shielding layer of signal cables at a single point while leaving the signal loop floating, with insulation resistance to ground maintained at no less than 50MΩ.
02
Interference Mitigation
Industrial environments are harsh, containing numerous high- and low-frequency interferences. These interferences are typically introduced into the PLC through cables connected to field devices.
Beyond grounding measures, anti-interference precautions should be implemented during cable selection and installation:
(1) Analog signals are small-signal and highly susceptible to external interference; double-shielded cables should be used.
(2) High-speed pulse signals (e.g., pulse sensors, digital counters) require shielded cables to prevent external interference and mitigate interference from high-speed pulses to low-level signals;
(3) Communication cables between PLCs operate at higher frequencies. Manufacturer-supplied cables are generally recommended; shielded twisted-pair cables may suffice for less critical applications;
(4) Analog signal lines and DC signal lines must not be routed in the same cable tray as AC signal lines;
(5) Shielded cables entering or exiting control cabinets must be grounded and connected directly to equipment without passing through terminal blocks;
(6) AC signals, DC signals, and analog signals must not share the same cable. Power cables should be laid separately from signal cables.
(7) During field maintenance, interference mitigation methods include: replacing affected lines with shielded cables and rerouting them; incorporating anti-interference filtering code into the program.
03
Eliminating Inter-wire Capacitance to Prevent False Triggers
Capacitance exists between all conductors within a cable. Qualified cables limit this capacitance value within specified ranges.
Even with qualified cables, when cable length exceeds a certain threshold, inter-wire capacitance may surpass required limits. When such cables are used for PLC inputs, inter-wire capacitance can cause PLC malfunctions, leading to numerous unexplained phenomena.
These phenomena primarily manifest as:
- Visibly correct wiring, yet no input detected by the PLC; Inputs that should be present are absent, while unwanted inputs appear-indicating mutual interference between PLC inputs. To resolve this issue:
(1) Use cables with twisted cores;
(2) Minimize cable length;
(3) Separate cables for inputs that interfere with each other;
(4) Employ shielded cables.
04
Output Module Selection
Output modules are categorized as transistor-type, bidirectional thyristor-type, or contact-type:
(1) Transistor-type modules offer the fastest switching speed (typically 0.2ms) but have the lowest load capacity (approx. 0.2–0.3A, 24VDC). They are suitable for rapid switching and signal interconnection equipment, commonly used with variable frequency drives and DC devices. Attention must be paid to the impact of transistor leakage current on the load.
(2) Thyristor-type modules offer contactless operation and AC load characteristics, though their load capacity is limited.
(3) Relay outputs support both AC and DC loads with high capacity. Relay contact outputs are typically the first choice for conventional control applications. Their disadvantage is slow switching speed (around 10 ms), making them unsuitable for high-frequency switching.
05
Inverter Overvoltage and Overcurrent Handling
(1) When reducing the setpoint to decelerate the motor, the motor enters regenerative braking mode. The energy fed back to the inverter is substantial, accumulating in the filter capacitors. This causes the capacitor voltage to rise rapidly, often reaching the DC overvoltage protection threshold and triggering an inverter trip.
Solution: Install an external braking resistor to dissipate the regenerative energy fed back to the DC side.
(2) When multiple small motors are connected to the inverter, an overcurrent fault in one motor triggers an alarm, causing the inverter to trip and halting all motors.
Solution: Install a 1:1 isolation transformer on the inverter's output side. When one or more small motors experience an overcurrent fault, the fault current impacts the transformer instead of the inverter, preventing tripping. After testing, the system operates reliably without recurring incidents of normal motors stopping.
06
Marking Inputs and Outputs for Troubleshooting
The PLC controls a complex system, presenting only staggered rows of input/output relay terminals, corresponding indicator lights, and PLC numbers-resembling an integrated circuit with dozens of pins. Without consulting the schematic, troubleshooting becomes impossible, and fault detection is extremely slow.
To address this, we created a reference table based on the electrical schematic. This table, affixed to the equipment's control console or cabinet, maps each PLC input/output terminal number to its corresponding electrical symbol and Chinese name-similar to the functional descriptions for each pin on an integrated circuit.
With this I/O table, electricians familiar with the operational process or the device's ladder diagram can proceed with troubleshooting.
However, for electricians unfamiliar with the operational process or unable to read ladder diagrams, an additional table is required: the PLC Input/Output Logic Function Chart. This chart essentially explains the logical correspondence between input circuits (triggering components, associated components) and output circuits (actuating components) for most operational processes.
Practice has proven: If you can skillfully utilize both the input/output correspondence table and the input/output logic function table, troubleshooting electrical faults becomes effortless-even without schematics.
07
Deducting Faults Through Program Logic
Numerous PLC types are commonly used in industry today. For low-end PLCs, ladder diagram instructions are largely similar. For mid-to-high-end models like the S7-300, many programs are written using language tables.
Practical ladder diagrams must include Chinese symbol annotations; otherwise, they are difficult to read. Understanding the equipment process or operational sequence beforehand makes interpreting ladder diagrams easier.
When performing electrical fault analysis, the reverse tracing method-also known as backtracking-is typically applied. This involves using the input/output correspondence table to locate the corresponding PLC output relay from the fault point, then tracing back to identify the logical relationships that caused its activation.
Experience shows that identifying a single issue usually resolves the fault, as simultaneous failures at two or more points are rare.
08
Diagnosing PLC Hardware Failures
Generally, PLCs are highly reliable devices with extremely low failure rates. The probability of hardware damage to the PLC or CPU, or software operational errors, is virtually zero. PLC input points rarely fail unless subjected to high-voltage intrusion. Similarly, the normally open contacts of PLC output relays exhibit exceptionally long lifespans unless compromised by external load short circuits, improper design, or current exceeding rated limits.
Therefore, when troubleshooting electrical faults, the focus should be on the peripheral electrical components connected to the PLC. Avoid immediately suspecting PLC hardware or programming issues, as this approach is crucial for rapid repair and swift restoration of production.
Consequently, the electrical fault diagnosis discussed here for PLC control circuits emphasizes not the PLC itself, but the peripheral electrical components within the circuits controlled by the PLC.
09
Optimizing Utilization of Software and Hardware Resources
(1) Instructions not involved in control cycles or those already activated before the cycle need not be connected to the PLC;
(2) When multiple instructions control a single task, they can be connected in parallel externally before being linked to a single input point;
(3) Make full use of the PLC's internal function blocks and intermediate states to ensure program integrity and coherence, facilitating development. This also reduces hardware investment and lowers costs;
(4) Where feasible, isolate each output channel for easier control and inspection, while protecting other output circuits. A fault in one output point will only cause the corresponding output circuit to lose control;
(5) For outputs controlling forward/reverse loads, implement interlocking not only within the PLC program but also externally to prevent the load from operating in both directions simultaneously;
(6) PLC emergency stops must be implemented via external switches to ensure safety.
10
Additional Considerations
(1) Never connect AC power lines to input terminals to prevent PLC damage;
(2) Ground terminals must be independently grounded, not connected in series with other equipment grounds. Ground wire cross-sectional area should not be less than 2mm²;
(3) The auxiliary power supply has limited capacity and can only drive low-power devices (e.g., photoelectric sensors);
(4) Some PLCs have reserved address terminals (i.e., empty address terminals). Do not connect wires to these terminals;
(5) When PLC output circuits lack protection, install protective devices such as fuses in series within the external circuit to prevent damage from load short circuits.