10 Practical Tips on PLC (Part 2)

6. Label input and output

Convenient maintenance PLC controls a complex system. What you can see are two rows of staggered input and output relay terminals, corresponding indicator lights and PLC numbers, just like an integrated circuit with dozens of pins. Anyone who does not look at the schematic diagram to repair the faulty equipment will be helpless and the speed of finding the fault will be very slow. In view of this situation, we draw a table based on the electrical schematic diagram and stick it on the console or control cabinet of the equipment, marking the electrical symbols and Chinese names corresponding to each PLC input and output terminal number, which is similar to the functional description of each pin of the integrated circuit.

With this input and output table, electricians who understand the operation process or are familiar with the ladder diagram of this equipment can start maintenance.

But for those electricians who are not familiar with the operation process and cannot read ladder diagrams, they need to draw another table: PLC input and output logic function table. This table actually explains the logical correspondence between the input circuit (trigger element, associated element) and the output circuit (actuator) in most operation processes.

Practice has proved that if you can skillfully use the input-output correspondence table and the input-output logic function table, you can easily repair electrical faults without drawings.

7. Infer faults through program logic

There are many types of PLCs commonly used in industry today. For low-end PLCs, the ladder diagram instructions are similar. For mid- to high-end machines, such as S7-300, many programs are written using language tables.

Practical ladder diagrams must have Chinese symbol annotations, otherwise it will be difficult to read. If you can have a general understanding of the equipment process or operation process before reading the ladder diagram, it will seem easier.

If an electrical fault analysis is to be performed, the reverse search method or reverse reasoning method is generally applied, that is, according to the input-output correspondence table, the corresponding PLC output relay is found from the fault point, and then the logical relationship that satisfies its action is reversed.

Experience shows that if one problem is found, the fault can be basically eliminated, because it is rare for two or more fault points to occur simultaneously in the equipment.

8. PLC self-fault judgment

Generally speaking, PLC is an extremely reliable device with a very low failure rate. The probability of damage to hardware such as PLC and CPU or software errors is almost zero. The PLC input point will hardly be damaged unless it is caused by strong electric intrusion. The normally open point of the PLC output relay will have a long contact life unless the peripheral load is short-circuited or the design is unreasonable, and the load current exceeds the rated range.

Therefore, when we look for electrical fault points, we should focus on the PLC's peripheral electrical components and not always suspect that there is a problem with the PLC hardware or program. This is very important for quickly repairing faulty equipment and resuming production.

Therefore, the electrical fault inspection and repair of the PLC control circuit discussed by the author does not focus on the PLC itself, but on the peripheral electrical components in the circuit controlled by the PLC.

9. Make full and reasonable use of software and hardware resources

(1) Instructions that do not participate in the control cycle or have been entered before the cycle do not need to be connected to the PLC;

(2) When multiple instructions control a task, they can be connected in parallel outside the PLC and then connected to an input point;

(3) Make full use of the PLC internal functional soft components and fully call the intermediate state to make the program complete and coherent and easy to develop. At the same time, it also reduces hardware investment and reduces costs;

(4) If conditions permit, it is best to make each output independent, which is convenient for control and inspection and also protects other output circuits; when an output point fails, it will only cause the corresponding output circuit to lose control;

(5) If the output is a forward/reverse controlled load, not only must the PLC internal program be interlocked, but measures must also be taken outside the PLC to prevent the load from moving in both directions;

(6) PLC emergency stop should be cut off using an external switch to ensure safety.

10. Other matters needing attention

(1) Do not connect the AC power cord to the input terminal to avoid burning the PLC;

(2) The grounding terminal should be grounded independently and not connected in series with the grounding terminal of other equipment. The cross-sectional area of ​​the grounding wire should not be less than 2mm²;

(3) The auxiliary power supply is small and can only drive low-power devices (photoelectric sensors, etc.);

(4) Some PLCs have a certain number of occupied points (i.e. empty address terminals), do not connect the wires;

(5) When there is no protection in the PLC output circuit, a protective device such as a fuse should be connected in series in the external circuit to prevent damage caused by load short circuit.