PLC, or Programmable Logic Controller, is extremely common in automation. To enhance your understanding of PLCs, this article will introduce PLC system integration, eight common error types during PLC use, and their troubleshooting methods. If you are interested in PLCs, please continue reading.
I. PLC System Integration
In manufacturing, there is a large amount of open-loop sequential control primarily using switching quantities, which performs actions sequentially according to logical conditions and timing. Additionally, there is control based on logical relationships for interlocking protection actions, independent of sequence and timing. Furthermore, there is a large amount of discrete data acquisition and monitoring, primarily using state quantities such as switching quantities, pulse quantities, timers, counters, and analog quantity over-limit alarms. Due to these control and monitoring requirements, PLCs have evolved to replace relay circuits and are primarily used for sequential control. PLC manufacturers have gradually added various communication interfaces to the original CPU modules, and fieldbus and Ethernet technologies have also developed simultaneously, making PLC applications increasingly widespread. PLCs possess advantages such as stability and reliability, low price, comprehensive functions, flexible and convenient application, and easy operation and maintenance, which are the fundamental reasons for their enduring market dominance.
The PLC controller itself adopts a modular hardware structure, including a motherboard, digital I/O modules, analog I/O modules, and special modules such as positioning modules and barcode recognition modules. Users can obtain the desired number of I/Os by expanding on the motherboard or by using bus technology to equip remote I/O slaves.
While enabling control of various numbers of I/Os, PLCs also have the ability to output analog voltages and digital pulses, allowing them to control various servo motors, stepper motors, variable frequency motors, etc., that can receive these signals. With the support of a touch screen human-machine interface, PLCs can meet your needs at any level of process control.
II. Eight types of PLC errors and their repair methods
1. CPU malfunction
When the CPU alarms abnormally, check all devices connected to the internal bus of the CPU unit. The specific method is to replace potentially faulty units one by one to identify the problematic unit and then take appropriate action.
2. Memory malfunction
If a memory error alarm occurs and the problem is with the program memory, and reprogramming from scratch does not resolve the issue, the problem may be caused by noise interference altering the program. Otherwise, the memory should be replaced.
3. Input/output unit malfunction, expansion unit malfunction
When this type of alarm occurs, you should first check the connection status of the input/output unit and expansion unit connectors, as well as the cable connection status, to determine which unit is causing the problem, and then replace the unit.
4. Do not execute the program.
Under normal circumstances, you can check the process by following the steps of input – program execution – output.
(1) Input checking is done using input LED indicators or an input monitor constructed with a writer. When the input LED is not lit, it can be initially determined that the problem is with the external input system, and then checked with a multimeter. If the output voltage is abnormal, it can be determined that the problem is with the input unit. When the LED is lit but there is no display on the internal monitor, it can be considered that the problem is with the input unit, CPU unit, or expansion unit.
(2) The program is viewed through the monitor on the writer. When the connection status of the ladder diagram is inconsistent with the result, it is a program error (such as the use of two layers of internal relays) or a problem with the calculation section.
(3) Output can be checked using the output LED indicator. If the calculation result is correct but the output LED indicates an error, it can be considered a problem with the CPU unit or I/O interface unit. If the output LED is lit but there is no output, it can be determined that there is a problem with the output unit or the external load system.
Because different PLC models have different methods for connecting I/O and LEDs (some connect to the I/O unit interface, and some connect to the I/O unit itself), the problem scale identified based on the LEDs also varies.
5. Some programs fail to execute.
The inspection method is the same as the previous one. However, if the input time of the counter, stepper controller, etc. is too short, there will be no response problem. In this case, the input time should be checked to see if it is large enough. The check can be performed according to the relationship between the input time (maximum response time of the input unit + operation scan time multiplied by 2).
6. If the power supply is interrupted for a short period of time, the program content will also be lost.
(1) First check if there is a problem with the battery.
(2) Check by repeatedly turning the PLC's power supply on and off. To ensure the microprocessor starts correctly, the PLC has an initial reset circuit and a program saving circuit when the power is off. If this circuit malfunctions, the program cannot be saved. Therefore, checking can be done by turning the power supply on and off.
(3) If the battery abnormality alarm still appears after replacing the battery, it can be determined that the leakage current of the memory or external circuit is abnormally increased.
(4) The switching on and off of the power supply always occurs synchronously with the machine system. At this time, the noise generated by the machine system can be checked. Since the disconnection of the power supply is a problem that often occurs at the same time as the operation of the machine system, most of the time it is caused by the strong noise generated by the motor or winding.
7. PROM is not working
First, check if the PROM connection is good, then determine if the chip needs to be replaced.
8. The operation stops after the power is restarted or reset.
This problem can be attributed to noise interference or poor internal contact within the PLC. Noise is generally caused by reduced capacitance of small capacitors on the circuit board or malfunctioning components. Poor contact can be checked by gently tapping the PLC body, as well as by inspecting the connections of cables and connectors.
