Siemens PLC Diagnostic Expert: Quickly Locating and Handling System Failures

Hello everyone, I am Xing Ge. Today, let’s talk about fault diagnosis for Siemens PLCs. As an automation engineer, encountering system failures is a common occurrence. However, being able to quickly locate and resolve issues is a true skill! Below, I will share my accumulated experience in PLC fault diagnosis over the years, hoping to help you remain calm when facing problems.

1.Basic Approach to Fault Diagnosis

Fault diagnosis is actually similar to a doctor diagnosing a patient. First, you need to “ask questions”—understand the fault phenomenon; then “examine”—collect various diagnostic information; and finally, “prescribe”—formulate a solution. Specifically, we can follow these steps:

1. Clarify the fault phenomenon
2. Collect diagnostic information
3. Analyze possible causes
4. Verify and eliminate
5. Resolve and retest

Sounds simple? In fact, each step has its own complexities. Let’s go through them step by step.

2.Clarify the Fault Phenomenon

The first step is to clarify what the problem is. This requires us to ask the right questions, such as:

• Is the fault sudden or gradual?
• Is it a complete shutdown or a localized anomaly?
• Is there any alarm information? What error is reported?
• Is the fault persistent or intermittent?

Remember not to easily trust the operator’s description; it’s best to go to the site and take a look yourself. I have encountered several instances where the operator said, “The machine is completely unresponsive,” but upon arrival, I found that the safety door was not properly closed.

3.Collect Diagnostic Information

Once the fault phenomenon is determined, the next step is to collect various diagnostic information. Siemens PLCs provide a wealth of diagnostic tools, mainly including:

1. CPU Diagnostic Buffer: Records system events and errors.
2. Forced Table and Monitor Table: Used to observe and modify variable values.
3. Online Status: Allows real-time viewing of program execution status.
4. Trace Function: Records the trend of variable changes over time.
5. System Status List (SSL): Contains hardware configuration information of the PLC.

In addition to these software tools,don’t forget to check the status of the PLC’s indicator lights. Is the SF (System Fault) light on? Is the MAINT (Maintenance Request) light flashing? These are all important diagnostic information.

4.Analyze Possible Causes

After collecting the information, it’s time to analyze. Common causes of PLC faults mainly fall into several categories:

1. Hardware Faults: For example, a faulty power module, loose communication cables, etc.
2. Software Issues: Program logic errors, data overflow, etc.
3. Communication Anomalies: Network disconnection, incorrect communication parameter settings, etc.
4. External Factors: For example, sensor failures, actuators getting stuck, etc.

When analyzing, be careful not to assume that the PLC is at fault just because an error is present. I have seen many beginners jump to the conclusion that the PLC is broken, only to find after much effort that it was a loose sensor connection.

5.Verify and Eliminate

With a preliminary judgment made, the next step is to verify and eliminate possible causes one by one. Here are a few tips:

1. From Simple to Complex: Start by checking the simplest possibilities, such as whether the power supply is normal.
2. Bisection Method: If the fault range is large, try dividing the system in half to gradually narrow down the range.
3. Replacement Method: Suspect a certain module is faulty? Try replacing it with a known good module.
4. Simulation Method: You can use the forced table to simulate input signals and see if the system responds normally.

Remember,be cautious during the verification process and do not introduce new problems due to diagnostic operations. I have made the foolish mistake of short-circuiting the emergency stop button to test it, only to burn out the button…

6.Resolve and Retest

Once the problem is identified, the next step is to resolve the issue and conduct comprehensive retesting. Solutions may include:

• Replacing faulty hardware
• Correcting program errors
• Adjusting system parameters
• Re-downloading the program, etc.

Regardless of the measures taken,be sure to conduct thorough testing after resolution to ensure:

1. The original fault has been eliminated
2. No new problems have been introduced
3. All system functions are normal

Don’t think that resolving the fault means everything is fine. I have experienced several times where fixing one fault led to another fault appearing the next day. Later, I found out it was because I hastily messed up the program.

7.Common Fault Case Analysis

Having discussed so much theory, let’s look at a few practical cases:

Case 1: CPU Continuous Restart

Phenomenon: The PLC’s running light continuously flashes between green and yellow, and the system cannot operate normally.

Analysis: This situation usually indicates that the CPU is caught in a start-exception-restart loop. Possible causes include:

1. Hardware failure, such as a damaged CPU.
2. Severe errors in the program that cause the CPU to crash shortly after each startup.
3. Storage card issues, preventing the program from loading properly.

Solution:

1. Check the CPU’s diagnostic buffer for specific error information.
2. Try clearing the CPU memory and re-downloading the program.
3. If the problem persists, the CPU module may need to be replaced.

Case 2: Communication Interruption

Phenomenon: The HMI displays “Communication Error” and cannot interact with the PLC.

Analysis: Communication issues may arise from multiple links:

1. Physical layer: Damaged network cables, poor connections, etc.
2. Network layer: Incorrect IP address configuration, erroneous gateway settings, etc.
3. Application layer: Incorrect parameter settings in the communication blocks of the PLC program.

Solution:

1. Check the network cable connections and replace the cable if necessary.
2. Verify that the IP settings of the PLC and HMI are correct and match each other.
3. Examine the communication function blocks in the PLC program to ensure parameter settings are correct.

Case 3: Output Not Acting

Phenomenon: A certain output point is set in the program, but the actual device does not respond.

Analysis: Possible reasons for the output not acting include:

1. Output module failure.
2. External wiring issues, such as blown fuses or disconnected lines.
3. Program logic issues, where the set statement is not actually executed.

Solution:

1. Use the forced table to force the output point to ON and check for output.
2. Measure the output voltage with a multimeter to confirm whether there is an output signal.
3. Use program status monitoring to check if the set statement was actually executed.

8.Prevention is Better than Cure

Rather than waiting for faults to occur and then diagnosing them, it’s better to take preventive measures:

1. Regularly back up programs: This is the most basic yet easily overlooked task.
2. Use program comparison functions: After each modification, compare with previous versions to avoid unintended changes.
3. Use comments wisely: Detailed comments can greatly improve fault diagnosis efficiency.
4. Establish a logging system: Record key data and events to provide a basis for fault analysis.
5. Regular maintenance: Check wiring, clean heat sinks, update firmware, etc.

9.Practical Exercise Recommendations

1. Build a small Siemens PLC control system that includes basic inputs and outputs and a simple HMI.
2. Write a program that includes common functions (such as timers, counters, communication, etc.).
3. Deliberately introduce some common faults, such as disconnecting an input or changing the IP address, and then try to diagnose and resolve them.
4. Familiarize yourself with various diagnostic tools in STEP 7 or TIA Portal.
5. Simulate a field environment and conduct fault diagnosis drills, setting time limits to improve diagnostic efficiency.

Remember, diagnostic skills are honed through practice. Try more, summarize more, and you too can become an outstanding PLC fault diagnosis expert!

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