PLC Programming in the Automotive Industry: The Quality Management System Behind the Prohibition of M/T

The automation systems on automotive manufacturing production lines have extremely high reliability requirements; a small programming error can lead to the entire production line shutting down, resulting in significant economic losses. A common guideline in automotive industry PLC programming standards is “prohibit the use of M/T (internal relays/timers) instructions as critical control”. This seemingly simple regulation actually contains rich quality management concepts.

1.

What are M/T and Their Potential Issues

M elements (internal relays) are internal contacts in a PLC, equivalent to a bit variable in CPU memory; T elements (timers) are used for delay control. The states of these two types of elements are lost after a power outage, leading to the following risks:

1. Power Outage Restart Hazard: Suppose there is a simple conveyor belt, and we use M0 to record “material has arrived”. When the production line suddenly loses power and is powered back on, the state of M0 is lost, and the system does not know the position of the material, which may lead to operational confusion.
2. State Non-Traceability: When issues arise on the production line that require debugging, these volatile elements cannot provide state information prior to the fault.
3. Poor Program Maintainability: Extensive use of internal elements complicates program logic, increasing maintenance difficulty.

Imagine if your navigation system suddenly restarted while driving, losing all route information – this is the state of a PLC system relying on M/T elements after a power outage.

2.

Solutions in the Automotive Industry

Using Data Registers (D) Instead of M Elements

Automotive industry standards typically require the use of D area (data registers) combined with power-off retention functions to replace M elements in critical control:

// Not recommended approach
LD X0        // Read sensor signal
OUT M0       // Store in internal relay
LD M0        // Read internal relay state
OUT Y0       // Control actuator

// Recommended approach
LD X0        // Read sensor signal
SET D100.0   // Store in data register bit 0
LD D100.0    // Read data register state
OUT Y0       // Control actuator

This way, even in the event of a power outage, critical state information can be retained, allowing the system to quickly recover to the correct state after a restart.

Batch Data Processing and State Management

In practical applications, state information is usually managed centrally:

// Centralized state management example
LD X0        // Sensor 1 triggered
SET D1000.0  // Production step 1 completion flag
LD X1        // Sensor 2 triggered
SET D1000.1  // Production step 2 completion flag

// Unified state logic processing
LD D1000.0
AND D1000.1
OUT Y0       // Only start the next phase if both steps are completed

Notes: When using the D area, be sure to configure the power-off retention range properly; otherwise, state information will also be lost!

3.

Backup and Recovery Mechanisms

The automotive industry also often requires the implementation of an automatic backup mechanism, periodically storing critical states in EEPROM or flash memory:

// Trigger backup once every hour
LD SM400     // 1-hour clock pulse
DMOV D1000 D2000 K50    // Copy 50 D units from D1000 to backup area D2000

This redundant design ensures that even in the worst-case scenario, the system can quickly recover to a state close to that before the fault.

4.

Program Quality and Traceability

Another reason for prohibiting M/T is to improve program traceability. The automotive industry typically requires:

1. State Visualization: All critical states must be monitorable, and a unified state table is usually established.
2. Variable Naming Standardization: For example, D1000~D1100 is specifically used to store state information for station 1.
3. Comment Completeness: Each state variable must have a clear comment explaining its function.

A PLC program that meets automotive industry standards can typically reflect the entire production process logic directly from the variable layout.

5.

Common Problem Solutions

Problem: Completely prohibiting M elements can lead to lengthy programs.Solution: Non-critical logic (such as HMI display control) can appropriately use M elements, but core control logic should adhere to using the D area.

Problem: The number of D area elements is limited; is it wasteful to use all D elements?Solution: Modern PLC storage capacity is quite sufficient, and the importance of resource utilization is far below that of safety and reliability.

6.

Practical Application Case

I once encountered a fault in an automotive parts factory: due to frequent power outages, the robotic arm controlled by M elements often lost its position information, leading to incorrect grabs. After switching to the D area with power-off retention functions, the problem was completely resolved, and the downtime rate dropped from an average of 5 times a month to 0.

The core of quality management is not about how well things are done, but how to avoid mistakes. The high standards for PLC programming in the automotive industry precisely reflect this point – by prohibiting volatile elements, unreliable factors are eliminated from the source.

Next time you write a PLC program, consider how the automotive industry’s standards can be applied to your project to enhance system reliability. You can start by trying to use only D area elements in a small test project to experience the benefits brought by this programming philosophy.

Recommended Reading

1. How to Build a PLC Program Version Management System? A Must-Study Topic for Automation Team Collaboration2. How to Build a Standard Function Block Library for PLC Programs? An Essential Path to Improve Team Efficiency3. How to Build a Standardized Debugging Process for PLC Programs? A Key Method to Shorten Project Cycles