When we buy things, we generally consider three factors: ease of use, affordability, and durability. For control systems, many people focus more on performance and price during the project design phase, neglecting stability and reliability during operation, which can also be understood as durability. In fact, the proportion of later maintenance in the total cost of ownership (TCO) of the entire system ( the PLC lifecycle includes equipment selection, procurement, design, installation, commissioning, maintenance, and service) is still very large.
PLC control reliability refers to the reliability of the system formed by the PLC and its controlled object. The system's purpose is to perform its intended function under specified conditions and within a specified time. This article briefly introduces the application of PLC automatic control systems, analyzes the reliability design methods of PLC control systems, and explores effective ways to improve the reliability of PLC automatic control systems.
The visible maintenance costs are just the tip of the iceberg.
Starting in the 1930s, in order to improve production efficiency, machining enterprises adopted mechanized assembly line production methods, setting up automated production lines for different types of parts. As product models were updated and replaced, the processing objects undertaken by the production lines also changed. This required changes to the control program so that the mechanical equipment on the production line could operate according to the new process. However, the relay-contactor control system used fixed wiring, which was difficult to adapt to this requirement.
1. Development of PLC in Automatic Control Systems
In 1968, General Motors (GM), the largest automaker in the United States, proposed combining the comprehensive functions, flexibility, and versatility of computers with the simplicity, ease of operation, and low cost of relay-contactor control systems to create a universal control device suitable for industrial environments. The programming methods and program input were simplified so that even those unfamiliar with computers could quickly master its use. Based on this concept, Digital Equipment Corporation (DEC) developed the first Programmable Logic Controller (PLC) in 1969, which was successfully tested on GM's automated assembly line. Since then, many renowned manufacturers in various countries have competed to develop their own PLCs, resulting in rapid product updates and continuously enhanced functions. From initially focusing on logic control, PLCs have evolved to include analog control, data processing, and communication networking capabilities. Another significant advantage of PLCs is their high reliability, with a mean time between failures (MTBF) exceeding 100,000 hours, significantly reducing equipment maintenance costs and economic losses caused by production downtime. Currently, PLCs have become the most widely used core device in electrical automatic control systems.
Reliability Design of 2PLC Automatic Control System
In the production design process, the work undertaken to ensure product reliability is called reliability design. Considering reliability issues throughout the design process is far better than discovering unreliable factors after the product has entered production and then making improvements. This is because the latter often requires significant costs in terms of changing tooling, materials, and processes.
The main contents of reliability design include the formulation of reliability indicators, reliability prediction, reliability allocation, as well as specific design work and reliability review related to improving reliability.
The requirements for reliability characteristics are called reliability indicators. Developing reliability indicators involves determining the indicator items and their values. The reliability requirements of a product often need to be reflected by several indicators. For exponentially distributed failures, reliability indicators can be represented by failure rate or MTBF; for early failures and wear-out failures, reliability and reliable life are more suitable; for repairable products such as control devices, effectiveness is also commonly used, which is the ratio of the product's working time to the sum of its working and non-working times. The reliability characteristics of the same product vary depending on conditions, time, and function; therefore, when specifying reliability indicators, it is essential to clearly define the conditions, time, and function to which the indicators correspond. For example, for contact reliability indicators, load voltage and current must be specified; specifying the random failure rate must simultaneously specify the random failure duration, i.e., the effective life.
3. Ways to improve the reliability of automatic control systems
1) Selection of automatic control system scheme
When selecting a solution, consideration should be given to minimizing the number of control elements, contacts, and solder joints to reduce system failure rate. Replacing a control cabinet composed of relays with a programmable logic controller (PLC) can improve system reliability. When comparing PLCs and relay control cabinets, in addition to purchase price, the advantages of the former, such as improved reliability, shorter development cycle, reduced workload, and saved maintenance time, should be fully considered.
2) Selection of control components
Correctly selecting the type and specifications of control components is crucial for improving their reliability. This requires a thorough and accurate understanding and analysis of the machine tool's requirements for the electrical control system and the system's requirements for the control components. Furthermore, it's essential to collect and digest the technical materials provided by the control component manufacturers. If this information does not meet the selection requirements, the machine tool manufacturer can test the control components under actual usage conditions to determine their suitability. Examples of reliability issues caused by improper selection include: neglecting input or output mechanical parameters; for example, failing to consider the force-stroke characteristics of the load when selecting an electromagnet; failing to consider the impact speed when selecting a limit switch; failing to consider the inching and reverse braking modes when selecting a contactor; failing to consider the rated minimum operating voltage and rated minimum operating current for reliable contact when selecting a relay; failing to consider the protection of contactor contacts when selecting short-circuit protection devices (including fuses); and failing to consider the voltage drop due to leakage impedance under electromagnetic system starting current conditions when selecting a control transformer.
3) Operating environment of control components
The working environment has a significant impact on reliability. Dust can cause electrical contact failures, reduce insulation performance, and increase the risk of AC magnetic system poles sticking together; therefore, necessary dustproof measures must be taken. For systems with high reliability requirements, control components should be housed in dust-proof enclosures or cabinets. Control components on machine tools often operate in environments with lubricating oil or cutting oil; in such cases, oil-resistant electrical components must be selected or oil-resistant measures must be taken.
4) Screening and preventative replacement
To reduce early system failures, machine tool manufacturers can screen control components, eliminating defective units after each component has been operated continuously a certain number of times under actual operating conditions. This reduces the likelihood of early failures in the system. Once components enter their wear-out and failure period, the failure rate increases significantly. To prevent this from affecting system reliability, components can be replaced at the end of their effective lifespan (or period of random failure), regardless of whether they are damaged. By employing screening and preventative replacement, it is ensured that components in the system operate within their random failure period. Only under this premise can the random failure rate of components be used to calculate the overall system failure rate.
5) On-site failure investigation
Fundamentally, improving reliability should begin with on-site failure investigation and analysis. Unlike general product design and calculation, reliability design cannot achieve the desired results solely through theory and calculation. The degree of reliability depends on the understanding (both in depth and breadth) of actual failures occurring in the field. Laboratory testing is also a method for obtaining failure data, but due to limitations in content, methods, and conditions, it can only provide an approximate simulation of on-site usage conditions and cannot replace the investigation and analysis of on-site failures. Reliability testing divorced from on-site conditions is meaningless.
PLC reliability does not equate to PLC control system reliability. A PLC control system is more complex than a standalone PLC. Increased system complexity increases the likelihood of malfunctions, thus reducing reliability. A PLC control system is not simply the sum of a PLC and its controlled object, but rather an organic combination of the two. A good combination is essential for reliability. Furthermore, a PLC control system is not simply a complete integration of the PLC and its controlled object; only those aspects relevant to the control process require integration.
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