To improve the reliability of PLC control system design method - Database & Sql Blog Articles

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1. Emphasize Installation

Enhancing the reliability of a PLC control system is an ongoing and long-term task. The first step is to ensure proper installation, which must be carefully inspected to minimize failure rates in production. Second, maintaining high-quality standards during maintenance is crucial, especially when making technical upgrades or system changes. If these are not properly documented, after several years of modifications, many wiring connections may be replaced, leading to lost line numbers, program changes, and no backup records. This increases the maintenance workload and reduces overall system reliability. This aspect is often overlooked but should be given serious attention.

2. Aging Screening Method

A common approach is the "aging screening" method, which aims to shorten the early failure period, extend the random failure phase, and replace components before the wear-out phase begins. This method is particularly effective for non-repairable parts. The failure rate of a PLC control system varies over time. We typically divide the failure rate into three phases: the early failure phase, the random failure phase, and the wear-out phase, as illustrated in Figure 2. This curve is commonly known as the bathtub curve.

(1) Early Failure Phase (O~t0): During this initial period, the failure rate is relatively high due to design flaws, component quality issues, or installation problems. However, the failure rate decreases rapidly as time passes. The main goal during this stage is to identify and eliminate potential weak points to stabilize the system quickly.

(2) Random Failure Phase (t0~t1): This phase is characterized by a stable and low failure rate. Failures occur randomly and are difficult to predict. This is the most reliable period of the system’s life cycle. During this time, it's important to extend maintenance intervals and perform regular checks to maintain system performance.

(3) Wear-Out Phase (after t1): As components age and degrade, the failure rate starts to increase. Mechanical wear, electrical aging, and insulation breakdown contribute to this phase. Most components will fail during this time. By identifying the onset of this phase, we can proactively replace parts to extend the system’s useful life and delay the inevitable wear-out stage.

3. Safety Design of PLC Control Systems

Although PLCs are generally safe and reliable, ensuring the stability and dependability of the entire system is essential. A well-designed safety strategy should leverage the inherent strengths of PLCs to guarantee secure and efficient operation.

(1) Hardware Protection: Includes interlock systems, limit switches, and emergency stop mechanisms to prevent dangerous situations.

(2) Software Protection: Involves interlocks, limit checks, time-based protection, and self-diagnostic functions. Since PLCs operate based on stored programs, they can execute only correct commands and reject invalid ones. Software protection uses built-in diagnostics to detect and resolve hidden issues, ensuring safe and stable operation through customized diagnostic routines.

4. Management of PLC System Reliability

1. Human Resource Management

With the advancement of automation technology, the demand for skilled personnel has increased. Employees need to be proficient in equipment operation, maintenance, planning, and design, as well as have computer skills and technical expertise. Therefore, investing in staff training is essential. Continuous education and development help improve both professional and ethical standards, ensuring that employees are fully capable of performing their roles effectively.

2. Maintenance Management

Maintaining the reliability of a PLC system largely depends on post-installation care. Key areas to focus on include power supply, environmental conditions, installation integrity, power supplies (PS), central processing units (CPU), signal modules (SM), and other critical components such as input/output relays. Regular inspections and timely repairs are vital to keeping the system running smoothly and safely.