In the world of PLCs, there are numerous manufacturers, each offering products with their own strengths and weaknesses. These systems can meet a wide range of user needs, but they often face challenges in terms of compatibility regarding form, structure, function, networking, and programming. The lack of a universal standard makes it difficult to compare different PLCs directly. In this article, we will share some insights on how to select the right PLC for an automatic control system, which can serve as a useful reference during the selection process.
When choosing a PLC, several factors should be considered:
**1. Workload**
This is one of the most critical aspects. At the beginning of designing an automatic control system, it's essential to accurately count the number of control points—both digital and analog. This is typically the primary factor in selecting a PLC. It’s common to choose a PLC with 10% to 30% more control points than required. Why? Because it allows for potential missing points during design, provides backup options in case of faults, and accommodates future expansion.
**2. Working Environment**
The environment where the PLC will operate is a crucial consideration. Automatic control systems are designed to handle harsh or complex environments, such as extreme temperatures, humidity, noise, and varying power supplies. Different PLC models have different environmental tolerances, so it’s important to choose a product that suits your specific conditions.
**3. Communication Network**
Modern PLCs are not just about local control—they need to communicate over networks. However, communication protocols vary widely among manufacturers, making interoperability challenging. Key considerations include:
- Communication between products from the same manufacturer.
- Compatibility between products from different manufacturers, especially if you're integrating multiple systems.
- Future-proofing: Choose protocols that are widely adopted, well-supported, and likely to remain relevant.
**4. Programming**
Programming is the core of any control system. The quality of the program directly impacts the performance of the entire system. Consider the following:
- **Programming methods**: Some use dedicated programmers, while others rely on PC-based software. Each has its pros and cons in terms of cost, portability, and functionality.
- **Programming languages**: Common types include ladder logic, sequential function charts, function block diagrams, structured text, and instruction lists. Most manufacturers offer multiple languages, and users should choose the one they are most comfortable with.
- **Memory capacity**: PLC memory stores both programs and data. Internal and external memory options are available, and sufficient space should be reserved for future growth.
- **Flexibility**: One of the main advantages of PLCs is the ability to easily modify control processes as needed.
- **Special modules**: Many PLCs come with specialized modules (e.g., communication, PID control) that simplify development and improve efficiency.
**5. Communication with Monitoring Systems**
PLCs must interact with monitoring systems for effective control. There are two main types:
- **Man-machine interfaces**: Early versions used simple panels, ideal for point-to-point control.
- **PC-based systems**: These are becoming more popular due to their advanced features, such as better graphics, animation, and network capabilities. They allow for more flexible and powerful monitoring solutions, provided the communication protocol is compatible.
**6. Scalability and Longevity**
Consider the product’s lifespan, continuity of support, and update cycle. A reliable PLC should have long-term availability and support, ensuring that spare parts and updates are accessible when needed.
**7. After-sales Service and Support**
Choose a reputable manufacturer and supplier with strong technical support. Good after-sales service can save time and reduce downtime.
**8. Cost**
While cost is an important factor, it should not be the only one. If other criteria are similar, then cost-performance becomes a key decision-making factor.
In practice, not all factors may apply equally. Some may take priority depending on the project. By evaluating the top five aspects, you can narrow down your choices and then refine your selection based on the remaining factors. As technology advances, the PLC industry is moving toward greater standardization, making future selections easier and more straightforward.
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