Programmable logic controllers ( PLCs ) and temperature controllers offer similar benefits in the manufacturing industry for ensuring effective process control, achieving consistent quality, and minimizing user error.
Process control equipment has taken on a variety of forms since the 17th century, such as temperature regulators in furnaces and, later, 18th and 19th century, flyball governors in steam engines, boiler valves, and other mechanical systems.
PLCs are a more modern tool that emerged in the 20th century (especially the 1970s) with the growing demand for factory automation. Before PLCs, their functions were achieved by a series of mechanical components that were extremely expensive to install and maintain, and very complex, making them difficult to modify when adjustments were needed in the factory (such as product line upgrades).
However, despite the many similarities between temperature controllers and PLCs, they still differ significantly in setup, programming, and application. Considering these differences, temperature controllers possess their own unique advantages:
1. Cost savings
Of course, this is relative. PLCs are designed for multi-tasking and are suitable for applications with multiple temperature loops. For some single-loop or few-loop control applications, many features of PLCs are unnecessary, making them expensive. In such cases, it is better to choose a controller specifically designed for temperature control.
2. Simple setup
As mentioned earlier, PLCs are designed for multitasking environments, thus requiring specialized programming skills and significant time to create solutions tailored to specific application needs. Temperature controllers, on the other hand (such as PID controllers), can be installed, set up, and optimized relatively quickly, requiring minimal experience.
Many temperature controllers can be panel-mounted, meaning they can be installed on the front panel of process machinery and have a display screen. Personnel with basic engineering knowledge can complete the setup in minutes. PLCs, on the other hand, are more complex. They are typically mounted on a rack behind the panel, do not have a display screen, and require a separate HMI (which also needs to be set up), thus significantly reducing their ease of setup.
3. Improved temperature control function
Temperature controllers are specifically designed to handle specific industrial processes, and therefore include features, outputs, and control functions directly related to these processes, such as specialized algorithms for applications requiring valve motor drive (VMD) control. PLCs, on the other hand, need to be suitable for a wide range of manufacturing and automation functions, thus their features for temperature control are limited. They can perform basic temperature control tasks, but their advantages are not as significant as those of dedicated temperature controllers. Furthermore, temperature control systems place very demanding requirements on microprocessors due to the need to process analog signals. Temperature controllers are designed specifically to handle these needs, while PLCs must be tested before their ability to meet these process requirements can be determined. Failure to meet these requirements will result in the PLC being unable to respond quickly to changes in the process, leading to either anticipation or lag, thus affecting product quality.
4. Clear and visual interface
Panel-mounted temperature controllers come in various sizes and levels of complexity, allowing operators to easily view process information and any warnings or alarms requiring attention. PLCs typically lack a direct interface, requiring a separate HMI (Hydraulic Interface), which needs to be configured independently. While an HMI displays essential process information, it often also shows various data related to other tasks managed by the PLC. This means panel-mounted temperature controllers offer significant advantages: a dedicated interface for viewing all relevant information and enabling quick adjustments. Many temperature controllers also provide additional data logging capabilities, allowing users to review previous changes and flag potential problems.
5. Accuracy
The temperature controller is part of a closed feedback loop that actively tracks the deviation between the process value and the setpoint, adjusting the power level as needed. Many controllers incorporate PID algorithms and automatic adjustment capabilities, enabling rapid setup and maintaining minimal deviation between the process value and the setpoint. This includes adjustments for unexpected process changes or interruptions (such as an oven door being opened). This is essential for maintaining consistently high product quality and reducing defective batches.
PLCs sometimes have PID functionality, but unlike temperature controllers, their PID capabilities are usually limited or require manual adjustment by engineers. This results in longer programming times and increases the risk of human error. As mentioned earlier, temperature control can place very high demands on the processor, and the PLC's ability to handle numerous other tasks simultaneously can lead to slower responses to process changes, resulting in lower accuracy compared to dedicated temperature controllers.
In process control applications, temperature controllers offer several advantages over PLCs. They are faster to install and set up, operate more quickly and simply, provide features specifically designed for temperature control, and, being designed for temperature control, can meet the high demands of process control.
If the system requires a higher level of automation that a temperature controller cannot achieve, then a PLC is a more reasonable solution. However, it is still recommended to use a dedicated temperature controller to compensate for the limitations of a PLC in temperature control. This allows you to utilize the specialized temperature control features while reducing the demands on the PLC, avoiding issues such as slow response and large deviations from the setpoint, thus ensuring high final product quality.
