The dyeing process occupies a crucial position in textile production, as dyeing quality directly determines the color and appearance of textiles—and even influences their production costs. In the dyeing process, key factors affecting dyeing include dye liquor concentration, temperature, liquid level, and more. Among these, temperature control is both critical and highly complex. Essentially, the dyeing process involves following a specific temperature curve meticulously designed by process engineers for different types of fabrics. Each dyeing process has stringent requirements for temperature settings as well as the heating and cooling procedures. Failure to meet these precise conditions can easily lead to defects such as color variations, batch-to-batch inconsistencies, and streaks, ultimately driving up the re-dyeing rate and increasing production costs. To address the complexities of temperature control during dyeing, a PLC-based temperature control system for dyeing machines has been developed. This system effectively manages the temperature throughout the dyeing process, helping minimize fabric defects, boost production efficiency, and reduce overall manufacturing costs.

　　System Control Requirements

　　1. Temperature curve storage requirements vary depending on the specific dyeing process, as each variety has unique temperature needs, resulting in distinct temperature profiles for each process. This system utilizes touchscreen configuration and recipe design to store multiple dyeing processes—and the data for each stage within those processes—directly on the touchscreen. This makes it easier for operators to manage and adjust process parameters at any time, saving changes instantly so that the data remains permanently stored on the screen. Operators input the customized temperature curves directly via the touchscreen, and the controller precisely executes temperature adjustments based on the received profiles. Both the controller and the touchscreen are equipped with a power-off feature that ensures long-term preservation of current data even during unexpected outages.

　　2. Temperature control requirements. During the dyeing process, the typical process curve is shown as follows:

　　Figure 1: Typical Process Curve

　　As shown in Figure 1, the dyeing process can be divided into several curve segments, with each segment corresponding to a specific temperature. Temperature control during the dyeing process primarily involves heating, maintaining, and cooling the dye solution inside the cups on the core. Additionally, based on actual production requirements, the heating phase is further refined by setting the temperature rise according to a predefined slope. In this setup, the PLC allows for slope-based heating rates ranging from 0 to 9°C, though the exact rate must be calculated based on the power output of the on-site heaters. During the recent field tests, when the heating rate was set to 4°C, the heater was already operating at full capacity. This user-friendly slope-setting feature in the PLC enables precise and efficient control over the heating phase. Temperature control is implemented through five subroutines: direct heating, slope-based heating, heat preservation, direct cooling, and slope-based cooling. Each of these five processes corresponds to a distinct program segment within the PLC, which then cycles sequentially based on the selected step number. Examples of these processes are illustrated in Figures 2 and 3.

　　Figure 2: Dyeing Formula Interface

　　Figure 3: Main Interface

　　3. Alarm Display and Dyeing Recipe Functionality. Figures 2 and 3 above show the two primary interfaces during operation. To help users better monitor various operational statuses, a curve graph has been integrated into the main screen, allowing for a more intuitive visualization of the process temperature trends. Meanwhile, the dyeing recipe interface enables users to conveniently set up individual process parameters and easily select which process to run. Each process in this system consists of 9 steps, and users can specify the exact step number from which the process should begin. Once the current process is complete, the main interface will display a notification indicating that the selected dyeing process has finished—please confirm before proceeding to the next one. The confirmation step ensures that the system moves smoothly to the next stage.

　　Figure 4: Confirmation of Process Completion

　　Figure 4 shows the right-hand interface, which primarily controls the motor speed and manages the variable-frequency forward/reverse operation. While the speed control is manually set, the forward/reverse functions—as well as the durations for forward rotation, reverse rotation, and stopping—are all implemented through PLC programming. In the event of any fault, the touch-screen indicator lights will flash, triggering a fault alarm that displays the cause of the issue on an alert screen.

　　The system's hardware components

　　This system employs the NA200 compact PLC introduced by Nanda Aotuo Company as the field controller. The NA series programmable controllers represent the latest generation of control devices developed by Nanda Aotuo Technology Co., Ltd., boasting internationally advanced features. Leveraging cutting-edge advancements in the industrial control field and backed by a brand-new software and hardware platform, these controllers deliver rapid processing capabilities, robust anti-interference performance, and highly flexible scalability. As a result, they effortlessly handle even the most complex environments and demanding applications with ease. With over two decades of experience in developing automation products and having undergone rigorous field testing across hundreds of projects, Nanda Aotuo’s PLCs have earned a reputation for unparalleled reliability. The product has successfully obtained CE certification, CCS certification, as well as passing stringent tests and inspections conducted by the Quality Inspection and Testing Center for Automation Equipment in Power Systems under the Electric Power Industry. All key performance indicators meet or exceed relevant industry standards, placing this product ahead of its competitors. The system utilizes the CPU2002 host module and connects to a touch screen via the RS232 port using the Modbus-RTU protocol. Additionally, the field controller is expanded with an intelligent temperature data acquisition module—specifically, an RTD input module featuring 2 channels. This versatile module includes 2 analog input points, eliminating the need for external transmitters altogether. With just one such module, the system can seamlessly perform both data collection and real-time processing tasks. For temperature signal detection, the system relies on platinum resistance PT100 sensors. Known for their exceptional measurement accuracy, stability, and reliability, PT100 sensors are widely used across industries for temperature measurements ranging from -200°C to +500°C.

　　Additionally, if needed, the PLC can be connected to factory monitoring via the RS485 interface using MODBUS-RTU, enabling remote operation and supervision.

　　The control system section is equipped with one CPU2002 (Nanda Aotuo), one 2-channel RTD input module (Nanda Aotuo), and a 7-inch touch screen (Nanda Aotuo).

　　System software design

　　As shown in Figure 1, the dyeing process can be divided into several curve segments, each corresponding to a specific target temperature, heating/cooling rate, and holding time. Temperature control during the dyeing process primarily involves managing the heating, holding, and cooling phases of the dye bath. Based on actual production requirements, heating is further categorized into two methods: direct heating to the desired temperature and precise ramp-rate heating to the specified temperature. Similarly, cooling is divided into direct cooling to the target temperature and accurate ramp-rate cooling tailored to the desired temperature profile. Consequently, temperature control is implemented through five subroutines: direct heating, ramp-rate heating, holding, direct cooling, and ramp-rate cooling. Whether to use direct or ramp-rate heating, or direct or ramp-rate cooling, is determined by the operator via the central control system and subsequently invoked by the main PLC program at the field level. Due to the large volume of the dyeing tank combined with relatively small heating pipes, the system exhibits significant thermal inertia. This can generally be characterized as a controlled object with substantial pure-lag and high inertia. Therefore, during the heating and cooling phases, a trend-based compensation method is employed: if the process involves heating, the system stops increasing the temperature once it reaches T minus the compensation value △Ti, where △Ti represents the temperature compensation. In the PLC’s main program, the system first evaluates data received from the touch screen before executing the appropriate subroutine. The incoming data includes critical parameters such as the target temperature, heating/cooling rate, and holding time. Based on this information, the PLC can accurately determine whether to initiate heating, hold the temperature, or proceed with cooling. However, relying solely on the target temperature, heating/cooling rate, and holding time would not yield precise decision-making. To address this, the system also incorporates the target temperature from the preceding curve segment as an additional reference for more accurate judgment.

　　Below, we'll use the example of ramp heating to illustrate the PLC's temperature-control process. During ramp heating, the heating curve is divided into several equal small intervals based on time, allowing the heating segment of the process curve to be represented in a stepwise manner. As long as each interval is sufficiently short, the deviation between the calculated temperature setpoint for each interval and the ideal value can be safely disregarded.

　　Due to the significant thermal hysteresis, when the actual temperature T approaches the setpoint, it’s advisable to implement a residual-heat-based temperature-raising strategy at this stage. This ensures that, during the transition from heating to holding, the temperature does not overshoot and remain excessively high.

　　Additionally, since the PT100 temperature measurement is taken at only one point in the dyeing tank, it doesn’t necessarily reflect the exact temperature of the dye solution inside the final container. Therefore, a temperature compensation factor △Ti has been introduced to better meet the process requirements. The heating curve is shown in the figure below:

　　Figure 5: Different heating rate slopes are shown in the figure (slopes of 3°C/min and 2°C/min). The curves during the holding phase can also be observed in the graph.

　　To better ensure system security, the system has added permission settings, offering a variety of roles such as Super Administrator (with full access rights) and Process Engineers, all of which can be freely customized according to your needs.

　　Figure 6: User Login Interface

　　The dyeing formula design significantly simplifies the issue of users finding it difficult to operate the simple, rudimentary computer interface of the dyeing machine. As shown in the figure below, you can now freely customize your process formulas directly on the touch screen.

　　Figure 7: Dyeing Process Formula Editing

　　This system not only enables temperature control but can also be easily expanded, as needed, to manage other dyeing process parameters such as water level and liquid level, showcasing excellent potential for future development. The system employs a PLC as the controller on the production floor, precisely managing temperature control to ensure timely and accurate quality control of dyed products—effectively eliminating potential quality issues right at the source, thereby guaranteeing consistent dyeing results and achieving first-time accuracy. Compared to traditional dyeing machine control computers, the primary advantages of using a PLC-based system lie in:

　　PLC control makes the system more stable and reliable;

　　Using a touchscreen for operation makes it remarkably convenient, delivers a more intuitive experience, and enhances the flexibility of your entire system.

　　The dyeing process formula significantly enhances operability and intuitiveness, saving you valuable time as well as manpower and resources.

　　The advantage of PLC control also lies in its greater flexibility, allowing you to improve or add new process steps to your system—simply by updating the program—thus saving you from having to spend a hefty sum on purchasing an entirely new dyeing-machine control computer! Plus, the touchscreen interface can be freely customized to meet your specific needs.