The cooling water temperature control system for the HPHT Hydraulic Cubic Press aims to control the outlet temperature of the cooling water precisely, thereby indirectly achieving effective control over the hammer head temperature during the diamond synthesis process, which in turn improves the quality of diamond synthesis and extends the service life of the top hammer.
Below is a detailed explanation of the system's principle for controlling cooling water temperature:
1、Control Objective and Challenges：
The HPHT Hydraulic Cubic Press is a key equipment for producing artificial diamonds, and the hammer head temperature directly affects the quality of diamond synthesis and the lifespan of the top hammer.
Since the hammer head temperature is not convenient to measure and control directly, the method of controlling the cooling water outlet temperature flowing through the hammer chamber is generally adopted to indirectly control the hammer head temperature.
Traditionally, manual and rough adjustment of the water outflow rate makes it difficult to respond to changes in water and hammer temperature during the synthesis process. This can lead to decreased diamond synthesis quality and even increase the risk of production accidents like "hammer cracking" or "explosion".
The cooling water temperature control system addresses the shortcomings of manual adjustment by precisely controlling the cooling water outlet temperature to achieve hammer temperature control during diamond synthesis.
The challenges faced by this cooling water temperature control system include its inherent lag, time-varying, and non-linear characteristics.
2、System Composition and Basic Principle：
The automatic water temperature control system consists of six completely independent and identical subsystems. Each subsystem is responsible for controlling the temperature of one cooling water circuit.
Each subsystem primarily comprises a controller, a stepper motor, and a flow control valve.
The core of the control process is: the upper computer sets a desired cooling water outlet temperature value.
The controller real-time acquires the actual cooling water outlet temperature.
By comparing the actual temperature with the set temperature, the controller calculates the temperature deviation and its rate of change.
Based on this deviation information, the system utilizes a multi-mode PID algorithm to determine the precise displacement the stepper motor needs to adjust.
Upon receiving the displacement command, the stepper motor precisely adjusts the opening of the connected flow control valve.
When a high cooling water outlet temperature is detected, the controller instructs the flow control valve to increase its opening, thereby increasing the cooling water flow, taking away more heat, and lowering the outlet temperature.
Conversely, when the outlet temperature is too low, the controller instructs the flow control valve to decrease its opening, reducing the cooling water flow, and raising the outlet temperature.
3、 Core Hardware Implementation：
Temperature Measurement: The system uses a DS18B20 single-wire digital temperature sensor to measure the cooling water outlet temperature. This sensor offers a wide measurement range (-55℃ to +125℃), high resolution (0.0625℃), long transmission distance, and strong anti-interference capability, making it suitable for harsh on-site environments. To further reduce interference, twisted-pair shielded cables are used for the measurement signal lines, and a median-average filtering method is employed in the software, which involves sampling temperature values multiple times, removing the maximum and minimum values, and then calculating the average to improve measurement accuracy.
Control Core: The entire water temperature controller uses an STM32 microcontroller as its core processor.
Communication Circuit: The controller communicates with the upper computer via an RS485 bus, following the MODBUS protocol in RTU mode, ensuring reliable data transmission between the upper and lower computers through a master-slave response mechanism. To enhance system reliability, the RS485 system is optically isolated from the microcontroller, and protection circuits are added to the communication lines to prevent surge currents from damaging the chips. Each subsystem has an independent communication address.
Stepper Motor Drive: The precise adjustment of the flow control valve's opening relies on the accurate movement of the stepper motor. The system incorporates an independent stepper motor drive circuit, with LV8727 as the core drive chip. LV8727 is a PWM current-controlled micro-stepping motor drive chip that supports various micro-stepping options, fast decay, slow decay, and mixed decay modes, and includes built-in temperature and overcurrent protection. The controller generates PWM waves with specific duty cycles, which are then filtered to produce a control voltage (Vref). This control voltage precisely determines the stepper motor's drive current, ensuring that its driving torque meets the requirements for the flow adjustment valve in its opening, closing, and regulating states.
4、Key Control Algorithms：
Stepper Motor Acceleration/Deceleration Algorithm: To ensure the stepper motor's speed, precision, and stability, and to prevent lost steps and overshoot, the system employs a uniform acceleration/deceleration algorithm. This algorithm divides the stepper motor's displacement adjustment process into three phases: uniform acceleration, uniform velocity, and uniform deceleration. The program calculates the number of steps for uniform acceleration, uniform velocity, and uniform deceleration based on given parameters such as initial speed, initial acceleration, target speed, and deceleration. By discretizing the acceleration and deceleration processes and calculating the pulse time for each step, the system achieves ideal control of the stepper motor, allowing the flow adjustment valve to quickly and smoothly reach the calculated opening position.
Cooling Water Temperature Control Algorithm (Multi-Mode PID): Addressing the inherent characteristics of large lag, time-varying, and non-linearity in the water temperature control system, the system utilizes a multi-mode PID control combined with conventional incremental PID single-mode control.
Control Mode Switching: When the temperature deviation is less than a preset value, the system uses a conventional incremental PID algorithm, which achieves good control performance.
Large Deviation Handling: When the temperature deviation is large, the system automatically switches to multi-mode PID control.
Mode Division: Multi-mode PID categorizes the control process into three modes—acceleration control, deceleration control, and steady-state control—by evaluating characteristic variables such as temperature deviation (e), its first derivative (ė), and second derivative (ë).
PID Parameters in Different Modes: During the acceleration control phase (energy storage process), primarily proportional action is used for fast response (Ki=0, Kd=0). In the deceleration control phase, integral and differential actions are mainly employed to suppress overshoot (Kp=0). In the steady-state control phase, primarily integral control is used to eliminate steady-state error (Kp=0, Kd=0). By applying different combinations of PID parameters in different control modes, the multi-mode PID algorithm effectively suppresses overshoot in systems with large lag and achieves steady-state with the shortest possible regulation time, enabling the system to adjust to the target temperature smoothly, quickly, and accurately.
5、System Performance:
Experimental results show that the system can control the outlet temperature of each cooling water circuit to within ±2℃ of the set value from the upper computer.
The system's maximum overshoot is only 2.5%.
The system can complete an adjustment from one temperature state to another within 18 seconds.
This fully meets the stringent requirements of the diamond synthesis process for outlet water temperature, enabling intelligent control of diamond cooling water, significantly reducing the possibility of production accidents, decreasing the labor intensity for operators, and greatly improving the quality of diamond production.