Abstract: Early injection molding machine control systems generally suffered from drawbacks such as aging components, high failure rates, and outdated performance. Developing precise, efficient, and energy-saving control systems using modern control technology to extend equipment lifespan and solve these practical problems during operation is a pressing issue for injection molding machine users in China. This paper takes a large injection molding machine imported in the 1970s as an example to explore the design issues of a novel injection molding machine control system.
Keywords: Programmable Computer Controller, Injection Molding Machine, Automation Studio
1. Methods and principles of injection molding machine control
The main problems to be solved in the control of injection molding machines include: mechanical motion control, power control, and temperature control.
1.1 Mechanical Motion Control
Mechanical hydraulic injection molding machines are widely used in the plastics industry. The key to their mechanical motion control is the mold opening and closing motion control. The motion principle of a double-elbow mold opening and closing mechanism is shown in Figure 1.
Figure 1. Schematic diagram of the motion principle of the double-curved toe clamping mechanism.
The object of the mold opening and closing motion control is the moving template C, which is driven by the hydraulic cylinder through the double toggle mechanism. Therefore, it is necessary to establish the relationship between the position of the hydraulic cylinder piston and the position of the moving template, as well as the relationship between the piston speed (determined by the hydraulic system's opening and closing cylinder flow rate) and the moving template speed when the hydraulic cylinder piston is at a certain position, that is, to perform the linearization work of the mold closing mechanism.
Based on the motion principle shown in Figure 1, the relationship between the position and velocity of the piston of the mold opening and closing cylinder and the moving template can be established. In the control system software, this part of the calculation is implemented using a general linearization subroutine to enable accurate control.
To reduce the computational load on the CPU and improve the real-time performance of the control system, a better approach is to develop computer software based on the above relationship to calculate the one-to-one correspondence between the piston and the moving template, forming a data module. During the mold opening and closing motion control, the relationship between the piston and the moving template can be directly read from the data module.
1.2 Power Control
The power transmission of injection molding machines is mainly hydraulic. The power control of injection molding machines needs to solve two problems: first, the control of pressure and flow, which are the main factors that determine the accuracy of the equipment; second, the design of a reasonable process flow and hydraulic action sequence diagram.
1.2.1 Pressure and Flow Control
Unlike modern injection molding machines that use proportional valves for continuous pressure and flow control, early injection molding machine hydraulic systems mostly employed discrete pressure and flow control. The multi-stage pressure control valves in large injection molding machines consist of six control solenoid valves that work in a linear superposition to control pressure. Therefore, it is first necessary to determine the contribution (pressure weight) of each control solenoid valve to the control pressure.
Experimentally, the pressure weights of the six solenoid valves were determined to be: P1 = 0.21875 MPa; P2 = 0.4375 MPa.
P3 = 0.875 MPa; P4 = 1.75 MPa; P5 = 3.5 MPa; P6 = 7.0 MPa.
The control operating pressure P of a multi-stage pressure control valve can be expressed by the following formula:
P = K1 P1 + K2 P2 + K3 P3 + K4 P4 + K5 P5 + K6 P6 (1)
K1 to K6 represent the opening coefficients of the six multi-stage pressure control solenoid valves, with values of 0 (no energization) and 1 (energization). According to equation (1), the cyclic judgment algorithm can easily realize the automatic determination of the action combination of the multi-stage pressure control solenoid valves based on the set pressure.
Flow control is similar to pressure control.
1.2.2 Process Motion Control
The process design for the equipment is shown in Figure 2.
Figure 2. Process flow of the injection molding machine
Designing a hydraulic system's timing diagram involves determining the logical sequence of valve actions corresponding to the process actions. This sequence varies between different systems.
1.3 Temperature Control
Temperature control in injection molding machines primarily refers to barrel temperature control. PID control with self-tuning parameters is a commonly used control method in modern injection molding machines. The system design involves the following steps:
(1) Design of temperature control system hardware. The barrel is divided into 7 control temperature zones. Each temperature zone uses a temperature sensor (K-type thermocouple) to detect the temperature. The signal is amplified and converted by A/D (implemented by the B&R PCC 2003 family of analog input modules AT 664) and then input to the PCC. The PCC performs calculations and processes the data to control 7 sets of control devices (contactors or contactless zero-crossing solid-state relays SSR), thereby controlling the resistance heater to heat the barrel.
(2) Identification of barrel temperature control model. The results show that the mathematical model of barrel temperature control can be expressed as a first-order inertial plus pure hysteresis element, as shown in equation (2).
(2)
In the formula, t is temperature, u is power, τ is system time delay, Kt is system time constant, K is system constant, and s is Laplace transform factor.
The parameters of the mathematical model can be obtained by step response method, as shown in Table 1.
Table 1 Mathematical model parameters for each heating section of the barrel
(3) Algorithm design. Determine the parameter self-tuning method and PID algorithm. The parameter relay self-tuning and incremental PID control algorithm are adopted. There are many materials that introduce the theory in detail.
2 Design of Injection Molding Machine Control System
Based on the previous analysis of injection molding machine control, the following section describes the construction of an injection molding machine control system using a programmable logic controller (PLC) from B&R.
2.1 Hardware System
The control system hardware configuration adopts a common upper and lower computer structure used in modern high-end equipment. The upper computer is the operation and monitoring part, using an IPC2001 with a 486DX2 CPU, running the B&R Automation Runtime V2.60 operating system, a TFT true-color screen, and a 30-key keypad specifically for injection molding machines. The lower computer includes a PCC controller and expansion modules, and its configuration mainly considers meeting the existing functional requirements of the control system, while appropriately reserving space for future functional expansion.
Figure 3 shows the hardware composition and interrelationship of a large injection molding machine control system consisting of upper and lower level mechanisms.
Figure 3 System hardware composition
2.2 Software System
2.2.1 Task Settings
The control system software mainly consists of upper-level computer software and lower-level computer software. The upper-level computer control system software needs to implement functions such as display, communication, data storage, recipe reading and setting, and includes four main parts: program module, system module, data module, and panel program module. The lower-level computer software system is the main part of the entire injection molding machine software system; it is an application program written according to the injection molding process flow. B&R PC operating system Automation Studio (AS) is a time-sharing multitasking operating system that uses a time-sharing multitasking mechanism to build its application software's operating platform. It can divide tasks into several levels with different priorities, where higher-priority tasks have shorter cycle scan times, and each task level can include multiple specific tasks, with further subdivisions of priority among these tasks. Under the management of the AS operating system, higher-priority tasks are always executed first, and lower-priority tasks are executed in the remaining time. Therefore, in software design, tasks can be set according to the importance of the application program, thereby optimizing the control system and achieving better real-time performance. The main functional task settings are shown in Table 2.
Table 2 Main Task Level Settings
2.2.2 Software Organizational Structure
The software system is written in C language and adopts a structured design. The organizational structure is a pyramid-shaped hierarchical structure with four levels from top to bottom (as shown in Figure 4).
The first layer is the organization layer (main program mode), which judges and processes the operation status of the injection molding machine; the second layer is the interlock operation (dynamic interlock and static interlock) for the motion organization and safety judgment of various states of the injection molding machine (fully automatic, semi-automatic, manual, setting, mold adjustment state); the third layer is the various actions or functions, which are combined and called by the second layer; the fourth layer refers to the system's pressure and flow control tasks.
Figure 4. Main program functional block diagram (partial)
3. Results
After being put into production, the newly designed control system for large injection molding machines has shown superior characteristics compared to the original control system in several aspects, including the following.
(1) The application of time-sharing multi-task programmable computer controller (PCC) solves the problem that the scanning speed of traditional PLC control system is limited by the program size, and ensures the real-time performance and accuracy of injection molding machine control.
(2) Based on the different importance and special requirements of each task, the task level is scientifically allocated to ensure the full realization of the functions of the injection molding machine while ensuring the speed and accuracy of key process control.
(3) High-level language (standard C) Structured programming makes the system more maintainable and scalable.
(4) It has relatively complete functions to meet the various process requirements of injection molding. The more scientific control algorithm makes the pressure and flow control accuracy higher than the original system; the perfect shaft interlock function makes the equipment have high safety and reliability.
(5) The machine has a high degree of automation, the temperature control system parameters are self-tuning, and the dynamic and static performance is superior to that of conventional PID control.
The development of this system has solved the problems existing in the use of old injection molding machines, and plays an important role in improving equipment functions and extending equipment service life.
