The development of modern injection molding machine focuses on improving the control level and the automation degree of the whole machine, as well as the optimization design of the whole machine and the improvement of processing technology, so as to meet the requirements for the quality and energy saving of injection molding products. With the development of microelectronics technology, embedded system chip is widely used because of its high integration, low price and strong performance. Using 32-bit ARM processor to realize the automatic control of injection molding production process, real-time display and online modify the process parameters of each stage, which not only improves the automation of injection molding machine, but also greatly improves the production efficiency and product quality of plastic products.

1. Composition and working process of injection molding machine

1.1 composition of injection molding machine

The controller takes the compound injection molding machine with the combination of electric and hydraulic as the control object. It is a typical system integrating machine, electricity and hydraulic. It is mainly composed of hydraulic oil circuit system, clamping system, injection system, heating and cooling system, lubrication system, electrical control system, safety protection device, monitoring system, feeding and other auxiliary systems. Each part of the injection molding machine controls and coordinates with each other to complete the circular action of the injection molding machine. The electrical control system is the “central nerve” of the injection molding machine. It controls various programs and actions of the injection molding machine, and effectively controls and adjusts the time, position, pressure, speed and speed. The controller realizes the control function of the electrical control system of the injection molding machine.

1.2 working process of injection molding machine

The controller of modern injection molding machine is mainly composed of injection molding machine process control, barrel temperature control, man-machine interface and so on; Control the flow, pressure, position and barrel temperature in the injection molding process; Display and online parameter modification of injection molding process parameters. The working process of injection molding machine is actually a process control. When an action is to be performed, the controller must output the corresponding control signal to energize the relevant solenoid valve, otherwise it cannot work normally. When the input signal of the corresponding travel switch or proximity switch is detected, the control signal can be removed to de energize the solenoid valve to end the action, and then the next action or stop the work according to the process requirements. The workflow is shown in Figure 1.

2. Design of system hardware circuit

The controller is composed of two control modules. The master-slave structure is used to control the injection molding process, temperature, man-machine interface and communication of the injection molding machine. The two control modules are managed by a processor. The processor adopts ARM processor LPC2138. One processor is responsible for keyboard and LCD display management to realize online parameter setting and display, so as to provide a friendly man-machine interface; Another processor is responsible for detecting each input of the injection molding machine, and realizing the injection molding process control and barrel temperature control. RS 232C serial interface is used for real-time communication between them to realize the exchange of information and data. The overall block diagram of system hardware is shown in Figure 2.

The keyboard circuit controller needs to realize the input of various actions and control parameters of the injection molding machine, so the keyboard circuit is essential. The keyboard circuit adopts ZLG7290 keyboard manager, which can sample 64 keys or sensors, detect the number of consecutive hits of each key, and has the functions of keyboard de jitter processing and double key interlock processing. Therefore, the preparation of keyboard scanning, de jitter and other related processing software is omitted, which not only saves program space, but also simplifies programming. Whenever a key is pressed, zi.g7290 will generate an interrupt signal. At this time, LPC2138 can read the key value from ZLG7290 through I2C interface, so as to judge the specific position of the key.

320 is selected for LCD module × For the 240 graphic dot matrix LCD module, the host is designed with an interface with the LCD module, which can be involved, so as to realize the real-time monitoring of the process parameters of the injection molding machine, the online modification of various key parameters and the command input prompt.

Digital input / output circuit the input and output of digital quantity pass through the optocoupler isolation circuit to electrically isolate the controller from the actuator, so as to prevent high voltage interference from damaging the operation of processor program. After shaping, the input signal is sent to the data bus and read by the arm processor. The output digital quantity directly drives the actuator after passing through the power amplification circuit.

The displacement detection circuit is designed with three displacement detection circuits to detect the screw position, switch mode position and support mode mechanism respectively. The detected displacement is used as a basis for switching pressure and speed. The sensor used for displacement detection is the displacement electronic ruler. The signal given by the displacement electronic ruler is an analog signal. The corresponding digital quantity is obtained through the A / D converter ADS7844. The LPC2138 can read the conversion result of ADS7844 through the SPI port.

The proportional valve control circuit controller is designed with two proportional valve control circuits to provide flow and pressure control. The proportional valve is controlled by PWM technology. The PWM signal is directly generated by the hardware circuit. After being amplified by the triode, the PWM signal drives the proportional valve through the Darlington tube.

Barrel temperature detection and heating control circuit this circuit is located in the slave part, which is the design focus of this controller, because the barrel temperature has a direct relationship with the quality of products. Due to the large inertia of the heating system and the influence of environmental factors such as power supply voltage fluctuation, it is often difficult to obtain ideal quality factors. Therefore, corresponding software and hardware measures need to be taken to obtain stable and reliable temperature control effect. The controller is designed with 6 temperature detection channels. The temperature detection sensor adopts K-type thermocouple, and its temperature measurement range is 0 ~ 400 ℃. The thermocouple signal is amplified by the instrument amplifier AD620, and then V / F conversion is carried out to realize analog-to-digital conversion. The use of V / F conversion circuit is due to the strong anti-interference ability of frequency signal and the high cost performance of analog-to-digital conversion. The switching of 6-channel temperature detection is controlled by relay to reduce the temperature measurement error caused by on resistance and realize the isolation between signal and system. The heating control circuit is also controlled by relay. The output of relay can directly drive electromagnetic relay or solid-state relay to control the power switch of heating device. In addition, the cold end compensation of thermocouple is realized by integrated temperature sensor. The specific temperature measurement circuit is shown in Figure 3.

3. Software design

The software design of the controller adopts modular design. The software of the host part mainly includes keyboard, display, communication and real-time clock modules. The software of the slave part is relatively complex, mainly including communication module, temperature detection control module, proportional valve control module, displacement detection module, process flow control module, etc.

3.1 host software design

The host software is divided into two modules, namely keyboard and display module and communication module. Among them, the keyboard and display module is responsible for managing the 64 key keyboard and LCD. Due to the large number of keys, the key control program of “state transition method” is adopted to execute the corresponding program according to the pressed keys, display the corresponding operation page on the LCD, set and display the parameters, and display the current working state of the injection molding machine in real time.

The communication module is responsible for communicating with the lower computer. The serial communication in the controller also adopts the master-slave structure, that is, the host sends communication requirements, such as parameter setting, manual operation, operation status reading, etc., while the slave CPU gives a response according to the requirements of the host CPU.

3.2 slave software design

The slave software mainly realizes the process control of the injection molding machine. The controller controls each process flow of the injection molding machine according to the current working state of the injection molding machine (manual, semi-automatic, full-automatic, etc.) and each input state (travel switch, electronic ruler, keyboard operation command, etc.). In the manual mode, each action of the injection molding machine can be performed separately, such as mold opening and closing, glue injection, pedestal action and mold supporting action. In the semi-automatic mode, the injection molding machine operates for one cycle under the control of the controller, that is, one product is produced. If the next cycle is required, the safety door can be opened and closed once. The full-automatic mode provides two modes: one is time automatic mode; The other is the electric eye automatic mode. The former judges the removal of products by timing. The timing starts from the time when the last mold support withdrawal action ends. This time can be set. As soon as the time comes, the injection molding machine will immediately enter the next cycle. The latter detects whether the products fall off according to the electric eye. If it is detected, it will immediately enter the next cycle, If no falling products are detected within the specified time, an alarm will be given.

The focus of slave software design is the temperature control module, which is also the focus of this paper. Temperature control is a necessary link of injection molding machine, and its control performance directly affects the quality of products. It includes temperature detection and heater control. The controller adopts the control algorithm combining fuzzy control and conventional PID control, and can be set not to use fuzzy control or PID control according to the actual needs. At this time, the temperature control is carried out according to the time cycle mode, that is, the temperature control is carried out by setting the heating time cycle and temperature control percentage.

According to fuzzy control theory, when the universe is discrete, the number of quantized inputs is limited. Therefore, the corresponding control quantity can be calculated offline according to different combinations of inputs, so as to form a control table. In actual control, just check this control table directly. How to implement the control rule table in arm processor is the key of the whole system. The system stores the control rule table in the form of two-dimensional matrix. For example, the two-dimensional matrix is defined as fuzzy Ctrl. Since the input deviation E and deviation change rate de are quantized as X, y respectively, and X, y = {- 6, – 5, – 4, – 3, – 2, – 1, 0, 1, 2, 3, 4, 5, 6}, they can be one-to-one corresponding to a, B = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12} in arm. For example, assuming that e is quantized as x = – 2 and y = 3, it corresponds to a = 4 and B = 9 in arm. When looking up the table, only fuzzyctrl needs to be looked up. This means that it has great advantages in terms of data management and convenience of looking up the table. This fuzzy control method of off-line calculation and on-line look-up table can easily meet the requirements of real-time control.

In addition, the controller also combines the traditional PID control method. The software judges the size range of deviation e, When e is + 6 ℃ or E is – 6 ℃, the system will automatically switch to the PID control program section to enable the system to respond quickly; when – 6 ℃, P At + 6 ℃, the system adopts fuzzy control algorithm to achieve better control effect. When realizing fuzzy control in arm, single point fuzzy set is adopted for fuzzification, discrete method is adopted for membership function, that is, continuous membership is simply discretized, and center of gravity method is still adopted for defuzzification. The flow of temperature detection program module is shown in Figure 4.

4. Conclusion

The reasonable division of the whole control task by using dual processors not only improves the processing speed and stability of the system, but also the software design is relatively simple. The processor combines the displacement sensor and controls and dynamically monitors the injection molding machine according to a certain pressure and speed, which greatly improves the accuracy and quality of the product. The barrel temperature control adopts fuzzy control combined with traditional control method PID control to achieve better control effect. The controller has been applied in practical production. After more than half a year of use, the system runs well and has stable performance.

Responsible editor: GT

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