Development of fully automatic cross-cutting machine based on single chip microcomputer technology

　　1. Research Purpose and Significance

　　With the rapid development of home appliance industry, automobile industry and decoration industry, the market demand for finishing thin materials, strips and foils is increasing, and the precision requirements for finished thin strips and foils are getting higher and higher. The development of high-precision finishing equipment for thin materials, strips and foils is a market need.

　　In the process of strip processing, it is often necessary to cut a roll of material to a fixed length with a small error; the number of blanks can be automatically counted and grouped; when cutting, it can be automatically pressed and cut continuously. Feeding, pressing, and cutting must be repeated in a certain order. For such control requirements, traditional control systems are difficult to achieve. Fortunately, with the development of single-chip microcomputer and PLC technology, traditional control systems are gradually replaced by new intelligent control systems. In view of the fact that PLC is more expensive than single-chip microcomputer and the number of input and output points is limited, in this article, the author mainly studies the fully automatic cross-cutting machine using single-chip microcomputer technology, and designs its mechanical structure and control system respectively.

　　2. General requirements of fully automatic cross-cutting machine

　　2.1 Mechanical system requirements

　　Due to the thin and soft characteristics of the cut strip, the feeding length of the feeding mechanism cannot be positioned by a mechanical stopper. It is required that the strip must be pressed during the cutting process. Due to different cutting lengths, the time spent by the feeding mechanism is different. Therefore, the speed of the cutter's back and forth movement is required to be adjustable.

　　2.2 Control system requirements

　　Manually input the required feeding length, the number of blanks in each group and the total number of blanking groups. Automatically control the feeding length; when the feeding length reaches the set value, it will automatically compact and then cut. When the number of blanks on the tray reaches the set number, it will automatically unload. During the processing, the total number of groups of cut strips and the number of cut strips in each group can be displayed in real time. The set parameters can be queried and abnormal alarms can be given. It has power-off protection and call recovery functions. It ensures processing accuracy and efficiency, and is simple and convenient to operate.

　　3 Specific solution design and implementation

　　3.1 Mechanical structure and electrical control working principle

　　The mechanical structure of the automatic cross-cutting machine is shown in Figure 1; the electrical control principle block diagram with the single chip microcomputer as the core is shown in Figure 2. Specific working process:

　　① Power on: Input the length of the strip to be cut, the number of blanks per group and other parameters through the keyboard. Click the RUN button and the system will run automatically.

　　② Start the main motor: The single-chip microcomputer sends a control signal, which is used to energize the intermediate relay KA1 through photoelectric isolation, and the inverter forward control terminal is connected to start the main motor, which drives the eccentric wheel to rotate, and then drives the upper cutter and the pressing mechanism to reciprocate up and down through the connecting rod.

　　③ Proximity sensor detects shear knife zero position: The shear knife zero position is set at the position where the clamping mechanism is just released during the upward movement of the shear knife. After the shear knife retreats to the zero position, it continues to move upward until the eccentricity passes the limit position and then returns.

　　④ Feeding: After the proximity sensor detects the zero position, it outputs a pulse signal to the microcontroller, and the microcontroller generates an interrupt. In the interrupt service program, the time it takes for the shear knife to retreat from the zero position to the upper limit position and then return to the zero position is used to start the stepper motor, drive the feeding roller to feed the material, and stop when it reaches the required length.

　　⑤ Pressing and cutting: After the upper shear knife passes the zero position, it continues to move downward, and the spring and the pressure block of the pressing mechanism first press the strip material, and then the shear knife moves downward to cut the strip material. After cutting the strip material, the shear knife continues to move downward to the lower limit position, and then returns. When it reaches the zero position, the pressing mechanism is released, and the next stepper motor feeding is started. This cycle repeats.

　　⑥ Counting: The microcontroller receives the pulse signal from the proximity sensor, which is both the zero position signal of the shearing knife and the number of blanks. The number of blanks can be recorded by recording the pulse number through the microcontroller counter.

　　⑦ Unloading: When the number of dropped sheets reaches the set value, the counter is interrupted, and a signal is sent to the intermediate relay KA2 in the interrupt service program to start the unloading motor to realize the rotary unloading and enter the next group of working cycles.

　　3.2 Control System Analysis and Design

　　3.2.1 Main motor control unit

　　In order to ensure the continuity and production efficiency of the processing process, the cutter must work continuously. At the same time, due to different cutting lengths, the time spent by the stepper motor to feed the material is different, so the main motor should be continuously powered and able to adjust the speed to meet the needs of different cutting lengths. Considering this requirement, the main motor in this design uses a three-phase AC motor, and a frequency converter with high-efficiency drive and good control characteristics is selected for speed control. The frequency of the frequency converter is given, and the single-chip microcomputer calculates the digital quantity according to the keyboard input length, and then converts it into a voltage signal through D/A and sends it to the voltage signal input end of the frequency converter control terminal. The main circuit and control circuit of the frequency converter are shown in Figure 3.

　　3.2.2 Feeding motor and unloading motor control unit　　

Figure 3 Inverter main circuit and control circuit [page]

　　The feeding needs to be precisely measured, so a stepper motor is used for feeding. The stepper motor drives the feeding roller to rotate, and the clamping roller clamps the strip between the two rollers under the action of the spring, and the feeding roller drives the feeding. During the feeding process, the factors that affect the feeding accuracy are: first, the stepper motor may lose steps; second, there may be slippage between the feeding roller and the strip. In order to overcome these two problems, considering that there is no slippage between the clamping roller and the strip, a rotary encoder is installed on the shaft of the clamping roller to detect the rotation angle of the clamping roller, so as to feedback the feeding length. If it is different from the set value, the stepper motor is started again to correct the feeding error caused by the loss of steps or slippage, so as to obtain higher feeding accuracy.

　　The unloading motor adopts an ordinary three-phase AC motor. When the number of sheets in each group reaches the set value, the single-chip microcomputer sends a signal to the intermediate relay KA2, and the normally open contact of KA2 closes to start the unloading motor and realize the indexing unloading.

　　3.2.3 MCU control module unit

　　The single-chip control module unit includes hardware design and software design. Considering the reliability, safety and installation convenience of the system, the controller hardware design is divided into three parts: mainboard, keyboard display and output driver.

　　① Hardware Design

　　The main board is responsible for completing signal acquisition, transmission and processing. Its main components include: single-chip microcomputer AT89C51, watchdog chip MAX692, expansion interface chip 8255 and digital-to-analog converter DAC0832. AT89C51 is mainly used for signal acquisition, data processing, control signal output, etc. It is the core of the entire control device. Due to the poor anti-interference ability of the single-chip microcomputer itself, especially in some places with harsh conditions and high noise, it often crashes due to external interference. Therefore, the watchdog chip MAX692 is used. MAX692 has functions such as backup battery switching, power-off judgment, and watchdog monitoring. When the system encounters interference and the program runs away, the system is reset. The interface expansion chip is mainly used for keyboard input and LED display. DAC0832 is used to convert the digital control signal output by the single-chip microcomputer into an analog voltage signal to give the frequency of the inverter.

　　The keyboard completes the input of processing parameters and intervention signals. Through the analysis of the entire production process of the automatic cross-cutting machine, 16 buttons are set, and a membrane switch matrix keyboard can be used. The emergency stop button is listed separately in a convenient location for operation. Considering that the display function requirements of the system are not high, the display adopts a general LED eight-segment code to meet the requirements. It needs to display 6 digits, of which 2 are used to display the number of groups, and the remaining 4 are used to display the number of cut sheets of each group in a timely manner.

　　The output drive part completes the output and power amplification of the control signal and drives each actuator to work. The circuits on the output board include transformer circuit, relay drive circuit, and stepper motor drive circuit. The transformer converts 220V strong electricity into 24V, 12V and 5V weak electricity to provide power for the microcontroller and stepper motor. The AC contactor is used to control the strong power circuit. Its contacts are connected between the power supply and the inverter. The control signal comes from the inverter start button SB2. The stepper motor drive circuit is used for photoelectric isolation and power amplification, and the weak current signal controls the strong current signal to achieve the drive of the stepper motor.

　　The main components required for the hardware are shown in Table 1.

　　② Software design:

　　According to the automation process of the automatic cross-cutting machine, a modular structure design is adopted. In order to ensure the accuracy of the processing size, the stepper motor is equipped with acceleration and deceleration control, and also has encoder feedback. A power-off protection program is designed to save the working status information and processing parameters in order to resume production. The program flow is shown in Figure 4:

　　3.3 System Power Configuration

Figure 4 Program flow

　　In order to ensure the reliable operation of the system, the power supply of the microcontroller must be isolated from the power supply of the external control channel. In this design, 220V AC is transformed by a transformer to obtain 5V, 12V and 24V AC. 24V AC is used for machine tool lighting, and 5V and 12V AC are rectified, filtered and stabilized to obtain 5V and 12V DC. Among them, 5V is used to power the microcontroller, and 12V is used to power the relay. The power supply of the stepper motor is configured separately according to needs.

　　Table 1 Components List

　　4 Conclusion

　　The fully automatic cross-cutting machine uses the programmable characteristics of the single-chip microcomputer to realize the complex action requirements of the cross-cutting machine through programming, simplifying the hardware and control circuits, and improving the cost performance of the whole machine. During the working process, accurate length cutting, automatic counting, and automatic grouping are performed without the participation of personnel, so the efficiency is high and the labor intensity of workers is reduced. It is especially suitable for cutting precious metals such as gold, silver, and copper.

References:

[1]. AT89C51 datasheet http://www.dzsc.com/datasheet/AT89C51_810155.html.
[2]. PLC datasheet http://www.dzsc.com/datasheet/PLC_1248813.html.
[3]. MAX692 datasheet http://www.dzsc.com/datasheet/MAX692_1058233.html.
[4]. DAC0832 datasheet http://www.dzsc.com/datasheet/DAC0832_253651.html.