Printing machine tension control system design (1)

Single chip microcomputer STM32L151CCU6

Based on the tension control principle of the printing industry, the composition of the tension control system is analyzed, and the closed-loop tension control system developed by STM32 as the main control chip and peripheral circuit is introduced. The system not only has high control precision, fast response, but also simple operation. With reasonable PID control, it is suitable for a variety of printed materials. It also calculates the diameter of the material barrel, making the entire tension control process more reasonable.

Tension control systems are widely used in light industry such as printing. It is important to maintain a constant tension to ensure the quality and efficiency of production when collecting and unwinding materials. In the printing process or after the printing is completed, the last process is generally to wind the processed product into a cylindrical shape. In this process, the quality of the winding will be the key to determine the quality of the product. The winding is too tight, and the material is easily deformed and broken. The winding is too loose and the material is not compact, which is not conducive to handling and transportation. In order to achieve compact winding and ensure the quality of the product, it is required to establish a certain tension on the material during the winding process and keep the tension constant. Sometimes the constant tension value must correspond to the diameter of the material winding, because the flexibility of different materials is also different, and when the soft material is wound with a fixed tension, the inner material is laminated to the outer layer. Deformation. In order to avoid this, the system needs to measure the diameter of the wound material and control the tension of the material in real time.

With the gradual structuring and systematization of the printing industry, the control of material tension has become more and more demanding. Because the printing process is different, the tension control method is very different. At present, the tension control system applied, according to the principle structure of its measurement control, mainly has the following three types: indirect method tension control system; direct method tension control system; combined with indirect method and direct method of composite tension control system. Indirect tension control eliminates the need to install a tension sensor, reducing the cost of control equipment. However, the indirect tension control can only meet the general tension control requirements, and it can not achieve satisfactory accuracy in practical applications. Although the direct method tension control has higher cost, it can complete the control process more accurately, and it is extremely fast. The response speed, here is the direct method of tension control. The system is designed with STM32RTC6 as the main control chip, and the running clock frequency is up to 72 MHz. The master chip integrates many high-performance peripheral resources, reducing the design trouble of the corresponding circuit, the normal working voltage is 3.3 V, and has strong power control function.

1 tension control system

In the printing industry, in order to meet the production requirements, it is often necessary to control the tension of some strip materials. The tension control system is a system developed by a single chip microcomputer or some embedded devices and peripheral circuits. Firstly, directly set the tension value required to be controlled, and let the signal collected by the tension sensor (generally millivolt level) be used as the tension feedback value. After comparing the deviation between the two, after the internal intelligent PID operation processing, adjust the actuator and automatically control the material. The tension of unwinding, intermediate guiding and winding is the fastest response of the system. In special cases, the user can also directly set a certain amount of output to the actuator (often the amount of current in the magnetic powder). Figure 1 shows the basic steps of the press tension control system.

1.1 Tension generation

As shown in Figure 1, first analyze the unwinding of the material on the left side, assuming that the tension of the material is released from the take-up reel is T, the linear velocity is the working speed v1 of the pulling roller, the linear velocity of the unwinding drum is v2, and the cross-sectional area of ​​the material For S, the elastic modulus of the material is E, the length from the traction roller to the unwinding drum is L, and t=L/v1 is the time when the material is transported from the traction shaft to the unwinding drum, according to Hooke's law:

It can be seen that if the tension needs to be controlled, the speed difference between the traction roller and the unwinding drum must be controlled. It can be seen that the tension control system is actually a linear speed tracking system. The tension of the material is an integral part of the control process. Under normal circumstances, the unwinding speed of the coil is less than the working speed of the traction shaft when the equipment is started, so that the tension is generated in the material. When the tension reaches our requirement, we stabilize the unwinding speed of the material, so that the material is It can run stably under this tension. The winding process of materials is similar. The actuator that controls the speed is magnetic powder. During the unwinding process, the magnetic powder brake is selected. During the winding process, the magnetic powder clutch is selected.

1.2 Measurement of tension

As shown in Figure 1, the measurement of the tension is mainly obtained by a tension sensor. In order to accurately measure the tension of the material, the material must pass the roller on the tension sensor at a wrap angle of 120. Through the principle of force synthesis (parallelogram principle), the force on the tension sensor is the sum of the tension of the material and the gravity of the roller. We use the calculation method to remove the gravity signal of the roller, and the collected tension signal participates in the subsequent control process.

2 implementation plan

2.1 hardware design

A tension sensor on the market, also called a pressure sensor. The range of the feedback voltage is based on the measured pressure. It is available in two models, a strain gauge pressure sensor and a differential pressure sensor. The strain gauge pressure sensor typically has a voltage range of 0 to 13 mV, while the differential pressure sensor typically has a voltage range of 0 to 150 mV. In order to meet the different choices of users, there must be two kinds of magnification in the signal amplifying circuit, so that the system can operate normally.

In order to make the analog voltage obtained more accurate, the amplifier circuit must have higher performance. The amplification of the millivolt signal must have higher common mode ratio, lower linearity error, and lower offset drift gain. A general amplifier does not meet this requirement. The instrumentation amplifier not only has the above high performance, but its closed-loop gain is determined by the external resistor connected between the inverting input and the output. Therefore, a closed-loop gain instrumentation amplifier, the AD620AN, with a single-ended output with differential inputs is used here. The external closed-loop amplification resistor selects two resistors with a precision of 1% to achieve two unused amplification factors. Then use the jumper cap switch to select the appropriate magnification.

ST's STM32 is currently the most popular control chip, using ARM's latest and advanced Cortex_M3 core; in terms of performance, it not only has outstanding power control functions, but also integrates a variety of useful peripherals, such as: Three 12-bit up to 1 MHz ADC peripherals, two 12-bit DACs, 256 K to 512 Kbytes of large program memory, 64 K of SRAM, etc.; its maximum operating frequency is 72 MHz. In terms of program design, ST provides a complete firmware library, which encapsulates each register operation with functions. The operation mode is very standardized, and the program design is relatively easy compared with other control chips. Among the many STM32 chip models, the STM32F103RCT6 is the most suitable one. It not only contains all the peripherals required, but also is inexpensive. During the tension control process, the analog voltage of the real-time sampling and amplification can be converted into a corresponding digital quantity by a 12-bit high-performance ADC, and the digital signal input from the angular velocity detector is coupled to the controller through the optical isolator. There are two kinds of angular velocity detectors, one is called proximity switch and the other is called coding switch. During the rotation of the roller, the teeth on the roller make them have high and low level digital changes, and the measurement effect is achieved. . For the convenience of users, a good human-machine interface is provided here. First, an input method of a combination of a button and an encoder is provided; secondly, the measured tension and the set tension are displayed by the liquid crystal, and the digital tube can also display the current output percentage.

In the tension control process, the control tension execution unit is magnetic powder, and the magnetic powder is divided into two types: one is called a magnetic powder clutch, and the other is a magnetic powder brake, which are used in the process of winding and unwinding the coil, respectively. The magnetic powder is connected to the take-up reel and the unwinding reel through a universal joint, and the magnetic powder is filled with fine ferromagnetic powder. Under the rated torque, the characteristic formula of the magnetic powder is Ma=K*If (K is a constant, If is the connected excitation current), and it can be seen that the torque of the magnetic powder is linear with the applied excitation current. The magnetic powder clutch has an active end and a driven end. In the absence of excitation current, the driven end does not rotate with the active end. When a certain excitation current is applied, the magnetic powder inside is magnetized, and the torque of the active end is coupled to a certain ratio. Slave. The magnetic powder clutch has only one output shaft. When the excitation current is not connected, the magnetic powder is not magnetized, the torque of the output shaft is large, and after the excitation current is connected, the output shaft of the magnetic powder clutch has a linear torque output.

The magnetic powder supply voltage often used in the market is around 24 V, and the full load current can reach 4 A to 6 A. This makes the constant current output circuit more difficult. The PWM wave of the STM32 control output is supplied to the magnetic powder by the amplification of the constant current module. Here, a field effect transistor with a withstand voltage of 36 V or more must be used. The PWM wave output by the controller must also be amplified by a certain voltage and current to control the breaking of the FET.