Gravure printing is named because of the printing plate characteristics of its printing process, and it occupies an important position in the flexible packaging industry with its exquisite printing quality. Gravure printing machine is generally composed of unwinding device, feed traction, host traction, discharge traction and rewinding device. Its standard configuration is a common seven-motor system (including automatic rewinding of rewinding and unwinding). During coil printing, it is necessary to use a certain tension to tension the material into the printing unit, and to ensure stable tension during startup, operation, roll change and parking, so as to ensure accurate registration of various colors and beautiful patterns. Therefore, the tension control is the technical core of the gravure printing machine drive control. If the tension control is stable and the tension fluctuation is small, the machine's overprinting accuracy and printing speed are high.
The structure and current situation of tension control system
In the printing process of the coil, the tension required by the system to control is mainly the unwinding tension, the feeding tension, the discharging tension and the winding tension. This is the four-stage tension we usually talk about. Generally, the coil material is in the printing color group. The tension control is realized through the incremental version. The shaftless system can also be implemented with the speed difference of the drive motors of each color group.
Tension control system is generally composed of detection, arithmetic control and driving device:
Detection part: This part is composed of a tension sensor or a floating roller. In recent years, since the floating roller absorbs the tension of the material better during the reel reeling process, the colleagues in the industry have used the floating roller to detect.
Operation control: completed by PLC, industrial computer or DSP. In recent years, due to the large-scale popularization of PLC, the performance-to-price ratio is good, and the calculation speed is increasing year by year, so most of them use PLC for tension control. Generally, the floating roller signal is directly converted into PLC after AD conversion, and the PLC performs curling diameter calculation, constant linear speed control and PID calculation of each section tension, and transmits the result to the vector frequency converter through analog DA.
The frequency converter acts as a drive actuator and a tension adjustment device in the entire control system. Therefore, the system has relatively high requirements on the key technologies such as frequency conversion speed range and accuracy, torque accuracy and dynamic response characteristics.
Limitations of traditional schemes and our reform ideas
Because the traditional system tension control is completed by a PLC, with the increase of printing speed (100-300m / min), the increase of the roll diameter ratio (600mm ~ 1500mm), the increase of the printing color set (the original 6 colors increase to 13 colors), and the increase in the number of auxiliary control points (pressure roller, fan, ink viscosity control, etc.), for the CPU of the PLC of the cyclic scan mode operation, it is gradually overwhelmed, even if some auxiliary control is moved to another PLC To complete, but the calculation time required for the actual four-segment tension with a more advanced PLC (such as Mitsubishi's FX-3U or SIEMENS's S7-300) should be nearly 20ms scan cycle.
So we began to try to reform the single CPU structure of traditional tension, the Dujiang algorithm that attracted multiple CPUs to calculate the tension of each segment and directly control the drive frequency conversion Song to adjust the tension, that is, according to the type of control function or need each CPU To improve the processing performance of important links. Mitsubishi launched the vector frequency conversion A740-A1 with its own coiling function. After researching Mitsubishi's equipment information, we believe that the introduction of the coiling frequency conversion is a good solution worth testing.
Mitsubishi winding special frequency conversion
1. There is a special PID software for float roller inside, the system can directly accept the signal of float roller potentiometer
(1) The PID of the floating roller can be set in stages according to the offset;
(2) The output of float roller control can be proportionally compensated with the main speed;
(3) It has the alarm function of material break.
2. Calculation function of winding diameter
(1) The curling diameter can be directly calculated: D = V / âˆnZ (V: linear speed, n: speed, Z: speed ratio);
(2) Various calculation force methods or direct setting of initial winding diameter;
(3) P and I at startup can be adjusted independently;
(4) The speed gain can be set in sections as the roll diameter changes, and the door can be switched;
(5) Automatically generate tension setting signal output for taper control;
(6) Detection and control output of unwinding / unwinding length.
3. Winding diameter calculation type constant tension control (torque control mode)
(1) STALL stall control;
(2) Acceleration and deceleration inertia compensation;
(3) Mechanical loss compensation;
(4) Regeneration avoidance function. [next]
Implementation of new scheme and system composition
The control system is controlled by seven motors, which are two rewinding (A740-7.5-A1), two unwinding (A740-7.5-A1), the host (A740-22K-CHI), and rewinding, Unwinding traction (A740-3.7-A1) composition. Among them, the floating roller signals of winding, unwinding and unwinding and unwinding traction can be directly connected to their respective frequency conversions, and the floating roller arithmetic modules built in the respective frequency conversions are respectively operated in parallel and controlled.
Control system features:
(1) Each frequency converter has the function of calculating and compensating the diameter of the roll, so we make each frequency converter operate in accordance with the requirements of the linear speed synchronization.
(2) The linear speed of the entire system has been synchronized in advance. The floating roller is only used for speed compensation control caused by tension fluctuations. The actual adjustment of the floating roller is only 0.1HZ ~ 0.3HZ, that is, the position fluctuation is small. Due to the independent calculation of multiple CPUs, the system response is significantly accelerated. During 250m / min high-speed non-stop feeding, the equipment tension fluctuation is very small. According to preliminary calculations, the system's response to the daily lead can be 10 to 20 times higher than that of a single-CPU PLC, about 1 to 2 ms.
(3) Since the initial winding diameter can be preset by parameters, according to the current actual linear speed and the preset winding diameter, we call this to control the pre-drive speed. The measured linear speed of the pre-drive frequency conversion is quite consistent with the old axis speed, and the fluctuation of the feed tension is small.
(4) Since A740-A1 comes with 485 communication interface, there are special communication instructions between FX3U and A740, so the whole system adopts 485 communication to set and modify parameters, and programming is quite convenient.
Future outlook
After our actual application and debugging on site, it has been proved that the tension control system of the standard seven motor of the printing machine formed by Mitsubishi's special winding frequency conversion has several advantages:
1. The system adopts multi-CPU parallel algorithm, which makes the range of the magazine increase when it is heartwarming, and the precision of tension control is also significantly improved, and the refueling work is reliable;
2. Because the floating roller signal directly enters the frequency conversion, PLC and frequency conversion are all connected by communication, so the system AD / DA module can be used as little as possible, which shortens the processing time and saves costs.
3. Mitsubishi special coil frequency conversion only adds the coil module to the soft state, and does not increase other costs. If the reduced AD / DA module is included, the cost performance of the entire system is improved and has a certain practical value.
4. Because the customer controls the cost on the price, we do not time the system and compare the actuarial cost. To further improve the performance, Mitsubishi A740 can be equipped with CC-Link communication card to improve the reliability of communication. In some high-speed synchronous printing presses, SSCNET-III (servo system control network) special accessories can also be selected as servo products, so that the A740-A1 can exert its excellent performance in the axisless control of the printing press. .
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