1, using PLC and frequency converter to realize speed synchronization control.
The main function of the film blow-molding printing unit is to extrude and blow-mold the plastic film, and then print the film with a gravure press. The printing process can adopt single-sided monochrome, single-sided multicolor, double-sided monochrome or double-sided multicolor according to different requirements. In the whole unit, it is necessary to control the speed of several motors, such as extrusion main drive motor, film stretching and traction motor, printing motor and finished product winding motor. There is a certain relationship between the speed of the motor. For example, the speed of the main extrusion motor is determined by the production requirements, but after the speed is determined, the corresponding traction speed is also determined according to the film thickness, so there is a certain relationship between extrusion speed and traction speed; At the same time, multiple groups of printing rollers must be synchronized, and the speed of printing motor and traction motor must be synchronized, otherwise the film quality, printing effect and production continuity will be affected; The speed of the winding motor is limited by the printing speed and changed accordingly to ensure that the printed film can be wound with constant tension.
In the transmission system of the above-mentioned unit, the synchronous transmission of multiple groups of printing cots can be connected by rigid mechanical shafts, and the whole printing cots are driven by a motor, thus ensuring their synchronization. The speed of the printing motor must be synchronized with the speed of the traction motor, otherwise the film between the two processes will be too tight or too loose, which will affect the printing quality and the continuity of production. However, the printing machine is far from the traction device, so it is impossible to adopt the method of mechanical rigid connection. In order to realize the synchronous control of traction and printing, the traction motor and printing motor are adjusted by frequency converters respectively, and then the two frequency converters are directly controlled by PLC.
The traction motor and printing motor adopt variable frequency speed regulation, and their control block diagram is shown in figure 1. In this closed-loop control, aiming at the speed of the traction roller, the speed of the printing roller is adjusted by the frequency converter of the printing motor to track the speed of the traction roller. Using rotary encoder 1 and rotary encoder 2, the pulse signals of the above two motors (encoder position is shown in Figure 3) are respectively collected and sent to the high-speed counting port of PLC or IR00000~IR00003 connected to CPU. Taking these two speed signal data as input, the PI control algorithm is carried out, and the operation result is sent to the analog module of PLC as the output signal to control the frequency converter of the printing motor. This can ensure that the printing speed changes with the change of traction speed, so as to synchronize the two speeds.
PI control algorithm is used to adjust the speed, and the program design block diagram is shown in Figure 2. The pulse signal collected by the encoder in the figure is converted into the speed data of the motor, which is stored in a DM area as the Y value in the operation after the upper and lower limits are processed. The calculated P value is sent to the analog output channel, and after the upper and lower limits are calibrated, it is converted into an acceptable current or voltage signal for the inverter to control the inverter of the printing motor.
In order to ensure that the film maintains constant tension during stretching and printing, a set of floating roller adjusting device is added between these two devices, and its structure is shown in Figure 3.
The above floating roller adjusting device is also to reduce external interference caused by factors such as power system fluctuation. However, the speed difference caused by fluctuation will make the positions of the two floating rollers rise too high or fall too low after a period of time. Therefore, when designing PI control algorithm, the influence of these interference factors is considered, and the accumulated error is adjusted by integrating link I, so that the traction roller and the printing roller are synchronously controlled with high synchronization accuracy, thus ensuring the stability of this control system.
2. Using PLC and frequency converter to realize stable speed ratio control.
In polypropylene (PP) spinning equipment, pre-stretched fibers need to be thermally stretched. Thermal stretching is carried out between two heating rollers and a pre-stretching roller, and each roller is driven by a motor respectively. The original motor speed regulation is driven by DC motor and adjusted by potentiometer. Speed fluctuation often occurs in production, the speed ratio is unstable, and the phenomenon of "winding" is easy to occur during processing, and the finished fiber appears "wool" and "hard head", which affects the quality of the finished chemical fiber. In the spinning process, the speed of the pre-stretching roller changes due to the technological requirements of linear orientation of PP raw materials and molecules, so it should be easy to adjust. After the drawing ratio is determined, the speed of the hot drawing roller should follow and change rapidly. Using programmable controller (PLC) and frequency converter control, the speed ratio between two hot stretching rolls and pre-stretching rolls can be well stabilized.
Fig. 4 is a schematic structural diagram of hot drawing in PP spinning machine. The pre-stretching rod and the two hot stretching rollers are driven by three motors respectively, and the speed of the two hot stretching rollers is the same, so the chemical fiber is not stretched, which plays a role in stabilizing the fiber performance; There is a certain speed ratio between hot stretching series and pre-stretching rolling. When one speed changes, the other needs to change according to the speed ratio. The pulse signal collected by rotary encoder is sent to the high-speed counting port of PLC or IR00000~IR00003 connected to CPU, and converted into speed data, which is used as the input parameter of proportional-integral (PI) control algorithm. The calculated results are used as output parameters, and after being calibrated by the analog output module of PLC, the speed-regulating frequency converter for controlling each motor is composed of current or voltage. In the control algorithm, the speed data of pre-stretching roller V 1 is multiplied by a certain speed ratio U (adjustable speed ratio) as the target value, so that the speed data V2 of hot stretching series can track the change of (V 1 u).
3. Concluding remarks
With the maturity of inverter technology and the expansion of its application range, programmable controller (PLC) can be used for control to meet the different requirements of speed control flexibility, accuracy and reliability of transmission system. The above two examples are examples of speed control by using PLC and frequency converter in actual production, both of which meet the expected requirements of synchronous or given speed ratio control.