In traditional transmission systems, maintaining a constant speed relationship between multiple actuators, including ensuring speed synchronization or a specific speed ratio, is often achieved using rigid mechanical transmission connections. However, if the mechanical transmission connections between the actuators are large or the distances between the actuators are significant, then a non-rigid connection transmission method with independent control must be considered. The following two examples illustrate control methods using PLCs and frequency converters to achieve speed synchronization and maintain a specific speed ratio between two motors.
1. Using PLC and frequency converter to achieve speed synchronization control
The main function of a film blow molding and printing unit is to process plastic films using extrusion blow molding, followed by printing on the films using a gravure printing machine. The printing process can employ single-sided monochrome, single-sided multi-color, double-sided monochrome, or double-sided multi-color printing methods depending on the requirements. Within the entire unit, the speeds of multiple motors need to be controlled, such as the extrusion main drive motor, the film stretching traction motor, the printing motor, and the finished product winding motor . The speeds of these motors are related. For example, the speed of the extrusion main motor is determined by production volume requirements, but once this speed is determined, the corresponding traction speed is also determined based on the film thickness; therefore, there is a definite relationship between the extrusion speed and the traction speed. Simultaneously, multiple printing rollers must be synchronized, and the speeds of the printing motor and the traction motor must also 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 is adjusted accordingly to ensure that the printed film can be wound with constant tension.
In the transmission system of the aforementioned unit, the synchronous drive of multiple printing rollers can be achieved using a rigid mechanical shaft connection. The entire printing roller drive is powered by a single motor, ensuring synchronization between them. The speed of the printing motor must be synchronized with the speed of the traction motor; otherwise, the film may become too tight or too loose between these two processes, affecting printing quality and production continuity. However, the printing unit and the traction device are far apart, making a rigid mechanical connection impractical. To achieve synchronous control between traction and printing, both the traction motor and the printing motor are speed-regulated using frequency converters, and a PLC directly controls both frequency converters .
The traction motor and printing motor employ variable frequency speed control, and their control block diagram is shown in Figure 1. In this closed-loop control, the speed of the traction roller is the target, and the printing motor's frequency converter adjusts the printing roller speed to track the traction roller speed. Rotary encoders 1 and 2 collect pulse signals from the two motors respectively (encoder positions are shown in Figure 3) and send them to the PLC 's high-speed counting port or connected to the CPU's IR00000~IR00003. Using these two speed signal data as inputs, a proportional-integral (PI) control algorithm is performed, and the result is sent as an output signal to the PLC's analog module to control the printing motor's frequency converter. This ensures that the printing speed tracks the changes in the traction speed, keeping the two speeds synchronized.
Speed regulation is achieved using a PI control algorithm, and the program design flowchart is shown in Figure 2. The pulse signal acquired from the encoder is converted into motor speed data, processed for upper and lower limits, and stored in a DM area as the y-value in the calculation. The calculated p-value is sent to the analog output channel, calibrated for upper and lower limits, and then converted into a current or voltage signal that the inverter can accept to control the inverter of the printing motor.
To ensure that the film maintains a constant tension between the traction and printing processes, a floating roller adjustment device is added between the two devices, the structure of which is shown in Figure 3.
The floating roller adjustment device mentioned above is also used to reduce external interference caused by factors such as power system fluctuations. However, the speed difference caused by fluctuations can, after a period of time, cause the two floating rollers to rise too high or fall too low. Therefore, when designing the PI control algorithm, the influence of these interference factors was considered. The integral element I is used to adjust the accumulated error, so that the traction roller and the printing roller can be synchronously controlled with high synchronization accuracy, thereby ensuring the stability of this control system.
2. Achieving stable speed ratio control using PLC and frequency converter.
In polypropylene (PP) spinning equipment, pre-stretched fibers require hot stretching. Hot stretching occurs between two heated rollers and a pre-stretching roller, each driven by a separate motor. The original motor speed control used DC motors, adjusted by potentiometers. During production, speed fluctuations frequently occurred, the speed ratio was unstable, and "roller entanglement" easily occurred during processing, resulting in "fuzzy" and "hard-end" fibers in the finished product, affecting the quality of the chemical fiber. During spinning, the speed of the pre-stretching roller is easily adjustable due to variations in PP raw materials, molecular linear orientation, and other process requirements. Once the stretching ratio is determined, the speed of the hot stretching roller must be rapidly monitored and adjusted. Using a programmable logic controller (PLC) and frequency converter for control can effectively stabilize the speed ratio between the two hot stretching rollers and the pre-stretching roller.
Figure 4 is a schematic diagram of the thermal stretching mechanism in a PP spinning machine. The pre-stretching roller and two thermal stretching rollers are driven by three motors. The two thermal stretching rollers operate at the same speed, resulting in no stretching of the chemical fiber and stabilizing its properties. There is a specific speed ratio between the thermal stretching roller and the pre-stretching roller; when the speed of one roller changes, the other must also adjust accordingly. Pulse signals collected by a rotary encoder are sent to the high-speed counting port of the PLC or connected to the CPU's IR00000~IR00003 pins. After being converted into speed data, this data serves as the input parameter for the proportional-integral (PI) control algorithm. The calculation result is used as the output parameter. After calibration by the PLC's analog output module, it is used as the current or voltage to control the speed of each motor via a frequency converter. In the control algorithm, the pre-stretching roller speed data V1 is multiplied by a certain speed ratio u (the speed ratio is adjustable) as the target value, causing the thermal stretching roller speed data V2 to track the change in (V1·u).
3. Conclusion
With the maturity of frequency converter technology and the expansion of its application scope, programmable logic controllers (PLCs) can be used to control them, thereby adapting to different requirements in drive systems for speed control flexibility, accuracy, and reliability. The two examples above are both instances of applying PLCs and frequency converters for speed control in actual production, and both have achieved the expected synchronous or given speed ratio control requirements quite well.
