To realize PLC, encoder and inverter synchronous control method - Database & Sql Blog Articles

Programmable Package SG-8018CA(SG7050C) 0.67M~170M

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This article introduces a synchronization control method using PLC, encoder, and frequency inverter. The system uses RS485 communication to connect the inverter and the programmable controller, allowing the PLC to adjust the inverter's frequency based on the speed measured by the encoder. A touch screen is used to set working parameters, providing an intuitive interface for operators.

1. Working Principle of the Device

The equipment is designed for post-printing processing, particularly for automatic lamination. Its main function is to apply a plastic film onto the surface of printed sheets, enhancing the visual appeal and protecting the printed graphics. Examples include packaging labels for salad oil or book covers.

The process starts with feeding the printed sheets straight into the laminating roller. The pressure from the rollers bonds the paper to the film. Finally, the paper is cut along the film's edge, though the exact cutting details are not described here.

2. Main Technical Challenges

Since each sheet is fed one at a time to the drum, the film must wrap around the drum’s surface without any gaps. In simple terms, this means attaching each sheet end-to-end on a roll of film. The paper is conveyed directly to the drum, while the film is unwound and pressed against the paper’s surface.

To ensure proper alignment, two variable-frequency motors drive the linear paper feeding mechanism and the drum separately. Their surface speeds must match precisely to avoid gaps between the sheets. The control accuracy must be less than 1 mm, and the paper length can be adjusted as needed. For example, one batch may require 10,000 sheets of the same length, while another batch might use a different size. The drum’s diameter remains constant throughout.

Two rotary encoders are installed to monitor the linear speed of the paper feeding mechanism and the drum. The PLC adjusts the inverter’s output frequency based on these measurements to maintain consistent speeds. During calculations, pi is involved, so data must be rounded. To improve accuracy, the data is expanded by 1000 or 10000 times during calculations.

The PLC’s dividend has limitations to prevent overflow. A 1000-pulse-per-revolution (p/r) encoder is used to divide the paper length into 1000 parts, making it easier to handle and expand the data by 1000 times. These design choices were made during the development phase, and further detailed calculations are omitted for clarity.

The system continuously compares the calculated data from both mechanisms. Based on the comparison, the inverter’s frequency is adjusted up or down by 0.01 Hz every 20 milliseconds to maintain synchronized speeds.

3. System Components

The system includes two encoders that feed high-speed counting signals into the PLC. Two inverters communicate with the PLC via the RS485 port, forming a closed-loop control system. While the specific wiring details are not provided here, they follow the Modbus protocol and are outlined in the inverter’s manual. Further technical details will be shared in future articles.