Abstract : This paper introduces a method for controlling a yarn tying machine and discusses the application and control instructions of PLC and frequency converter in the yarn tying machine. Keywords : Programmable Logic Controller, Frequency Converter, Yarn Tying Machine. In the control of a cable forming machine for optical fiber to optical cable, the yarn tying machine is an important component: it binds the stranded loose tubes and central reinforcing members into an envelope shape with yarn. Since the speed of the cable forming machine production line changes online according to production requirements, the yarn tying machine must respond promptly to this speed requirement by precisely adjusting the rotation speed of its own jet to ensure that the envelope-shaped yarn is not affected. At the same time, the stability of the yarn tying machine pitch also requires the yarn tying machine to maintain a constant jet speed during the yarn tying process; therefore, this places higher technical demands on the motor speed regulation and stable operation of the yarn tying machine. The following describes the use of PLC and frequency converter to implement yarn tying machine control. 1 Design Concept The yarn tying machine consists of a three-phase asynchronous motor, air brake, holding brake, yarn bundle shaft, jet, hub, solenoid valve, etc. Air brakes and solenoid valves are used to open or close the holding brake. When the holding brake is closed, a three-phase asynchronous motor drives the aircraft to rotate, and the yarn package on the yarn bundle shaft is wound around the loose tube and reinforcing center piece in the hub through one port of the aircraft. As long as the speed of the three-phase asynchronous motor is controlled in a timely and accurate manner, the speed of yarn release on the yarn bundle shaft can be adjusted, thereby realizing the yarn tying machine's response to the production line speed and stable yarn tying. 2. Implementation Method and Control Structure PLC has strong processing capabilities in sequential control, logic operation, numerical operation, and communication. Frequency converter has strong speed regulation performance for asynchronous motors and has very complete functions in motor thermal protection and alarm/notification. Therefore, we effectively combine PLC and frequency converter to achieve effective control of the yarn tying machine. [b][align=center]For details, please click: Application of PLC and Frequency Converter in Yarn Tying Machine[/align][/b]