Abstract:

Objective The existing research primarily focuses on real-time detection of the working status of a single species of actuator, resulting in poor generality and inability to ensure the accuracy of the terminal control process in the weft knitting machine. This study investigates the forced motion characteristics between knitting needles and cam during the knitting process, providing theoretical and experimental evidence for the real-time detection of the working status and faults in the weft knitting equipment from a vibration perspective.

Method Combining the actual weaving process of the weft knitting machine, which involves the knitting needle impacting the cam and undergoing forced motion along the cam track, a testing platform was designed to investigate the vibration response characteristics of the cam during the weaving process. PZT (piezoelectric ceramic) was attached to the surface of the piezoelectric cam, creating a 'cam + PZT' coupling system. Vibration characteristic curves of the coupling system were obtained at different rotational speeds and knitting techniques.

Results The vibration characteristics (amplitude, frequency) under different rotational speeds and knitting techniques, as well as their mathematical relationship with the rotational speed and needle selection method, yield the following three results for fault diagnosis and online detection of weft knitting equipment from a vibration perspective. Firstly, the moment when the knitting needle strikes the cam, the vibration signal experiences a rapid increase in amplitude, reaching its maximum value within a short timeframe (approximately 200 microseconds). The amplitude then gradually diminishes to zero. Importantly, the decay time period remains consistent and can be considered negligible compared to the needle selection cycle. This finding holds true irrespective of the impact velocity(Fig. 3). When the weft knitting machine is in the fully selected needle state, the needle selection frequency is equal to the vibration frequency detected by the "cam + PZT" coupling system. Conversely, when employing an n₁×n₂knitting technique, the needle selection frequency becomes n₁+n₂ times the vibration frequency detected by the "cam + PZT" coupling system. These results highlight the relationship between needle selection and the vibration response of the knitting apparatus(Fig. 4-8). The maximum value of the vibration signal detected by the "cam + PZT" coupling system exhibits a linear increase in relation to the rotational speed of the cylinder for different knitting techniques. Notably, this behavior remains independent of the specific knitting technique utilized, emphasizing the importance of rotational speed as a contributing factor to the vibration characteristics (Fig. 9 and Fig. 10 ). These results provide essential insights into the vibration-based real-time detection of working status and faults in weft knitting equipment. They pave the way for developing efficient fault diagnosis and online monitoring systems for weft knitting machinery. The findings contribute to the advancement of the weft knitting industry by improving the accuracy and reliability of detecting and addressing operational issues in real-time.

Conclusion The vibration attenuation characteristics of the "cam + PZT" coupling system can be utilized to detect faults during the weaving process for impact frequencies below 5 kHz. This implies the potential for implementing fault detection mechanisms based on the vibration decay pattern of the coupling system. Understanding the relationship between needle selection frequency and vibration frequency across different knitting techniques allows for the diagnosis of jacquard weaving faults. The consistent patterns observed in needle selection and vibration frequencies provide insights into specific needle selection methods. Additionally, the correlation between the maximum value of the vibration signal and the vibration speed enables real-time monitoring of rotational speed in the weaving process. This offers a means to assess the timeliness and stability of rotational speed in weft knitting machines. In summary, these conclusions highlight the practical applications of the results, including fault detection, jacquard weaving fault diagnosis, and online monitoring of rotational speed in weft knitting machines through vibration analysis.

Key words: weft knitting machine, cam, vibration response characteristic, fault diagnosis