Effect Of Wrapping Density On Electromechanical Performance Of Wrapped Yarn Strain Sensor And Optimization

With the continuous development and innovation of science and technology,yarn-shaped strain sensors with good embeddability,flexible wearability,and harmlessness to the skin have received widespread attention.Yarn-shaped strain sensors mainly include three types,monofilament strain sensors,yarn strain sensors and rope strain sensors.Among them,the yarn strain sensor of the wrapping structure has become one of the yarn strain sensors concerned because of its large extensibility,flexibility,controllability,and high sensitivity.The wrapping structure yarn sensor can be made of wrapped yarn through conductive treatment or spinning with conductive yarn.Its sensing performance depends not only on conductive sensitive materials,but also on the structure of the wrapped yarn.Most of the existing researches use the former method,but only verify the feasibility of the wrapped yarn as the sensor sensitive material substrate,and have not explored the specific impact of the wrapping structure on the sensor;Individual studies have noticed that the structure of the wrapped yarn affects the sensor performance.However,the relationship between the process parameters of the wrapped yarn structure and the sensor performance has not been thoroughly analyzed.Obviously,no matter which method is used to prepare the wrapped yarn strain sensor,clarifying the relationship between the process parameters of the wrapped yarn structure and the sensing performance is of great significance to promote the development and application of such sensors.In view of the insufficient research on the relationship between the structure and process parameters of the wrapped yarn strain sensor and the sensing performance,this topic mainly studies the influence of the wrapping density and structure of the wrapped yarn on the sensing performance of the strain sensor.The instability of the sensing performance caused by the intrinsic mechanical properties of the yarn is discussed.An optimization method to improve the performance of the wrapped yarn strain sensor is discussed.The specific research content is as follows:(1)The effects of different wrapping density and wrapping structure on the electromechanical properties of the wrapped yarn were found,and the wrapping density had a greater impact on repeatability,sensitivity,and zero drift,and had a smaller effect on threshold and response time,and the single-layer and the double-layer wrapping structure affects sensitivity and repeatability.In response to the requirements of the core yarn and outer yarn of the wrapped yarn strain sensor,two different core yarns and one outer yarn were tested on the sample wrapping machine,and the 6 different wrapping density of 20-45 twist/cm were selected for wrapped yarns,then those were subsequently polymerized in situ by pyrrole to prepare a resistive strain sensor.At the same time,a double-layer wrapped yarn was prepared with the same wrapping density,and a strain sensor was formed by in-situ polymerization of pyrrole.For in-situ polymerized conductive wrapped yarns,the tensile fracture properties and strain-resistance properties of the wrapped yarns after polymerization were tested.The study found that the conductive treatment reduces the breaking strength and elongation of the wrapped yarn to a certain extent,but does not affect the use of the sensor.By observing the resistance response of the in situ polymerized conductive wrapped yarn under different strain conditions,the sensor performance indicators such as sensitivity,repeatability,and zero drift are calculated,and the influence of wrapping density of the wrapped yarn on the electromechanical performance of the sensor is discussed.The results show that the sensitivity of both single-layer and double-layer yarns gradually increases with the increase of the wrapping density,and the wrapping density is 45 twists/cm when the maximum tensile strain is 30%.The sensitivity of single-layer wrapped yarn can reach 3.04,and the sensitivity of 45 twist/cm double-layer wrapped yarn can reach 4.23.The sensitivity of double-layer wrapped yarn is significantly higher than that of single-layer wrapped yarn.As for the sensor threshold,a method of gradually reducing the strain was used to sequentially collect the resistance of the wrapped yarn at a tensile speed of 1 mm/s and a strain of 5%,2%,1%,0.5%,0.2% and 0.1%.The results show that the single-layer and the resistance change of double-layer wrapped yarn is most obvious at 0.5% strain.For the repeatability of the sensor,the repeatability was calculated with 10 stretching cycles and the repeated stability error was calculated with 100 stretching cycles.The results show that with the increase of the wrapping density,the repeatability of the single-layer wrapped yarn gradually increases,the repeatability error is 5.9% at 45 twists/cm.The repeat stability error decreases first and then increases,and it is the smallest at 35 twists/cm.The repeatability error of the double-layer wrapped yarn decreases first and then increases,and it is the smallest at 30 twists/cm,which is 2.04%,but the repeat stability error shows a gradual decrease.Under the same wrapping density,the repeatability error of the double-layer wrapped yarn is smaller than that of the single layer,but the repeat stability error is larger than that of the single-layer.Secondly,in order to characterize the response time of the sensor,by measuring the resistance transient transition time of the sensor under high speed and low tensile strain,the experimental results show that with the increase of the wrapping density,the difference in response time of the double-wrapped yarn is small,both are about 120 ms.Finally,the zero drift of the sensor was evaluated.By characterizing the relative change of the resistance of the double-wrapped yarn under 30% tensile strain,the results show that as the wrapping density increases,the zero-point drift increases first and then decreases.It reached the maximum at 30 twists/cm,15.94%.(2)Aiming at the problem of insufficient repeat stability of single-layer and double-layer wrapped yarns in(1),the effects of pre-stretched conductive polymerization treatment and PDMS setting on the sensing performance of double-layer wrapped yarn sensors were found.Both the stretch polymerization treatment and the PDMS setting treatment can significantly enhance the repeat stability,but will not significantly change the response time and the amount of zero drift.In view of the foregoing research,it is found that the conductive layer is easily damaged during the stretching process.This experiment proposes to apply a pre-stretched polymer conductive treatment.Considering that the human body's monitoring sensor generally has a tensile deformation of about 20%,30% pre-stretching was used for in-situ polymerization of pyrrole,and a tensile cycle test and apparent morphology observation were performed.For the repeated stability of the sensor,the maximum tensile strain is 30% and the characterization is performed after 100 cycles.The results show that the pre-stretched polymer conductive treatment significantly improves the repeated stability of the sensor,the repeated stability error appears as the wrapping density increases.The trend of increasing after decreasing is the smallest at 35 twists/cm,which is 13.3%;There is no significant difference in the sensor response time between different wrapping density,and the change in the amount of zero drift compared to unstretched polymerization is not obvious.However,with the increase of the wrapping density,the amount of zero drift increases first and then decreases.The PDMS setting treatment is used to further improve the sensing performance.The in-situ polymerized conductive wrapped yarn passed a coater to obtain a uniform and good surface coating,and was subjected to a tensile cycle test and an apparent morphological observation.After the coated yarn was stretched for 100 times,the sensor repeated stability was characterized.The results showed that the PDMS setting shape significantly improved the repeated stability of the wrapped yarn sensor,and the repeated stability error decreased as the wrapping density increased.After the increase,it is the smallest at 30 twists/cm,which is 12.8%,and the repeatability is the best.Relatively speaking,the PDMS process does not change the response time and zero drift of the sensor.(3)Evaluate the feasibility of applying conductive wrapped yarns to monitor human body posture.The results show that the optimally selected double-layered wrapped yarn sensors have accurate dynamic responses to different joints and different angles of movement,and have real-time monitoring function.The pre-stretched 35 twist/cm double-layer wrapped yarn was selected and adhered to the forefinger and knee joint.By controlling the bending angle of the limb,the resistance response was observed.The results show that the sensor can respond to the movement angle in real time in response to changes in the bending angle of different parts,and has initially achieved the ability to monitor in body.In summary,this topic studies the influence of the wrapping density and structure on the sensing performance of the wrapped yarn sensor,and further discusses how to improve the sensing performance of the wrapped yarn sensor through pre-stretch polymerization and PDMS setting,clarifies the influence of wrapping density on sensing repeatability,sensitivity,response time,and zero drift,and identifies measures to improve sensing performance,namely pre-stretch polymerization and PDMS setting to improve sensing repeatability and poor stability.These research conclusions lay the foundation for the subsequent design and development and application of wrapping structure sensors,and also provide process guidance for the development of other textile electronic devices using wrapped yarns.