Preparation And Performance Evaluation Of Resistive Braided Rope Flexible Strain Sensor

With the rapid development of wearable devices,the key component of flexible sensors had been paid more and more attention.The key to the development of flexible sensors was to meet the needs of human consumption and exercise.With the textile process' s natural advantages,the onedimensional flexible strain sensor produced by the textile process could meet the needs of skinfriendly and various secondary processing,and textile had become one of the important platforms for developing flexible sensors.At present,typical one-dimensional flexible strain sensors included yarn sensor and rope sensor.Among them,the preparation of yarn sensor had been widely studied.Generally,the conductive yarn with a sheath-core structure was prepared by the wrapping method,or the non-conductive yarn was directly conductively treated.The preparation process of this type of yarn sensor was relatively simple,but the strain sensing performance of this type of sensor,such as repeatability and stability,largely depended on the yarn preparation process and sensitive material preparation method.In contrast,rope strain sensors had attracted attention in recent years,and the possibility of using braided ropes as strain sensors had been initially confirmed: the braided process was integrally formed by cross molding,and the stability was high,and the cross circuit formed by it was beneficial to the stretching process.The change in contact resistance formed different conductive paths.However,little attention had been paid to the relationship between the processing technology of the braided rope and the sensing performance.In view of the current research status of rope-shaped one-dimensional flexible strain sensors,this subject prepared a one-dimensional flexible strain sensor based on a braided rope structure.Polyester multifilament was wrapped on a highly elastic spandex core yarn using a weaving process to form a core structure.The braided rope was then conductively treated by polypyrrole in-situ polymerization to obtain a braided rope flexible strain sensor.In this topic,the main forming structural parameters of the braided flexible strain sensor—the number of braids and braiding angles—affected the sensing performance of the sensor.The main research contents of the subject were as follows:(1)Aiming at the influence of the braided rope forming structure parameter—the number of braided ropes on the sensing performance of the flexible strain sensor of braided ropes,the electromechanical performance of the sensor under different braided ropes was evaluated.The change of the number had a significant impact on the sensing performance of the braided rope sensor.The stretchable sheath core rope was braided with 4,6 and 8 yarns,and the sensor was prepared by in-situ polymerization of polypyrrole.The optical morphology of the flexible strain sensor of the braided rope after polymerization was observed with an optical microscope to confirm.The film was evenly and well coated on the yarn surface,and then the sensor performance analysis test was performed.In the time-dependent test,time constant analysis was performed for creep and stress relaxation within 5 minutes,and the results showed that the response time constant of the 8 yarn braided rope sensor was the smallest,because the number of braids was large.And the sheath-core structure was tightly wrapped.In the sensitivity test,the flexible strain sensors of three types of braided ropes were subjected to 50% strain elongation.The tests showed that the more the number of braided yarns,the better the sensor's sensitivity.In comparison,the sensitivity of GF of the 8 knitting yarns was the best,which can reach 0.045,but the response range of the strain resistance was very small that only 16%,which couldnot meet the requirements of human motion amplitude.This was mainly because when the number of knitting is 8,the initial modulus of the yarn is large,and the resistance is liable to abruptly change during stretching.On the other hand,the hysteresis of the sensor increases with the number of braided ropes under a tensile cycle of 50% strain.When the number of braids is eight,the hysteresis was 18%.The reason was that the 8 braided ropes had high rigidity,and the softness of the yarn was reduced,resulting in a decrease in the tensile recovery rate of the yarn and an increase in the difference in the stretching cycle.In addition,in the repeatability test,when the cyclic tensile strain was 50%,the greater the number of weaving roots,the better the repeatability.According to SEM observation,the main reason for the poor repeatability of the 4 was that the polypyrrole was cracked during stretching.Comprehensively comparing the strain response range,sensitivity,and repeatability,this study selected a braided flexible strain sensor with six braids to continue its sensing performance research.(2)Aiming at another forming parameter of the braided rope,the influence of the braiding angle on the sensing performance of the flexible strain sensor of the braided rope was compared.The degree of influence of the two formation parameters on the braided flexible strain sensor was compared.Compared with the two structural parameters,the number of braids had a more significant effect on the sensing performance of braided rope sensors.On the basis of six braided flexible strain sensors with braided ropes,braided conductive ropes with three specifications of 20 °,30 °,and 40 ° were prepared.In the sensitivity test,the sensitivity of the three types of braided conductive ropes was not much different.A significant analysis was made on the impact of the braid angle and the number of braids on the sensitivity,and it was found that the number of braids had a greater impact on the sensitivity.Moreover,as the weaving angle increased,the hysteresis decreased.Comparing and analyzing the effects of the weaving angle and the number of weaving roots on the hysteresis,we found that the number of weaving roots had a greater effect on the hysteresis of the sensor.Further,the repeatability of flexible strain sensors for braided ropes under different stretching rates and different stretching amplitudes was investigated.The strain-resistance test with a strain rate of 10 mm / min,20 mm / min,50 mm / min,and 100 mm / min was performed at 50% of the maximum strain.The results showed that the resistance change trend of the flexible strain sensor of the braided rope did not show a significant change with the strain rate.This showed that the braided flexible strain sensor could meet the stability test requirements at different speeds.On the other hand,at a strain rate of 50 mm / min,the maximum strain was 5%,10%,20%,30%,and 50%.Respectively,the change in resistance with time was recorded,and the extension of the resistance with strain was observed.The release cycle changed regularly and the response was highly repeatable.Then,the M-shaped peak of the strain-resistance curve during the stretching process was analyzed.As the strain of the braided rope increased,the resistance first increased and then decreased.The main reason for this trend was that during the stretching process,the contact resistance was caused by surface contact to point contact,and then to surface contact to formed a series-parallel circuit.(3)Evaluated the effect of coating setting on the sensing performance of braided rope strain sensors and its application in human limb monitoring.The results showed that the coating setting had a certain improvement on the sensing performance of flexible rope braided sensors,and proved that it could meet the need of human limb monitoring.After the in-situ polymerization,the conductive rope was subjected to PDMS coating shaping treatment to evaluate the resistance environment stability and apparent morphology of the sensor after the coating.The PDMS coating had a good coating effect on the surface of the braided conductive rope.At the same time,the resistance change of the PDMS coating on the polypyrrole conductive layer of the braided rope was compared with the resistance change after 30 days.Further testing and comparing the sensing performance of the braided rope sensor before and after the coating,compared with the electromechanical performance before the coating,the sensitivity and repeatability of the sensor had not improved significantly,but the hysteresis had become significantly smaller,only 1.11%,which proved that PDMS was good for braiding.The lag of the rope sensor had been significantly improved.On this basis,the feasibility of the braided rope flexible strain sensor to monitor the state of human joint motion was further tested.The braided rope flexible strain sensor with the best braiding number of 6 was selected for full finger bending motion test at different rates.The results showed that under different exercise frequencies,the resistance of the sensor changed with the exercise frequency in real time,and could clearly distinguish the exercise frequency,which showed that the braided rope flexible strain sensor could meet the limb movement state monitoring.To sum up,for the flexible strain sensor of braided rope,by studying the influence of braided structure parameters-braided number and side braided angle on the sensing performance,the relationship between braiding process and structure and sensing performance was established.The mechanism of change in tensile resistance was clarified,and the improvement of the sensing performance of the coating sizing was confirmed,and the application effect of the sensor in the detection of human limb movement status was verified.In addition,these research results have application value for other flexible wearable devices based on braided rope structure.