Abstract:

Objective With the rapid development of micro-electromechanical systems, torsion sensor has been widely used for effectively monitoring mechanical behavior in complex environments. The current torsion sensor is not satisfactory because of its rigidity, structural complexity and high price. Therefore, in order to meet different requirements and further expand its application space, a capacitive torsion sensor based on electrospinning fiber core-spun yarn (EFCY) with excellent flexibility and transduction properties was proposed.

Method A four-needle water bath electrospinning method was used to prepare the EFCY with silver coated nylon (SCN) as the core, and polyacrylonitrile (PAN) electrospinning fiber as the sheath. The EFCY was prepared using the following electrospinning conditions: the mass fraction of the spinning solution was 12%, the spinning rate was 0.36 mL/h, the voltage was 18 kV, the drawing distance was 100 mm, the winding rate was 33 cm/min, and 4 needles were arranged in a straight line above the core yarn. The performance of the EFCY was analyzed, and a capacitive torsion sensors was constructed by using two EFCYs (Sensor 1# with the initial distance between the two EFCYs was 0 mm and Sensor 2# with the initial distance between the two EFCYs was 4 mm) with the SCN as electrode and. The effects of the initial distance and twisting speed on the capacitance of the sensors were discussed, and its repeatability was tested.

Results The coating layer of EFCY was complete in structure and uniform in thickness (about 21.4 μm). The average diameter of the electrospinning PAN fibers was about 249.60 nm. Compared with SCN, the mechanical properties of EFCY were improved to some extent. The torsion sensor shows good sensing performance. When the initial distance between the two EFCYs in the sensor increases from 0 mm (Sensor 1#) to 4 mm (Sensor 2#), the initial capacitance of the sensor decreases from 7.28 pF to 2.63 pF. With the increase of twist, the capacitance of the two sensors showed a trend of gradual increase. When the twist of the two sensors exceeds 4 twist/cm, the capacitance changes tend to be consistent. When the twist reached 13-14 twist/cm, the electrospinning fiber coating layer would be destroyed under the action of extrusion pressure and friction, thus damaging the sensor structure and making it ineffective. In the range of 60-160 r/min, continuous twist-untwist-reverse twist-untwist cycle tests were carried out on the sensor at 6 different speeds. The results showed that the maximum value of C_p/C₀(real-time capacitance/initial capacitance) increased significantly after 2-3 cycles at the speed of 60 r/min, showing initial instability. Subsequently, under different speeds, the maximum value of C_p/C₀ is basically stable at about 3.4, which is almost not affected by the increase of speed. A cycle test of about 7 000 s at a speed of 160 r/min and a maximum twist of 5 twist /cm showed that the sensor maintained high sensing stability after the first few cycles.

Conclusion EFCY with SCN as core yarn and PAN electrospinning fiber as coating layer was prepared by four-needle water bath electrospinning method, and the capacitive torsion sensor was constructed based on EFCYs. The properties of yarn are analyzed, and the sensing principle and properties of torsion sensor are discussed. The results show that the surface of SCN core yarn is covered by electrospinning fiber coating layer completely and uniformly, which can provide an ideal dielectric layer for the sensor. The capacitance value of torsion sensor increases with the increase of twist, and the limit twist can reach about 13 twist /cm. Under different test conditions, the sensor shows excellent repeatability. The sensor has an excellent application prospect in the field of flexible electronics for torsion monitoring.

Key words: electrospinning, electrospinning fiber cored yarn, capacitance, torsion sensor, sensing performance