Research On Design And Preparation Of Stiffness-tunable Composites And Application In Soft Actuators

Many organisms in nature can change their stiffness to adapt to complex environmental changes,such as elephant trunks can grasp heavy objects,octopuses create virtual joints in the process of movement.Inspired by these biological systems,researchers have begun to experiment with applying stiffness-tunable systems to soft actuator technology.They are expected to alternate reversibly between a rigid state that is subjected to high loads and a flexible state that can deform in an unstructured environment.For existing materials and systems with stiffness tunability,thermally responsive materials have a wide range of stiffness variation,are universal and easy to control.Therefore,in this thesis,we design and prepare a stiffness-tunable composite material based on thermally responsive materials and investigate its properties in practical applications.The primary work and conclusions are as follows.(1)The successful preparation of variable stiffness composites is achieved by using physical blending method with thermosetting epoxy resin as matrix material and low melting point alloy as filler phase.Continuous change of material stiffness by changing the temperature from 20? to 70?.Changing the ratio between different materials to obtain composites with different gradient alloy content.SEM analysis,thermogravimetric analysis,differential scanning calorimetric analysis,and mechanical analysis are used to characterize the internal structure,thermal properties,and mechanical properties of the composites.The stiffness of the prepared composites is predicted using a class of composite mixture models to verify the accuracy of experiments and analyze.(2)Based on the previous research,the structures are designed.The original pneumatic actuator preparation process is improved and appropriate variable stiffness composites are selected to produce soft pneumatic actuators with stiffness adjustable capability.The deformation of variable stiffness pneumatic actuator at different temperatures and air pressures is analyzed using finite element simulation software.The simulation results show that the bending angle and tip displacement of stiffness-tunable pneumatic actuator exhibit an exponential-like relationship with temperature.At low temperatures,the stiffness-tunable layer is stiffer and takes up most of force,and the pressure acting on the cavity surface is small,resulting in high stresses in the variable stiffness layer and low stresses on the inner cavity surface and sidewall surfaces.As the temperature rises,the stiffness of stiffness-tunable layer decreases sharply,the force assumed decreases,and the pressure acting on the inner cavity suddenly increases,resulting in a decrease in the stress in the stiffness-tunable layer and an increase in the stress on the inner cavity surface and the sidewall surface.(3)Building a bench to test and analyze the performance of stiffness-tunable pneumatic actuators and traditional silicone pneumatic actuators at different temperatures(20?,70?)to compare the performance differences between the three actuators.The comparison shows that the stiffness-tunable actuator is able to achieve a large deformation at 70?,which the bending angle can reach 153°.The use of stiffness-tunable material makes the initial stiffness of pneumatic actuator much higher than the initial stiffness of conventional silicone actuator,proving that the use of stiffness-tunable composite can greatly improve the stiffness of flexible actuator and thus substantially increase the output force of the flexible actuator.A three-finger gripper is designed and fabricated based on stiffness-tunable pneumatic actuator,and a series of different shapes and weights of objects are tested for gripping ability.The results show that the maximum gripping mass of three-finger gripper is about 800 g,which is 8 times the weight of the actuator itself.Whether it is a spherical ping pong ball,a cylindrical reagent bottle,or an irregularly shaped heavy object such as a mouse or wrench,the gripper can be wrapped around the surface and successfully lifted.This further demonstrates that stiffness-tunable materials can greatly increase output force without sacrificing the flexibility of pneumatic actuator.(4)The variable stiffness materials are introduced into the wheel structure design and two types of variable stiffness wheels are successfully prepared.The surface temperature of wheel increases gradually under the action of a constant current of 1.5A.After powering on for 10 min,the temperature up to 70?.Then the power is cut off and the temperature slowly decreased.Relevant performance tests are conducted for stiffness-tunable wheels to test their radial deformations and grounding areas.Constructing a friction experimental bench to test the friction of wheel during motion under different conditions.The results show that composites can enhance the wheels' performance,change the degree of wheel deformation and improve the wheel motion.