Operating principle Schematic diagram of the motor structure The main part is a room temperature vulcanized two-component silicone rubber rod. The rod body has four parallel holes, and the middle diameter is slightly larger to isolate the thermal coupling between the SMA springs, and the connecting wires are placed at the same time. The other three smaller diameter holes are evenly distributed along the circumference for placing the SMA spring. The SMA spring is slightly smaller in diameter than the placement hole, and is freely movable in the hole. The two ends are fixed on the fixing piece, and the fixing piece is also mechanically fixed on both ends of the rod.
The spring is heated by the current. When the temperature of the spring rises to the phase transition temperature point As, a phase change from martensite to austenite occurs, and the spring contracts. Since the spring axis does not coincide with the rod axis, the rod bends. Conversely, if the spring temperature drops to the martensite phase transition temperature point Ms, the spring becomes soft. Under the action of the rod spring force, the spring is stretched, the motor returns to the default state, and the spring length change is the power of the motor. . When two springs are energized and heated, the bending deformation in the space of 120° can be realized, and the motor has the space running capability.
The elastic rod deformation theoretical length L, the diameter of 2h rods correspond to different bending moments, as shown. The deformed shape of the rod is approximated as an arc. If l1, l2, θ1, θ2, R1, and R2 represent the length of the spring in two states, the angle between the ends of the rod and the radius of curvature at the spring, the spring deformation and the rod bending The relationship between the angular changes can be derived as follows: 1l LRhR θ = + (1) 2l LRhR θ (2) Small changes in the length of the spring will result in a large bending of the rod.
The analysis rod assumes the following assumptions when stressing: 1 ignoring the bar shearing and twisting deformation; 2 ignoring the axial deformation of the rod; 3 the direction of the distributed load of the spring and the rod is perpendicular to the spring; 4 the distributed load is evenly distributed along the length of the rod; The shape of the rod after bending is approximately an arc. For the force analysis diagram of the rod and the spring, P is the concentrated end concentrated load, L is the rod length, θ is the rod bending angle, Ï is the radius of curvature at the axis after the rod is bent, and ÏSMA is the radius of curvature at the spring axis after the rod is bent; Ρin is the radius of curvature at the center of the hole where the spring is placed after the rod is bent; q is the load density uniformly distributed along the length of the rod; qSMA is the load density uniformly distributed along the length of the spring; qin is the load density along the central axis of the spring. The force balance equation of the SMA spring in the y-axis direction is SMASMA0sincos(d)0YPqθ
The SMA spring is designed with a 30mm long SMA wire. If the recovery strain is 1%, the length change after recovery is only 0.3mm. In comparison, the spiral SMA spring with wire diameter 1mm, number of turns 30, diameter 8mm, maximum axial deformation Up to 50mm. It can be seen that the axial recovery of the SMA spring is large. Although the shear modulus is not constant for SMA filaments, the design can still draw on the traditional spring design principle. For Ti-NiSMA, the shear modulus changes from high temperature phase to low temperature phase by 300%.
The prototype was fabricated into a SMA wire wound into a spring shape for memory processing, and thus a processing mold was designed as shown. This mold limits the SMA wire to a spring-like placement in the furnace as follows: 1 The SMA wire is cut to a suitable length and wound into the positioning thread. 2 In a heating furnace, keep it at 600 ° C for 1 h. 3 and take it out and immerse it in cold water for rapid quenching.
The length of the silicone rod is 60mm, and the radius is determined by the deformation model. The principle is to make the bending deformation of the SMA spring as long as it is heated and contracted. At the same time, the rod can quickly return to the default linear state when the SMA spring is cooled. Finally, the rod radius is chosen to be 5mm.
The size of the center hole has a great influence on the temperature distribution of the rod. The larger the hole radius, the smaller the thermal coupling effect between the springs, but the worse the rod elasticity. Therefore, under the condition of ensuring the temperature coupling limitation of the SMA spring, take a small value as much as possible. If the distance between the center line of the SMA spring placement hole and the center through hole axis is set to 1 mm, the finite element analysis is performed on the steady temperature distribution of the rod when the single SMA spring is heated, showing that when the through hole radius is 1.5 mm, the heating spring is used. The temperature effect to the unheated spring is 30 ° C, which is less than the austenite transformation temperature As of the SMA spring. Therefore, the through hole radius is finally selected to be 1.5 mm.
Temperature influence when the center hole radius is 1.5mm 2=1.5mm In addition, the eccentricity d of the SMA spring placement hole axis is also a very important parameter. It can be seen that the larger the d, the greater the bending curvature of the motor, and the smaller the thermal coupling effect between the SMA springs. Therefore, under the premise of satisfying the mechanical strength of the silica gel rod, the maximum value is 3.5 mm.
The silica gel rod has good elasticity, and a two-component RTV515 was selected to make a prototype. The prototype is made as shown. Firstly, the mold is fixed, and three wires having a diameter slightly larger than the SMA spring are sequentially passed through the upper fixing piece, the circular sleeve, and the lower fixing piece, and then fixed on the bracket.
Then, the copper tube with the same diameter as the central through hole is fixed through the circular sleeve to ensure that the axes of the two are coincident. Finally, the liquid two-component rubber and the catalyst are uniformly mixed at a volume ratio of 10:1, and then injected into a circular sleeve by a spray gun to be solidified. After the silica gel is vulcanized, the wire and the copper tube are taken out to obtain a silica gel rod. The three springs are placed in the placement hole, and the two ends are fixed on the fixing piece, and the fixing piece is mechanically fixed on both ends of the elastic rod. The prototype specifications are shown in the table below.
This paper describes the structure of a compact flexible motor. Three SMA springs are eccentrically embedded in a two-component silicone rod that is vulcanized at room temperature. The spring and the wire are freely moved relative to each other except that the ends are connected by a fixing piece. The SMA spring is heated by a current pulse with adjustable duty cycle to control its contraction deformation, and the flexible space operation of the motor can be realized. This low-power, high-power-to-weight ratio, compact, flexible motor is expected to be practical in anthropomorphic robots and other micro-miniature robot systems. The future research direction is to further improve the material preparation process level and ensure the consistency of motor performance.
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