Research On The Force Transfer Mechanism And Arm Stiffness In Soft Robots Designed For Gripping Using SMA

The technical approach of embedded shape memory alloy actuators (ESMAA) is presented herein. Two forms of shape memory alloy (SMA) actuators, that is, SMA wire and SMA spring were embedded in two different soft robots, namely:(1) a robot gripper for picking complex objects and (2) an octopus robot arm. They were designed to analyze the force distribution mechanism on the gripping fingers as well as the stiffness on the octopus arm. The robot gripper is capable of displaying humanoid actions and was designed basically to investigate the force transfer occurring on the fingers cast in form of rods. The second design is an octopus robot-arm used in analyzing the stiffness of an octopus on the arms when relaxed in its natural habitat or when gripping objects. The rods cast as human fingers and octopus arms were made from silicon elastomers using molds. The casting was done at room temperature and allowed to harden after which they became flexible and soft. They were thereafter, embedded with SMA actuators as artificial muscles. On the soft robot gripper fingers, the insertion of SMA actuators was done symmetrically to create the equilibrium needed to balance the force transfer that occurs during actuation for effective gripping.On the other hand, the SMA actuator was inserted linearly in the octopus robot arm in line with the hydrostatic nature of the octopus and the longitudinal and transverse oblique muscle-array of the arms which aids muscular elongation and contraction. In testing the designs, the DC motor with a voltage gauge, was connected to make for duty ratio adjustment of the current and displayed controllable actuations. The soft robot gripper gripped and lifted all kinds of objects with complex shapes. Tests showed that the gripping force on the soft robot fingers increased with a corresponding increase in current and voltage. Investigations also revealed a slight difference between the embedded and unembedded SMA actuators, probably due to variations in interface stress in the host medium. Similarly, comparative observations from the robot gripper showed that the two forms of SMA actuators on the designed octopus arms displayed deformations short of what is required of an octopus arm. This is because the gripper, the arm with embedded spring, and the arm with embedded wire produced very low actuation and return force, and were quick to reach rupture points at a given voltage. Nonetheless, it was observed that the SMA springs gave better deformations than the SMA wires in all the scenarios tested. Thus, in order to go on with the search for further solution, the voltage supply to the SMA wire was withdrawn and the wire connected to a driver. The driver then received the voltage and transmitted it to the SMA wire in form of torque. This, no doubt, produced excellent deformations semblance to an octopus arm. However, the SMA wire was chosen in preference for SMA springs or other tested materials because of its high tensile strength, elasticity, flexibility and ability to bend around obstacles when compared to SMA springs, steel wires, copper wires or ordinary rope. The modulus of elasticity deduced from the robot arms gave small values of Young’s moduli, an indication that the stiffness on the robot arms are small. This may also be used to suggest that the stiffness on the arm of a live octopus is small. The two kinds of structures designed herein are simple to manipulate, with no negative impact on the environment. The silicone elastomer used as flesh, are chemically-inert and are now being used as medical grade materials in hospitals.