Abstract
Shape memory alloys (SMAs) are smart materials widely used as actuators for their high power to weight ratio despite their well-known low energy efficiency and limited mechanical bandwidth. For robotic applications, SMAs exhibit limitations due to high power consumption and limited stroke, varying from 4% to 7% of the total length. Hysteresis, during the contraction and extension cycle, requires a complex control algorithm. On the positive side, the small size and low weight are eminently suited for the design of mini actuators for robotic platforms. This paper describes the design and construction of a light weight and low power consuming mini rotary actuator with on-board contact-less position and force sensors. The design is specifically intended to reduce (i) energy consumption, (ii) dimensions of the sensory system, and (iii) provide a simple control without any need for SMA characterisation. The torque produced is controlled by on-board force sensors. Experiments were performed to investigate the energy consumption and performance (step and sinusoidal angle profiles with a frequency varying from 0.5 to 10 Hz and maximal amplitude of ). We describe a transient capacitor effect related to the SMA wires during the sinusoidal profile when the active SMA wire is powered and the antagonist one switched-off, resulting in a transient current time varying from 300 to 400 ms.
Highlights
The design of a mini robotic system with several degrees of freedom (DOFs), is challenging mainly for the choice of the mini actuators
Rotary actuators with Shape memory alloy (SMA) wires in antagonistic configuration can be designed by using the following configurations: (i) one SMA wire to actively rotate the actuator in one direction and one spring material to passively rotate it in the opposite direction by stretching the SMA wire; (ii) two SMA wires to actively rotate the actuator in both directions
The frame was designed by using finite element analysis (FEA) and stressing the structure with two forces of 5 N for each beam, above than the maximal force produced by the thicker SMA wire (150 μm of diameter), which is 3.21 N
Summary
The design of a mini robotic system with several degrees of freedom (DOFs), is challenging mainly for the choice of the mini actuators. The challenge is exacerbated when the number of DOFs become high, i.e. in hyper-redundant robots These can be designed by using external actuators (i.e. DC motors) and cables or tubes to transmit the motion [1,2,3]. An alternative solution is the use of internal (embedded) actuators to provide multiple DOFs. The advantages of external actuators design includes low weight and high force to weight ratio. The advantages of external actuators design includes low weight and high force to weight ratio With this design the number of DOFs is limited by the number of cables/tubes required [1,2,3,4,5,6,7,8]. Another important requirement in the design of a robotics actuator is the control of the output force/torque for safe interaction with the surrounding environment, considered a challenge when the size of the robot becomes small
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