The design of a macroscopic SMA actuator has to deal with the time needed for one contraction/stress cycle. An SMA element is usually heated by passing an electric current through it and cooled by the heat transfer to the environment.
The maximum force of a SMA-wire is proportional to its diameter. To use SMA as an actuator for (humanoid) robots high forces are needed, but the cycle time highly increases with the wire’s diameter. To avoid this property the new actuator/sensor design presented combines many single SMA wires in a new way to one muscle-like actuator. A high possible field of application is one of the primary object targets. By keeping a minimum distance between each single wire, a short cool-down time of the thin wires can be ensured for the entire actuator.
Using two of these actuators in an antagonistic flexor-extensor muscle-like manner offers the possibility to generate a defined force at every time. Another possibility is to use a spring in combination with one SMA actuator.
One advantage of this new design is the scalability of pull force and length. The pull force depends directly on the number of wires. One type of actuator used is made up of 10 pairs of SMA wires with a thickness of 100 mircometer and 180gf pull-force each. The total pull force is 3.6kgf. The actuator has a length of 22cm and a maximal displacement of 1.5cm.
The experimental rig consists of an actuator fixed in the centre of the top bar. A platform to place different weights is base-fixed at the actuator followed by an inductive linear position encoder used for evaluation of the resistant-based position encoding. There is the option to fix a second actuator from the bottom of the rig to the position encoder. A regulated power supply is provided by the use of a pulse width modulation.
To get the connection between resistance and contraction the platform was loaded with different weights and the current to heat the actuator was varied at each weight. The results show that there is a close connection between the resistance and contraction. In addition the results show that the connection between resistance and contraction is independent of the load and depends only on the applied current. The curve of the resistance is inverted and scaled in the diagram. The change in the resistor curve after 65s is related to a temperature higher than 90 degree celsius.  The interrelation between current and resistance is linear and can be retrieved easily. The total change of the actuators resistance between both states is 0.711 ohm and the values can be reproduced to a deviation of 0.002 ohm.
Connection between resistance and contraction (contraction red line, resistance blue line)
Test Rig
Actuators placed at the Lara´s lower leg
Example of the used actuators