Raboroko equipped with muscles of sewing thread

American and Chinese engineers developed programmable soft manipulator with actuators of twisted and coiled nylon fibers. The engineers have refined the existing technology of manufacture of such actuators and now they do not have pre-tensioners. In addition, the new actuators are cheap to manufacture, are driven by electricity and can be used for the propulsion of Autonomous soft robots, says the work, published in the journal Soft Robotics.

Soft robots are well suited for use in areas where flexibility and security interaction with the environment or a person, for example, in medicine. In order for such robots to fully realize their benefits, they need appropriate soft actuators — mechanisms responsible for the movement of robots and their parts.

Today there are many designs of actuators based on different principles from conventional electric, pneumatic and hydraulic, to more exotic variants, for example, uses for work the phase transition liquid-gas.

One of the promising options — twisted and rolled into a spiral polymer fibers that shrink when under a small voltage of several volts. Artificial muscles, based on this type of actuators cheap to produce, can be reduced by 49% and lift weights almost 100 times superior to their own, and “feel” their deformation through the change in the electrical properties of the fibers (e.g., resistance).

One of the drawbacks of the previous technological actuators of this type was the necessary pre-load, as rolled into a spiral fiber must be slightly stretched to reduce. This significantly limits their applicability for soft robots due to the resulting deformation. Engineers from Colorado state University and the Harbin Institute of technology under the leadership of Zhao Jianguo (Jianguo Zhao) improved production technology of twisted and coiled actuators, eliminating the need for additional load.

The actuator is made of conductive nylon sewing thread, coated with silver. First, the thread is twisted using stepper motors, and then turns into a spiral around a special mandrel with copper spiral guide. After annealing for several hours in a furnace, the billet is removed from the mandrel and was “trained” — it connects the current source and the spiral goes through several cycles of slow heating and cooling. This is necessary to relieve internal stress in the actuator. As a result, the step between the turns of the spiral is maintained even without the prior tension.

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