Phase change drive: soft robot runs with water vapor actuators
Robots with "steam drive"? It may sound outdated, but it also offers advantages in a modern form.
A four-legged robot is driven by phase change actuators.
(Image: Diogo Fonseca)
A research team from the University of Coimbra has developed a soft phase change actuator for robots that uses the transition of water from liquid to gas to generate a mechanical movement. The actuator develops high force and precision.
Electric actuators require complex gears and therefore many vulnerable components in order to increase the force of an electric motor so that it can be used to drive a robot. Pneumatic systems, which can be used as an alternative, are noisy and dirty. The scientists at the University of Coimbra are therefore pursuing a different approach that works quietly and precisely while developing a high force: the phase change actuator.
The actuator uses a small heater in the form of a coil to convert water into steam. This generates an internal pressure that sets soft, flexible structures in motion and can thus drive a robot. The actuator works with a voltage of just 24 V. It generates a force of over 50 N at pressure rates of up to 100 kPa/s, as the researchers write in the study "Electrically-driven phase transition actuators to power soft robot designs", which has been published in Nature Communications.
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The scientists had to contend with a number of problems when developing the phase transition actuator. For example, the strain rates were initially very slow, the performance was not high enough and the design was too complex. The researchers therefore concentrated on the basic technical requirements: fluid selection, optimum heat transfer and simple control.
Steam-powered robots
When it came to the liquid, the researchers quickly settled on water, as it offers the best thermodynamic efficiency, is safe to handle and easily available. However, the use of water also has a disadvantage because the enthalpy of vaporization, the energy required to convert the water from a liquid to a gaseous state, is relatively high. The researchers write that although there are enthalpically better liquids, the use of water is safer.
The actuators created by the scientists consist of a silicone elastomer. This enabled expansion rates of 16.6 percent per second to be achieved. Even after more than 1000 actuation cycles, the performance remained constant.
The scientists developed various actuators with linear and curved drive elements as well as a combination of both. This is possible because the heating element, the fluid chamber and the soft drive structure are separate from each other. This means that the design of the actuator can be easily adapted and quickly produced using 3D printing and casting.
The scientists also succeeded in reducing mechanical vibrations caused by subcooled boiling. The researchers were able to mitigate these instabilities, which are caused by thermodynamic imbalances inside the actuator, using non-linear control algorithms to create a stable boiling state.
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The scientists built various robots around the phase change actuators. These include a gripper and a type of walking robot with four gripper legs, which it can use to pull itself forward. However, the actuators do not work quickly due to the slow cooling and therefore slow pressure reduction. In the optimum case, the scientists achieved stable movement cycles of 25 seconds.
The scientists are now working on making the phase change actuators even smaller in order to improve heat dissipation. They also want to increase the operating pressure.
(olb)
