All soft robots require an actuation system to generate reaction forces, to allow for movement and interaction with its environment. Due to the compliant nature of these robots, soft actuation systems must be able to move without the use of rigid materials that would act as the bones in organisms, or the metal frame that is common in rigid robots. For actuation that involves bending, some sort of stress difference must be created across the component, such that the system has a tendency to bend towards a certain shape to relieve this said stress. Nevertheless, several control solutions to soft actuation problem exist and have found its use, each possessing advantages and disadvantages. Some examples of control methods and the appropriate materials are listed below.
Electric field One example is utilization of
electrostatic force that can be applied in: •
Dielectric Elastomer Actuators (DEAs) that use
high-voltage electric field in order to change its shape (example of working DEA). These actuators can produce high forces, have high specific power (W kg−1), produce large strains (>1000%), possess high energy density (>3 MJ m−3), exhibit self-sensing, and achieve fast actuation rates (10 ms - 1 s). However, the need for high-voltages quickly becomes the limiting factor in the potential practical applications. Additionally, these systems often exhibit leakage currents, tend to have electrical breakdowns (dielectric failure follows
Weibull statistics therefore the probability increases with increased electrode area ), and require pre-strain for the greatest deformation. Some of the new research shows that there are ways of overcoming some of these disadvantages, as shown
e.g. in Peano-HASEL actuators, which incorporate liquid dielectrics and thin shell components. These approach lowers the applied voltage needed, as well as allows for self-healing during electrical breakdown.
Thermal •
Shape memory polymers (SMPs) are smart and reconfigurable materials that serve as an excellent example of thermal actuators that can be used for actuation. These materials will "remember" their original shape and will revert to it upon temperature increase. For example,
crosslinked polymers can be strained at temperatures above their
glass-transition (Tg) or melting-transition (Tm) and then cooled down. When the temperature is increased again, the strain will be released and materials shape will be changed back to the original. This of course suggests that there is only one irreversible movement, but there have been materials demonstrated to have up to 5 temporary shapes. One of the simplest and best known examples of shape memory polymers is a toy called
Shrinky Dinks that is made of pre-stretched
polystyrene (PS) sheet which can be used to cut out shapes that will shrink significantly when heated. Actuators produced using these materials can achieve strains up to 1000% and have demonstrated a broad range of energy density between −3 and up to 2 MJ m−3. Definite downsides of SMPs include their slow response (>10 s) and typically low force generated. Although made of metal, a traditionally rigid material, the springs are made from very thin wires and are just as compliant as other soft materials. These springs have a very high force-to-mass ratio, but stretch through the application of heat, which is inefficient energy-wise.
Pressure difference •
Pneumatic artificial muscles, another control method used in soft robots, relies on changing the pressure inside a flexible tube. This way it will act as a muscle, contracting and extending, thus applying force to what it's attached to. Through the use of valves, the robot may maintain a given shape using these muscles with no additional energy input. However, this method generally requires an external source of compressed air to function. Proportional Integral Derivative (PID) controller is the most commonly used algorithm for pneumatic muscles. The dynamic response of pneumatic muscles can be modulated by tuning the parameters of the PID controller. == Sensors ==