In the field of soft robotics, a variety of pneumatic actuators are frequently used in order to achieve softness. The problem is that an external compressor and supply tubes are required to maintain the supply of compressed air. This means that soft robots have to be tethered to external devices and the mobility of robots are limited by those tethers. In designing mobile soft robots, dealing with this problem is an inevitable challenge.
Tolley et al. developed an untethered soft robot using a small commercial air compressor. In this system, two central compressors supply air to all actuators installed on the robot. Each individual actuator is controlled by its own control valve. Small control valves can turn on and off only.
A team of two students (one undergrad, one masters) from Japan designed a solution based on a hydraulic system that can replace pneumatic systems. Since liquid is much less compressible in comparison to gas, it is easy to control actuators more accurately. The difference from conventional hydraulic systems is that the pump directly controls the actuator. The system is compact because it can get rid of complex pipes and control valves. Tethers are not required because the pump can work using batteries. Kaminaga et al. developed EHA(Electro-Hydraulic Actuator)-like actuators which have the same concept, using direct control pumps, for oil hydraulic systems.
In the team's approach, actuators are not solid cylinders but soft actuators such as McKibben actuators and alcohol is used as the working fluid. To achieve this concept, the pump should be specially designed to directly control soft actuators on a tether-less robot.