A series elastic actuator using air and water and its force control

This study proposes a new type of series elastic actuator (SEA) utilizing a mixture of air and water. Instead of simply mixing air and water, a control method for its output force based on piston position control is suggested using the stiffness model of the mixture. The actuator is driven by the pressure generated when a stiff linear actuator compresses or decompresses the mixture in the syringe. The pneumatic medium makes the actuator compliant but slow, whereas the hydraulic one has a high bulk modulus, allowing for a fast but stiff response from the actuator. These characteristics can be adjusted by changing the air-to-water mixing ratio in the preparation stage before operation. After applying this actuator to the parallel jaw gripper system, grasping force control was performed without a load cell by utilizing the stiffness model of the mixture and the output force model. In an additional experiment, a very flexible object (a rose flower) and a very hard object (a glass vase) were successfully grasped by the gripper, which confirms the effectiveness of the actuator in controlling the grasping force.