Conventional robotic grippers with rigid “fingers” tend to be expensive, have limited capabilities and aren’t particularly suited for safely handling delicate objects. To overcome such hurdles, researchers at the Robotics and Biology Laboratory (RBO Lab) at the Technical University Berlin are developing soft robotic grippers that are adaptive, simple and inexpensive. The ultimate goal is to closely mimic the actions of a human hand.
Among other projects, the RBO Lab performs basic research on how to create and control what they term Soft Hands. The main focus is to design robust, customizable and effective soft actuators and related control technology.
According to lab officials, traditional electromechanical actuators built from components like motors, gears, tendons and links are prone to wear, require many parts, and are difficult to assemble. This makes the resulting robots expensive and, for most applications, unaffordable.
Soft Hands represent a departure from classical robot-hand design because they specifically exploit mechanical compliance coupled with sophisticated control strategies. Finger movements are powered by compressed air. The aim is to make grasping simple, flexible and adaptable while sacrificing ultra-precise positioning that isn’t necessary in many applications.
The lab has developed several prototypes. The latest version is dubbed the RBO Hand 2, reportedly an inexpensive, highly compliant and dexterous anthropomorphic hand. The fingers, called PneuFlex actuators, are made of fiber-reinforced silicone rubber using additive-manufacturing and molding processes. In the future, the soft actuators may be 3D printed in a single manufacturing step using a variety of materials and designs, to provide specific user-defined capabilities.
Finger construction includes a rubber top section, and rubber embedded with inelastic fibers in the bottom section. Inﬂating the finger with pressurized air forces the top to extend while the bottom half does not. The resulting diﬀerences in length between top and bottom causes the actuator to bend. Helically wound reinforcement ﬁbers strengthen and stabilize the actuator’s shape, so inﬂation leads to bending rather than to radial expansion.
The team is also investigating embedding soft sensors into the PneuFlex actuators. Due to high deformability, most existing sensor technologies are not compatible with flexible actuators. To add tactile feedback capabilities, the researchers are exploring alternative sensor technologies such as: liquid-metal strain sensors for sensing deformation; grating of optical fibers to sense shape; conductive thermoplastic-elastomer fibers to measure strain; and touch sensing with stretchable multi-layer capacitive surfaces.
The RBO Hand 2 is controlled using a relatively inexpensive PneumaticBox, a system developed for real-time synchronization and control of the pneumatic fingers. The PneumaticBox hardware includes of an array of 5/3 valves, and embedded computer (Beaglebone Black) along with valve drivers, pressure sensors, and a 24-V power supply. It uses widely used, open-source robotic software (such as ROS, RoboticsLab RLab and Python scripting) and can be controlled remotely via a TCP/IP network.
The RBO Hand 2 was developed to investigate the capabilities and limits of robotic hands when relying only on soft, deformable structures. The device’s unique adaptability offers several benefits, such as:
- Readily withstands blunt collisions
- Offers low impact energies
- Passively compliant fingers and palm decouple contact from the robot arm, stabilizing force control
- Adaptability to various object shapes simplifies finger control
- Pneumatic actuation permits complex hand and actuator geometries
Finally, another important aspect of the work, according to RBO Lab officials, is that soft-robot engineering is still in its infancy, compared to electromechanical hands. Continued research into designs, controls and technologies related to soft hands should result in further breakthroughs.
Technical University Berlin