Soft robots, or those made from materials such as rubber, gels and cloth, have advantages over their harder, heavier counterparts, especially when it comes to tasks that require direct human interaction. Robots that could safely and gently help people with limited mobility to shop, prepare meals, dress or even walk would undoubtedly change lives.
However, soft robots currently lack the power needed to perform these types of tasks. This longstanding challenge – making soft robots stronger without compromising their ability to interact with their environment gently – has limited the development of these devices.
With the relationship between strength and softness in mind, a team at Penn Engineers has designed a new electrostatically controlled clutch that allows a soft robotic hand to carry 4 pounds – about the weight of a sack of apples – which is 40 times more than the hand could lifting without the coupling. In addition, the ability to perform this task that requires both a gentle touch and force was achieved with just 125 volts of electricity, one third of the voltage required for current clutches.
Their safe, low-power approach could also enable wearable soft robotic devices that would simulate the sensation of holding a physical object in augmented and virtual reality environments.
James PiculAssistant Professor of Mechanical Engineering and Applied Mechanics (MEAM), Kevin Turner, Professor and President of MEAM with a secondary appointment in Materials Science Engineering, and their Ph.D. students, David Levine, Gokulanand Iyer and Daelan Roosa into a study Science Robotics describes a new fracture mechanics-based model of electroadhesive couplings, a mechanical structure that can control the stiffness of soft robotic materials.
With this new model, the team was able to realize a coupling that is 63 times stronger than current electro-adhesive couplings. Not only did the model increase the force capacity of a clutch used in their soft robots, it also lowered the voltage required to drive the clutch, making soft robots stronger and safer.
Current soft robotic hands can hold small objects, such as an apple, for example. Being soft, the robotic hand can gently grasp objects of different shapes, understand the energy required to lift them, and become stiff or tense enough to pick up an object, a task similar to how we grasping and holding things in our own hands. An electro-adhesive coupling is a thin device that enhances the change of stiffness in the materials, enabling the robot to perform this task. Similar to a clutch in a car, the coupling is the mechanical connection between moving objects in the system. In the case of electro-adhesive couplings, two electrodes coated with a dielectric material are attracted to each other when voltage is applied. The attraction between the electrodes creates a frictional force at the interface that keeps the two plates from sliding past each other. The electrodes are attached to the flexible material of the robot hand. By switching on the coupling with an electrical voltage, the electrodes stick together and the robot hand can carry more weight than before. Disengaging the clutch allows the plates to slide past each other and relaxes the hand, allowing the object to be released.
Traditional coupling models are based on a simple assumption of Coulombic friction between two parallel plates, where friction keeps the two plates of the coupling from sliding past each other. However, this model does not represent how the mechanical load is unevenly distributed in the system, and therefore cannot accurately predict the capacity of the coupling force. It is also not robust enough to develop stronger couplings without using high voltages, expensive materials or intensive manufacturing processes. A robotic hand with a clutch created using the friction model could potentially pick up an entire bag of apples, but would require high voltages that make it unsafe for human interaction.
“Our approach addresses the force capacity of couplings at the model level,” says Pikul. “And our model, the fracture mechanics-based model, is unique. Instead of making parallel plate joints, we based our design on lap joints and investigated where fractures could occur in these joints. The friction model assumes that the stress on the system is uniform, which is not realistic. In reality, stress is concentrated at different points and our model helps us understand where those points are. The resulting coupling is both stronger and safer as it requires only a third of the tension compared to traditional couplings.”
“The framework and model for fracture mechanics in this work have been used for decades for the design of bonded joints and structural components,” says Turner. “What’s new here is the application of this model to the design of electro-adhesive couplings.”
The researchers’ improved linkage can now be easily integrated into existing devices.
“The fracture mechanics-based model provides fundamental insight into the operation of an electroadhesive coupling, allowing us to understand them better than the friction model ever could,” says Pikul. “We can already use the model to improve current couplings by making only small changes to the geometry and thickness of the material, and we can continue to push the boundaries and improve the design of future couplings with this new insight.”
To demonstrate the strength of their coupling, the team attached it to a pneumatic finger. Without the researchers’ clutch, the finger could hold the weight of one apple while inflated in a curled position; with that, the finger could hold an entire bag.
In another demonstration, the coupler was able to increase the strength of an elbow joint to support the weight of a mannequin arm at the low power consumption of 125 volts.
Future work the team would like to explore includes using this new pairing model to develop wearable augmented and virtual reality devices.
“Traditional couplings require about 300 volts, a level that can be unsafe for human interaction,” says Levine. “We want to keep improving our couplings by making them smaller, lighter and more energy efficient to bring these products into the real world. Ultimately, these couplings could be used in wearable gloves that simulate object manipulation in a VR environment.”
“Current technologies provide feedback through vibration, but simulating physical contact with a virtual object is limited with current devices,” says Pikul. “Imagine having both the visual simulation and the feeling of being in a different environment. VR and AR can be used in training, remote work or just to simulate touch and movement for those who don’t have those real world experiences. This technology brings us closer to those possibilities.”
Improving human-robot interactions is one of the main goals of Pikul’s lab and the immediate benefits that this research provides fuel their own research passions.
“We haven’t seen many soft robots in our world yet, and that’s partly because of their lack of power, but now we have one solution to that challenge,” says Levine. “This new way of designing couplings could lead to applications of soft robots that we cannot imagine now. I want to make robots that help people, make people feel good, and enhance the human experience, and this work brings us closer to that goal. I am very excited to see where we go next.”
A mechanics-based approach to realize high-capacity electro-adhesives for robots
Article publication date
November 30, 2022