Soft robots or those made from materials like rubber, gel, 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 for groceries, prepare meals, get dressed or even walk would undoubtedly be life-changing.
However, soft robots currently lack the power to perform such tasks. This long-standing challenge—making soft robots stronger without compromising their ability to gently interact with their environment—has limited the development of these devices.
By considering the relationship between strength and softness, a team at Penn Engineers has developed a new electrostatically controlled clutch that allows a soft robotic hand to hold 4 pounds – about the weight of a bag of apples – which is 40 times the hand’s weight could lift without the clutch. Additionally, the ability to perform this task, which requires both a gentle touch and force, was achieved with only 125 volts of power, one-third the voltage required for current clutches.
Their safe, low-power approach could also enable wearable soft robotic devices that would simulate the feeling of holding a physical object in augmented and virtual reality environments.
James Pikul, Assistant Professor of Mechanical and Applied Mechanics (MEAM), Kevin Turner, Professor and Chair of MEAM with a sideline in Materials Science Engineering, and their Ph.D. Students, David Levine, Gokulanand Iyer and Daelan Roosa published a study in Scientific Robotics Description of a new fracture mechanics-based model of electro-adhesive couplings, a mechanical structure that can control the stiffness of soft robotic materials.
With this new model, the team was able to realize a clutch that is 63 times stronger than current electro-adhesive clutches. The model not only increased the force capacity of a clutch used in their soft robots, it also reduced 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. Because the robotic hand is soft, it can delicately grip objects of different shapes, understand the energy required to lift it, and become stiff or tense enough to pick up an object, a task similar to the way we grip things in our own hands and hold.
An electro-adhesive coupling is a thin device that enhances the change in stiffness in materials, allowing the robot to perform this task. The clutch, similar to a clutch in a car, is the mechanical connection between moving objects in the system. In electro-adhesive couplings, two electrodes coated with a dielectric material attract each other when a voltage is applied. The attraction between the electrodes creates a frictional force at the interface that prevents the two plates from slipping past each other. The electrodes are attached to the flexible material of the robotic hand.
By turning on the clutch with an electrical voltage, the electrodes stick to each other and the robotic hand holds more weight than before. Turning off the clutch allows the plates to slide past each other, relaxing the hand so the object can be released.
Traditional clutch models are based on a simple assumption of Coulomb friction between two parallel plates, where friction prevents the two plates of the clutch from sliding past each other. However, this model does not capture how mechanical stress is unevenly distributed in the system and therefore does not predict clutch force capacity well. Nor is it robust enough to be used to develop stronger clutches without using high voltages, expensive materials, or intensive manufacturing processes. A robotic hand with a clutch created with the friction model can potentially pick up a whole bag of apples, but requires high voltages that make human interaction unsafe.
“Our approach addresses the force capacity of clutches at the model level,” says Pikul. “And our model, the fracture mechanics-based model, is unique. Instead of creating parallel plate clutches, we based our design on lap joints and investigated where fractures might occur in these joints. The friction model assumes that the loading on the system is uniform, which is not realistic. In reality, tension is concentrated at different points, and our model helps us understand where those points are. The resulting clutch is both stronger and safer, requiring only one-third the tension of traditional clutches.”
“The fracture mechanics framework and model in this work have been used for the design of bonded joints and structural members for decades,” says Turner. “What’s new here is the application of this model to the construction of electro-adhesive couplings.”
The researchers’ improved coupling can now be easily integrated into existing devices.
“The model based on fracture mechanics provides fundamental insights into how an electroadhesive clutch works and helps us to understand it better than the friction model ever could,” says Pikul. “We can already use the model to improve current clutches by making very subtle changes to material geometry and thickness, and with this new understanding we can continue to push the boundaries and improve the design of future clutches.”
To demonstrate the strength of their clutch, the team attached it to a pneumatic finger. Without the researchers’ grasp, the finger was able to support the weight of an apple while being inflated into a curled-up position; With that, your finger could hold a whole bag of it.
In another demonstration, the clutch could increase the strength of an elbow joint to support the weight of a mannequin arm at the low power requirement of 125 volts.
Future work the team is excited to delve into is using this new clutch model to develop wearable augmented and virtual reality devices.
“Conventional clutches require about 300 volts, a level that can be unsafe for human interactions,” says Levine. “We want to keep improving our clutches by making them smaller, lighter and less energy intensive to bring these products into the real world. Eventually, these clutches could be used in wearable gloves that simulate object manipulation in a VR environment.”
“Current technologies provide feedback through vibrations, but simulating physical contact with a virtual object is limited with today’s devices,” says Pikul. “Imagine having both the visual simulation and the feeling of being in a different environment. VR and AR could be used for training, remote work, or just to simulate touch and movement for those who lack 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 direct benefits this research provides fuels her own research passion.
“We haven’t seen many soft robots in our world, and that’s partly because of their lack of strength, but now we have a solution to that challenge,” says Levine. “This new way of designing clutches could lead to soft robotic applications that we cannot currently imagine. I want to develop robots that help people, make them feel good, and enhance the human experience, and this work brings us closer to that goal. I’m really excited to see where we’re going next.”
David J. Levine et al, A mechanics-based approach to realizing high force capacity electro-adhesives for robots, Scientific Robotics (2022). DOI: 10.1126/scirobotics.abo2179
Provided by the University of Pennsylvania
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