Soft robots, or those made with materials like rubber, gels, and fabrics, have advantages over their harder, heavier counterparts, especially when it comes to tasks that require direct human interaction. . Robots that could safely and smoothly help people with reduced mobility with grocery shopping, meal preparation, dressing or even walking would undoubtedly change their lives.
However, soft robots currently lack the necessary strength to perform these kinds of tasks. This long-standing challenge – to make soft robots stronger without compromising their ability to smoothly interact with their environment – has limited the development of such devices.
With the relationship between strength and softness in mind, a team of Penn engineers designed a new electrostatically controlled clutch that allows a soft robotic hand to be able to hold 4 pounds – about the weight of a bag of apples – that’s 40 times more than the hand could lift without the clutch. Additionally, the ability to perform this task requiring both soft touch and force was accomplished with only 125 volts of electricity, one-third of the voltage required for today’s clutches.
Their safe, low-power approach could also enable wearable soft robotic devices that simulate the sensation of holding a physical object in augmented and virtual reality environments.
James Pikul, Assistant Professor in Mechanical Engineering and Applied Mechanics (MEAM), Kevin Turner, Professor and Chair of MEAM with a secondary appointment in Materials Science Engineering, and their Ph.D. students, David Levine, Gokulanand Iyer and Daelan Roosa, published a study in Scientific robotics describing a new model of electroadhesive clutches based on fracture mechanics, a mechanical structure capable of controlling the stiffness of soft robotic materials.
Thanks to this new model, the team was able to achieve a clutch 63 times stronger than current electro-adhesive clutches. The model not only increased the force capacity of a clutch used in their soft robots, but it also decreased the voltage required to power the clutch, making the soft robots stronger and safer.
Current flexible robotic hands can hold small objects, such as an apple. Being soft, the robotic hand can delicately 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 grasp and hold things in our own hands. An electro-adhesive clutch is a thin device that improves the change in stiffness of 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 the case of electro-adhesive clutches, 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 which prevents the two plates from sliding past each other. The electrodes are attached to the flexible material of the robotic hand. By activating the clutch with electrical voltage, the electrodes stick together and the robotic hand supports more weight than it could before. Deactivating the clutch allows the plates to slide over each other and the hand to relax, so the object can be released.
Traditional models of clutches are based on a simple assumption of Coulomb friction between two parallel discs, where the friction prevents the two clutch discs from sliding past each other. However, this model does not capture how the mechanical stresses are unevenly distributed in the system and therefore does not correctly predict the clutch force capability. It is also not 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 using the friction model may be able to pick up an entire sack of apples, but will require high voltages that make it unsafe for human interaction.
“Our approach addresses the force capability 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 looked at where fractures might occur in those 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 these points are. The resulting clutch is both stronger and safer as it requires only a third of the tension compared to traditional clutches.
“The fracture mechanics framework and model in this work have been used for the design of bonded joints and structural components for decades,” says Turner. “What’s new here is the application of this model to the design of electro-adhesive clutches.”
The researchers’ improved clutch can now be easily integrated into existing devices.
“The fracture mechanics-based model provides fundamental insight into how an electro-adhesive clutch works, helping us understand them more than the friction model ever could,” says Pikul. “We can already use the model to improve current clutches simply by making very small changes to geometry and material thickness, and we can continue to push the boundaries and improve the design of future clutches with this new understanding.”
To demonstrate the strength of their clutch, the team attached it to a pneumatic finger. Without the researchers’ clutch, the finger was able to support the weight of an apple while inflated in a curled position; with it, the finger could hold an entire bag.
In another demonstration, the clutch was able to increase the force of an elbow joint to support the weight of a mannequin arm at the low power demand of 125 volts.
Future work the team is excited to pursue includes using this new clutch model to develop wearable augmented and virtual reality devices.
“Traditional clutches require around 300 volts, a level that can be dangerous for human interaction,” says Levine. “We want to continue to improve our clutches, making them smaller, lighter and less energy-intensive to bring these products to the real world. Eventually, these clutches could be used in wearable gloves that simulate the manipulation of objects in a VR environment.
“Current technologies provide feedback through vibration, 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 another environment. Virtual reality and augmented reality could be used in training, remote working or simply to simulate touch and movement for those who don’t have those experiences in the real world.This technology brings us closer to those possibilities.
Improving human-robot interactions is one of the primary goals of Pikul’s lab, and the direct benefits of this research fuel their own research passions.
“We haven’t seen many soft robots in our world yet, and that’s partly due to 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 robot applications that we can’t imagine right now. I want to create robots that help people, make them feel good, and improve the human experience, and that work brings us closer to that goal. I’m really excited to see where we go next.
The title of the article
A mechanics-based approach to making high-strength-capacity electroadhesives for robots
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