When you think of robots, you might imagine human-like droids from science fiction movies. But robots are actually used in many practical applications today. A robot is simply a machine that can execute a task with little human interaction. This includes anything from programmable vacuum cleaners to automated manufacturing arms in an assembly line.
One physical quality in a robotic system is how much a component can stretch or bend when under a physical force, called its mechanical stiffness. Scientists have demonstrated they could improve the performance of robotic systems, such as prosthetics and walking robots, by programming or altering the mechanical stiffness of a joint-like connection piece called a clutch. Scientists have programmed rotational clutches to mechanically connect and disconnect elements and control energy transfer. However, these clutches were made of rigid materials and restricted to rotational movement, meaning they lacked stretchability. A flexible clutch would improve robotic systems, such as the swimming efficiency of robotic fish or mechanical grippers that can gently lift objects.
To adjust stiffness in a flexible robotic system, scientists recently developed a clutch made of a special type of insulator sandwiched between two thin conductors, called an electroadhesive clutch. When there is no voltage applied across an electroadhesive clutch, the top and bottom layers can slide relative to each other, keeping the joint flexible. When a voltage is applied, the two conductive layers are attracted to each other due to electrostatic forces. The top and bottom layers can no longer slide and the clutch becomes stiff. Although these clutches show promise, the adhesion between the two conductive layers is broken under relatively low forces. In other words, it has a low force capacity.
Scientists from University of Pennsylvania recently developed a new model to better predict the force capacity of electroadhesive clutches. They suspected the current model did not accurately predict electroadhesive behavior, limiting improvements on clutch force capacity. Their goal was to demonstrate the force capacity could be increased simply by changing the clutch’s shape and thickness.
The previous model assumed static friction was holding the conductive plates together, meaning the clutch was failing because the plates were slipping apart. However, the scientists in this study assumed the clutch was failing due to cracks forming between the conductor and insulator layers. Using calculations based on the study of cracking in materials, or fracture mechanics, they demonstrated the force capacity depended on two factors related to the clutch’s shape. Specifically, the force capacity increased if the overlap between conductors, or the contact area, also increased. They also showed the force capacity depended on how much the clutch would change in volume under an applied force, or the compliance. A clutch with a lower compliance had a stronger force capacity.
In a robotic system, it is not always possible to increase the contact area, so the scientists focused on decreasing the compliance of the clutch. By assuming each component of the clutch behaved like a spring, they demonstrated the clutch compliance decreased when the clutch width increased and the length decreased.
The scientists measured the force capacity of electroadhesive clutches of different geometries. They designed clutches with a flexible polymer called Parylene-C as the insulator layer and thin aluminum contacts on a polyester film as conductors. Keeping contact area at a constant value of 16 cm2, or about 6 inches2, they varied the width and length of each clutch. They attached each clutch in a special machine that slowly stretched each sample until it broke and recorded the corresponding force. They successfully measured higher force capacity in the wider, or less compliant, clutches. This result confirmed the scientists’ model.
The scientists then tested their clutches in a real robotic system. They designed a robotic hand with flexible fingers that could hold a bag with one apple. However, when more apples were added to the bag, the fingers bent away from the palm and dropped the bag. The scientists attached an electroadhesive clutch to the fingers and demonstrated the robotic hand could hold a full bag of apples when a voltage was applied to the clutch. Comparing the weight of the bags, they concluded the robotic hand with an electroadhesive clutch could hold 27 times more weight.
The scientists in this study developed a fracture mechanics based model to better predict the force capacity of electroadhesive clutches. They concluded the improved clutch performance will enable wearable robotics in prosthetics and robotics with gentle yet strong gripping abilities. However, the scientists cautioned their model may not apply to clutches with low electrostatic pressure or in situations where the clutch is loaded at a different angle.