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Robot hand uses light to sense force, paving way to collaborative humanoids

Optical sensors may be the key to highly dexterous robots that can safely interact with humans.

Robotics researchers at Carnegie Mellon University are experimenting with fiber optic sensors. The researchers created a highly sensitive robotic hand that uses optical sensors and light to sense force. The sensors may play an important role in a coming class of highly dexterous collaborative humanoids for industry and personal assistance.

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The history of robotics is a history of sensors. Industrial robots are capable of executing tasks with extreme precision, but until recently they have been confined to cages. The combination of extraordinary power and a limited ability to sense the world around them have made them too dangerous for humans to work around. More recently, collaborative robots, such as Rethink Robotics' Baxter, allow for flexible task planning and are designed to interact with humans. In large part this is because they rely on force sensors that can detect an unexpected obstacle, prompting the robot to change course before any damage is done.

But we're only at the beginning of human-robot collaboration, and as robots become increasingly dexterous, and as more powerful AI enables machines to do their own task planning, new sensors that greatly improve tactile and force sensing on multi-joint robots will be essential.

"If you want robots to work autonomously and to react safely to unexpected forces in everyday environments, you need robotic hands that have more sensors than is typical today," says Yong-Lae Park, assistant professor of robotics at CMU. "Human skin contains thousands of tactile sensory units only in the fingertip and a spider has hundreds of mechanoreceptors on each leg, but even a state-of-the-art humanoid, such as NASA's Robonaut, has only forty-two sensors in its hand and wrist."

One problem with simply increasing the number of conventional pressure or force sensors in a robot is that these sensors require wiring that's prone to breaking and susceptible to interference from electric motors. That's why Park and his team have been looking at optical solutions. Optical fiber is compact, wire-free, and impervious to electromagnetic interference.

The optical sensors are able to detect strain by measuring shifts in the wavelength of light reflected by the optical fiber. CMU researchers embedded 14 strain sensors into each finger of a robotic hand they constructed, giving it the ability to sense forces of less than a tenth of a newton (a newton is roughly 1/5 pounds of force, or the crude equivalent of the downward force of a small apple sitting on a table on earth).

The lack of complex wiring and the compact form factor means a large number of sensors can fit into a small area, providing more detailed force feedback. But in spite of their advantages, conventional optical sensors have a major limitation: They don't stretch well. That's a big constraint in a device such as a hand, where a wide range of motion is essential.

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Stretchable optical sensor

To solve the problem, Park and his researchers are developing a highly stretchable optical sensor. The silicone strands on their flexible sensors are lined with reflective gold. As the silicone is stretched, cracks in the reflective layer permit light to escape. By measuring the loss of light, these sensors can calculate the strain or deformation of the finger.

Intriguingly, these flexible optical sensors could be incorporated into soft, life-like skins that are able to detect contact and also measure force. It's an important step away from the multi-axis robots on today's shop floors and toward the sorts of humanoid machines of popular imagination.