A robot’s soft touch

Delft researchers develop gentle robot hands that curl around any object they encounter.

The under-actuated titanium finger moves more naturally than a regular robotic finger. But it needs to become a bit stronger. (Photo: Just Herder)
The under-actuated titanium finger moves more naturally than a regular robotic finger. But it needs to become a bit stronger. (Photo: Just Herder)

Gently a finger curls around my hand. It smoothly follows every movement I make. Only the cold metal touch reveals that this is a robotic finger.

PhD student, Gert Kragten, of the biomechanical engineering department (Mechanical, Maritime and Materials Engineering), makes the finger curl by pulling a rod at the other end, just like a puppeteer. Specially designed pulleys at the joints of the finger ensures that the power is distributed over the three phalanxes, although not per se evenly. The way the power is distributed depends on the shape of the object the finger encounters, yet it always results in an accurate grip.

This technique is called under-actuated grasping. Within robotics this is a rather new discipline. The fingers or gripers (if you combine more fingers) can move in many more directions than there are motors. “In robotics, we are accustomed to developing machines that generate a desired movement and we put a motor at every hinge”, says Kragten’s colleague, dr. Just Herder. “In contrast, here we only use one motor. Therefore, in a sense you could say that you can’t really steer an under-actuated finger.”

Herder has been working on under-actuated fingers at the TU for about ten years. He is one of the pioneers in the field, which is still very small. Last weekend he, Kragten and a couple of other colleagues were in Montreal to attend a conference dedicated to this subject. There were about 40 attendees. “Almost everybody who works in this field was present,” says Kragten, who organized the conference. “We are a small group of rebels within robotics”, Herder adds.

Earlier this year, the research on gentle robot hands resulted in a spin-off company, called Lacquey, which was set up by dr. Richard van der Linde and dr. Martijn Wisse. This company is specialized in gripers that can handle soft vegetables and fruit for industrial purposes.

Herder focuses his research on prostheses: “With an under-actuated prosthesis, people only need to learn how to make one motor move, which is quite simple. And if you don’t use the prostheses - if you are resting - the robotic hand is soft, just like a normal hand. So it looks and feels much more natural than a regular robotic hand.”

Together with Peter Steutel, a student, Herder has developed a finger consisting of one solid piece of titanium. The thin structure is very flexible and does not require hinges. He now wants to develop the other fingers as well, combining them to form a hand. But that is still very challenging. Among other things, the construction needs to become stronger. Herder: “With the type of finger we have now, a person could only hold something as heavy as a glass of milk.”
Ultimately Herder’s goal is to make soft, inexpensive plastic hands.  

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