Posted in | Cybernetics | Humanoids

German Researchers Build an Anthropomorphic Robot Hand

German researchers have built may have just built the toughest robot hand yet. It is an anthropomorphic robot hand that can endure collisions with hard objects and even strikes from a hammer without breaking into pieces.

In designing the new hand system, researchers at the Institute of Robotics and Mechatronics, part of the German Aerospace Center (DLR), focused on robustness. The DLR hand has the shape and size of a human hand, with five articulated fingers powered by a web of 38 tendons, each connected to an individual motor on the forearm.

The high point that makes the DLR hand different from other robot hands is that it can control its stiffness. The motors can tension the tendons, allowing the hand to absorb violent shocks. In one test, the researchers hit the hand with a baseball bat—a 66 G impact. The hand survived.A video on the internet shows the fingers moving and the hand getting hit by a hammer and a metal bar.

The DLR team didn’t want to build just another human hand, look alike as other teams have. They wanted a hand that can perform like a human hand both in terms of dexterity and resilience.

The hand has a total of 19 degrees of freedom, and it can move the fingers independently to grasp varied objects. The fingers can exert a force of up to 30 Newton at the fingertips, which makes this hand also one of the strongest ever built.

Another key element in the DLR design is a spring mechanism connected to each tendon. These springs give the tendons, which are made from a super strong synthetic fiber called Dyneema, more elasticity, allowing the fingers to absorb and release energy, like our own hands do. This capability is key in achieving robustness and for mimicking the kinematic, dynamic, and force properties of the human hand.

During normal operation, the finger joints can turn at about 500 degrees per second. By tensioning the springs, and then releasing their energy to produce extra torque, the joint speed can reach 2000 degrees per second. This means that this robot hand can do something few others, if any, can: snap its fingers.

Markus Grebenstein, the hand's lead designer, says that existing robot hands built with rigid parts, despite their Terminator-tough looks, are relatively fragile. Even small collisions, with forces of a few tens of newtons, can dislodge joints and tear fingers apart.

“If every time a robot bumps its hand, the hand gets damaged, we’ll have a big problem deploying service robots in the real world,” Grebenstein says.

To change its stiffness, the DLR hand uses an approach known as antagonistic actuation. The joints of each finger are driven by two tendons, each attached to one motor. When the motors turn in the same direction, the joint moves; when they turn in opposite directions, the joint stiffens.

Other hands, such as the Shadow hand designed in the U.K., also use antagonistic actuation. But the Shadow uses pneumatic artificial muscles, which have limitations in how much they can vary their stiffness.

Before developing the new hand, Grebenstein designed the hand of another advanced robot, the humanoid Justin. He says that in one experiment they would throw heavy balls and have Justin try to catch them. “The impact would strain the joints beyond their limits and kill the fingers,” he says.

The new hand can catch a ball thrown from several meters away. The actuation and spring mechanisms are capable of absorbing the kinetic energy without structural damages.

But the hand can’t always be in a stiff mode. To perform manipulative tasks that require accuracy, it is better to have a hand with low stiffness. By adjusting the tendon motors, the DLR hand can do just that.

To operate the hand, the researchers use special sensor gloves or simply send grasping commands. The control system is based on monitoring the joint angles. It doesn’t need to do impedance control, Grebenstein says, because the hand has compliance within the mechanics.

To detect whether an object is soft and must be handled more gently, the hand measures force by keeping track of the elongation of the spring mechanisms.

“In terms of grasping and dexterity, we’re quite close to the human hand,” he says, adding that the new hand is “miles ahead” of Justin’s hands.

Grebenstein insists it’s hard to estimate the cost of the project. But he says that the hardware for one hand would cost between 70,000 and 100,000 Euros.

The researchers are now building a complete two-arm torso called the DLR Hand Arm System. Their plan is to study innovative grasping and manipulation strategies, including bimanual manipulations.

Grebenstein hopes that their new approach to hand design will help advance the field of service robots. He says that current robot hardware has limited new developments, because of being costly and researchers can't afford to do experiments that might damage them.

“The problem is," he says, "you can’t learn without experimenting.”

As robotic systems and applications become more and more complex, the danger of costly damage to robots hinders development of radically different motion control and planning strategies. Furthermore the dynamic properties of current robotic systems are not sufficient for human tasks such as throwing or running. Typical actuators cannot provide the required power during peak loads without getting too bulky and heavy. Important advances in space and service robotics would be feasible if future robotic systems are:

  • Robust against “every-day” impacts
  • Able to store energy on short-term

This motivated the design of the DLR Hand-Arm-System which mimics the kinematic, dynamic and force properties of the human arm using variable passive compliance actuators. This highly integrated mechatronic system consists of 52 drives and more than 100 sensors.

Mechatronics is the utmost integration of mechanics, electronics, and information technology up to “intelligent mechanisms” and robots which interact with their environment. Here, the “integral” design optimization and 3D simulation of such systems and components before they are built plays a decisive role. Accordingly, research carried out in the DLR Institute of Robotics and Mechatronics is based on the interdisciplinary (virtual) design, computer-aided optimisation and simulation, as well as implementation of complex mechatronic systems and man-machine interfaces.The central aim is the development of innovative robot systems as far as future robonauts that should relieve the astronauts and in the long run also replace them. In 1993 the institute sent the first remote-controlled robot ROTEX into outer space with the space shuttle COLUMBIA, in 1999 it remotely programmed the Japanese ETS VII robot which was the first to fly freely in the universe and is now planning to test the joints of its latest lightweight robot generation on the space station.

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