Machinedesign 5994 Grippers Modeled After Nature 1

Robot imitates an elephant’s trunk

Sept. 13, 2010
The light and flexible Bionic Handling Assistant mimics the motion of an elephant’s trunk. FinGrippers weigh significantly less than metal grippers and are suited for delicate objects.

The Bionic Handling Assistant boasts an ultralight structure, nimble grippers, and servopneumatic control.

The elephant’s trunk is a remarkable tool — capable of lifting heavy loads, yet dexterous and sensitive enough to pick up a peanut. By analyzing its structure and motion, and taking advantage of new manufacturing techniques and high-speed servopneumatic controls, engineers at Festo, Esslingen, Germany, have developed a new lightweight, biomechatronic system that could change the way manufacturers handle products.

At first glance, the Bionic Handling Assistant resembles not so much a robotic arm but an organic structure with three basic elements: an arm, hand axis, and gripper with adaptive fingers.

The arm is made up of three segments, each comprised of three polyamide bellows actuators with a 3° taper running the length of the segment. Individual bellows cover about a third of the arm’s circumference, so actuating all three with compressed air linearly extends the arm. Pressurizing just one or two produces angular displacement. The bellows act like a spring and return to their original position when compressed air exhausts. Bowden cable potentiometers mounted outside the actuators track position and permit closed-loop control of movements.

The hand axis contains three additional bellows actuators surrounding a ball joint. Activating them displaces the gripper by angles up to 30°. Festo SMAT-8M position transmitters — compact, magnetic-proximity sensors with ±0.1-mm repeatability — track movements and make for precise alignment.

Finally, the FinGripper, with a design based on a fish’s tail fin, is what actually grasps the workpiece. Unlike conventional metal grippers or vacuum handlers, the FinGripper is light, flexible, and readily adapts to an object’s shape, which makes it particularly well suited for fragile and irregularly shaped products. It consists of a pneumatic bellows actuator (or, for longer life, a simple pneumatic cylinder) and three fingers. Compressed air actuates the bellows which, via lever action, opens and closes the fingers

The basic finger consists of two plastic bands which meet at one end to form a triangular structure. Intermediate stays that add strength connect to the bands via articulated joints. This flexible design lets the fingers adapt to the shape of a workpiece when pressure is applied laterally — just like a human hand, but much faster, according to company officials.
The Bionic Handling Assistant is 0.75 m long with a maximum extension of 1.1 m and an operating range (diameter) of 1.2 m. Thirteen actuators give the unit 11 degrees of freedom. It weighs only 1.8 kg but handles weights to 500 gm. Operating pressure is 1.5 to 3.0 bar.

Motion control

Festo’s VPWP proportional directional-control valves control flow to the actuators in the servopneumatic system. The valves have 5/3-way operation for varying the direction and speed of movement and feature digital data transmission, built-in pressure sensors, an adaptive, “self-tuning” control algorithm, and diagnostic capabilities.
Overall motion control is via the CMXR robot controller that is also used with the company’s tripod handling robots and high-speed gantries. The CMXR, according to company officials, combines mechanical, drive, and control elements into a complete kinematic system that coordinates highly dynamic 3D motion.

It offers features such as motion-path smoothing, ramp shapes for acceleration, and constant path speeds, but it also protects the handling device — say from overloads. CMXR interpolates and positions all axes, so it can define the tooling end position in three dimensions. This gives devices the capability to trace contours along a centerline, as is required for bonding, laser welding, and water-jet cutting. The CMXR is the interface to the master controllers, as well as to valve terminals, servoaxis motor controllers, and vision systems.

Rapid manufacturing
A particular challenge for Festo’s engineers was how to efficiently manufacture the precise geometric structure of the pneumatic actuators and grippers. And, in fact, “this completely new handling concept is only made possible by rapid-manufacturing technology,” explains Klaus Müller-Lohmeier, head of Advanced Prototyping Technology at Festo. “The FinGrippers and other components of the Bionic Handling Assistant are all made by additive manufacturing. Otherwise, they wouldn’t be practical to fabricate,” he explains.

Rapid manufacturing, an offshoot of rapid prototyping, permits efficient “printing” of customized parts. In practice, the 3D CAD model of a component is cut into virtual slices. These slices are then physically built up, layer by layer, using selective laser sintering.

In this process, successive layers of polyamide powder just 0.1-mm thick are applied onto a base platform. Each new layer is fused to the underlying layer by a laser beam, which hardens the powder to form a solid structure. This avoids the expensive molds necessary to produce parts through conventional injection molding.

The high flexibility, excellent long-term resilience, and low density (0.95 gm/cc) of polyamide make it well suited for the Bionic Handling Assistant, says Festo. The material provides an unprecedented ratio of weight-to-force transmission capacity, a prerequisite for the device’s high performance. For instance, the FinGripper weighs about 90% less than comparable metal grippers, letting it hold and transport workpieces efficiently.

Gentle handling

The Bionic Handling Assistant is currently in its prototype phase, undergoing test and evaluation. A third-generation version is due out early next year. However, the underlying design principles lend themselves to future innovations, says Festo.

Possible applications extend beyond the automated gripping of delicate objects such as fruit, vegetables, eggs, and plants. They might be used to help automate dairy operations or to securely handle medical devices. Support for the physically handicapped and the elderly is another possible application, according to the company.

The FinGrippers, however, are currently in production and being used on more-conventional robots by nearly 20 customers to quickly and reliably handle fruit and pressure-sensitive food.

For instance, the Dutch company Total Systems, a manufacturer of equipment for handling flowers and bulbs, opted for FinGrippers on a machine that sorts bulbs by size and quality — work that was previously done laboriously and less efficiently by hand.

And attempting to move soft, hollow, chocolate eggs with conventional grippers has, in the past, resulted in a significant amount of damage and waste. On a sorting station with FinGrippers, in contrast, the fingers fully enclose the chocolate eggs without crushing them or damaging the aluminium wrapping. The flexible design even lets the fingers grip eggs that are leaning or improperly positioned.

© 2010 Penton Media, Inc.

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