Robotic systems often excel at performing one function. One type has a light touch enabling it to locate and gently grasp an object whereas another type has high strength so it can lift a heavy object and move it to a precise location. This specialization makes it difficult for a robot to perform both tasks without a sophisticated control system.

To get around this limitation, an advanced flexible coupling gives robotic manipulators adjustable stiffness so they can perform both tasks. Called a variable compliance coupling, this device contains cable segments that deflect under load, providing a resilient action when the robot contacts an object to be manipulated.

Developed by NASA Goddard Space Flight Center, Greenbelt, Md., the coupling can be used in wrist joints of robotic manipulators that grasp, lift, and move heavy objects. And the concept is being adapted to commercial applications.

Compliance in robots

To visualize robotic compliance, consider the action of a human arm in opening a drawer. First, the hand approaches in a limber manner until it finds the handle. Then, the hand, wrist, and shoulder stiffen slightly as they translate and rotate in three planes until they are aligned for the pull. Finally, the fingers tighten as the arm pulls the drawer open.

A robotic arm with a gripping device or end effector performs similar tasks. A flexible wrist joint with 6 degrees of freedom (translation in the X, Y, and Z directions and rotation about these axes), makes the action more like that of a human arm.

Compliance is a low stiffness characteristic that makes a robotic arm more forgiving so that it contacts objects gently rather than bumping into them. It is incorporated in the wrist joint in the form of a shock-isolating compliant coupling, Figure 1, that contains short flexible cables clamped at their ends in brackets. These brackets are either manually adjusted or motor-driven to vary the cable flexing and thereby adjust their stiffness for different tasks. This adjustment feature is called variable compliance.

Cable bending characteristics

Most compliant mechanisms use marine and aircraft- type cables ranging in size from ¹/₁₆ to ³/₈-in. diameter. These cables are generally mounted between opposing brackets or holders with the cables initially straight. As they deflect under load, Figure 2, they become stiffer and provide damping, caused by friction between cable strands. High loads increase the stiffening effect through a combination of bending in the cable ends and tension in the center.

Stiffness can be modified by varying cable parameters — length, diameter, stranding, and material — and by changing their configuration — the number and spacing of cable sets and the angle between cable segments.

In one type of cable, called right regular lay, the inner strands are wound counterclockwise and the outer strands are wound clockwise. Damping is increased (to limit vibration) by twisting the cable in the direction of the outer strands or layers. This causes the outer layers to contract and the inner layers to expand so that they rub against each other.

In another type, inner and outer layers are wound in the same direction. Called lang-lay, this configuration is generally much less stiff.

King-size coupling

Goddard built one of the earliest (and largest) compliant coupling systems, Figure 3, in 1972. This unit was used as part of a vibration system on the end of a 450,000-lb centrifuge that whirls around at 200 mph. Attached to an outer frame, the cables support a vibrating 10,000-lb casting in the center.

This system keeps the Launch Phase Simulator centrifuge stable and vibration- free despite being subjected to 30 g acceleration. For this reason, cables in a pneumatic hammer can support the weight of a person leaning on the rigid frame while the chipper bangs into the concrete.

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