Compliant couplings give robotic manipulators gentle touch and muscle

April 1, 2000
A coupling developed by NASA uses flexible cables to adjust the stiffness of robotic manipulators so that they can offer both a light touch and high strength.

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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Weight lifting

NASA also developed simple versions of the variable compliance coupling for two robotic arms, capable of lifting 200 and 800 lb, respectively. The smaller version contains 16 cables of ¼-in. stainless steel. The larger, 64-cable version, Figure 1, uses threaded rods to adjust the distance between opposing brackets that support the cable ends. In a horizontal plane, the brackets move inward or outward to vary cable tension and stiffness. In the vertical direction, slots in the brackets allow the cables to move up and down to vary stiffness.

When a rotary clamping mechanism on the end effector contacts a mating device, the cables bend to accommodate misalignment. Then, the clamping mechanism rotates and grips the target in preparation for lifting.

A smaller, more efficient version of the 200-lb unit was installed between the arm and end effector of a NASA T3 robot, which has simulated tasks to be performed on Space Station Freedom.

Ball-screw drive

In a more sophisticated design, a step motor moves the cable support brackets via a gearbox and ball-screw assembly, Figure 4. Using position feedback data, a computer controls the motor and applies a brake to stop the motor shaft when the brackets reach the required position.

This system has 320 cables of ¼-in. stainless steel to support a 4,000 lb. load. However, the same principle can be applied to a much smaller mechanism. The difference is that a small system usually has only two positions — stiff and limber. By contrast, the large motor-driven mechanism achieves many positions and degrees of compliance.

Commercial applications

Engineers in the private sector are evaluating (or developing) compliant couplings for various applications including machine tool assemblers, orthopedic devices, joysticks, and high-wire inspectors.

Assembler. Here, a small robot uses servo motors to put a socket wrench on a nut, back it off, and remove it, although the wrench and nut are at different angles and their centerlines don’t coincide. The robot works even if the nut is vibrating.

This system could also be used in a handicapped patient’s station to perform tasks such as supplying food, drink, reading material, telephone, desk computer, and washing or hair-combing equipment.

Walkers and knee joints. Among the uses for this technology is a mobile support system for elderly and handicapped persons having limited use of their lower extremities.

A compliant joint can also be used in prosthetic and robotic devices that permit controlled rotational movement in three planes where the cables are coupled into a common mounting joint.

Joysticks. Hand-operated joysticks control mobile equipment such as cranes, small vehicles, remote handling apparatus, robots, and aircraft. Some types have variable response: the harder or farther one pushes, the more response is achieved. But, they move either too little or too much in response to operator input. And they lack sensory feedback to tell the operator how hard or fast the machine is moving.

A compliant cable mechanism provides a nonlinear response in which the joystick gets stiffer with increasing force, providing better control. When a machine controlled by a joystick moves, a force sensor measures reaction forces and converts them to tactile feedback, so the operator knows when the target is contacted, its position, and the force exerted on the target.

One company is incorporating this technology in a device that can be used with 3D CAD systems to enhance the manipulation of objects in space.

High-wire inspector. In 1 9 9 1 , engineers at NASA and NSI Technology Services Corp., Fairfax, Va., developed a robot to inspect utility power lines for defects. This remote-controlled robot climbs up a rod onto an electrical power line, then crawls along the line between a generating plant and substation. Compliant cables allow the robot to bend around curves in any direction.

Moving at 7 mph, the robot uses a camera to look for right-of-way infringement, tower corrosion, broken or burnt-out insulators, and frays or nicks in the conductor. An operator in a nearby vehicle controls the robot speed and direction (climbing around suspension fittings) and the camera operation (pan, tilt, and zoom).

Feedback loop

In compliant systems, feedback data for computer control is usually provided by linear variable-differential transducers (LVDTs) that measure translation and rotation of a robotic gripper (in up to 6 degrees of freedom).

The LVDT signals go to a computer that calculates the positions of both robot arm and target object, then aligns the arm with the object.

Information for this article was obtained from James A. Kerley, Goddard Space Flight Center, and NASA Technical Paper 3216, “Cable compliance.”

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