ISR research accomplishments

Six-Degree-of-Freedom Parallel "Minimanipulator" with Three Inextensible Limbs
US Patent 5,279,176   January 18, 1994

Research team

Lung-Wen Tsai (ME/ISR)
Farhad Tahmasebi

Accomplishment

The need for the minimanipulator. Most six-degree-of-freedom parallel manipulators contain six limbs. The Stewart platform is a flight simulator that contains six limbs and has been used and studied as a parallel manipulator.

These mechanisms have some distinct disadvantages. Their direct kinematics analyses are very complicated. There is a high possibility of mechanical interference between their limbs. Many parts are needed in their construction.

The only six-limbed, six-DOF parallel manipulators for which closed-form direct kinematics solution has been reported in the literature are special forms of the Stewart platform. In these special forms, pairs of spherical joints are concentric on either the platform or both the base and the platform. However, pairs of concentric spherical joints may very well present design problems.

In a Stewart platform, if the prismatic joints are actuated by electric power, then the actuators are not fixed. As a result, the weight of each actuator is a load for the other actuators. As a result, payload capacity is reduced, actuator sizes are increased and more power is dissipated.

Finally, the limbs in the Stewart platform and the mechanisms introduced by Kohli et al. and Pierrot et al. are extensible (each limb contains a revolute or a prismatic joint). As a result, their resolutions, accuracies and stiffness properties leave much to be desired.

What does the minimanipulator accomplish? This invention is a six-degree of freedom parallel "minimanipulator," with three inextensible limbs. It is designed to provide high resolution, accuracy and stiffness for fine position and force control in a hybrid serial-parallel manipulator system.

The term "minimanipulator" is used because this mechanism is not designed to provide very large platform displacements. All of its actuators are mounted on its fixed base without using any power transmission devices such as gears or belts. The minimum possible number of limbs is used, and there is symmetry between the limbs.

The lower end of each limb is connected to a two-DOF driver (e.g., a bidirectional linear stepper motor, an X-Y position table, a pantograph or a five-bar linkage) and can be moved freely on the base plate. The desired "minimanipulator" motion is obtained when the drivers move the lower ends of its three limbs on its base plate. Pantograph or five-bar linkage drivers further improve resolution, accuracy, and stiffness.

Compared to the six-limbed, six-DOF parallel manipulators, a "minimanipulator" has the following advantages. Closed-form solution can be obtained for its direct kinematics. There is a lower possibility of mechanical interference between its limbs. Fewer parts are needed. Payload capacity can be made higher. Actuators can be made smaller. It dissipates less power.

For more information

View this patent at Delphion.

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