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| Learn More | Robot Design: Hybrid Robotic Structures |
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| What is a Hybrid Robotic Structure? |
| Why build Hybrid Robotic Structures? |
| Hybrid Architecture Robotics |
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| What is a Hybrid Robotic Structure? |
A Hybrid Robotic Structure consists of both serial and parallel robotic joints.
Most robots are basically serial or parallel, but several robotic architectures can produce a hybrid robot, including end effector attachments, robotic hands, multiple limb robots or a few special architecture robots. These systems are usually developed in an attempt to take advantage of the benefits of both types of robotic structures.
However, the result usually incorporates some of the disadvantages, as well as some of the advantages of the different structures.
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| Why build Hybrid Robotic Structures? |
As was discussed in the basic topics section on serial and parallel robotic structures, the different architectures offer different advantages and disadvantages. For instance, a serial robot is typically far more dexterous than a parallel robot, while a parallel robot can support greater loads without significant deflections than a comparably sized serial robot.
Furthermore, some structures are inherently hybrid structures. Robotic hands and multiple limb robots (whether arms or legs) experience a serial to parallel transformation when their limbs make and maintain contact with a common object.
The common object acts as if it was an additional link in the robotic structure, and the contact points act as if they were additional joints. As a result, what were once independent serial robots becomes a single parallel robot whose movements are now constrained by the connection maintained between the links and the common object.
The Robotics Research Group Dual Arm Robot is engaged in just such a task (Fig. 1 below) by manipulating a hinged beam with both arms. The "toy" acts as an additional set of links and joint and when combined with the arms becomes a part of a closed kinematic chain.
When one gripper releases the beam, the chain is broken and the arms are once again serial structures.
The Robonaut hand conceptual model (Fig. 2 below) experiences the same phenomenon whenever it grasps an object. In addition to the hand, other end effectors make use of active and passive robotic structures for precision assembly tasks.
These structures are often simply parallel mechanisms mounted on serial machines. Dr. Delbert Tesar of the Robotics Research Group holds US Patent #4,505,166 for a related device.
Human beings experience the same serial to parallel transformation when they walk. Each serial leg comes in contact with the ground, and the legs become a parallel base for the serial torso. One cannot move one leg without moving the other leg or changing the contact condition with the ground.
When a person walks, they repeatedly undergo this transformation with every step. Robotic walking machines and mobile robots experience the same effect, but for a robot, the control problem associated with this effect, (specifically the computation of forward and inverse kinematics) becomes a serious concern.
For further information comparing serial and parallel robotic structures and the problems associated with Serial to Parallel Transformations, please refer to the Serial to Parallel Structures Basic Topics.
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Fig. 1 |
Fig. 2 |
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| Hybrid Architecture Robotics |
Aside from multiple limb robots, robotic hands and walking or mobile robots, there is one additional type of hybrid robotic structure under development, the variable geometry truss. Trusses are used extensively in buildings, bridges, and other quasi-static structures because they can span greater distances for their size and weight than simple members alone.
A variable geometry truss robot uses trusses whose geometry can be reconfigured to allow for a limited amount of movement. Each truss set becomes in effect, a parallel robotic structure.
By combining multiple layers of these truss sets, one after another in series, a reasonable amount of movement can be achieved at the end effector position, while providing an extremely stiff serial arm structure.
The Robotics Research Group at The University of Texas at Austin has a Variable Geometry Truss, or VGT, robot in its High Bay Laboratory. Because of its high payload and stiffness, the Manufacturing Cells Team has incorporated the VGT into their research plans.
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| Images of the VGT robot |
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