A parallel manipulator is a mechanical system that uses several computer-controlled serial chains to support a single platform or end effector. The most famous parallel robot is composed of 6 linear actuators that support a movable base used for flight simulators and other equipment.
It is often said that parallel robots are harder, faster, and more accurate than serial robots. However, the facts are much more complicated, because there are huge differences between parallel robots.
Finally, it can be said that most of the multi-axis precision positioning devices are based on parallel robots, mainly hexapods and tripods. However, the reason for this phenomenon has little to do with the error accumulation of serial robots (while the errors of parallel robots are average), and it has more to do with the rigidity of the hexapod and the tripod.
These systems are also called parallel robots. They are articulated robots that use a similar mechanism to move the robot's base or one or more manipulators. Their "parallel" difference, compared to the serial manipulator, is that the end effector (or "hand") of this link (or "arm") is connected to it through some (usually three or six) independent parallel mechanisms on the pedestal.
What is used here is “parallel” in the topological sense, not “parallel” in the geometric sense; these links interact, but this does not mean that they are parallel lines.
Design Features
Compared with serial manipulators, each chain of parallel manipulators is usually shorter and simple in structure, so it can resist unnecessary movement. The positioning error of one chain and the positioning errors of other chains are average, not cumulative. For serial robots, each actuator must move within its own degrees of freedom; however, in parallel robots, the off-axis flexibility of joints is also affected by other chains. It is this closed-loop stiffness that makes the entire parallel robot rigid relative to its components, unlike the serial chain, which gradually reduces its rigidity as the number of components increases.
This mutual reinforcement also allows a simple structure: the Stewart platform hexapod chain uses linear actuators of prismatic joints between ball joints in any axis direction. Ball joints are passive: they simply move freely, without actuators or brakes; their positions are completely restricted by other chains. The Delta robot has a rotary actuator mounted on a base, which can move a lightweight, rigid parallelogram arm. The actuator is installed between the tops of three of the arms. Similarly, it can also be installed on a simple ball joint. The static representation of a parallel robot is usually similar to an articulated truss: the connecting rods and their actuators only feel tension or compression, without any bending or torque, which again reduces the influence of any flexibility on the external force of the shaft.
Another advantage of the parallel manipulator is that the heavy-duty actuators are often installed on a single basic platform, and the movement of the arm is only carried out through the pillars and joints. This reduction in mass along the arm allows for a lighter arm structure, resulting in a lighter actuator and faster movement. This concentration of mass also reduces the overall moment of inertia of the robot, which may be an advantage for mobile or walking robots.
All these characteristics make the manipulator have a wide range of motion capabilities. Since their speed of action is often limited by rigidity rather than pure force, they can move quickly compared to serial manipulators.
Compared with serial manipulators, most robotic applications require rigidity. Serial robots can achieve this by using high-quality rotary joints that allow movement on one axis but are rigid for movement outside the axis. Any movement allowed by the joint must also be carried out under the deliberate control of the actuator. A movement requires several axes, so many such joints are needed. Unnecessary flexibility or sloppiness in one joint can cause similar sloppiness in the arm: there is no opportunity to support the movement of one joint to another. The inevitable hysteresis and off-axis flexibility continue to accumulate along the kinematic chain of the arm; precision arms are a compromise between the accuracy, complexity, and cost of these joints.
Compared with serial robots, one of the main disadvantages of parallel robots is that their working space is limited, because their legs may collide, and (for hexapod robots) each leg has five passive joints, and each joint has its own mechanical limits. Another disadvantage of parallel robots is that they completely lose their stiffness in singular positions (the robot gains uncontrollable finite or infinite degrees of freedom; it can become swaying or moving). This means that the Jacobian matrix, the mapping from the joint space to the Euclidean space, becomes singular (the rank decreases from 6).
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