Parallel manipulator
Encyclopedia
Articulated robot
An articulated robot is a robot with rotary joints . Articulated robots can range from simple two-jointed structures to systems with 10 or more interacting joints....
s that use similar mechanisms for the movement of either the robot on its base, or one or more manipulator
Manipulator
In robotics a manipulator is a device used to manipulate materials without direct contact. The applications were originally for dealing with radioactive or biohazardous materials, using robotic arms, or they were used in inaccessible places. In more recent developments they have been used in...
arms. Their 'parallel' distinction, as opposed to a serial manipulator
Serial manipulator
Serial manipulators are by far the most common industrial robots. Often they have an anthropomorphic mechanical arm structure, i.e. a serial chain of rigid links, connected by joints, forming a "shoulder", an "elbow", and a "wrist"....
, is that the end effector (or 'hand') of this linkage (or 'arm') is connected to its base by a number of (usually three or six) separate and independent linkages working in parallel. 'Parallel' is used here in the topological sense, rather than the geometrical; these linkages act together, but it is not implied that they are aligned as parallel lines.
Most robot applications require rigidity. Serial robots may achieve this by using high-quality rotary joints that permit movement in one axis but are rigid against movement outside this. Any joint permitting movement must also have this movement under deliberate control by an actuator. A movement requiring several axes thus requires a number of such joints. Unwanted flexibility or sloppiness in one joint causes a similar sloppiness in the arm: there is no opportunity to brace one joint's movement against another. Their inevitable hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
and off-axis flexibility accumulates along the arm's kinematic chain
Kinematic chain
A kinematic chain is the assembly of several kinematic pairs connecting rigid body segments. The complexity of the chain is determined by the following factors:...
; a precision arm is a compromise between precision, complexity and cost of these joints.
Parallel robots take an alternate approach. Each chain is usually short, simple and can thus be rigid against unwanted movement (compared to the serial arm). Errors in one chain's positioning are averaged in conjunction with the others, rather than being cumulative. Each actuator must still move within its own degree of freedom, as for a serial robot; however in the parallel robot the off-axis flexibility of a joint is also constrained by the effect of the other chains. It is this closed-loop stiffness that makes the overall parallel manipulator stiff relative to its components, unlike the serial chain that becomes progressively less rigid with more components.
This mutual stiffening also permits simple construction: Stewart platform
Stewart platform
A Stewart platform is a type of parallel robot that incorporates six prismatic actuators, commonly hydraulic jacks. These actuators are mounted in pairs to the mechanism's base, crossing over to three mounting points on a top plate. Devices placed on the top plate can be moved in the six degrees...
hexapods chains use prismatic joint
Prismatic joint
A prismatic joint provides a linear sliding movement between two bodies, and is often called a slider, as in the slider-crank linkage. A prismatic joint is formed with a polygonal cross-section to resist rotation...
linear actuator
Linear actuator
A linear actuator is an actuator that creates linear motion .Mechanical and hydraulic actuation are the most common methods of achieving the linear motion...
s between any-axis universal ball joint
Ball joint
In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles.More specifically, a ball joint is a steel bearing stud and socket enclosed in a steel casing. The bearing stud is tapered and threaded. It fits into a tapered hole in the steering knuckle. A...
s. The ball joints are passive: simply free to move, without actuators or brakes; their position is constrained solely by the other chains. Delta robot
Delta robot
A delta robot is a type of parallel robot. It consists of three arms connected to universal joints at the base.The key design feature is the use of parallelograms in the arms, which maintains the orientation of the end effector...
s have base-mounted rotary actuator
Rotary actuator
A rotary actuator is an actuator that produces a rotary motion or torque.The simplest actuator is purely mechanical, where linear motion in one direction gives rise to rotation. The most common actuators though are electrically powered...
s that move a light, stiff, parallelogram arm. The effector is mounted between the tips of three of these arms and again, it may be mounted with simple ball-joints. Static
Statics
Statics is the branch of mechanics concerned with the analysis of loads on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at a constant velocity...
representation of a parallel robot is often akin to that of a pin-jointed truss: the links and their actuators feel only tension or compression, without any bending or torque, which again reduces the effects of any flexibility to off-axis forces.
A further advantage of the parallel manipulator is that the heavy actuators may often be centrally mounted on a single base platform, the movement of the arm taking place through struts and joints alone. This reduction in mass along the arm permits a lighter arm construction, thus lighter actuators and faster movements. This centralisation of mass also reduces the robot's overall moment of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
, which may be an advantage for a mobile or walking robot.
All these features result in manipulators with a wide range of motion capability. As their speed of action is often constrained by their rigidity rather than sheer power, they can be fast-acting, in comparison to serial manipulators.
Their major drawback is their limited workspace, because the legs can collide and, in addition (for the hexapod) each leg has five passive joints that each have their own mechanical limits. Another drawback of parallel robots is that they lose stiffness in singular positions completely (The robot gains finite or infinite degrees of freedom which are uncontrolable; it becomes shaky or mobile). This means that the Jacobian matrix, which is the mapping from joint space to Euclidian space, becomes singular (the rank decreases from six).
Applications
Major industrial applications of these devices are:- flight simulatorFlight simulatorA flight simulator is a device that artificially re-creates aircraft flight and various aspects of the flight environment. This includes the equations that govern how aircraft fly, how they react to applications of their controls and other aircraft systems, and how they react to the external...
s - automobile simulators
- in work processes
- photonicsPhotonicsThe science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. The term photonics thereby emphasizes that photons are neither particles nor waves — they are different in that they have both particle...
/ optical fiberOptical fiberAn optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
alignmenthttp://www.pb.izm.fhg.de/p2sa/020_Technologies/Optik/FundE_Faserjustage.html
They also become more popular:
- in high speed, high-accuracy positioning with limited workspace, such as in assembly of PCBsPrinted circuit boardA printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
- as micro manipulators mounted on the end effector of larger but slower serial manipulatorSerial manipulatorSerial manipulators are by far the most common industrial robots. Often they have an anthropomorphic mechanical arm structure, i.e. a serial chain of rigid links, connected by joints, forming a "shoulder", an "elbow", and a "wrist"....
s - as high speed/high-precision milling machineMilling machineA milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
s
Parallel robots are usually more limited in the workspace; for instance, they generally cannot reach around obstacles. The calculations involved in performing a desired manipulation (forward kinematics) are also usually more difficult and can lead to multiple solutions.
Two examples of popular parallel robots are the Stewart platform
Stewart platform
A Stewart platform is a type of parallel robot that incorporates six prismatic actuators, commonly hydraulic jacks. These actuators are mounted in pairs to the mechanism's base, crossing over to three mounting points on a top plate. Devices placed on the top plate can be moved in the six degrees...
and the Delta robot
Delta robot
A delta robot is a type of parallel robot. It consists of three arms connected to universal joints at the base.The key design feature is the use of parallelograms in the arms, which maintains the orientation of the end effector...
.