A Stewart platform is a type of parallel manipulator that has six prismatic actuators, commonly hydraulic jacks or electric linear actuators, attached in pairs to three positions on the platform's baseplate, crossing over to three mounting points on a top plate. All 12 connections are made via universal joints. Devices placed on the top plate can be moved in the six degrees of freedom in which it is possible for a freely-suspended body to move: three linear movements x, y, z (lateral, longitudinal, and vertical), and the three rotations (pitch, roll, and yaw).
Stewart platforms are known by various other names. In many applications, including in flight simulators, it is commonly referred to as a motion base. [1] It is sometimes called a six-axis platform or 6-DoF platform because of its possible motions and, because the motions are produced by a combination of movements of multiple actuators, it may be referred to as a synergistic motion platform, due to the synergy (mutual interaction) between the way that the actuators are programmed. Because the device has six actuators, it is often called a hexapod (six legs) in common usage, a name which was originally trademarked by Geodetic Technology [2] for Stewart platforms used in machine tools. [3]
This specialised six-jack layout was first used by V E (Eric) Gough of the UK and was operational in 1954, [4] the design later being publicised in a 1965 paper by D Stewart to the UK Institution of Mechanical Engineers. [5] In 1962, prior to the publication of Stewart's paper, American engineer Klaus Cappel independently developed the same hexapod. Klaus patented his design and licensed it to the first flight simulator companies, and built the first commercial octahedral hexapod motion simulators. [6]
Although the title Stewart platform is commonly used, some have posited that Gough–Stewart platform is a more appropriate name because the original Stewart platform had a slightly different design, [7] while others argue that the contributions of all three engineers should be recognized. [6]
In industrial applications, linear actuators (hydraulic or electric) are typically used for their simple and unique inverse kinematics closed form solution and their good strength and acceleration.
For prototyping and low budget applications, typically rotary servo motors are used. A unique closed form solution for the inverse kinematics of rotary actuators also exists, as shown by Robert Eisele [8]
Stewart platforms have applications in flight simulators, machine tool technology, animatronics, crane technology, underwater research, simulation of earthquakes, air-to-sea rescue, mechanical bulls, satellite dish positioning, the Hexapod-Telescope, robotics, and orthopedic surgery.
The Stewart platform design is extensively used in flight simulators, particularly in the full flight simulator which requires all 6 degrees of freedom. This application was developed by Redifon, whose simulators featuring it became available for the Boeing 707, Douglas DC-8, Sud Aviation Caravelle, Canadair CL-44, Boeing 727, Comet, Vickers Viscount, Vickers Vanguard, Convair CV 990, Lockheed C-130 Hercules, Vickers VC10, and Fokker F-27 by 1962. [9]
In this role, the payload is a replica cockpit and a visual display system, normally of several channels, for showing the outside-world visual scene to the aircraft crew that are being trained.
Similar platforms are used in driving simulators, typically mounted on large X-Y tables to simulate short term acceleration. Long term acceleration can be simulated by tilting the platform, and an active research area is how to mix the two.
James S. Albus of the National Institute of Standards and Technology (NIST) developed the Robocrane, where the platform hangs from six cables instead of being supported by six jacks.
The Low Impact Docking System developed by NASA uses a Stewart platform to manipulate space vehicles during the docking process.
The Computer Assisted Rehabilitation Environment developed by Motek Medical uses a Stewart platform coupled with virtual reality to do advanced biomechanical and clinical research. [10]
Dr. J. Charles Taylor used the Stewart platform to develop the Taylor Spatial Frame, [11] an external fixator used in orthopedic surgery for the correction of bone deformities and treatment of complex fractures.
The Ampelmann system is a motion-compensated gangway using a Stewart platform. This allows access from a moving platform supply vessel to offshore constructions even in high wave conditions.
A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolecules, such as molecular machines. Machines can be driven by animals and people, by natural forces such as wind and water, and by chemical, thermal, or electrical power, and include a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement. They can also include computers and sensors that monitor performance and plan movement, often called mechanical systems.
An industrial robot is a robot system used for manufacturing. Industrial robots are automated, programmable and capable of movement on three or more axes.
An actuator is a component of a machine that produces force, torque, or displacement, usually in a controlled way, when an electrical, pneumatic or hydraulic input is supplied to it in a system. An actuator converts such an input signal into the required form of mechanical energy. It is a type of transducer. In simple terms, it is a "mover".
In robotics, robot kinematics applies geometry to the study of the movement of multi-degree of freedom kinematic chains that form the structure of robotic systems. The emphasis on geometry means that the links of the robot are modeled as rigid bodies and its joints are assumed to provide pure rotation or translation.
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A mirror mount is a device that holds a mirror. In optics research, these can be quite sophisticated devices, due to the need to be able to tip and tilt the mirror by controlled amounts, while still holding it in a precise position when it is not being adjusted.
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Six degrees of freedom (6DOF), or sometimes six degrees of movement, refers to the six mechanical degrees of freedom of movement of a rigid body in three-dimensional space. Specifically, the body is free to change position as forward/backward (surge), up/down (heave), left/right (sway) translation in three perpendicular axes, combined with changes in orientation through rotation about three perpendicular axes, often termed yaw, pitch, and roll.
A delta robot is a type of parallel robot that 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. In contrast, a Stewart platform can change the orientation of its end effector.
A six-legged walking robot should not be confused with a Stewart platform, a kind of parallel manipulator used in robotics applications.
A parallel manipulator is a mechanical system that uses several computer-controlled serial chains to support a single platform, or end-effector. Perhaps, the best known parallel manipulator is formed from six linear actuators that support a movable base for devices such as flight simulators. This device is called a Stewart platform or the Gough-Stewart platform in recognition of the engineers who first designed and used them.
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Acceleration onset cueing is a term for the cueing principle used by a simulator motion platform.
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots. Robotics is related to the sciences of electronics, engineering, mechanics, and software.
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A full motion racing simulator, sometimes called a full motion sim rig, is a motion simulator that is purposed for racing, and must provide motion simulation in all six degrees of freedom, as defined by the aviation simulator industry many decades ago. The six degrees of freedom coincide with Earth physics, and are commonly referred to as:
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Moshe Shoham is a professor emeritus in the faculty of mechanical engineering at the Technion - Israel Institute of Technology.