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Stewart Platform
Published in Rajesh Singh, Anita Gehlot, Intelligent Circuits and Systems, 2021
A Stewart platform, formerly known as the Gough-Stewart platform, is a parallel manipulator having six prismatic actuators which are fixed to the baseplate in three positions in pairs, which cross over to three positions on the top plate. The platform is flexible enough to move in six different degrees of freedom which is similar to the motion of a freely suspended object or body. The Stewart platform is a one of a kind design which has a variety of applications which involve high precision and accuracy for a work having a limited workspace. Some of them have been studied in this paper. This paper has been divided into several parts. The first few parts give a brief about the basics regarding manipulators. The middle part explains about the Stewart platform, its applications and evolution. The last part describes the mathematics of the platform, which includes its inverse kinematics, followed by control of the platform.
Physical Motion
Published in Alfred T. Lee, Vehicle Simulation, 2017
Without question, the most common motion platform system in use today is some variation of the Stewart platform* (Stewart, 1965). The Stewart platform is a multiaxis, synergistic motion platform typically consisting of six drive legs (hexapod) supporting a platform on which a cab is mounted (Figure 3.3). The drive legs typically have maximum extensions of 0.5–3 m with the midpoint of the extension used as a neutral point. The typical configuration has extensions of ±0.6 m for training platforms and ±1.2 m or more for simulators used in research. The design is an elegant solution to the problem of providing motion in all the 6 DOF within the smallest possible platform workspace. The very limited motion envelope that the design offers eliminates the need for large facilities, thereby dramatically reducing the facility-related costs as well as the cost of the system itself. The lower cost of the Stewart platform has made it the system of choice in deployment of large numbers of simulators used in training and testing. The Stewart platform has gained rapid popularity primarily because it fulfilled the low cost requirement that vehicle operators desired. Moreover, it filled the criteria, particularly of pilots, that favor at least some motion cueing, however minimal, to none at all. This user acceptance element of simulator motion systems does not mean that the system is effective in providing motion cueing, only that it may support a certain level of subjective realism. Indeed, its effectiveness as a motion device in the training regime is still very controversial.
Vibration isolation of a double-layered Stewart platform with local oscillators
Published in Mechanics of Advanced Materials and Structures, 2022
Shurui Wen, Jianying Jing, Ding Cui, Zhijing Wu, Wenyu Liu, Fengming Li
To fully isolate two bodies from each other, isolators exploit six single-axis that can be controlled in a centralized or decentralized manner. One of the most accepted parallel manipulators known as the Stewart platform. The Stewart platform is a system that includes three major parts: top or moving plate, base or fixed plate, and six identical stretchable legs. The ends of all the legs are attached to the moving and fixed plates by spherical, universal or flexible joints [17,18]. This mechanism was first suggested by Gough in 1949 as a universal tire-testing machine [19], which was re-discovered and presented to academia in 1965 by Stewart and these structures were developed and used as simulators for training pilots [20]. Gough was the first to realize the benefits of this mechanism and the research in this area was carried out after Stewart’s Paper. Since then, Stewart platforms have attracted significant attention in aerospace engineering [21] due to their high stiffness, strong bearing capacity, and non-accumulation of position errors [22].
Determination of particular singular configurations of Stewart platform type of fixator by the stereographic projection method
Published in Inverse Problems in Science and Engineering, 2021
Doğan Dönmez, İbrahim Deniz Akçalı, Ercan Avşar, Ahmet Aydın, Hüseyin Mutlu
External fixators or fixators in short are medical devices used widely in orthopaedic clinical practice for the treatment of complex bone deformities, reduction of old, unreduced fractures, limb lengthening, etc. Because of their capability of six-degree-of-freedom bone displacement coupled with suitable stiffness characteristics, the Stewart platform (SP) provides a good basis for their employment as a fixator. This potential created by two rings interconnected by six rods having adjustable lengths has been utilized by researchers in the fixator types frequently referred to as ‘hexapod’ [1–5]. However, a major limitation in all these SP-based fixators is the singularity problem that results in an uncontrollable state for the device. In view of the fact that the function of the robotic frame is not only to hold the fragments in stable equilibrium but also to move them from the displaced configuration to the final aligned configuration securely upon suitable actuation, the question of singularity becomes of great importance for reliable use of these devices. Under such circumstances, it is also not possible to compute forces and displacements in the device.
Parallel manipulator dynamics embedded in singularity free domains of functionality
Published in Mechanics Based Design of Structures and Machines, 2021
A spatial manipulator that has received much attention in the literature (Merlet 2006) is the Stewart platform that is used in aircraft flight and ground vehicle driving simulators. The driving simulator shown at the left of Fig. 4, with a vehicle cab and graphics system in the dome, was used for off-road applications. The schematic on the right of Fig. 4 shows six hydraulic actuators that control position and orientation of the upper platform, relative to the lower platform. Details of attachment points on the lower platform of diameter 5 m and the upper platform of diameter 4 m are shown in Fig. 5. Lower bounds of m and upper bounds of m on the lengths of the actuators were selected by the manufacturer to assure that there are no kinematic singularities in the operational envelope of the simulator. While this geometry is a slight idealization of the actual simulator, it is adequate to illustrate performance of the manipulator.