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The Stewart platform consists of two plates connected by six mobile and extensible legs. The lower or base plate is immobile. The upper or mobile plate has six degrees of freedom, three rotational and three translational. The platform is a six-degree-of-freedom (DoF) mechanical system used for accurate positioning applications. It is highly stable and easy to control.
The platform's six legs each have two parts, an upper and a lower leg, with a piston-like cylindrical DoF between each pair of parts. The legs are connected to the base plate and the top plate by universal joints at each end of each leg. (These universals are not just sets of abstract DoFs. Each also contains a spider-like body, while also having two DoFs.) The upper part of each leg can slide into and out of the lower leg, allowing each leg to be varied in length. The position and orientation of the mobile platform (top plate) varies depending on the lengths to which the six legs are separately adjusted.
Once the top is connected to the legs, the entire Stewart platform assembly has 36 DoFs. Only six DoFs are independent, the same as the top plate would have if it were disconnected. You can think of these independent DoFs as the six adjustable leg lengths or as equivalent to the six DoFs of the mobile plate.
The following example uses a complex computer-aided design (CAD) assembly that models the Stewart platform.
Note The Stewart platform assembly in this example is an advanced example of computer-aided design. You should work through the previous case studies before attempting to work with this assembly.
Look for the 45 CAD files of this case study in the smlink/smlinkdemos directory. The master assembly file is:
Open the master assembly file, stewart_platform.ASSEMBLYFILETYPE. Click the assembly and rotate it to view the top and bottom plates and the legs.
Stewart Platform CAD Assembly
The CAD hierarchy for the Stewart platform contains assemblies for the top and base plates, as well as assemblies for the six legs. All the constraints on the assembly parts are grouped into one group, containing 30 constraints. There are 448 component parts and 38 subassemblies, which you can open individually to examine the separate parts.
The base plate is about 24 centimeters (cm) in diameter; the top plate about 16.5 cm. When centered and oriented flat, the top plate is about 20 cm above the base. The assembly models the platform material as aluminum (about 2.7 grams per cubic cm).
Apply any changes you want to the assembly configuration or settings. If you change the assembly or any subassemblies, you need to rebuild the assembly before exporting it to XML.
Using the SimMechanics™ Link interface to your CAD platform, export the assembly into Physical Modeling XML. Because the assembly is so complex, the export process takes longer than it does for simpler assemblies. As the export proceeds, various parts and subassemblies are highlighted. When the highlighting stops, the export is finished.
The exported model appears as stewart_platform.xml in your working CAD folder.
Once you have exported the CAD assembly, you can generate the corresponding SimMechanics model. Using the mech_import command, you import the Physical Modeling XML file that SimMechanics Link generated during export, and let SimMechanics automatically generate the model for you. Then, review the model and make any required changes to the model. For a step-by-step description of the CAD Import procedure, see Import a CAD Stewart Platform Model.