Design of Stewart Platform for Wave Compensation
Translated title
Design af Stewart Platform for Bølgekompensering
Authors
Madsen, Anders Lolk Lohmann ; Kristensen, Søren Giessing
Term
4. term
Education
Publication year
2012
Submitted on
2012-06-01
Pages
82
Abstract
Kraner monteret på skibe må ofte afbryde arbejdet i bølger, fordi den ophængte last begynder at svinge. Denne afhandling undersøger, om en Stewart-platform – et bevægelsessystem med seks frihedsgrader (6-DOF) – kan modvirke både skibets bevægelser og lastens pendulering. At designe en sådan parallel robot er vanskeligt, fordi kinematikken (hvordan systemet bevæger sig) er kompleks. Arbejdet opstiller en designproces: vælge platformtype, sikre at konstruktionen kan generere kræfter og hastigheder i alle retninger, sikre at det krævede arbejdsrum kan nås, og overholde fysiske begrænsninger som aktuatorgrænser og undgåelse af kollision mellem ben. Afhandlingen giver en grundig gennemgang af kinematik og kinematiske ydelsesindeks (mål for bevægelighed og præcision, herunder manøvredygtighed). Den bedste geometri til bølgekompensation findes ved matematisk optimering, nærmere bestemt Sequential Quadratic Programming (SQP). Der udvikles metoder til at holde aktuatorbevægelser inden for realistiske grænser og til at undgå benkollisioner, og disse integreres i optimeringen af de kinematiske ydelsesindeks. Metoderne kan også bruges til andre anvendelser end bølgekompensation. To platformtyper udvælges og optimeres; resultaterne viser, at en traditionel type er ringere end en lidt mere kompleks type. Den mere komplekse er mindre og har 13,5 % bedre manøvredygtighed. Kravene til et hydraulisk aktueringssystem fastlægges ved hjælp af optimering, og der opbygges en komplet simuleringsmodel af både det hydrauliske og det mekaniske system.
Ship-mounted cranes often have to stop work in waves because the suspended load starts to swing. This thesis investigates whether a Stewart platform—a motion system with six degrees of freedom (6-DOF)—can cancel both ship motion and load pendulation. Designing such a parallel robot is difficult because its kinematics (how it moves) is complex. The work sets out a design process: select the platform type; ensure the mechanism can generate forces and speeds in all directions; verify the required workspace can be reached; and satisfy physical constraints such as actuator limits and avoiding leg collisions. The thesis provides a thorough review of kinematics and kinematic performance indices (measures of mobility and precision, including dexterity). The best geometry for wave compensation is found using mathematical optimization, specifically Sequential Quadratic Programming (SQP). Methods are developed to keep actuator motions within feasible limits and to prevent leg collisions, and these are integrated into the optimization of the kinematic performance indices. The methods are applicable beyond wave compensation. Two platform types are selected and optimized; results show that a traditional layout is inferior to a slightly more complex one. The more complex design is smaller and has 13.5% higher dexterity. Requirements for a hydraulic actuation system are determined using optimization, and a complete simulation model of the hydraulic and mechanical system is built.
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