טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLasker Lior
SubjectAnalysis and Path Planning of a Planar Parallel
Manipulator with Joint Clearances
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Yizhar Or
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

In modern industry there is a large variety of robots in use, many of them are parallel manipulators that contain closed kinematic chains, where each chain is made from links connected by joints. These robots can carry large payloads and are very robust and accurate. The basic parallel manipulator consists of several kinematic chains, connected from the ground link (base) to an upper platform, which functions as the robot's end effector. Moments or forces (depending on the link's types) are transmitted through those chains to the end effector, and change its position and orientation. All the mechanical parts of the manipulator are manufactured by machining or sheet metal technologies, depending on the accuracy requirements for each part. As accurate as it may be, there is still an inherent clearance existing in the joints, which may affect the relative position between the links. As the number of links in each chain increases the relative position error between links due to clearances is growing, and can cause significant inaccuracies in the platform's position and orientation. Additionally, it affects the ability of the platform to resist external loads, and may affect the smoothness of the movement. The influence of clearances grows significantly when the robot's dimensions get smaller, approaching the micro scale mechanical structures, called MEMS (Micro-Electrical-Mechanical-Systems). Micromechanical structures are commonly used for sensors such as accelerometers in modern cars for airbag deployment in collisions, gyroscopes in robotic systems for loop closing in the dynamic stability control loop, strain gauges, etc.

The manufacturing processes in MEMS are different from in the macro world. The common manufacture procedures are based on the lithography process followed by wet or dry etching. The results of these procedures are planar two-dimensional figures. The tolerances in the lithography and etching procedures can reach high values (ten percents), for instance the tolerance in rotational joints of a simple planar mechanism is comparable to its links' lengths. Thus, the clearances at the joints of micro mechanisms are something unavoidable and their consequences may cause inaccuracies and failure, which can be crucial for the micro mechanism’s regular operation.

The main goal of this work is to analyze feasible static forces in a simplified planar parallel mechanism model with joint clearances, and develop an algorithm for planning a feasible path of quasi -static motion that accounts for the joint clearances under a given external load.