서비스 로봇 주행 플랫폼을 위한 2-자유도 변형 바퀴 메커니즘

학위논문(박사)--서울대학교 대학원 :공과대학 기계항공공학부,2019. 8. 김종원.This thesis presents a new 2-degree of freedom (DOF) transformable wheel mechanism for service robot mobile platforms. On flat grounds, conventional wheel is a simple and effective structure for the mobile platforms. However, it is difficult for the mobile platforms with conventional wheels to overcome obstacles in human living environment due to high required friction and large fluctuations on main body while overcoming obstacles. By 2-DOF transformation, the wheel can effectively reduce friction dependency as the transformed wheel maintain contact only with a bottom surfaces of various obstacles. Also, bottom surface contact condition allows the platform to climb various obstacles regardless of the existence of vertical surfaces of obstacles. In addition, fluctuation of a main body while overcoming various obstacles is also reduced by the effect of the curved shaped transformed wheel. Consequently, the platform with 2-DOF transformable wheel can overcome various obstacles in human environment at high speed up to 40 m/min which is average walking speed of adult human. The concept and required functions of 2-DOF transformable wheel was defined and a preliminary simulation and experiments were performed to verify performance of transformed wheel shape. 36 Types of 2- DOF transformable were explored by type synthesis of 2-bar serial and 5-bar parallel mechanisms. By excluding mechanisms that do not satisfy kinematic requirements, and by applying mechanisms synchronizing 3 lobes, five different types of design alternative mechanisms were created. Through the step overcoming simulation, kinematic characteristics analysis and evaluation of design alternatives was performed in terms of range of transformation, interference, and singularity. 5-bar mechanism with two P joint at base (PRRRP) was selected as an optimal mechanism. To achieve both control simplicity and design simplicity, a pair of RRPP 4-bar mechanism was synthesized to the selected PRRRP mechanism to ensure independencies between wheel rotation DOF and transformation DOFs. The design variables were selected to satisfy the required transform range through kinematic analysis, and power transmission parts were designed based on static analysis. Selected mechanism based mobile platform STEP was designed and fabricated. The size and the weight of the platform is 1150 mm x 250 mm x 420 mm and 18 kg respectively. CompactRio was used as the main controller, and was controlled by the PC's labview wirelessly. The performance of the 2 DOF wheel based mobile platform was verified by experiments on step obstacles of various heights (70 mm, 110 mm) and stair obstacles of various sizes (300 mm x 100 mm, 320 mm x 140 mm, 300 mm x 160 mm). In addition, field tests had been performed on actual stairs in our living environment. As a result, the platform could overcome various obstacles at high speed up to 40 m/min. The transformable wheel based mobile platform STEP that is proposed in this study can be used as a mobile platform of service robots so that the service robots could expand their applications by increasing their range of activity.본 논문은 새로운 2-자유도 변형 바퀴 기반의 주행 플랫폼(STEP)에 관한 연구이다. 기존의 원형 바퀴는 구조가 단순하고 구동이 간단하여 평지 주행에 효과적인 대안이지만 인간 생활 환경에 존재하는 다양한 장애물을 극복하기엔 마찰력에 대한 의존도가 높으며 흔들림이 크다는 한계가 존재한다. 본 논문에서는 2-자유도의 변형 메커니즘을 제안하여, 변형된 바퀴가 바닥 면과의 접촉만을 유지하게 함으로써 장애물 극복 시 마찰력에 대한 의존도를 효과적으로 감소시킨다. 또한 변형된 곡선 형상의 바퀴를 통해 본체의 흔들림도 감소시켜, 결과적으로는 사람이 걷는 속도인 40 m/min 의 주행 속도에서도 안정적으로 다양한 크기의 장애물 극복이 가능하다. 본 연구에서는 기구학을 기반으로 2-자유도 변형 바퀴의 개념 및 기구학적 요구조건을 정의하였다. 변형된 바퀴 형상을 3D 프린팅을 통해 제작하였으며, 예비 실험 및 해석을 통해 계단 장애물 극복 성능을 검증하였다. 2-자유도의 변형 기능을 수행할 수 있는 직렬 2절 링크 및 병렬 5절 링크 메커니즘이 총 36가지 탐색 되었다. 요구조건을 만족하지 못하는 메커니즘을 제외하고, 동기화 메커니즘을 결합하여 최종적으로 다섯 가지 유형의 설계 대안 메커니즘을 도출하였다. 설계대안들은 장애물 극복 과정의 기구학 시뮬레이션 및 변형 범위, 간섭 및 특이점 회피 여부를 통해 평가가 수행되었으며, 최종적으로 3R2P 5절 링크 메커니즘이 선정되었다. 설계를 단순화하면서 바퀴의 회전에 대해 바퀴의 변형 자유도를 독립시키기 위해, 한 쌍의 4절 RRPP 메커니즘을 합성하였다. 최종 변형 메커니즘의 기능 및 주행 플랫폼의 성능을 검증하기 위해, 모바일 플랫폼 STEP 이 설계되고 제작되었다. 설계 변수는 기구학 해석을 통해 필요한 변형 범위를 충족하도록 선정되었으며, 모터 선택 및 보강 설계는 정적 해석을 통해 수행되었다. 제작된 플랫폼의 크기는 1150 mm x 250 mm x 420 mm 이며 무게는 18 kg 이다. 주 제어기로 CompactRio 가 사용되었으며, PC 의 labview 로 무선으로 구동되었다. 2-자유도 변형 바퀴 기반 모바일 플랫폼 STEP 의 주행 성능은 다양한 크기의 계단 장애물 (300 mm x 100 mm, 320 mm x 140 mm, 300 mm x 160 mm)과 턱 장애물에 대한 실험을 통해 검증되었다.1. Introduction 1 1.1. Mobile platform for service robots 1 1.2. Limitations of wheeled mobile platforms 1 1.3. Previous researches 4 1.3.1. Wheel linkage mechanisms 4 1.3.2. Transformable wheel mechanisms 6 1.4. Performance indices for mobile platforms 9 1.4.1. Required friction coefficient 9 1.4.2. Trajectory smoothness 10 1.5. Research objevtices 12 2. The Concept Transformable Wheel 13 2.1. The Concept of 2-DOF transfomable wheel 13 2.1.1. Definition of transformable wheel 13 2.1.2. Objective obstacles in human environment 15 2.1.3. Necessity of 2-DOF transformation 17 2.2. Obstacle overcoming process of transformable wheel 18 2.3. Preliminary experiments with curved wheel 19 2.3.1. Trajectory analysis 19 2.3.2. Required friction coefficient 23 2.3.3. Preliminary experiments results 26 3. Type Synthesis of 2-DOF Transform Mechanism 29 3.1. Requirements of transform mechanisms 29 3.2. Exploration of 2-dof mechanisms 33 3.2.1. Systematic process of developing design alternative mechanisms 33 3.2.2. Exploration of 2-dof mechanisms 34 3.2.3. Exclusion of alternatives with passive prismatic joints 37 3.2.4. Design alternatives by combining synchronization mechanism 39 3.3. Characteristic analysis of alternative mechanisms 40 3.4. Step overcoming simulation and evaluation 47 3.4.1. Simulation conditions 47 3.4.2. Inverse kinematics of alternatives 48 3.4.3. Simulation results 48 3.4.4. Analysis and evaluations 51 3.5. 5-bar 3R2P mechanism 55 4. Design of the STEP Platform 56 4.1. Concept design of the STEP platform 56 4.1.1. Issues of actuator location 56 4.1.2. Final concept of the mobile platform STEP 57 4.2. Design of the STEP platform 60 4.2.1. Description of mechanical parts 61 4.2.2. Kinematics based parameter selection 66 4.2.3. Static analysis 69 4.3. Fabricated STEP Platform 75 4.4. Design modifications 80 4.4.1. Link supporting ball roller 80 4.4.2. Clearance of a screw axis 81 4.4.3. Gap between motor-boxes 82 4.4.4. Timing belt deviation 83 5. Experiments 84 5.1. Experimental setup 84 5.1.1. Testbench : various sizes of stairs and steps 84 5.1.2. Active supporter 86 5.1.3. Initial condition 87 5.1.4. Guide to constrain pitch motion & trajectory 87 5.2. Transformation tests 89 5.3. Tests on flat surfaces 90 5.4. Stair overcoming experiments 91 5.5. Step overcoming experiments 98 5.6. Size varying stairs overcoming experiments 100 5.7. Field tests 102 6. Conclusion 104Docto