gazebo:build a robot（系统学习系列）

1.model structure and requirements
文件夹结构

 Database         database.config  -->版本信息，现在由CMakeLists.txt生成。。（在线模型库需要此文件，本地模型不需要此文件）         model_1 : 文件夹                model.config                 model.sdf --->SDF description of the model                model.sdf.erb                 meshes                materials                       textures                       scripts                plugins

2.how to contribute a model
https://bitbucket.org/osrf/gazebo_models/pull-requests/241/added-model-demo-joint-types/diff
3.Make a model
model结构：一个模型文件是1.links（ collision碰撞检测？ visual外观 inertial惯性 sensor传感器） 2.Joints A joint connects two links. 3.Plugins 控制模型。
STEP 1: COLLECT YOUR MESHES
STEP 2：MAKE YOU MODEL SDF file
STEP 3：ADD TO THE MODEL SDF FILE
4.Make a mobile Robot
5.Import Meshes
6.Attach Meshes
7.Add a sensor to a Robot
8.Make a Simple Gripper
9.Attach Gripper to Robot
10.Nested model
11.Model Editor
12.Animated Box
13.Make an animated model(actor)
14.Inertial parameters of triangle meshes
15.Visibility layers