Joint——在不同物體間加入關節
在我們周圍有許多關節，例如，手機的鉸鍊，門的鉸鍊，等等。一個接頭連接配接兩個物體。換句話說，關節是限制兩個物體運動的限制。ODE使用與限制含義相同的關節。
How to use a joint
1、Create a joint
dJointCreate***()
2、connect bodies with the joint
dJointAttach ()
3、Set an anchor of the joint
dJointSet***Anchor()
4、Set an axis of the joint
dJointSet***Axis()
How to set parameters of the joint
5、Set range of motion
dJointSetHingeParam (dJointID, dParamLoStop, the minimum range of motion);
dJointSetHingeParam (dJointID, dParamHiStop, the maximum range of motion);
6、Set angular velocity and maximum torque
dJointSetHingeParam (dJointID, dParamVel, target angular velocity);
dJointSetHingeParam (dJointID, dParamFMax, maximum torque);
你可以設定關節的參數，dParamLoStop是遠動最小範圍，dParamHiStop是運動最大範圍。dParamVel是期望速度，dParamFMax是關節上的最大力矩，下面的例子展示了一個跳躍機器人。
1 // Joint —— a hopping robot
2 #ifdef dDOUBLE
3 #define dsDrawSphere dsDrawSphereD
4 #define dsDrawCapsule dsDrawCapsuleD
5 #endif
6
7 typedef struct {
8 dBodyID body; // a body
9 dGeomID geom; // a geometry
10 dReal radius, length, mass; // radius[m], length[m], mass[kg]
11 } myLink;
12
13 myLink ball, pole;
14
15 static void simLoop (int pause)
16 {
17 const dReal *pos1,*R1,*pos2,*R2;
18 dSpaceCollide(space,0,&nearCallback); // Collision detection
19 dWorldStep(world,0.01); // Step a world for 0.01 [sec]
20 dJointGroupEmpty(contactgroup); // Empty the joint group
21
22 // Draw a ball
23 dsSetColor(1.0,0.0,0.0); // Set red color
24 pos1 = dBodyGetPosition(ball.body); // Get a position
25 R1 = dBodyGetRotation(ball.body); // Get an orientation
26 dsDrawSphere(pos1,R1,ball.radius); // Draw a sphere
27
28 // Draw a capsule
29 pos2 = dBodyGetPosition(pole.body); // Get a position
30 R2 = dBodyGetRotation(pole.body); // Get an orientation
31 dsDrawCapsule(pos2,R2,pole.length,pole.radius); // Draw a capsule
32 }
33
34 // Create an object
35 void createBallPole()
36 {
37 dMass m1, m2;
38 dReal x0 = 0.0, y0 = 0.0, z0 = 2.5; // a ball
39 ball.radius = 0.2;
40 ball.mass = 1.0;
41 ball.body = dBodyCreate(world);
42 dMassSetZero(&m1);
43 dMassSetSphereTotal(&m1,ball.mass,ball.radius);
44 dBodySetMass(ball.body,&m1);
45 dBodySetPosition(ball.body, x0, y0, z0);
46 ball.geom = dCreateSphere(space,ball.radius);
47 dGeomSetBody(ball.geom,ball.body);
48
49 // a capsule
50 pole.radius = 0.025;
51 pole.length = 1.0;
52 pole.mass = 1.0;
53 pole.body = dBodyCreate(world);
54 dMassSetZero(&m2);
55 dMassSetCapsuleTotal(&m2,pole.mass,3,pole.radius,pole.length);
56 dBodySetMass(pole.body,&m2);
57 dBodySetPosition(pole.body, x0, y0, z0 - 0.5 * pole.length);
58 pole.geom = dCreateCapsule(space,pole.radius,pole.length);
59 dGeomSetBody(pole.geom,pole.body);
60
61 // hinge joint
62 joint = dJointCreateHinge(world, 0); // Create a hinge joint
63 dJointAttach(joint, ball.body,pole.body); // Attach joint to bodies
64 dJointSetHingeAnchor(joint, x0, y0, z0 - ball.radius); // Set a joint anchor
65 dJointSetHingeAxis(joint, 1, 0, 0); // Set a hinge axis(1,0,0)
66 }
67
68 int main (int argc, char **argv)
69 {
70 setDrawStuff(); // Set a draw function
71 dInitODE(); // Initialize ODE
72 world = dWorldCreate(); // Create a world
73 space = dHashSpaceCreate(0); // Create a collision space
74 contactgroup = dJointGroupCreate(0); // Create a joint group
75 dWorldSetGravity(world,0,0,-9.8); // Set gravity
76 ground = dCreatePlane(space,0,0,1,0); // Create a ground
77 createBallPole(); // Create ball and capsule
78
79 dsSimulationLoop (argc,argv,352,288,&fn); // Simulation loop
80
81 dWorldDestroy (world); // Destroy the world
82 dCloseODE(); // Close ODE
83 return 0;
84 } 在本例中，機器人由連接配接軀幹和腿的鉸鍊關節組成。讓我們閱讀源代碼。createBallandPole函數是為軀幹建立一個球，為腿建立一個圓柱體，并通過dJointCreateHinge()建立一個關節。主體通過dJointAttach()與接頭連接配接。然後，dSetHingeAnchor()設定關節的錨點，dJointSetHingeAxis()設定鉸節點的軸。在本例中，旋轉軸向量設定為(1,0,0)，即，軸向量是x軸。我沒有設定關節的參數來簡化源代碼。通常，您可以設定關于關節的許多參數。請詳細檢視ODE使用者手冊。