Joint——在不同物体间加入关节
在我们周围有许多关节，例如，手机的铰链，门的铰链，等等。一个接头连接两个物体。换句话说，关节是约束两个物体运动的约束。ODE使用与约束含义相同的关节。
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);
你可以设置关节的参数，dParamLoStop是远动最小范围，dParamHiStop是运动最大范围。dParamVel是期望速度，dParamFMax是关节上的最大力矩，下面的例子展示了一个跳跃机器人。
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，机器人由连接躯干和腿的铰链关节组成。让我们阅读源代码。createBallandPole函数是为躯干创建一个球，为腿创建一个圆柱体，并通过dJointCreateHinge()创建一个关节。主体通过dJointAttach()与接头连接。然后，dSetHingeAnchor()设置关节的锚点，dJointSetHingeAxis()设置铰节点的轴。在本例中，旋转轴向量设置为(1,0,0)，即，轴向量是x轴。我没有设置关节的参数来简化源代码。通常，您可以设置关于关节的许多参数。请详细查看ODE用户手册。