7. Joint

This is the 7th ODE (Open Dynamics Engine) tutorial.

This tutorial explains a joint.

There are a lot of joints around us. For example, a hinge of a cellular phone, a hinge of a door, and so on. A joint connect two bodies. In other words, a joint is a constraint which restricts a movement of two bodies. ODE uses a joint in the same meaning of a constraint.

How to use a joint

Create a joint
- dJointCreate***()
connect bodies with the joint
- dJointAttach ()
Set an anchor of the joint
- dJointSet*** Anchor()
Set an axis of the joint
- dJointSet***

*** means a type of joint, such as a Hinge, Ball, Slider, Universal, and so non. Please read theUser Manual of ODE.

How to set parameters of the joint

Set range of motion
dJointSetHingeParam (dJointID, dParamLoStop, the minimum range of motion);
dJointSetHingeParam (dJointID, dParamHiStop, the maximum range of motion);
Set angular velocity and maximum torque
dJointSetHingeParam (dJointID, dParamVel, target angular velocity);
dJointSetHingeParam (dJointID, dParamFMax, maximum torque);

You can set parameters of a joint. dParamLoStop is the minimum and dParamHiStop is the maximum range of motion. dParamVel is target speed and dParamFMax is maximum torque to apply the joint.

Sample program 7

Now, a sample program of a hopping robot (above the figure) is shown as follows.

// sample7.cpp  by Kosei Demura 2006-2009
#ifdef  dDOUBLE
#define dsDrawSphere dsDrawSphereD
#define dsDrawCapsule  dsDrawCapsuleD
#endif

typedef struct {
  dBodyID body;   // a body
  dGeomID geom;　//  a geometry
  dReal   radius,　length,　mass;　// radius[m], length[m], mass[kg]
} myLink;

myLink ball, pole;

static void simLoop (int pause)
{
  const dReal *pos1,*R1,*pos2,*R2;　    
  dSpaceCollide(space,0,&nearCallback); 　// Collision detection
  dWorldStep(world,0.01);　// Step a world for 0.01 [sec]
  dJointGroupEmpty(contactgroup);  // Empty the joint group

  // Draw a ball
  dsSetColor(1.0,0.0,0.0);   // Set red color
  pos1 = dBodyGetPosition(ball.body);　// Get a position
  R1   = dBodyGetRotation(ball.body);　// Get an orientation
  dsDrawSphere(pos1,R1,ball.radius);　 // Draw a sphere

 // Draw a capsule
  pos2 = dBodyGetPosition(pole.body);　// Get a position
  R2   = dBodyGetRotation(pole.body);　// Get an orientation
  dsDrawCapsule(pos2,R2,pole.length,pole.radius);　// Draw a capsule
}

// Create an object
void createBallPole()
{
  dMass m1, m2;
  dReal x0 = 0.0, y0 = 0.0, z0 = 2.5;    // a ball
  ball.radius = 0.2;
  ball.mass   = 1.0;
  ball.body = dBodyCreate(world);
  dMassSetZero(&m1);
  dMassSetSphereTotal(&m1,ball.mass,ball.radius);
  dBodySetMass(ball.body,&m1);
  dBodySetPosition(ball.body, x0, y0, z0);
  ball.geom = dCreateSphere(space,ball.radius);
  dGeomSetBody(ball.geom,ball.body);

  // a capsule
  pole.radius = 0.025;
  pole.length = 1.0;
  pole.mass   = 1.0;
  pole.body =  dBodyCreate(world);
  dMassSetZero(&m2);
  dMassSetCapsuleTotal(&m2,pole.mass,3,pole.radius,pole.length);
  dBodySetMass(pole.body,&m2);
  dBodySetPosition(pole.body, x0, y0, z0 - 0.5 * pole.length);
  pole.geom =  dCreateCapsule(space,pole.radius,pole.length);
  dGeomSetBody(pole.geom,pole.body);

  // hinge joint
  joint = dJointCreateHinge(world, 0);　// Create a hinge joint
  dJointAttach(joint, ball.body,pole.body);　// Attach joint to bodies
  dJointSetHingeAnchor(joint, x0, y0, z0 - ball.radius);　 // Set a joint anchor
  dJointSetHingeAxis(joint, 1, 0, 0);　// Set a hinge axis(1,0,0)
}

int main (int argc, char **argv)
{
  setDrawStuff();　　// Set a draw function   
  dInitODE();       // Initialize ODE
  world = dWorldCreate();　// Create a world
  space = dHashSpaceCreate(0);　// Create a collision space
  contactgroup = dJointGroupCreate(0);　// Create a joint group
  dWorldSetGravity(world,0,0,-9.8);  // Set gravity
  ground = dCreatePlane(space,0,0,1,0); // Create a ground
  createBallPole();　// Create ball and capsule

  dsSimulationLoop (argc,argv,352,288,&fn);  // Simulation loop

  dWorldDestroy (world);　// Destroy the world 　
  dCloseODE();  // Close ODE
  return 0;
}

In this example, a robot is composed of a hinge joint which connects a torso and a leg.

Let’s read the source code. createBallandPole function is to create a ball for a torso , and a cylinder for a leg, and create a joint by dJointCreateHinge(). Bodies were connected with the joint by dJointAttach().

Then, dSetHingeAnchor() sets an anchor of the joint and dJointSetHingeAxis() sets an axis of the hinge joint. This example, the rotation axis vector is set to (1,0,0), i.e., the axis vector is the x-axis.

I did not set parameters of the joints to simplify the source code. Usually, you can set many parameters about joints. Please see the ODE user manual in detail.

You can download the sample program, sample7-090309.zip, from here. Please download and enjoy it.

See you !

demu

APIs

void dJointAttach (dJointID joint, dBodyID body1, dBodyID dBody2)
Attach the joint between body1 and body2

dJointID dJointCreateHinge (dWorldID world, dJointGroupID)
Create a hinge Joint. dJointGroupID is set to zero.

void dJointSetHingeAnchor (dJointID joint, dReal x, dReal y, dReal z)
Set an anchor point (x, y, z) of the hinge joint.
void dJointSetHingeAxis (dJointID joint, dReal x, dReal y, dReal z)
Set an axis vector (x, y, z) of the hinge joint.