Creating a Simple Mechanism Using Various Types of Joints

Create geometry for all parts of the mechanism.
Assign bodies and collision shapes to the parts.
Set up masses for the parts.
Notice
It is very important to ensure mass balance – avoid connection of too heavy bodies to light ones, otherwise the joints may become unstable!
Connect all parts of the mechanism using appropriate types of joints. Set up joint parameters.
Animate the mechanism using joint motors.

Creating Geometry and Adding Some Physics

The first thing we are going to address in this tutorial is the geometry of our mechanism (see the picture above). We are going to create the following parts:

2 blue guide bars
a red piston
a green rod
a black wheel
2 dummy objects with dummy bodies to attach the parts of our mechanism to.

We are also going to add some physical parameters to the geometry such as mass and collision shapes. So, we need a set of basic functions:

Source code (C#)

/// method, creating a named dummy body of a specified size at pos
ObjectDummy createBodyDummy(String name, vec3 size, dmat4 transform)
{
	// creating a dummy object
	ObjectDummy dummy = new ObjectDummy();
        
	// setting parameters
	dummy.setWorldTransform(transform);
	dummy.setName(name);

	//assigning a dummy body to the dummy object and adding a box shape	to it
	new BodyDummy(dummy.getObject());
	dummy.getBody().addShape(new ShapeBox(size).getShape(), MathLib.translate(0.0f, 0.0f, 0.0f));

	return dummy;
}

/// method, creating a named box having a specified size, color and transformation with a body and a shape
ObjectMeshDynamic createBodyBox(String name, vec3 size, float mass, vec4 color, dmat4 transform)
{
    // creating geometry and setting up its parameters (name, material, property and transformation)
    ObjectMeshDynamic OMD = Primitives.createBox(size);
    OMD.setWorldTransform(transform);
    OMD.setMaterial("mesh_base", "*");
    OMD.setMaterialParameter("albedo_color", color, 0);
    OMD.setProperty("surface_base", "*");
    OMD.setName(name);

    // adding physics, i.e. a rigid body and a box shape with specified mass
    BodyRigid body = new BodyRigid(OMD.getObject());

    body.addShape(new ShapeBox(size).getShape(), MathLib.translate(new vec3(0.0f)));
    OMD.getBody().getShape(0).setMass(mass);

    return OMD;
}

/// method, creating a named cylinder having a specified size, color and transformation with a body and a shape
ObjectMeshDynamic createBodyCylinder(String name, float radius, float height, float mass, vec4 color, dmat4 transform)
{
    // creating geometry and setting up its parameters (name, material, property and transformation)
    ObjectMeshDynamic OMD = Primitives.createCylinder(radius, height, 1, 32);
    OMD.setWorldTransform(transform);
    OMD.setMaterial("mesh_base", "*");
    OMD.setMaterialParameter("albedo_color", color, 0);
    OMD.setProperty("surface_base", "*");
    OMD.setName(name);

    // adding physics, i.e. a rigid body and a cylinder shape with specified mass
    BodyRigid body = new BodyRigid(OMD.getObject());
    body.addShape(new ShapeCylinder(radius, height).getShape(), MathLib.translate(new vec3(0.0f)));
    OMD.getBody().getShape(0).setMass(mass);

    return OMD;
}

Now using these functions we can create our mechanism. We are going to use DynamicMesh objects for the parts and Dummy objects for mounting points.

Source code (C#)

// creating parts of the mechanism
dmat4 transform = new dmat4(MathLib.translate(0.0f, 0.0f, 10.0f) * MathLib.rotateY(90.0f));

piston = createBodyBox("piston", new vec3(1.0f, 2.0f, 0.5f), 15.0f, new vec4(1.0f, 0.1f, 0.1f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, 3.5f, 0.0f)));
guide_bar1 = createBodyBox("guide_bar1", new vec3(1.0f, 9.0f, 0.5f), 15.0f, new vec4(0.0f, 0.1f, 0.7f, 1.0f), transform * new dmat4(MathLib.translate(1.0f, 3.0f, 0.0f)));
guide_bar2 = createBodyBox("guide_bar2", new vec3(1.0f, 9.0f, 0.5f), 15.0f, new vec4(0.0f, 0.1f, 0.7f, 1.0f), transform * new dmat4(MathLib.translate(-1.0f, 3.0f, 0.0f)));
wheel = createBodyCylinder("wheel", 3.0f, 0.25f, 25.0f, new vec4(0.1f, 0.1f, 0.1f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, -15.0f, -0.5f)));
rod = createBodyCylinder("rod", 0.1f, 15.0f, 5.0f, new vec4(0.1f, 1.1f, 0.1f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, -5.0f, 0.0f) * MathLib.rotateX(90.0f)));

//creating mounting points
dummy1 = createBodyDummy("dummy1", new vec3(1.0f, 1.0f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, -15.0f, -1.5f)));
dummy2 = createBodyDummy("dummy2", new vec3(1.0f, 1.0f, 0.5f), transform * new dmat4(MathLib.translate(0.0f, 7.0f, 0.0f)));

Adding and Setting Up Joints

Now that we have created all parts of our mechanism with physical bodies and shapes, let us link them together with joints.

First, we are going to attach the wheel to the first mounting point(dummy1). The wheel is going to rotate around its axis. Let us use a cylindrical joint here. As the center of the wheel is aligned with the center of dummy1, we may use a simple constructor JointCylindrical() and specify only bodies, the anchor point will be placed automatically between their centers of mass.

Then we must set the coordinates (in the world space) of the axis of wheel rotation. In our case it is (1.0f, 0.0f, 0.0f). And finally set other joint parameters.

Source code (C#)

// creating a cylindrical joint
jc = new JointCylindrical(dummy1.getBody(), wheel.getBody());

// setting rotation axis in world coordinates
jc.setWorldAxis(new vec3(1.0f, 0.0f, 0.0f));

// setting common joint constraint parameters
jc.setLinearRestitution(0.4f);
jc.setAngularRestitution(0.4f);
jc.setLinearSoftness(0.4f);
jc.setAngularSoftness(0.4f);

// setting linear and angular damping
jc.setLinearDamping(4.0f);
jc.setAngularDamping(2.0f);

// setting small linear limits as we are not going to use this degree of freedom to the full extent
jc.setLinearLimitFrom(-0.0005f);
jc.setLinearLimitTo(0.0005f);

// setting number of iterations
jc.setNumIterations(16);

Now let us attach the rod to the wheel and to the piston. This is where we need a hinge joint. For both of these joints we have to specify anchor point and joint axis coordinates in the JointHinge() constructor

Source code (C#)

// creating hinge joints
jh = new JointHinge(wheel.getBody(), rod.getBody(), new dvec3(0.0f, -12.5f, 10.0f), new vec3(1.0f, 0.0f, 0.0f));
jh2 = new JointHinge(rod.getBody(), piston.getBody(), new dvec3(0.0f, 2.5f, 10.0f), new vec3(1.0f, 0.0f, 0.0f));

// setting number of iterations
jh.setNumIterations(8);
jh2.setNumIterations(8);

The next thing we are going to do is to attach 2 guide bars to the second mounting point(dummy2) using a pair of fixed joints. For both of these joints we will specify anchor point coordinates in the JointFixed() constructor

Source code (C#)

// creating fixed joints
jf1 = new JointFixed(dummy2.getBody(), guide_bar1.getBody(), new dvec3(0.0f, 7.0f, 9.0f));
jf2 = new JointFixed(dummy2.getBody(), guide_bar2.getBody(), new dvec3(0.0f, 7.0f, 11.0f));

// setting number of iterations
jf1.setNumIterations(1);
jf2.setNumIterations(1);

And the last joint we are going to use is a prismatic joint to attach the piston to the second mounting point(dummy2). Here we will specify joint axis coordinates, linear limits to determine the range of motion for the piston and linear softness

Source code (C#)

// creating a prismatic joint
jp = new JointPrismatic(piston.getBody(), dummy2.getBody());
jp.setWorldAxis(new vec3(0.0f, 1.0f, 0.0f));

// setting linear limits [-5.0; 0.0] and softness
jp.setLinearLimitFrom(-5.0f);
jp.setLinearLimitTo(0.0f);
jp.setLinearSoftness(0.5f);

// setting number of iterations
jp.setNumIterations(8);

Using Joint Motors

To animate the mechanism we are going to use the motor of our cylindrical joint. In order to make it move we must set angular velocity and torque:

Source code (C#)

// setting up motor parameters for a cylindrical joint to animate the whole mechanism
jc.setAngularVelocity(1000.0f);
jc.setAngularTorque(150.0f);

Putting it All Together

In this section let us sum up all described above. The final code for our tutorial will be as follows:

Source code (C#)

// AppWorldLogic.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Unigine;

namespace UnigineApp
{
	class AppWorldLogic : WorldLogic
	{
		PlayerSpectator player;
		
        // parts of the mechanism
        ObjectDummy dummy1;
        ObjectDummy dummy2;
        ObjectMeshDynamic rod;
        ObjectMeshDynamic wheel;
        ObjectMeshDynamic piston;
        ObjectMeshDynamic guide_bar1;
        ObjectMeshDynamic guide_bar2;

        // joints of the mechanism
        JointCylindrical jc;
        JointHinge jh;
        JointHinge jh2;
        JointFixed jf1;
        JointFixed jf2;
        JointPrismatic jp;

        /// method, creating a named dummy body of a specified size at pos
        ObjectDummy createBodyDummy(String name, vec3 size, dmat4 transform)
        {
	        // creating a dummy object
	        ObjectDummy dummy = new ObjectDummy();
        
	        // setting parameters
	        dummy.setWorldTransform(transform);
	        dummy.setName(name);

	        //assigning a dummy body to the dummy object and adding a box shape	to it
	        new BodyDummy(dummy.getObject());
	        dummy.getBody().addShape(new ShapeBox(size).getShape(), MathLib.translate(0.0f, 0.0f, 0.0f));

	        return dummy;
        }

        /// method, creating a named box having a specified size, color and transformation with a body and a shape
        ObjectMeshDynamic createBodyBox(String name, vec3 size, float mass, vec4 color, dmat4 transform)
        {
	        // creating geometry and setting up its parameters (name, material, property and transformation)
	        ObjectMeshDynamic OMD = Primitives.createBox(size);
	        OMD.setWorldTransform(transform);
	        OMD.setMaterial("mesh_base", "*");
	        OMD.setMaterialParameter("albedo_color", color, 0);
	        OMD.setProperty("surface_base", "*");
	        OMD.setName(name);

	        // adding physics, i.e. a rigid body and a box shape with specified mass
	        BodyRigid body = new BodyRigid(OMD.getObject());

	        body.addShape(new ShapeBox(size).getShape(), MathLib.translate(new vec3(0.0f)));
	        OMD.getBody().getShape(0).setMass(mass);

	        return OMD;
        }

        /// method, creating a named cylinder having a specified size, color and transformation with a body and a shape
        ObjectMeshDynamic createBodyCylinder(String name, float radius, float height, float mass, vec4 color, dmat4 transform)
        {
	        // creating geometry and setting up its parameters (name, material, property and transformation)
	        ObjectMeshDynamic OMD = Primitives.createCylinder(radius, height, 1, 32);
	        OMD.setWorldTransform(transform);
	        OMD.setMaterial("mesh_base", "*");
	        OMD.setMaterialParameter("albedo_color", color, 0);
	        OMD.setProperty("surface_base", "*");
	        OMD.setName(name);

	        // adding physics, i.e. a rigid body and a cylinder shape with specified mass
	        BodyRigid body = new BodyRigid(OMD.getObject());
	        body.addShape(new ShapeCylinder(radius, height).getShape(), MathLib.translate(new vec3(0.0f)));
	        OMD.getBody().getShape(0).setMass(mass);

	        return OMD;
        }

		/* .. */
		
		public override int init()
		{

            // setting up physics parameters
	        Physics.get().setGravity(new vec3(0.0f, 0.0f, -9.8f * 2.0f));
	        Physics.get().setFrozenLinearVelocity(0.1f);
	        Physics.get().setFrozenAngularVelocity(0.1f);
	
	        // setting up player
	        player = new PlayerSpectator();
	        player.release();
	        player.setPosition(new dvec3(22.0f, -2.0f, 10.0f));
	        player.setDirection(new vec3(-10.0f, -2.0f, 0.0f), new vec3(0.0f, 0.0f, -1.0f));
	        Game.get().setPlayer(player.getPlayer());
            // creating parts of the mechanism
            dmat4 transform = new dmat4(MathLib.translate(0.0f, 0.0f, 10.0f) * MathLib.rotateY(90.0f));

            piston = createBodyBox("piston", new vec3(1.0f, 2.0f, 0.5f), 15.0f, new vec4(1.0f, 0.1f, 0.1f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, 3.5f, 0.0f)));
            guide_bar1 = createBodyBox("guide_bar1", new vec3(1.0f, 9.0f, 0.5f), 15.0f, new vec4(0.0f, 0.1f, 0.7f, 1.0f), transform * new dmat4(MathLib.translate(1.0f, 3.0f, 0.0f)));
            guide_bar2 = createBodyBox("guide_bar2", new vec3(1.0f, 9.0f, 0.5f), 15.0f, new vec4(0.0f, 0.1f, 0.7f, 1.0f), transform * new dmat4(MathLib.translate(-1.0f, 3.0f, 0.0f)));
            wheel = createBodyCylinder("wheel", 3.0f, 0.25f, 25.0f, new vec4(0.1f, 0.1f, 0.1f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, -15.0f, -0.5f)));
            rod = createBodyCylinder("rod", 0.1f, 15.0f, 5.0f, new vec4(0.1f, 1.1f, 0.1f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, -5.0f, 0.0f) * MathLib.rotateX(90.0f)));

            //creating mounting points
            dummy1 = createBodyDummy("dummy1", new vec3(1.0f, 1.0f, 1.0f), transform * new dmat4(MathLib.translate(0.0f, -15.0f, -1.5f)));
            dummy2 = createBodyDummy("dummy2", new vec3(1.0f, 1.0f, 0.5f), transform * new dmat4(MathLib.translate(0.0f, 7.0f, 0.0f)));


            // creating a cylindrical joint
	        jc = new JointCylindrical(dummy1.getBody(), wheel.getBody());

	        // setting rotation axis in world coordinates
	        jc.setWorldAxis(new vec3(1.0f, 0.0f, 0.0f));

	        // setting common joint constraint parameters
	        jc.setLinearRestitution(0.4f);
	        jc.setAngularRestitution(0.4f);
	        jc.setLinearSoftness(0.4f);
	        jc.setAngularSoftness(0.4f);

	        // setting linear and angular damping
	        jc.setLinearDamping(4.0f);
	        jc.setAngularDamping(2.0f);

	        // setting small linear limits as we are not going to use this degree of freedom to the full extent
	        jc.setLinearLimitFrom(-0.0005f);
	        jc.setLinearLimitTo(0.0005f);

	        // setting number of iterations
	        jc.setNumIterations(16);


            // creating hinge joints
	        jh = new JointHinge(wheel.getBody(), rod.getBody(), new dvec3(0.0f, -12.5f, 10.0f), new vec3(1.0f, 0.0f, 0.0f));
	        jh2 = new JointHinge(rod.getBody(), piston.getBody(), new dvec3(0.0f, 2.5f, 10.0f), new vec3(1.0f, 0.0f, 0.0f));

	        // setting number of iterations
	        jh.setNumIterations(8);
	        jh2.setNumIterations(8);


            // creating fixed joints
	        jf1 = new JointFixed(dummy2.getBody(), guide_bar1.getBody(), new dvec3(0.0f, 7.0f, 9.0f));
	        jf2 = new JointFixed(dummy2.getBody(), guide_bar2.getBody(), new dvec3(0.0f, 7.0f, 11.0f));

	        // setting number of iterations
	        jf1.setNumIterations(1);
	        jf2.setNumIterations(1);	

            // creating a prismatic joint
            jp = new JointPrismatic(piston.getBody(), dummy2.getBody());
            jp.setWorldAxis(new vec3(0.0f, 1.0f, 0.0f));

            // setting linear limits [-5.0; 0.0] and softness
            jp.setLinearLimitFrom(-5.0f);
            jp.setLinearLimitTo(0.0f);
            jp.setLinearSoftness(0.5f);

            // setting number of iterations
            jp.setNumIterations(8);

            // setting up motor parameters for a cylindrical joint to animate the whole mechanism
            jc.setAngularVelocity(1000.0f);
            jc.setAngularTorque(150.0f);	

			return 1;
		}
		
		/* .. */
		
	}
}

Last update: 2018-08-10

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Creating a Simple Mechanism Using Various Types of Joints

In This Article:

Creating Geometry and Adding Some Physics

Adding and Setting Up Joints

Using Joint Motors

Putting it All Together