public DoublePendulum() {
    super("DoublePendulum");

    j1 = new PinJoint("j1", new Vector3d(0, 0, 2), this, Axis.X);

    Link l1 = new Link("l1");
    l1.setComOffset(0, 0, 0.5);
    l1.setMassAndRadiiOfGyration(1.0, 0.05, 0.05, 0.3);
    l1.addEllipsoidFromMassProperties(YoAppearance.Pink());
    j1.setLink(l1);

    j2 = new PinJoint("j2", new Vector3d(0.0, 0.0, 1.0), this, Axis.X);
    Link l2 = new Link("l2");
    l2.setComOffset(0, 0, 0.5);
    l2.setMassAndRadiiOfGyration(1.0, 0.05, 0.05, 0.3);
    l2.addEllipsoidFromMassProperties(YoAppearance.Purple());

    j2.setLink(l2);

    j1.addJoint(j2);
    addRootJoint(j1);
  }
  /** Joints */
  public Step2Robot() {
    super("v2Robot");
    this.setGravity(-9.81);

    // Body (2DOF = Z + Pitch)
    bodyJoint1 = new SliderJoint("bodyZ", new Vector3d(0.0, 0.0, 0.0), this, Axis.Z);
    bodyJoint1.setDynamic(true);
    Link bodyLinkSlider = setNullLink();
    bodyJoint1.setLink(bodyLinkSlider);
    this.addRootJoint(bodyJoint1);
    bodyJoint1.setInitialState(legHeight, 0.0);

    bodyJoint2 = new PinJoint("bodyPitch", new Vector3d(0.0, 0.0, 0.0), this, Axis.Y);
    bodyJoint2.setDynamic(true);
    Link bodyLinkPitch = body();
    bodyJoint2.setLink(bodyLinkPitch);
    bodyJoint1.addJoint(bodyJoint2);

    // Upper Joint
    hipJoint = new PinJoint("hip", new Vector3d(0.0, 0.0, 0.0), this, Axis.Y);
    hipJoint.setDynamic(true);
    hipJoint.setLimitStops(0.0, 1.0, 1e6, 1e3);
    Link upperLink = upperLink();
    hipJoint.setLink(upperLink);
    bodyJoint2.addJoint(hipJoint);

    // Lower Joint
    kneeJoint = new SliderJoint("knee", new Vector3d(0.0, 0.0, -upperLinkLength), this, Axis.Z);
    kneeJoint.setDynamic(true);
    kneeJoint.setLimitStops(0.0, 0.6, 1e9, 1e2);
    Link lowerLink = lowerLink();
    kneeJoint.setLink(lowerLink);
    hipJoint.addJoint(kneeJoint);

    // Visible ground contact point
    GroundContactPoint contactPoint =
        new GroundContactPoint("Foot", new Vector3d(0.0, 0.0, gcOffset), this);
    kneeJoint.addGroundContactPoint(contactPoint);
    Graphics3DObject graphics = kneeJoint.getLink().getLinkGraphics();
    graphics.identity();
    graphics.translate(0.0, 0.0, gcOffset);
    double radius = 0.03;
    graphics.addSphere(radius, YoAppearance.Orange());

    // Ground Model
    GroundContactModel groundModel =
        new LinearGroundContactModel(
            this, 1500, 150, 50000.0, 1e5, this.getRobotsYoVariableRegistry());
    GroundProfile3D profile = new FlatGroundProfile();
    groundModel.setGroundProfile3D(profile);
    this.setGroundContactModel(groundModel);
  }
  /** Joints */
  public Step6IDandSCSRobot_pinKnee() {
    super("Robot");

    /** **************** ID ROBOT ********************** */
    elevator = new RigidBody("elevator", elevatorFrame);

    bodyJointID = new SixDoFJoint(JointNames.BODY.getName(), elevator, elevatorFrame);
    createAndAttachBodyRB(LinkNames.BODY_LINK, bodyJointID);

    for (RobotSide robotSide : RobotSide.values) {
      // HIP ID (location, joint, and rigidBody)
      Vector3d hipOffset = new Vector3d(0.0, robotSide.negateIfRightSide(hipOffsetY), 0.0);
      RevoluteJoint hipJointID =
          ScrewTools.addRevoluteJoint(
              JointNames.HIP.getName(),
              bodyJointID.getSuccessor(),
              hipOffset,
              jointAxesPinJoints); // The parent rigid body of the hip joint is:
                                   // bodyJointID.getSuccessor()
      allLegJoints.get(robotSide).put(JointNames.HIP, hipJointID);
      createAndAttachCylinderRB(LinkNames.UPPER_LINK, JointNames.HIP, robotSide);

      // KNEE ID
      Vector3d kneeOffset = new Vector3d(0.0, 0.0, kneeOffsetZ);
      RevoluteJoint kneeJointID =
          ScrewTools.addRevoluteJoint(
              JointNames.KNEE.getName(), hipJointID.getSuccessor(), kneeOffset, jointAxesPinJoints);
      allLegJoints.get(robotSide).put(JointNames.KNEE, kneeJointID);
      createAndAttachCylinderRB(LinkNames.LOWER_LINK, JointNames.KNEE, robotSide);

      // ANKLE ID (location, joint, and rigidBody)
      Vector3d ankleOffset = new Vector3d(0.0, 0.0, ankleOffsetZ);
      RevoluteJoint ankleJointID =
          ScrewTools.addRevoluteJoint(
              JointNames.ANKLE.getName(),
              kneeJointID.getSuccessor(),
              ankleOffset,
              jointAxesPinJoints);
      allLegJoints.get(robotSide).put(JointNames.ANKLE, ankleJointID);
      createAndAttachFootRB(LinkNames.FOOT_LINK, JointNames.ANKLE, robotSide);
    }

    /** **************** SCS ROBOT ********************** */
    // BODY SCS
    bodyJointSCS = new FloatingPlanarJoint(JointNames.BODY.getName(), this);
    this.addRootJoint(bodyJointSCS);
    createAndAttachBodyLink(LinkNames.BODY_LINK);
    bodyJointSCS.setCartesianPosition(0.0, initialBodyHeight);

    for (RobotSide robotSide : RobotSide.values) {
      // HIP SCS
      PinJoint hipJointSCS =
          new PinJoint(
              robotSide.getSideNameFirstLetter() + JointNames.HIP.getName(),
              new Vector3d(0.0, robotSide.negateIfRightSide(hipOffsetY), 0.0),
              this,
              jointAxesPinJoints);
      hipJointSCS.setLimitStops(-0.75, 0.75, 1e3, 1e1);
      // It is NOT necessary to set limits in the ID description because if the SCS description
      // doesn't let the robot move passed a point the ID robot won't be able to pass it either

      if (robotSide == RobotSide.LEFT) {
        hipJointSCS.setQ(-0.6);
      } else {
        hipJointSCS.setQ(0.1);
      }

      bodyJointSCS.addJoint(hipJointSCS);
      idToSCSLegJointMap.put(allLegJoints.get(robotSide).get(JointNames.HIP), hipJointSCS);
      createAndAttachCylinderLink(LinkNames.UPPER_LINK, JointNames.HIP, robotSide);

      // KNEE SCS
      PinJoint kneeJointSCS =
          new PinJoint(
              robotSide.getSideNameFirstLetter() + JointNames.KNEE.getName(),
              new Vector3d(0.0, 0.0, kneeOffsetZ),
              this,
              jointAxesPinJoints);
      kneeJointSCS.setLimitStops(-0.01, 1.8, 1e5, 1e3);
      hipJointSCS.addJoint(kneeJointSCS);
      idToSCSLegJointMap.put(allLegJoints.get(robotSide).get(JointNames.KNEE), kneeJointSCS);
      createAndAttachCylinderLink(LinkNames.LOWER_LINK, JointNames.KNEE, robotSide);

      // ANKLE SCS
      PinJoint ankleJointSCS =
          new PinJoint(
              robotSide.getSideNameFirstLetter() + JointNames.ANKLE.getName(),
              new Vector3d(0.0, 0.0, ankleOffsetZ),
              this,
              jointAxesPinJoints);
      ankleJointSCS.setLimitStops(-0.7, 0.7, 1e3, 1e2);

      if (robotSide == RobotSide.RIGHT) {
        ankleJointSCS.setQ(-0.1);
      }

      kneeJointSCS.addJoint(ankleJointSCS);
      idToSCSLegJointMap.put(allLegJoints.get(robotSide).get(JointNames.ANKLE), ankleJointSCS);

      // FEET SCS
      createAndAttachFootLink(LinkNames.FOOT_LINK, JointNames.ANKLE, robotSide);
      GroundContactPoint gcHeel =
          new GroundContactPoint(
              robotSide.getSideNameFirstLetter() + "gcHeel",
              new Vector3d(-0.1, 0.0, gcOffset),
              this);
      GCpointsHeel.set(robotSide, gcHeel);
      ankleJointSCS.addGroundContactPoint(gcHeel);
      Graphics3DObject graphicsGCheel = ankleJointSCS.getLink().getLinkGraphics();
      graphicsGCheel.identity();
      graphicsGCheel.translate(-0.1, 0.0, gcOffset);
      graphicsGCheel.addSphere(gcRadius, YoAppearance.Orange());

      if (robotSide == RobotSide.RIGHT) {
        setFStoTrue(robotSide);
      }

      GroundContactPoint gcToe =
          new GroundContactPoint(
              robotSide.getSideNameFirstLetter() + "gcToe", new Vector3d(0.4, 0.0, gcOffset), this);
      GCpointsToe.set(robotSide, gcToe);
      ankleJointSCS.addGroundContactPoint(gcToe);
      Graphics3DObject graphics = ankleJointSCS.getLink().getLinkGraphics();
      graphics.identity();
      graphics.translate(0.4, 0.0, gcOffset);
      graphics.addSphere(gcRadius, YoAppearance.DarkOliveGreen());
    }

    /** ************** SCS Ground Model ************************ */
    GroundContactModel groundModel =
        new LinearGroundContactModel(this, 1e3, 1e3, 5e3, 1e3, this.getRobotsYoVariableRegistry());
    GroundProfile3D profile = new FlatGroundProfile();
    groundModel.setGroundProfile3D(profile);
    this.setGroundContactModel(groundModel);
  }
 public double getBodyPitchVel() {
   return bodyJoint2.getQD();
 }
 // ************************* Part 2
 public double getBodyPitch() {
   return bodyJoint2.getQ();
 }