joint.md

Joint

Joint {
  SFNode jointParameters NULL   # {JointParameters, HingeJointParameters, PROTO}
  SFNode endPoint        NULL   # {Solid, Slot, PROTO}
}

Description

The Joint node is an abstract node (not instantiated) whose derived classes model various types of mechanical joints: hinge (HingeJoint), slider (SliderJoint), ball joint (BallJoint), hinge2 (Hinge2Joint). Apart from the ball joint, joints can be motorized and endowed with PositionSensor nodes.

The Joint node creates a link between its Solid parent and the Solid placed into its endPoint field. Using a SolidReference inside endPoint enables you to close mechanical loops within a Robot or a passive mechanical system.

Field Summary

• jointParameters: this field optionally specifies a JointParameters node or one of its derived classes. These nodes contain common joint parameters such as position, stops, anchor or axis if existing. This field must be filled with an HingeJointParameters node for an HingeJoint or an Hinge2Joint, with a JointParameters node for a SliderJoint (anchor-less) and with a BallJointParameters node for a BallJoint.

  For an Hinge2Joint, the jointParameters field is related to the first rotation axis while an additional field called jointParameters2 refers to the second rotation axis.

  For a BallJoint, the jointParameters field is related to the first rotation axis while two additional fields called jointParameters2 and jointParameters3 refer to the second and third rotation axes.

  3D-vector parameters (e.g axis, anchor) are always expressed in relative coordinates with respect to the closest upper Pose's frame using the meter as unit. If the jointParameters field is not specified, parameters are set with the default values defined in the corresponding parameter node.

• endPoint: this field specifies which Solid will be subjected to the joint constraints. It must be either a Solid child, or a reference to an existing Solid, i.e. a SolidReference. Alternatively, a Slot node can be inserted in the endPoint field, but this Slot should be connected to another Slot whose endPoint is either a Solid or a SolidReference.

Joint's Hidden Position Fields

If the jointParameters is set to NULL, joint positions are then not visible from the Scene Tree. In this case Webots keeps track of the initial positions of Joint nodes by means of hidden position fields. These fields, which are not visible from the Scene Tree, are used to store inside the world file the current joint positions when the simulation is saved. As a result joint positions are restored when reloading the simulation just the same way they would be if JointParameters nodes were used.

For HingeJoint and SliderJoint nodes containing no JointParameters, Webots uses the hidden field named position. For a Hinge2Joint node, an additional hidden field named position2 is used to store the joint position with respect to the second axis. For a BallJoint node, an additional hidden field named position3 is used to store the joint position with respect to the third axis.