Ball Joint

This section describes the Ball joint entity of MotionView and shows the various usage, creation, and editing methods.

Theory/Background

A Ball joint(also known as a spherical joint; socket joint) is a three degree-of-freedom kinematic pair used in mechanisms. Ball joints provide three-axis rotation function used in many places such as:

•	Steering rack to knuckle via tie-rods

•	Knuckle-to-Control arm joints.

ball_joint_example_mv

Ball joint construction

Joint Definition

The Ball joint is represented using co-incident Markers when exported to an MBD Solver. The representation can be visualized as shown in the figure below:

ball_joint_diagram_mv

Ball joint marker representation

The bodies that are constrained by the ball joint are represented using two markers: Marker I and Marker J. Marker I belongs to Body 1 and Marker J belongs to Body 2. The constraints are applied on the Marker I with respect to the Marker J. Both of the markers will have a common point of origin, the same as the Joint’s Origin point. Marker I is free to rotate about the X, Y, and Z axis of Marker J.

Entity Data Members

The topological information required to define an Ball Joint is shown in the figure below:

ball_joint_diagram2_mv

The data members of the Ball Joint can be classified into the following members:

Connectivity

An Ball Joint needs the following:

•	Body 1 and Body 2 - Specify two bodies between which the joint is to be created.

•	Origin - Specify Point, which defines the location of the joint.

•	Compliant Mode (Optional) - If the Allow Compliance option is selected while adding the joint to a model, the joint's compliance state can be toggled between Compliant and Non-Compliant.

•	Friction Properties (Optional) - can also be assigned (see Properties below).

The joint can be modeled as a Single entity or as a Pair entity.

Properties

Optional Friction properties can also be specified for the joint:

ball_joint_friction_props_diagram_mv

Activating the Use Friction option on the Friction Properties tab will display the following options in the panel:

Coefficient Dynamic

Coefficient Static

Stiction Transition Velocity

Max. Stiction Deformation (Adams only)

Active for Static/Quasi-Static Analysis

True/False

Effect

•	Stiction and Sliding

•	Stiction Only

•	Sliding Only

Input Forces

•	Preload (True/False)

•	Reaction Force (True/False)

Torque Preload

Ball Radius

LuGre Parameters

•	Bristle Stiffness

•	Damping Coefficient

•	Viscous Coefficient

Creating and Editing Joints

To learn how to add an Ball Joint to a model, please see the Joints topic.

1.	Once an Ball Joint has been added to the model using any of the "entity" creation methods, the panel for the joint will automatically be displayed in the panel area. See the panel examples below:

ball_joint_single_panel_mv

Joints Panel (Ball Joint) – Connectivity Tab – Single Entity

ball_joint_pair_panel_mv

Joints Panel (Ball Joint) – Connectivity Tab – Pair Entity

2.	The joint definition needs two bodies which are connected by the joint. Select Body 1 by picking the body from the graphics area, or double click the Body 1 collector to open to the model tree (from which the desired body can be selected).

3.	Similarly, pick Body 2 from the graphics area by clicking on the desired body (or use the collector and model tree).

4.	Select the point of Origin from the Project Browser, or the graphics area, to specify a location for the joint.

5.	Optionally, you can specify frictional properties for the joint. Activate the Use Friction check box on the Friction Properties tab. The attributes related to the Ball joint friction are shown below:

ball_joint_panel_friction_props_tab_mv

Joints panel - Friction Properties tab

Checking the Use Friction option displays a set of input fields which you need to specify as needed by the friction model of the solver. In addition, the LuGre Parameters tab also appears if the SolverMode is set to MotionSolve.

ball_joint_panel_lugre_param_tab_mv

Joints panel - LuGre Parameters tab

Based on the joint selection, the relevant fields are displayed in the Friction Properties tab. These fields are populated by default values as shown in the figure above. There is a option provided in the LuGre Parameters tab which allows you to populate the default LuGre parameters.

For additional details on the various fields in the Friction Properties and LuGre Parameters tab, please refer to the Joints Panel – Friction Properties Tab and Joints Panel - LuGre Parameters Tab topics.

1.	Left click the Joints panel icon on the Constraint toolbar.

The Project Browser will filter the entities and display only the joints in the model.

2.	Select the joint in the Project Browser.

The corresponding panel is automatically displayed.

3.	From the Connectivity tab, use the Joint type drop-down menu to change the joint type, or use the collectors to change the bodies and origins/alignment points of the joint.

ball_joint_panel_select_joint_menu_options_mv

Joints panel - Connectivity tab - joint type drop-down menu

Ball Joint in MDL and XML Formats

The model containing the Ball Joint can be saved in MDL format from MotionView and exported in the MotionSolve XML format.

The Ball Joint can be of the following types:

1.	Non-Compliant - Single and Pair

2.	Compliant – Single and Pair

These four types of entities can be added to the model using the MDL Statements shown below:

Syntax:

*BallJoint(joint_name, "joint_label", body_1, body_2, origin, [ALLOW_COMPLIANCE])

To understand the complete syntax of the MDL statement please refer to the *BallJoint() topic.

To set the friction properties of the joint, the MDL statements to be used are: *SetJointFriction() and *SetJointFrictionLugre().

Syntax:

*SetJointFriction(varname, use_friction, mu_static, mu_dynamic, use_static, transition_vel, max_deformation, effect, preload_input, reaction_force_input, bending_moment_input, torsional_moment_input, f_preload, reaction_arm, initial_overlap, overlap_delta, rot_constraint, t_preload, pin_radius, friction_arm, bending_reaction_arm, ball_radius )

*SetJointFrictionLugre(varname, use_default, bristle_stiffness, damping_effects, viscous_effects )

Note - These two statements for friction are common for all of the joints where friction can be applied. However, a few of the fields might not be applicable to certain joints.

Syntax:

*BallJointPair(joint_name, "joint_label", body_1, body_2, origin, [ALLOW_COMPLIANCE])

To understand the complete syntax of this MDL statement please refer to the *BallJointPair() topic.

To set the friction properties of the joint pair, the MDL statements to be used are: *SetJointFriction() and *SetJointFrictionLugre().

Syntax for Symmetric Joint Pair:

*SetJointFriction(varname, LEFT|RIGHT, use_friction, mu_static, mu_dynamic, use_static, transition_vel, max_deformation, effect, preload_input, reaction_force_input, bending_moment_input, torsional_moment_input, f_preload, reaction_arm, initial_overlap, overlap_delta, rot_constraint, t_preload, pin_radius, friction_arm, bending_reaction_arm, ball_radius )

*SetJointFrictionLugre(varname, use_default_l, bristle_stiffness_l, damping_effects_l, viscous_effects_l,

use_default_r, bristle_stiffness_r, damping_effects_r, viscous_effects_r)

Syntax for Asymmetric Joint Pair:

*SetJointFriction(varname, , use_friction_l, mu_static_l, mu_dynamic_l, use_static_l, transition_vel_l, max_deformation_l, effect_l, preload_input_l, reaction_force_input_l, bending_moment_input_l, torsional_moment_input_l, f_preload_l, reaction_arm_l, initial_overlap_l, overlap_delta_l, rot_constraint_l, t_preload_l, pin_radius_l, friction_arm_l, bending_reaction_arm_l, ball_radius_l, use_friction_r, mu_static_r, mu_dynamic_r, use_static_r, transition_vel_r, max_deformation_r, effect_r, preload_input_r, reaction_force_input_r, bending_moment_input_r, torsional_moment_input_r, f_preload_r, reaction_arm_r, initial_overlap_r, overlap_delta_r, rot_constraint_r, t_preload_r, pin_radius_r, friction_arm_r, bending_reaction_arm_r, ball_radius_r )

Note - These syntax for the LuGre parameters statement for an symmetric joint pair is identical to its symmetric counterpart. MotionView will ignore the part of the statement that belongs to the dependent side of the joint and replicate it with the parent side parameters.

To learn how to create a complete model using MDL Statements please refer to tutorial MV-1060: Introduction to MDL.

The Ball Joint when exported to the MotionSolve XML format is defined as a Constraint_Joint statement.

Syntax:

<Constraint_Joint

id = "integer"

label = "Name of Joint"

type = "JOINT TYPE"

i_marker_id = "integer"

j_marker_id = "integer"

Example:

<Constraint_Joint

id = "301001"

label = "Ball Joint"

type = "SPHERICAL"

i_marker_id = "30103050"

j_marker_id = "30101050"

In the above XML Model statement the i_marker_id and j_marker_id represent the I and J markers of the Joint which belong to Body 1 and Body 2 respectively. To understand the complete syntax of the Constraint_Joint XML model statement, please refer to the MotionSolve Reference Guide Page for Constraint_Joint.

In addition, the friction on the joint is written out as a Force_JointFriction statement in MotionSolve.

Creating and Editing Ball Joints using Tcl

In MotionView, Tcl can be used to add any MDL entities to the model. There are two Tcl commands that can be used to add and edit an entity:

Syntax:

mdlmodel_handle InterpretEntity new_handle keyword varname label

Example:

mdlmodel_handle InterpretEntity joint_ball_handle BallJoint j_ball "\"Ball Joint\"" b_1 B_Ground p_0

Syntax:

mdlmodel_handle InterpretSet keyword tokens

Example:

mdlmodel_handle InterpretSet SetJointFriction j_ball true mu_static mu_dynamic use_static transition_vel max_deformation effect preload_input reaction_force_input bending_moment_input torsional_moment_input f_preload reaction_arm initial_overlap overlap_delta rot_constraint t_preload pin_radius friction_arm bending_reaction_arm ball_radius

mdlmodel_handle InterpretSet SetJointFrictionLugre j_ball use_default bristle_stiffness damping_effects viscous_effects

The InterpretEntity command is used to add entities to the model and the InterpretSet command is used to set the entity properties. So, in the case of the Ball Joint, the properties that can be set are the Joint Initial Conditions. Extended definitions for InterpretEntity and InterpretSet can be found in the HyperWorks Desktop Reference Guide.

Note - When using the InterpretEntity and InterpretSet commands, it is important to also use the Evaluate command in order for the changes to take effect immediately.

To learn how to create a complete model using Tcl commands, please refer to tutorial MV-1040: Model Building Using Tcl.

Example Model

The example file below shows a Ball Joint connecting two bodies:

*BeginMDL( the_model, "Model" )

*StandardInclude(FILE)

*Point( p_0, "Point 0" )

*PointPair( p_1, "Point 1" )

*SetPoint( p_1, LEFT, , -100 )

*Body( b_0, "Body 0", p_0, , , , )

*BodyPair( b_1, "Body 1", p_1, , , , )

*Set( b_0.usecm, true )

*Set( b_1.usecm, true )

*SetBodyInertia( b_1, LEFT, 1, 10000, 10000, 10000 )

//Example - Single Ball Joint

*BallJoint( j_bjsingle, "Ball Joint Single", b_0, B_Ground, p_0 )

//Example - Ball Joint Pair

*BallJointPair( j_bjpair, "Ball Joint pair", b_1, B_Ground, p_1 )

//Example - Setting single Ball Joint properties

*SetJointFriction( j_bjsingle, true, 0.35, 0.26, ACTIVE_STATIC, 0.15, , STICTION_AND_SLIDING, PRELOAD, REACTION_FORCE, , , , , , , , 1000, , , , 10 )

*SetJointFrictionLugre( j_bjsingle, true, 100, 0.316, 0.0004 )

//Example - Setting Ball Joint pair properties

*SetJointFriction( j_bjpair, , true, 0.35, 0.26, ACTIVE_STATIC, 0.15, , STICTION_ONLY, PRELOAD, REACTION_FORCE, , , , , , , , 1000, , , , 10

, true, 0.35, 0.26, ACTIVE_STATIC, 0.15, , STICTION_ONLY, PRELOAD, REACTION_FORCE, , , , , , , , 1000, , , , 10 )

*SetJointFrictionLugre( j_bjpair, false, 100, 0.316, 0.0004, false, 100, 0.316, 0.0004 )

*EndMDL()