You have established the correct hierarchy for all components in the suspension. Now you must specify which axes each object can rotate about. If an object is free to move in space, it is said to have 6 degrees of freedom. This means that it has 6 ways it can move. It can translate along each of the X, Y, and Z-axes, and it can rotate about each of the X, Y, and Z-axes. In a practical mechanism, such as this suspension, each object must have less than 6 degrees of freedom; otherwise, it would simply fall apart. Therefore you have to specify motion restrictions for each object.
In this section, you will be working on the IK portion of the Hierarchy panel. It will be helpful at this point to look at the rollouts available in IK.
Set up:
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Continue from the previous lesson, or open tut_mech_03b.max from the \tutorials\mech_design directory.
When you create an object that is intended to be part of an IK hierarchy, and you want to set joint parameters before you have applied IK to the object, you must change the object's Position controller to make the joint limits available. This task has already been performed for the objects in the scene you initially opened, but has not been performed for the new dummy objects.
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In the Assign Controller rollout, highlight the Position track, and click Assign Controller.
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Choose the Bezier Position controller.
This controller makes both the Sliding Joints and Rotational Joints rollouts available for the object. You will use these rollouts in the procedures that follow.
Specify active joint axes:
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On the Hierarchy panel, click IK. The IK section of the panel has many controls.
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Close each of the rollouts to get an overall view of the categories.
There are 5 rollouts within IK. For specifying active axes we will use the lower two rollouts, the Sliding Joints and Rotational Joints. The Sliding Joints rollout controls translation along X, Y and Z while the Rotational Joints rollout controls rotation about X, Y, and Z.
Each object’s motion is specified in terms of its parent’s coordinate system. Therefore the reference coordinate system should be set to Parent prior to setting active joint axes.
There are two kinematic chains in the suspension. The upper kinematic chain has the following hierarchy:
UPPER FRAME -> UPPER ARM -> UpperArmDummy -> CARRIER -> WHEEL
The active axes for each object in the hierarchy are indicated as follows:
UPPER ARM: Rotational Joints in Y axis
UpperArmDummy: Rotational Joints in XYZ axes
WHEEL: Rotational Joints in X axis
No other objects have active axes for either sliding or rotational joints.
In the following steps, the active joint axes are set for each object in the upper kinematic chain.
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On the toolbar, click Reference Coordinate System, then choose Parent.
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Right-click in the Perspective viewport to make it active, then click Min/Max toggle.
Set up UPPER FRAME:
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Press H and select the UPPER FRAME from the list. The UPPER FRAME is fixed and will not move in the animation.
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In the Hierarchy panel > IK > Sliding Joints rollout, verify that the Active check box is turned off for each of the X, Y and Z-axes.
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In the Hierarchy panel > IK > Rotational Joints rollout, turn off the Active check boxes for each of the X, Y and Z-axes.
Set up UPPER ARM:
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Press H and select UPPER ARM from the list. The UPPER ARM should rotate about the Y-axis only.
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In the Hierarchy panel > IK > Rotational Joints rollout, turn off the Active check box on both the X and Z-axes.
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The UPPER ARM should not be free to slide along any axis direction. As a default, Sliding Joints is turned off for all object’s axes.
Next the active joint axes for the lower kinematic chain are set.
The lower kinematic chain has the following hierarchy:
LOWER FRAME -> LOWER ARM -> LowerArmDummy
Joint constraints for the lower kinematic chain are summarized as follows:
LOWER ARM: Rotational joint in Y-axis
No other objects have active axes for either sliding or rotational joints.
Set the active joint axes for the lower kinematic chain:
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Press H and select LowerArmDummy from the list. The LowerArmDummy should be fixed with respect to its parent, the LOWER ARM.
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In the Hierarchy panel > IK > Rotational Joints rollout, turn off the Active check box for each of the X, Y and Z-axes.
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Press H and select LOWER ARM from the list. The LOWER ARM should be free to rotate only about the Y-axis.
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In the Hierarchy panel > IK > Rotational Joints rollout, clear the Active check box for both the X and Z-axes.
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In the Hierarchy panel > IK > Rotational Joints rollout, turn off the active check boxes for LOWER FRAME.
Now you will establish the relationship between the two kinematic chains. You will be animating the LOWER ARM, and you want the upper kinematic chain to follow. To accomplish this, you will bind the CARRIER to the LowerArmDummy.
