Configurable Joint

Configurable Joints are extremely customisable since they incorporate all the functionality of the other joint types. You can use them to create anything from adapted versions of the existing joints to highly specialised joints of your own design.

Properties

Details

Like the other joints, the Configurable Joint allows you to restrict the movement of an object but also to drive it to a target velocity or position using forces. However, there are many configuration options available and they can be quite subtle when used in combination; you may need to experiment with different options to get the joint to behave exactly the way you want.

Constraining Movement

You can constrain both translational movement and rotation on each axis independently using the X, Y, Z Motion and X, Y, Z Rotation properties. If Configured In World Space is enabled then movements will be constrained to the world axes rather than the object’s local axes. Each of these properties can be set to Locked, Limited or Free:

• A Locked axis will allow no movement at all. For example, an object locked in the world Y axis cannot move up or down.
• A Limited axis allows free movement between predefined limits, as explained below. For example, a gun turret might be given a restricted arc of fire by limiting its Y rotation to a specific angular range.
• A Free axis allows any movement.

You can limit translational movement using the Linear Limit property, which defines the maximum distance the joint can move from its point of origin. (measured along each axis separately). For example, you could constrain the puck for an air hockey table by locking the joint in the Y axis (in world space), leaving it free in the Z axis and setting the limit for the X axis to fit the width of the table; the puck would then be constrained to stay within the playing area.

You can also limit rotation using the Angular Limit properties. Unlike the linear limit, these allow you to specify different limit values for each axis. Additionally, you can also define separate upper and lower limits on the angle of rotation for the X axis (the other two axes use the same angle either side of the original rotation). For example, you could construct a “teeter table” using a flat plane with a joint constrained to allow slight tilting in the X and Z directions while leaving the Y rotation locked.

Bounciness and Springs

By default, a joint simply stops moving when it runs into its limit. However, an inelastic collision like this is rare in the real world and so it is useful to add some feeling of bounce to a constrained joint. You can use the Bounciness property of the linear and angular limits to make the constrained object bounce back after it hits its limit. Most collisions will look more natural with a small amount of bounciness but you can also set this property higher to simulate unusually bouncy boundaries like, say, the cushions of a pool table.

The joint limits can be softened further using the spring properties: Linear Limit Spring for translation and Angular X/YZ Limit Spring for rotation. If you set the Spring property to a value above zero, the joint will not abruptly stop moving when it hits a limit but will be drawn back to the limit position by a spring force (the strength of the force is determined by the Spring value). By default, the spring is perfectly elastic and will tend to catapult the joint back in the direction opposite to the collision. However, you can use the Damper property to reduce the elasticity and return the joint to the limit more gently. For example, you might use a spring joint to create a lever that can be pulled to the left or right but then springs back to an upright position. If the springs are perfectly elastic then the lever will tend to oscillate back and forth around the centre point after it is released. However, if you add enough damping then the spring will rapidly settle down to the neutral position.

Drive forces

Not only can a joint react to the movements of the attached object, but it can also actively apply drive forces to set the object in motion. Some joints simple need to keep the object moving at a constant speed as with, say, a rotary motor turning a fan blade. You can set your desired velocity for such joints using the Target Velocity and Target Angluar Velocity properties. You might also require joints that move their object towards a particular position in space (or a particular orientation); you can set these using the Target Position and Target Rotation properties. For example, you could implement a forklift by mounting the forks on a configurable joint and then setting the target height to raise them from a script.

With the target set, the X, Y, Z Drive and Angular X/YZ Drive (or alternatively Slerp Drive) properties then specify the force used to push the joint toward it. The Drives’ Mode property selects whether the joint should seek a target position, velocity or both. The Position Spring and Position Damper work in the same way as for the joint limits when seeking a target position. In velocity mode, the spring force is dependent on the “distance” between the current velocity and the target velocity; the damper helps the velocity to settle at the chosen value rather than oscillating endlessly around it. The Maximum Force property is a final refinement that prevents the force applied by the spring from exceeding a limit value regardless of how far the joint is from its target. This prevents the circumstance where a joint stretched far from its target rapidly snaps the object back in an uncontrolled way.

Note that with all the drive forces (except for Slerp Drive, described below), the force is applied separately in each axis. So, for example, you could implement a spacecraft that has a high forward flying speed but a relatively low speed in sideways steering motion.

Slerp Drive

While the other drive modes apply forces in separate axes, Slerp Drive uses the Quaternion’s spherical interpolation or “slerp” functionality to reorient the joint. Rather than isolating individual axes, the slerp process essentially finds the minimum total rotation that will take the object from the current orientation to the target and applies it on all axes as necessary. Slerp drive is slightly easier to configure and fine for most purposes but does not allow you to specify different drive forces for the X and Y/Z axes.

To enable Slerp drive, you should change the Rotation Drive Mode property from X and YZ to Slerp. Note that the modes are mutually exclusive; the joint will use either the Angular X/YZ Drive values or the Slerp Drive values but not both together.