Configurable Joint

Configurable Joints incorporate all the functionality of the other joint types and provide greater control of character movement. They are particularly useful when you want to customize the movement of a ragdoll and enforce certain poses on your characters. You can also use them to adapt jointsA physics component allowing a dynamic connection between Rigidbody components, usually allowing some degree of movement such as a hinge. More info
See in Glossary into highly specialized joints of your own design.

Properties

Details

Like the other joints, you can use the Configurable Joint to restrict the movement of an object but you can also use it to drive it to a target velocity or position with forces. Because there are a lot of configuration options,you may need to experiment with them to get the joint to behave exactly the way you want.

Constraining movement

You can constrain both the translational movement and rotation on each joint axis independently using the X, Y, Z Motion and X, Y, Z Rotation properties. Enable Configured In World Space to constrain movements 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 restricts all movement, so the joint cannot move 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 can 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 is then constrained to stay within the playing area.

You can also limit rotation using the Angular Limit properties. Unlike the linear limit, you can specify different limit values for each axis with this property. 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 can construct a “teeter table” by 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 stops moving when it runs into its limit. However, an inelastic collisionA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
See in Glossary like this is rare in the real world and so it is useful to add some feeling of bounce to a constrained joint. To make the constrained object bounce back after it hits its limit, use the Bounciness property of the linear and angular limits. Most collisions look more natural with a small amount of bounciness but you can also set this property higher to simulate unusually bouncy boundaries like the cushions of a pool table.

To soften the joint limits even more, use 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 does not abruptly stop moving when it hits a limit, but is 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 catapult the joint back in the opposite direction to the collision.

You can use the Damper property to reduce the elasticity and return the joint to the limit more gently. For example, you can use a spring jointA joint type that connects two Rigidbody components together but allows the distance between them to change as though they were connected by a spring. More info
See in Glossary to create a lever that you can pull to the left or right, but then it springs back to an upright position. If the springs are perfectly elastic then the lever will 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 object it is attached to, but it can also actively apply drive forces to set the object in motion. Some joints need to keep the object moving at a constant speed, such as a rotary motor turning a fan blade. Use the Target Velocity and Target Angular Velocity properties to set your desired velocity for such joints.

You might need to use joints that move their object towards a particular position in space,or a particular orientation. Use the Target Position and Target Rotation properties to set this functionality. For example, to implement a forklift, mount the forks on a configurable joint and then set 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 specify the force to use 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 they seek a target position. In velocity mode, the spring force depends 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. For example, the formula for the XDrive force is:

force = PositionSpring * (target position - position) + PositionDamper * (targetVelocity - velocity)

So, the force grows proportional to the difference between the current value and the target value, and reduces by the damper proportional to the difference between the current velocity and the target one. Unity applies the force to both positional and rotational drives.

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 a joint that you stretch far from its target from rapidly snapping the object back in an uncontrolled way.

When you use any drive force (except for Slerp Drive, described below), the joint applies the force to the object separately in each axis. For example, you can implement a spacecraft that has a high forward flying speed but a relatively low speed in sideways steering motion.

Slerp Drive

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

To enable Slerp drive, change the Rotation Drive Mode property from X and YZ to Slerp. A joint can use either the Angular X/YZ Drive values or the Slerp Drive values, but not both.

• 2019–05–23 Page amended