Here we will walk through how the UR3 in the ArticulationRobot scene was put together.
When building a robot arm with articulation joints, each movable part of the robot should be a child of the previous part. You can see these successive parent-child relationships by expanding the UR3 object in the Hierarchy window.
Next, you must add an ArticulationBody component to each of the game objects that compose the robot arm. You will not need to add Rigidbody components to any parts of the robot arm, but you may still want to add them to other objects in the environment that the robot will interact with. In our example, there are Rigidbody components only on the cube and the table. You do still need to add colliders though.
If you examine the ArticulationBody component on the base of the robot (the UR3 game object), you will notice that it has a very simple interface. This is the root body of the articulation, which plays a special role. The immovable property should be checked if you do not intend this part of the robot to move around.
The successive ArticulationBody components on your robot arm are much more customizable. We will dive into these settings next.
The most important setting here is Articulation Joint Type. The available types are:
Fixed- does not allow any relative movement of the connected bodiesPrismatic- only allows relative translation of the connection bodies along one specified axisRevolute- allows rotational movement around the X axis of the parent's anchorSpherical- allows relative rotations (but not translations) of the two connected bodies
Since all the joints on our robot arm rotate on only one axis, all of our arm's articulation joints are Revolute. Although Revolute joints are contrained to only rotate around the X axis, you can use the Angular Rotation property to rotate the entire articulation body such that its X axis points in the desired direction. The two pincher fingers on the hand are Prismatic because they slide back and forth on one axis.
All the joint types have the following physical parameters:
Linear Damping- Resistance that will slow down linear motion of the jointAngular Damping- Resistance that will slow down angular motion of the jointJoint Friction- Amount of friction that is applied as a result of connected bodies moving relative to this body
There are two ways to move articulation joints - by applying forces, or by using drives. In this project, we use drives. A drive attempts to move the joint to the specified target or at the specified target velocity. Here, we move our revolute joints by updating the target to the desired rotation in degrees. You can see this being done in the RotateTo method in the ArticulationJointController script.
The drive acts like a spring in its attempt to achieve and maintain the target.
On the drive, you can also specify:
Stiffness- The stiffness of the spring connected to this drive.Damping- The damping of the spring attached to this drive.Force Limit- The maximum force this drive can apply to a body.
By default, the Motion property on the articulation components will be set to Free. For a revolute joint, this means that the joint can rotate indefinitely. However, real systems rarely act this way. In the real UR3 robot, safeguards prevent the joints from moving beyond two full revolutions in either direction. Exceeding this limit in the real robot would twist the wires inside and damage the hardware.
We can mimic this in simulation by adding limits to our joints. To do this, change the Motion selection to Limited in the dropdown. Doing so on the revolute joint will add two new properties to the drive - a Lower Limit and an Upper Limit, defined in degrees.