A revolute joint is the rotating hinge used in many robot arms, especially at the shoulder, elbow, and wrist. It connects two rigid links while allowing one link to rotate relative to the other about a fixed axis. This single rotational degree of freedom makes motion easier to model, control, and build with motors and bearings.
Understanding revolute joints is essential for predicting how a robot arm reaches, lifts, and positions objects.
Inside a revolute joint, a shaft, bearing, and actuator guide rotation around the joint axis while resisting unwanted translation or sideways motion. The joint angle, often written as theta, describes the orientation of one link relative to the next. Motors apply torque to change theta, while sensors such as encoders measure the actual angle for feedback control.
By combining several revolute joints, a robot can create complex paths from simple rotations.
Key Facts
- A revolute joint has 1 rotational degree of freedom and ideally 0 translational degrees of freedom.
- The joint variable is an angle, commonly written as theta, measured in degrees or radians.
- Angular displacement in radians is theta = s / r, where s is arc length and r is radius.
- Angular speed is omega = Delta theta / Delta t.
- Torque about the joint axis is tau = rF sin(theta), where theta is the angle between the lever arm and force.
- Mechanical work done by a rotating joint is W = tau Delta theta when torque is constant and Delta theta is in radians.
Vocabulary
- Revolute joint
- A mechanical joint that allows rotation about one fixed axis like a hinge.
- Degree of freedom
- An independent way a part can move, such as one rotation or one translation.
- Joint axis
- The imaginary line about which a revolute joint rotates.
- Torque
- A turning effect produced by a force acting at a distance from an axis of rotation.
- Range of motion
- The allowed interval of joint angles between the mechanical or programmed limits.
Common Mistakes to Avoid
- Treating a revolute joint like a sliding joint. A revolute joint changes angle about an axis, while a sliding or prismatic joint changes position along a line.
- Forgetting to use radians in rotational equations. Formulas such as W = tau Delta theta require Delta theta in radians, not degrees.
- Assuming the whole joint can move in any direction. An ideal revolute joint only allows one rotation and constrains the other motions.
- Measuring the joint angle from the wrong reference link. The angle theta must be defined consistently between two connected links or the kinematics will be incorrect.
Practice Questions
- 1 A revolute elbow joint rotates from 20 degrees to 95 degrees in 3.0 s. What is its average angular speed in degrees per second and in radians per second?
- 2 A motor applies a constant torque of 12 N m to a revolute joint through an angular displacement of 1.5 rad. How much mechanical work is done?
- 3 A robot elbow uses a revolute joint instead of a prismatic joint. Explain how this choice affects the motion of the forearm link and why it is useful for a robot arm.