A SCARA robot is a common industrial arm used for fast pick-and-place, assembly, dispensing, and packaging tasks. SCARA stands for Selective Compliance Assembly Robot Arm, which describes how the arm is flexible in the horizontal plane but stiff in the vertical direction. This combination lets it move quickly across a work surface while pressing or inserting parts with good vertical accuracy.
Its simple joint layout also makes it easier to model than many six-axis robots.
A typical SCARA arm has two parallel-axis revolute joints that swing two horizontal links, plus a vertical prismatic joint that raises and lowers the end effector. Many SCARA robots also include a wrist rotation about the vertical axis for orienting grippers or tools. The two rotary joints determine the x and y position of the tool, while the vertical slide determines z position.
This makes the configuration well suited for planar motion, short cycle times, and repeated operations over a table or conveyor.
Key Facts
- SCARA stands for Selective Compliance Assembly Robot Arm.
- A basic SCARA position system uses two revolute joints for x-y motion and one prismatic joint for z motion.
- Degrees of freedom for a common SCARA arm: 4, including shoulder rotation, elbow rotation, vertical translation, and wrist rotation.
- Planar reach condition: |L1 - L2| <= r <= L1 + L2, where r = sqrt(x^2 + y^2).
- Forward kinematics in the plane: x = L1 cos(theta1) + L2 cos(theta1 + theta2), y = L1 sin(theta1) + L2 sin(theta1 + theta2).
- Vertical motion is often modeled as z = d3, where d3 is the extension of the prismatic joint.
Vocabulary
- SCARA
- A robot arm configuration that is compliant in the horizontal plane and rigid along the vertical axis.
- Revolute joint
- A joint that rotates about an axis, like a hinge or motorized pivot.
- Prismatic joint
- A joint that moves in a straight line, such as a vertical slide.
- End effector
- The tool attached to the robot wrist, such as a gripper, suction cup, or dispenser.
- Workspace
- The region of space that the robot end effector can reach.
Common Mistakes to Avoid
- Treating a SCARA arm like a full six-axis robot is wrong because most SCARA arms are designed mainly for planar positioning plus vertical motion, not arbitrary 3D orientation.
- Forgetting that the two rotary joint axes are parallel is wrong because this parallel-axis layout is what creates the characteristic x-y motion of the SCARA configuration.
- Assuming the arm can reach every point inside a circle is wrong because the inner unreachable region depends on the difference between the two link lengths.
- Confusing compliance with weakness is wrong because SCARA compliance is selective, allowing small horizontal give while maintaining high stiffness in the vertical direction.
Practice Questions
- 1 A SCARA arm has link lengths L1 = 30 cm and L2 = 20 cm. What are the maximum and minimum radial distances the end effector can reach from the base axis?
- 2 For a SCARA arm with L1 = 0.40 m, L2 = 0.30 m, theta1 = 0 degrees, and theta2 = 90 degrees, calculate the x and y position of the end effector using x = L1 cos(theta1) + L2 cos(theta1 + theta2) and y = L1 sin(theta1) + L2 sin(theta1 + theta2).
- 3 Explain why a SCARA robot is often preferred over a six-axis robot for high-speed pick-and-place tasks on a flat conveyor.