SCARA
The SCARA (Selective Compliance Assembly Robot Arm) is a type of industrial robot with four degrees of freedom: two parallel revolute joints for compliance in the horizontal XY plane, a prismatic joint for rigidity along the Z axis, and a rotational wrist joint for end-effector orientation. This configuration enables precise, high-speed operations like pick-and-place and assembly in manufacturing.[1] Developed in Japan in the late 1970s by Professor Hiroshi Makino at Yamanashi University, inspired by designs from the 1977 International Symposium on Industrial Robots, the first SCARA prototype was built in 1978 through the SCARA Robot Consortium involving Yamanashi University and 13 Japanese companies. An improved prototype followed in 1980, with initial commercial production beginning in 1981 by companies such as Sankyo Seiki, Pentel, and NEC.[2][3][1] SCARA robots offer high speeds (typically up to 8 m/s horizontally), repeatability within 0.01 mm, and payloads of 1–50 kg, with a compact design suited for cylindrical workspaces. They integrate easily with end-effectors like grippers or vision systems and are used in electronics assembly, automotive handling, packaging, and semiconductor processing. Modern variants include collaborative models and AI-enhanced controls for flexible automation.[1][4]Overview
Definition and Etymology
A SCARA robot, short for Selective Compliance Articulated Robot Arm, is a type of industrial robotic arm designed primarily for high-speed and high-accuracy operations in assembly and material handling tasks.[1] Originally, the acronym stood for Selective Compliance Assembly Robot Arm, reflecting its initial focus on assembly processes, though it has since evolved to encompass broader articulated applications while retaining the core concept of selective compliance.[5] This design allows the robot to exhibit compliance—or flexibility—in response to forces in the horizontal plane, enabling it to adapt to minor positional variations during insertion or picking tasks, while remaining rigid along the vertical axis to ensure precise force application.[6] The term "SCARA" was coined in 1978 by a team led by Professor Hiroshi Makino at Yamanashi University in Japan, during the development of the first prototype, to describe an articulated robotic structure that selectively complies with external forces in designated directions rather than being fully rigid or compliant across all axes.[7] This etymology underscores the robot's innovative balance between flexibility and stability, distinguishing it from earlier rigid manipulators and drawing inspiration from human arm kinematics for efficient, task-oriented motion.[8] Physically, a typical SCARA robot features a configuration with two parallel revolute joints that provide planar movement in the horizontal (X-Y) plane, a prismatic joint for vertical (Z-axis) translation, and an additional rotational joint at the wrist for end-effector orientation, resulting in four degrees of freedom suited to planar tasks.[9] This arrangement supports rapid cycle times and repeatability on the order of 0.01 mm, making it ideal for operations requiring speed without sacrificing accuracy in select compliance scenarios.[10]Design Principles
The SCARA robot's arm consists of two linked revolute joints at the shoulder and elbow, connected by parallel links that form a parallelogram linkage to enable planar motion in the XY plane.[9] This configuration, combined with a prismatic joint for the Z-axis and a revolute joint for end-effector wrist rotation, allows for four degrees of freedom optimized for assembly tasks.[11] The parallelogram linkage ensures that the end-effector maintains a consistent orientation during horizontal movements, facilitating precise positioning without complex recalibration.[9] The selective compliance mechanism is central to the design, providing rigidity along the Z-axis for accurate vertical insertion and positioning, while allowing controlled flexibility—or "play"—in the XY plane to accommodate tolerances in parts during assembly and prevent jamming.[12] This compliance arises from the parallel rotary joints, which permit slight deflections under load in the horizontal directions but maintain structural stiffness vertically through direct prismatic actuation.[9] Such a setup enhances operational efficiency in high-speed pick-and-place operations by balancing precision and adaptability.[11] SCARA robots typically support payloads ranging from 1 to 50 kg, depending on the model and arm length, with the capacity decreasing as the horizontal reach extends.[9] Their workspace forms a cylindrical envelope, with horizontal reaches commonly up to 1,000 mm, enabling operation within compact industrial footprints.[13] Drive systems generally employ servo motors for each joint—often AC servo types for high torque and precision—paired with belt drives or direct gearing to achieve rapid accelerations while minimizing backlash.[11]History
Invention and Early Prototypes
The SCARA (Selective Compliance Assembly Robot Arm) robot was invented by Professor Hiroshi Makino at Yamanashi University in Japan between 1977 and 1978. Makino's concept emerged from discussions at the 7th International Symposium on Industrial Robots held in Tokyo in 1977, where limitations of existing industrial robots for precise assembly tasks were highlighted.[8][1] The primary motivation was to address challenges in automated assembly, particularly the "peg-in-hole" insertion problem, by designing a robot with selective compliance—rigid in the vertical (Z) axis for accurate positioning and compliant in the horizontal (X-Y) plane to accommodate tolerances in electronics manufacturing. This contrasted with earlier robots like the Unimate, introduced in 1961, which relied on hydraulic actuation for heavy-duty operations such as welding and material handling but lacked the speed and compliance needed for light, high-precision pick-and-place tasks in electronics.[8][14][2] In April 1978, Makino formed the SCARA Study Group, a collaborative consortium involving five Japanese companies, to fund and develop the technology, with an initial budget of approximately ¥5,000,000. The first prototype was completed in October 1978, featuring a two-link arm driven by DC printed motors and optical encoders for position feedback. This early model demonstrated the core SCARA kinematics, enabling horizontal compliance while maintaining vertical stiffness, and was inspired by a byobu (folding screen) structure for its flexible yet stable design.[8][15][2] Fundamental testing of the first prototype focused on verifying selective compliance, revealing satisfactory insertion performance but issues with vibrations during high-speed motions. A second prototype, built in May 1980 in collaboration with Yaskawa Electric Corporation, incorporated DC motors with tachometer-generators for enhanced feedback and digital servo controls to mitigate these vibrations. Usability evaluations emphasized assembly tasks, assessing key characteristics such as motion speed (targeting up to 2000 mm/s horizontally), positional repeatability (in the range of microns), and seamless integration with pick-and-place operations for electronics components. These prototypes laid the groundwork for SCARA's suitability in compliant, high-throughput assembly environments.[8][2][16]Commercialization and Evolution
The transition from research prototypes to commercial SCARA robots began shortly after the initial development at Yamanashi University, where Professor Hiroshi Makino created the first prototype in 1978. This innovation was supported by a collaborative consortium formed in 1978, involving the University of Yamanashi and 13 Japanese companies, aimed at standardizing and advancing the technology through industry-university joint research and development. By 1981, consortium members such as Shibaura Machine (then Toshiba Machine), Sankyo Seiki, Pentel, and NEC introduced the first commercial SCARA models to assembly lines, marking the entry into widespread industrial use. These early models focused on high-speed, precise operations suited for electronics manufacturing, quickly gaining traction among Japanese firms like those in the burgeoning consumer electronics sector during the 1980s economic boom. Throughout the 1990s, SCARA robots evolved with the integration of advanced computer controls, enabling more sophisticated programming and real-time adjustments that improved reliability and adaptability in production environments. This period saw enhancements in servo systems and digital interfaces, allowing for smoother trajectory planning and reduced downtime in automated lines. By the 2020s, further advancements incorporated lightweight materials such as advanced composites and aluminum alloys, reducing overall mass while maintaining structural integrity, alongside AI-driven features for predictive maintenance and adaptive learning in dynamic tasks. These developments have broadened SCARA applications beyond initial assembly roles to versatile handling in sectors requiring flexibility. Over the decades, SCARA technology has shifted from specialized assembly to multi-purpose handling, with notable improvements in performance metrics; modern models achieve speeds up to 3 m/s and repeatability below 0.01 mm, enabling higher throughput in precision-demanding operations. Globally, adoption expanded in the 1990s as Japanese manufacturers like Epson and Yamaha entered European and North American markets, while European firms such as ABB (building on its 1987 IRB 300 SCARA introduction via predecessor ASEA) facilitated integration into Western automotive and electronics industries. This international proliferation solidified SCARA's role as a cost-effective solution for high-volume automation.Kinematics and Mechanics
Degrees of Freedom and Configuration
The standard SCARA robot possesses four degrees of freedom (4-DOF), enabling precise positioning in assembly tasks through a combination of rotational and translational motions.[17] This configuration includes two revolute joints in the horizontal plane for planar movement, one prismatic joint for vertical adjustment, and one revolute joint at the wrist for tool orientation.[18] The joints are arranged in an RRPR sequence: Joint 1 is a revolute joint at the base, providing rotational motion around the vertical z-axis to sweep the arm across a circular horizontal area. Joint 2 is a second revolute joint at the elbow, allowing the arm to extend and retract radially within the workspace. Joint 3 is a prismatic joint that delivers linear translation along the vertical axis, typically with a range of 100-300 mm depending on the model. Joint 4 is a revolute joint at the wrist, rotating the end-effector around the vertical axis to adjust tool pitch without tilting.[19] This setup ensures high speed and accuracy in the horizontal plane while maintaining rigidity in that direction and selective compliance vertically.[17] Configuration variations extend beyond the standard 4-DOF to include 5-DOF or 6-DOF models, often by incorporating an additional prismatic joint at the base for greater vertical reach or extra revolute joints for enhanced dexterity, sometimes paired with integrated vision systems for adaptive positioning.[20] For instance, a 5-DOF SCARA adds base translation to expand the workspace height, while 6-DOF versions introduce further rotational freedom akin to serial manipulators but retaining SCARA's horizontal efficiency.[21] The robot's coordinate systems follow the Denavit-Hartenberg convention, with the base frame {0} anchored at Joint 1, its z_0 axis aligned vertically along the rotation axis and x_0-y_0 in the horizontal plane. Subsequent frames {1} to {4} are assigned at each joint, culminating in the end-effector frame {4} at the tool tip, where z_4 remains parallel to the vertical for consistent downward-facing operations. This frame hierarchy produces a cylindrical workspace, with radial extent governed by the combined reach of the first two links (typically 300-600 mm) and axial height by the prismatic joint.[19] To model the geometry, the link parameters are captured using Denavit-Hartenberg (DH) parameters for the standard 4-DOF SCARA:| Joint i | a_i | \alpha_i (rad) | d_i | \theta_i (rad) |
|---|---|---|---|---|
| 1 | L_1 | 0 | 0 | \theta_1 (variable) |
| 2 | L_2 | 0 | 0 | \theta_2 (variable) |
| 3 | 0 | 0 | d_3 (variable) | 0 |
| 4 | 0 | 0 | 0 | \theta_4 (variable) |