SCARA robot technology benefits
Selective compliance articulated robot arm (SCARA) robots are capable of many types of movement.
Learning Objectives
- A selective compliance articulated robot arm (SCARA) robot is designed for small part assembly and pick-and-place applications.
- SCARA robots excel in repeatability and accuracy.
- SCARA robots require a control system, which may house all the servo amplifiers, programming logic and functional safety.
SCARA insights
- A selective compliance articulated robot arm (SCARA) robot has relatively high speed and high precision and is designed for pick-and-place applications.
- SCARA robots are a versatile and simple solution to a variety of automated assembly applications.
A selective compliance articulated robot arm (SCARA) robot is a mechanism with relatively high speed and high precision, designed for small part assembly and pick-and-place applications.
SCARA mechanisms
Motion of the tool in a SCARA robot is possible in three-dimensional Cartesian space; X, Y, Z with a 4th dimension which is rotation about the vertical z axis. The robot is mechanically compliant in the XY plane, and rigid in the z direction. This is the basis of the selective compliance articulated robot arm (SCARA). Note however that positioning repeatability is high in all directions X, Y, Z and Rz.
Figure 1: SCARA mechanism motion in X,Y,Z,Rz. Courtesy: Yaskawa America Inc.
SCARA servo axes movement
The SCARA mechanism is driven by 4 servo axes. The first axis moves the first section of the arm which pivots at the base. The second axis moves the second section of the arm. The end of this section supports the tool quill at the far end, which can move up and down with the third axis and rotate with axis four.
Figure 2: Second servo axis and section of SCARA robot. Courtesy: Yaskawa America Inc.
SCARA peripherals
Expect a SCARA to have a main connection for motors and encoders, plus vertical axis holding brake connectors and integrated air lines. Often, the tool quill is hollow, to allow routing of air lines or wiring to the tool.
SCARA motion
Naturally, the axes can all move together and in coordination to move the tool along a trajectory or from one position to another. This can be achieved using the axis coordinates, also called joint coordinates for the fastest positioning with an arced path trajectory. Inverse kinematics also can be applied to command a specific trajectory of the tool center point (TCP) for example, to perform a linear move.
Figure 3: Fast positioning of joints results in arced path trajectory. Courtesy: Yaskawa America Inc.
Figure 4: Inverse kinematics for specific path trajectory. Linear trajectory shown. Courtesy: Yaskawa America Inc.
The work envelope for the robot is nearly cylindrical, with the exception of the area directly behind the robot. And the depth limited by the vertical axis.
Figure 5: Typical SCARA work envelope. Courtesy: Yaskawa America Inc.
It is interesting to note that in much of this work envelope, the TCP position can be achieved with exactly two different poses, depending on the orientation of the second joint.
Figure 6: SCARA with two poses for same tool position. Courtesy: Yaskawa America Inc.
SCARA strengths
This table summarizes the SCARA robot compared to other articulated robot mechanisms such as a delta robot or the common 6-axis robot arm.
Figure 7: SCARA vs. other robot technologies. Courtesy: Yaskawa America Inc.
Repeatability is where SCARA robots excel. A repeatability of 10 microns is not uncommon; that’s 1 hundredth of a millimeter, the best repeatability of all articulated robots. While a DELTA robot is still a faster way to move light loads, the SCARA can be bolted down with a simple floor mount rather than an overhanging structure.
Figure 8: SCARA with simple floor mount. Courtesy: Yaskawa America Inc.
SCARA applications
These characteristics make SCARA a great choice for fast pick and place of small parts with high repeatability. The same applies to assembly processes involving small part insertion, pressing, or even driving screws with the rotational axis. Dispensing is another good application for SCARA robots. They are often used in inspection, sorting and other processes involving parts moving on conveyors.
Figure 9: Pick and place with SCARA and 6-axis manipulator. Courtesy: Yaskawa America Inc.
Note that SCARA robots are not often a good choice for applications involving welding and plasma, due to their limited degrees of freedom, nor do they have the rigidity required for computer numerical control (CNC) or machine tool applications.
SCARA product selection
SCARAs come in a range of sizes for light to medium payloads. Like other robots, the principle specifications to look for are payload and radial reach.
Figure 10: Radial reach of a SCARA robot. Courtesy: Yaskawa America Inc.
For applications that require pressing, look for the z axis force. Also consider ingress protection requirements, operating temperature and ratings for food grade, collaborative, or explosion proof.
On the controller side, look for a conveyor tracking feature, which allows the robot to synchronize with a moving conveyor to pick and place.
Figure 11: Conveyor tracking with a SCARA. Courtesy: Yaskawa America Inc.
Functional safety is also a controller feature, which keeps every part of the robot and the tool clear of interference.
Figure 12: Functional safety prevents the robot from entering an interference zone. Courtesy: Yaskawa America Inc.
System integration strategies
SCARA robots require a control system. The most common and straight-forward setup is to use a standalone robot controller. Such a unit may house all the servo amplifiers, programming logic and functional safety. It is programmed with a pendant and can interface to an upper-level machine controller or other devices using digital and analog input/output (I/O) or network connections.
Figure 13: SCARA with standalone controller and pendant. Courtesy: Yaskawa America Inc.
In some cases, the network connection may allow for complete programming and control to be hosted by the upper-level controller. Rather than programming through the pendant, this setup promotes centralized programming of a variety of robots, servos, inverter drives and other devices in the system.
Figure 14: SCARA motion interface to machine controller. Courtesy: Yaskawa America Inc.
Or it may be possible for the machine controller to solve the robot kinematics internally and drive individual servos directly without a dedicated robot controller. This approach works well for the most flexibility in panel layout, control and customization.
Figure 15: Direct control of SCARA by machine controller. Courtesy: Yaskawa America Inc.
SCARA robots are versatile and simple solution to a variety of automated assembly applications that require rapid and precise positioning of small parts.
Matt Pelletier is product training engineer, Yaskawa America Inc. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, cvavra@cfemedia.com.
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Keywords: SCARA, robotics
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