Horizontal articulated robots, also called SCARA Robots (Selective Compliance Assembly Robot Arm) or swivel arm robots, have an articulated arm that can only swivel in the horizontal direction. The Scara robot has serial kinematics with typically four degrees of freedom, which only allows handling in plane-parallel surfaces. The first axis of rotation is formed by the swivel arm, which can be rotated around the robot base, and its joint forms the second axis. Attached to the end of the swivel arm is the vertical Z-axis for holding a gripper or tools, which can also be rotated around its longitudinal axis as the fourth axis. Due to their design, SCARA Robots offer high rigidity in the vertical direction, but can only absorb lower forces in the XY direction.
As a variation of this principle, double-arm SCARA Robots have parallel kinematics consisting of two articulated arms connected at the ends, which enable very high speed and positioning accuracy in a small working field.

Scara Robots usually have four axes and four degrees of freedom. The axes are designed as serial kinematics, where each arm link is connected to only one other arm link. The disadvantage of this design is that the position errors add up at the end of the chain. 
The SCARA robot stands on a fixed robot base around which the first arm link of the two-part horizontally lying swivel arm rotates. The links of the swivel arm are movably connected to each other via the second vertical axis of rotation. At the end of the second arm, the rotatably mounted Z-axis is attached, at the lower end of which grippers or tools, for example, are mounted as end effectors. The drives are integrated into the individual arm links. Due to the design with vertically positioned rotary axes, high rigidity is achieved in the vertical direction without the robot's own weight acting on the drives, as is the case with vertical articulated arm robots. The working area is often kidney-shaped, but can also extend circularly around the robot base. 
In the double-arm SCARA Robot, the two articulated arms connected at the ends form a closed kinematic chain, which enables handling at very high speed and high positioning accuracy in the XY direction, but only allows a small working space.