When humanoid robots can accurately replicate human limb movements, and when specialized robots can operate flexibly in high-risk environments, a core component underpins it all — the Robot Joint Motor.

In quality inspection and performance evaluation of integrated robot joints, second-harmonic (2×) frequency testing has become an indispensable key step. Although it may look highly specialized, this test is directly linked to core performance indicators such as joint accuracy, stiffness, and reliability. This document explains—covering technical principles, engineering practice, and quality control—why second-harmonic testing is necessary and why it is increasingly important in modern robot manufacturing.