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• Vibration of 6 kinds of PV module under robot cleaning were measured. • A module's size was the dominant factor in how much it vibrated. • A module's frame and glass thickness had little influence on its vibration. • Overall the amount of vibration by robot cleaning was small – less than 1 mm. • Strong wind caused more severe module vibrations than the cleaning robot. • Large-format modules and robots that run on the modules may present greater vibration risk. Automatic PV cleaning machines ("robots") are becoming widely used in desert regions. The motion of the robot and its brushing action can vibrate the PV modules being cleaned. This study measured the vibration of six different kinds of PV module, excited by a single kind of commercial cleaning robot. The robot's brushes were straight, rather than helical, which imparted a periodic excitation to the modules at approximately 7 Hz. For comparison, module vibration by strong wind was also measured. It was found that (i) among similar-sized PV modules, the amount of vibration was largely independent of the module construction (glass thickness and frame), (ii) modules tended to vibrate at their natural frequency rather than the brush (excitation) frequency, and (iii) the module deflection range by robot cleaning was approximately 0 mm upward to 1 mm downward, while in strong wind it was approximately 2 mm upward to 1 mm downward. In this study, which used modules up to 2 × 1 m, the particular cleaning robot did not cause significant vibration, however as commercial modules become larger they may also become more flexible and prone to larger deflections. [ABSTRACT FROM AUTHOR]