A robot arm is provided with an end effecter for grasping an object and a force sensor for detecting a force acted upon the end effecter. Whereby snapping the fastening assembly onto the frame will fasten the adapter cards at the frame easily and quickly. An engaging groove is defined in the receptacle connector and has the same shape as the engaging element for receiving the engaging element therein. As a result, the lamp unit is positioned with respect to the resin frame and fixed to the latter. The spherical spacers prevent or reduce damage to the pixel defining layer caused by inconsistencies or unevenness in the deposition mask by maintaining a spacing between the pixel defining layer and the deposition mask.