Ao map baking

hi all … i need some help pleas

in the process of baking ao map

is it better to bake it in the default way ((bake ao map no from selected to active or with cage …)

or using the ao node on the high resolution mesh and bake it as emit …

am asking that beause am facing this problem : when i try to bake ao map it give me an all black map i try to flip normal apply scale … try with different low resolution mesh … but nothing … ((any idea why this happen ?? is it because am using very small object ?? i mean 20 cm in real world scale))