US Patent 9211920 Magnetically coupled accessory for a self-propelled device

A system comprising a self-propelled device and an accessory device. The self-propelled device includes a spherical housing, and a drive system provided within the spherical housing to cause the self-propelled device to roll. When the self-propelled device rolls, the self-propelled device and the accessory device magnetically interact to maintain the accessory device in contact with a top position of the spherical housing relative to an underlying surface on which the spherical housing is rolling on.