hammock rocker

[VARISTORS]

Looking for ideas (schematic) on how to build an automatic , hands free, hammock rocker.The hammock is suspended between two trees - so a rocking chair with a hammock for a seat is a no and the kits (a stake and some rope are a no. Perhaps a rope attached to an eccentric wheel run by a motor -or -a solenoid to actuate the rope attached to the hammock activated by a stress gauge? Ideas need to consider the hammock-potato's mass (varies relative to relative) and how to establish the desired frequency.