motorised pounding mallet

[msimon785]

I can tell you right away that a string/motor system will prove entirely ineffective. If the mallet requires functionality, or at least any actual level of practical force, you will need to stay away from flexible parts and move towards either dynamic parts or a CAM. Either of these will work just fine. If you choose to implement a CAM, you will need to design a solid system in which bearings are placed around the main rotating shaft.
By a dynamic system, I infer something with two main positions/stages, or in other words, a linear actuator. Among these, a motorized linear actuator could work, or if you have the expertise and will, you may choose to attempt a pneumatic actuator. Automation direct sells some good actuators.

[Kyoshirin]

In non-technical language, msimon785 is saying that if you want this to be a tool instead of just a moving sculpture (I believe just a moving sculpture is what you wanted) you should use an air powered piston (with a recommendation in his post of where to get some) or any of the previous design ideas that Joe G. or I have given.