motorised pounding mallet

[Joe G.]

I agree, a cam powered system sounds like it will achieve what you're looking for.

http://www.youtube.com/watch?v=1wjjRYgDZT0

[Kyoshirin]

In the 2D view, you see a trapezoid with a circle inside it. That is a single plate that holds the pivot axle. In the 3D view, you see how it is implemented. With the two plates mounted inside the supports, you can hold up the arm/lever on that pivot axle. In the video you posted, this is accomplished by two pillow blocks (grey omega-shaped things) on top of the pillar. What I am describing for use by you is called a bearing plate, which can be easily made out of sheet metal, a bearing, and three screws.
Does this make sense, or should I try another approach?
Joe G.'s video is another way you could make this automated mallet. How you want to make it is up to you.

[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.