Motorizing an elliptical trammel mechanism

[azizlulu]

I need help building a motorized elliptical trammel, can someone help me?
The idea is, I need to keep the basic design"educational", and keep the main purpose of it.
I'll attach a photo of the basic idea http://dcu.ie/mechanical_engineering/technical/undergrad/mechanics/Elliptical%20Trammel/htm61.gif ,

 

[Baluncore]

The engineering will depend on why.
Why motorised, what will it do and why will it do it?
Is it required to draw the full ellipse, only half or a quadrant?
How big do you want it to be?

The two guides that cross may be in the same plane if the followers can cross without effect from the cross track.

If it will draw an ellipse continuously then can the point that traces the ellipse be the driven part, or is that a pointer, pencil or cutter?