Robot # of DOFs and Configurations

The project prompt states the following:
"In liver radiofrequency ablation, a surgeon places a needle into a lesion (e.g. cancer) located in the liver, starts radiofrequency ablation, removes that needle, after which he places another needle and repeats the procedure as many times as needed (sometimes, over 10 times).
The goal of this project is to develop a robotic device to consecutively place series of needle in a lesion and allow for surgeon to perform radio frequency ablation."

I highlighted the text to show that we need more that 2 DOF to perform the task for the following reasons.

1. 2-DOFs (of the RP & PP type shown below) allows for only a 2D work-space.  We need a 3D work-space to ensure accurate needle placement.
2. Since we are required to perform multiple ablations per lesion, Yaw and Pitch of the needle should be considered if we would like to keep the procedure as minimally invasive as possible.  Depending on the size and number of lesions in the liver, it would be ideal to be able to completely ablate these lesions from the least number of incision points. (See fig 1).

Fig 1: Yaw and pitch of ablation needle

 So, how many DOFs do we need?

I think a 5-DOF design would be the best option. This allows the needle to reach any place in the workspace at any yaw or pitch angle.

What are some configuration options?

Hoi's suggestion of 2 Wrist + Prismatic Joint configuration would work.  We may also want to consider the SCARA configuration which from some prelim research seems popular in robotic surgical systems (Figure 2 and 3).

Fig 2: SCARA + Wrist (5 DOF) Robot CAD model

Fig 3: Coordinate Frame