I started to look into the different areas when I first joined Intelligent Machine Dynamics Laboratory and the ideas of haptics and haptic interfaces were very interesting to me. Some examples are shown below,

Human extender by Kazerooni, UC Berkeley

Haptic backhoe by Frankel, Kontz and Book, Georgia Tech

Rutgers Ankle, Rutgers University

I finally chose to focus my research on dissipative passive haptic interface after some initial work. A haptic interface is a human-machine interface that constructs a sensation of touch for the human operator. Its applications can be found in various industries, hospitals etc. (For more information please go to http://www.imdl.gatech.edu/mkontz/research/) A dissipative passive haptic interface contains only passive actuators which include passive actuators that can remove energy from the system by resisting motion in the system, like brakes. Here are two examples of dissipative passive haptic interfaces, P-TER (Passive Trajectory Enhancing Robot) and MR (Magneto-rheological) P-TER.

The advantage of a passive haptic display is better safety compared to an active display. Its disadvantage is the limited control ability from the passive actuators. My research focuses on the control ability and limitation brought by dissipative passive actuators on passive haptic interfaces.

Steerability concept is defined based on the force generation analysis to identify the limitation. Passive steerability angle is defined to quantify steerability. Two-sided steerability is required for a planar robot to redirect motion to both left and right of the current velocity. Non-redundant 2R manipulator does not have two-sided steerability at all situations. Redundancy brought by serial or parallel structures could improve manipulator’s steerability. Steerability theorem is developed to determine two-sided steerability for a multi-joint manipulator.

Please refer to the following publication for details on planar steerability

D. Gao and W. J. Book, "Steerability for Planar Dissipative Passive Haptic Interfaces," IEEE/ASME Transactions on Mechatronics, V11, I2, pg179-184

More detail on dynamics and 3D steerability analysis could be found in my thesis

D. Gao, “Control Limitation analysis for dissipative passive haptic interfaces,”
Ph.D. Thesis, Georgia Inst. Tech., Atlanta, GA, 2005, p. 137.