Dynamic coupling between a human user and haptic virtual environment

In a teaching lab focused on embedded control, students create and interact with virtual environments using a haptic interface. Coupling physical (in particular physiological) environments to virtual environments gives rise to many interesting phenomena, one of which is the appearance of dissipativity in the coupled dynamics, the source of which is difficult to identify. Simple harmonic oscillators without damping exhibit damped behavior and diminished peak amplitudes when students excite them with their best manual approximations of step inputs. Motivated in part by our desire to develop teaching materials, we seek a simple human user model that describes the observed phenomena. We have found that a second order spring-mass-damper model describes the source impedance with which a human user is able to impose a position input on the haptic device, and that this impedance model can be incorporated into a model of the user's neuromotor intent by placing the spring as a series elastic element with a motion source. We use simple models to describe smooth inputs generated by the user's neuromotor system, and these are expressed as displacements of the motion source. We use the same haptic device to conduct system identification experiments using frequency domain techniques to estimate the driving point impedance of the human hand, and have recently incorporated these experiments into lab exercises.