Task instruction: The largest influence on human operator motion control dynamics

The dynamics of the human operator while interacting with a haptic interface are often either neglected or at best modeled as a constant mass-spring-damper model. However, it is well known that humans are able to substantially modify their dynamic behaviour. The goal of this study is to show the extent to which humans can modify their endpoint dynamics while interacting with a haptic manipulator. In an experimental study the endpoint dynamics of the ankle-foot complex were estimated as an admittance using closed-loop identification techniques. Subjects were perturbed with continuous torque perturbations and required to perform three tasks: to maintain a constant position (position task), to maintain a constant force (force task) and to relax. Results indicate that subjects substantially decrease their admittance during a position task compared to a force task (up to a factor of 40 at frequencies below 0.5 Hz.), indicating they could choose to resist the torque perturbations or not. It is concluded that modeling the human as a constant mass-spring-damper system can lead to substantial errors in the design of haptic systems.