Dynamic Reconfiguration of Redundant Haptic Interfaces for Rendering Soft and Hard Contacts

There are conflicting objectives between required characteristics of haptic interfaces such as maximum force feedback capability versus back-drive friction, which can be optimally traded-off in a redundant haptic interface; a redundant haptic interface has more degrees of freedom than minimally required ones for a given task. In this article, a contact-aware null-space control approach for redundant haptic interfaces is proposed to address these trade-offs. First, we introduce a task-dependent null-space controller in which the internal motion of the redundant haptic interface is appropriately controlled to achieve a desired performance; i.e., low back-drive friction in case of free-space motion and soft contact or large force feedback capability in case of stiff contact. Next, a transition method is developed to facilitate the adaptation of the null-space controller's varying objectives according to the varying nature of the task. The transition method prevents discontinuities in the null-space control signal. This transition method is informed by a proposed actuator saturation observer that monitors the distance of joint torques from their saturation levels. The overall outcome is an ability to recreate the feelings of soft contacts and hard contacts with higher fidelity compared to what a conventional non-redundant haptic interface can achieve. Simulations are provided throughout the paper to illustrate the concepts. Moreover, experimental results are reported to verify the effectiveness of the proposed control strategies. It is shown that the proposed controller can perform well in the soft-contact, hard-contact, and transition phases.