Adapting haptic guidance authority based on user grip

Haptic guidance systems support the operator in task execution using additional forces on the input device. Scaling of the guidance forces determines the control authority of the support system. As task complexity may vary, one level of the guidance scaling may be insufficient, and adaptation of the control authority may be helpful. The available literature mostly proposed to adapt the authority based on external cues (e.g. actual performance or safety) and the user had no direct way to modulate the desired level of support. In this paper we investigated a variable authority guidance scheme based on the user grip force. During a user study (with 8 subjects) we explored two opposite approaches to trade the control authority (i.e. increasing or decreasing guidance force magnitude with increased user grip). To simulate increased task difficulty and imperfections of the haptic guidance system, at random times either an unpredictable force disturbance was added or subjects were presented with incorrect guidance. While the performance differences between the fixed- and the variable-authority schemes were not significant, the "decreasing guidance with increased user grip" scheme allowed to substantially reduce the user control effort (steering force), especially when the guidance system was incorrect. The presented method essentially provides additional control over the guidance system without reducing the performance.