This paper discusses stability problem of haptic device in haptic display systems. Based on the analysis of impedance display device,the interaction between tools and arbitrary-shaped objects is simplified as a dynamic unilateral constraint contact problem. Using Phantom®
as the experiment platform,force distribution relative to time and to device position in several typical unilateral constraints is compared,and the reasons leading to vibration in complex unilateral constraint are uncovered. Compared with passivity-based stability analysis method,a force filtering method based on force-position gradient is proposed. Experiment results indicate that the proposed method can effectively eliminate vibration and stable interaction can be realized.