Nobuharu Mimura

Stability analysis of 3D grasps by a multifingered hand

We discuss stability of 3D grasps by using a three-dimensional spring model (3D spring model). Many works have described stability of a frictionless grasp. These works used a one-dimensional spring model (1D spring model) for representation of frictionless contact as a matter of course. However, a 1D spring model involves the following problems: (i) displacement of a finger is restricted to one dimension along the initial normal at contact point; and (ii) it is not clearly considered that a finger generates contact force to the object along the normal direction only. To overcome the problems and provide accurate grasp stability, we introduce a 3D spring model. Then a fingers displacement is relaxed and a fingers contact force is precisely formulated. We analyze grasp stability from the viewpoint of the potential energy method. From numerical examples, we show that there exists an optimum contact force for stable grasp. Moreover, we also analyze a frictional grasp by using contact kinematics of pure rolling. By comparing frictional grasp stability with a frictionless one, we prove that friction enhances stability of a grasp.

Parameter identification of contact conditions by active force sensing

When assembly tasks are accomplished by robot manipulators, it is necessary to identify interactions between grasped objects and external environments, such as contact positions and contact type, and also contact directions. This paper discusses the identification of unknown parameters such as mentioned above. Point contact, line contact and plane contact are formulated in contact conditions. Applying this formulation, a simple scheme is proposed for identification of contact conditions by using active force sensing method. In this algorithm, contact type, contact direction and contact point can be identified.<<ETX>>

Identification of contact conditions from contaminated data of contact moment

When a grasped object is in contact with external environment, it is required to identify contact conditions prior to performing the assembly tasks. This paper discusses a method for identification of contact conditions from the information of force sensor equipped with the robot hand. This paper treats the practical case where sensing data are contaminated with noise. We propose an efficient and analytical algorithm for identifying contact conditions by using an active force sensing method. The algorithm can identify not only contact position and contact force, but also contact type such as soft finger contact type, line contact type, and plane contact type. These contact types are characterized by a standard deviation of contact moments. The contact position is estimated by a least-squares method. The contact moment is then estimated from noisy observations and its eigenvalues are analyzed. The contact type can be judged by the eigenvalues of estimated contact moment. The effectiveness of the algorithm is demonstrated by simulations.

A new analytical system applying 6 DOF parallel link manipulator for evaluating motion sensation

A 6 DOF motion system using a new parallel link mechanism is developed for purposes of evaluating human motion sensation. This motion system is made up of three five-bar mechanisms, each of which has 2 degrees of freedom (DOF) and is driven by AC servo motors. Therefore, this system is very small and low cost, but its motion area is larger and its response is better than previous hydraulic Stewart platform. Furthermore, we have developed a high-speed parallel signal-processing controller for motion control and a new 6 DOF acceleration sensing system based on the parallel sensing concept. Combining the new motion and sensory system with multivideo system, we have built up a new analytical system for evaluating wide band-width of motion sensation in getting on such vehicles as automobile or airplane.

Grasp Stability Analysis of Two Objects in Two Dimensions

This paper analyzes grasp stability of two objects in two dimensions. In the previous researches on the grasp stability, the number of grasped objects is restricted to 1. For efficiency of tasks, multiple objects had better been grasped and manipulated by a robot hand. In order to design stable grasp, grasp stability of multiple objects must be quantitatively evaluated. In case of two grasped objects in two dimensions, the number of parameters of object displacement is 6. However, the displacement of the objects is constrained by each other, because the contact between the objects must be maintained. Independent parameters of the object displacement are provided and then finger displacement is derived. The grasp stability is evaluated from the viewpoint of the potential energy method. From numerical examples, the effect of curvature at contact point between two objects is demonstrated. The effect of this curvature stands for the feature of multiple objects.

Identification of contact conditions from contaminated data of contact force and moment

This paper discusses a method for identification of contact conditions from the information of 6-axes force sensor equipped with a robot hand. The previous paper (1999) has the following problems: the noise of sensing force was not considered, and hence the estimates obtained by the previous method are biased from true value, and the identification of contact types depends on an unknown contact position. In this paper we consider the noise of force. We propose a method of removing the bias from the estimates. Hence, it is guaranteed that asymptotically unbiased estimates are obtained. The contact types can be judged by eigenvalues of a covariance matrix, which is derived from estimates of the contact moment. The effectiveness of the algorithm is demonstrated by simulations.

Parameter identification in the grasp of an inner link mechanism

The identification of unknown parameters, such as external force and contact points, once an unknown object is grasped stably by an inner link mechanism is discussed. Once an unknown object is grasped stably, it is considered that the grasp will be made more stable and manipulations will thus be performed if unknown parameters can be identified by using data obtained during the initial grasp. The minimal structure required for grasping and manipulating an object is a three-fingered, nine degree-of-freedom (DOF) hand in three-dimensional space. In this case, the unknown parameters cannot be identified only by grasping an unknown object. It is shown that unknown parameters can be identified by appropriate changes in the distribution of joint stiffness and torques, using a minimal structured hand.<<ETX>>

Grasp stability analysis of two objects with any friction property in two dimensions

This paper analyses the grasp stability of two objects with any friction property in two dimensions. In the grasp of two objects, there exist multiple contact points which include one contact point between two objects and plural contact points between objects and fingers. In the previous work, all these contact points are assumed to be either only with friction or only without friction. This paper discusses a more practical grasp, in which there exist both friction contact and frictionless contact. In this case, both rolling motions and sliding motions occur simultaneously due to both friction and frictionless contact in the grasp system. The grasp stability is evaluated by the potential energy stored in the grasp system. Using numerical examples, the effect of friction is demonstrated. This evaluation method is applicable to fixture position planning.

An efficient calibration method for a novel 6-DOF acceleration sensor system and application to measurement of a vehicle motion

This paper describes a calibration method of 6-DOF acceleration sensor system using dual-axis type linear accelerometers. In our sensor, 6-DOF acceleration can be resolved using the geometrical condition of the accelerometers. Therefore, the measurement error results from the alignment error of the accelerometers. The error analysis shows to be able to identify the error only using some limited linear accelerations. Moreover, in dual-axis type accelerometers, there exit some dependency among each axis-sensitivities. Then, we a new calibration method to above case, and investigate the influence of the error margin included in the calibration. According to the analysis, it is shown that the geometrical alignment error to be identified, which absolute value is small or near zero in comparison with other, can be assumed zero. As a result, more efficient calibration is made possible by decreasing the number of calibration data needed in the calibration. Finally, the availability of the proposed calibration method and our sensor system are verified through vehicle motion experiments. The results show that it has good performance for a complex motion.

Uncertainty of estimated parameters of contact conditions by active force sensing

When a grasped object is in contact with an external environment, it is required to identify contact conditions prior to performing assembly tasks. The contact conditions include contact position, contact force, contact type, direction of contact normal, and direction of contact line between the object and the environment. The contact type is assumed to be a point, soft-finger, line, or planar contact type with friction. These contact parameters are identified by active force sensing. Moreover, uncertainty of the estimated parameters is derived. This paper formulates least-squares functions including contact position, direction of contact normal, and direction of contact line. The uncertainty is derived by using an amount of each function. The effectiveness of this method is verified by using numerical examples. As a result, we obtain the novel knowledge that contact types can be distinguished by the uncertainty.