Toru Omata

Rigid body analysis of the indeterminate grasp force in power grasps

The grasp force in power grasps is not necessarily determined uniquely by joint torques when Coulomb friction is assumed in the contact points between a grasped object and finger links. This paper analyzes the indeterminate grasp force with their rigid body models. This paper shows that contact sliding directions are constrained in power grasps. The frictional force acts only in the opposite direction of sliding. This characteristic of friction restricts the grasp force in power grasps although it is indeterminate in general. First the authors show an example to illustrate this restriction imposed on a grasp force. A special case where it is unique is shown.

Fast dextrous re-grasping with optimal contact forces and contact sensor-based impedance control

This paper presents an approach to fast object manipulation by dextrous re-grasping of multi-fingered hands. The approach is based on a real-time grasping force optimization (GFO) algorithm and a fingertip impedance control scheme. Both the controller and the GFO make use of 6D contact force sensor data at run-time. The latter keeps the contact forces as small as possible while considering friction limits at the contact points during (re)grasping. The impedance controller is used to impose the optimized contact forces onto the grasped object while simultaneously enabling active control of the fingertip positions. Experiments demonstrate the robustness of the approach and the increase in task speed during multi-fingered manipulation.

Planning reorientation of an object with a multifingered hand

A multifingered hand can reorient an object by regrasping it in a hand. This paper presents a planner which plans a sequence of repositioning of fingers for horizontal rotation of an object for a desired angle. The authors previously developed an algorithm for computing contact positions of fingers where they can maintain equilibrium. The algorithm is used to compute a new contact position of a removed finger where there is a finger which can be removed next. Based on these computations, the authors develop an algorithm for searching for a sequence of repositioning of fingers. Examples show that the algorithm can efficiently find sequences which require the number of repositioning steps.<<ETX>>

Regrasps by a multifingered hand based on primitives

This paper studies regrasping by a multifingered hand to achieve a desired reorientation of an object by using regrasp primitives. Among them is pivoting in which two fingers grasp an object with their fingertips forming an axis of rotation and the third finger rotates the object about this axis. This paper presents an algorithm for calculating the region for the axis for which the third finger can rotate the object at given angles. Pivoting is also efficient to achieve three dimensional reorientation of an object when it is used together with other primitives. This paper also discusses how sequential executions of primitives achieve three dimensional reorientation.

Load-sensitive continuously variable transmission for robot hands

This paper presents a load-sensitive continuously variable transmission (CVT) for finger joints. The fingers of a robot hand require power when grasping an object and speed when opening and closing. Therefore a CVT is ideal to improve the power transmission of a finger joint. Existing friction CVTs are too big and heavy to be installed in a finger joint. By focusing on the fact that finger joints do not necessarily rotate 360 degrees, a remarkably simple and small load-sensitive CVT can be developed based on a crank CVT. The mechanism consists of a five-bar linkage and a torsion coiled spring. We have developed one joint with the CVT and have experimentally verified that the CVT can increase its reduction ratio from 0.5 to 3.3 in response to a load.

Assemblable three-fingered nine-degree of freedom hand for laparoscopic surgery

This paper proposes an assemblable hand that can be inserted through trocars for robotic hand assisted laparoscopic surgery (HALS). When it is difficult to perform surgery using only slender laparoscopic surgery instruments, surgeons often apply HALS, which makes an incision about 7-8 cm through which their hand is inserted. This is invasive compared with complete laparoscopic surgery. We proposed robotic HALS to replace a human hand with a robotic hand. We previously developed a three-fingered five-degree of freedom assemblable hand. It is challenging to an assemblable hand with more degrees of freedom (DOF) that can be assembled with a simple assembly procedure. This paper presents an assemblable hand with three fingers and nine degrees of freedom-the 3f9d-hand. Its power transmission mechanisms and assembly procedure are completely different from those of our previous 3f5d-hand. The new hand consists of center, right, and left finger units. The center finger unit connects the operational part at its end and the right and left finger units connect to the operational part outside the abdominal cavity. This facilitates assembly and improves safety, which is a significant improvement compared with the previous hand. Although the hand has no wrist joint, its three finger joints play the role of a wrist joint. A preliminary experiment with a plastic model verified that the proposed assembly procedure was feasible and the hand was easily asembled and disassembled.

Three-Fingered Eight-DOF Hand That Exerts 100-N Grasping Force With Force-Magnification Drive

This paper presents a three-fingered, eight-DOF hand, 100 N Hand II, which can exert a grasping force of 100 N. A force-magnification drive, which can maintain a large grasping force without energy consumption, allows the hand to exert such large grasping force, and a high-speed driving mechanism enables all joints to perform high-speed motions of over 400°/s. In our previous prototype (100 N Hand I), a screw and nut mechanism was used to implement force magnification. The linear motion of this mechanism was translated to rotational joint motion via a linkage mechanism, producing a relatively small motion range for each joint and with large palm dimensions. 100 N Hand II employs a worm gear instead of a screw and nut mechanism, so it can magnify the torque at an arbitrary joint angle. This improved force-magnification drive increases the motion range of each joint and reduces the palm size. The force and speed are confirmed experimentally.

Sensor based control for the execution of regrasping primitives on a multifingered robot hand

We demonstrate the robust execution of two regrasping primitives, rotation and pivoting on a four-fingered hand, based on the real-time sensors data based control. We indicate frequently occurring grasp errors and faults during execution of the two regrasping primitives. The sensors data is obtained from two sources. One is force-torque sensors mounted on the finger tips of the four fingers. The other is a laser range sensor mounted on the palm of the hand. The grasp information acquired through these two sources is useful for both sensing the errors and controlling the fingers. The paper proposes strategies to recover or prevent faulty grasps during manipulation, which may lead to unwanted object movements or grasp failures. The object to be manipulated is a polygonal prism, the model of which is known a priori.

100g-100N finger joint with load-sensitive continuously variable transmission

This paper presents a 99 g finger joint that can exert a very strong fingertip force of more than 100 N. We have shown that a simple five-bar linkage can be used as a load-sensitive continuously variable transmission (CVT) for a finger joint. The maximum fingertip force of our previous finger was limited by the mechanical strength of its links and bearings, not by the power of its DC motor. If the machine strength can be improved, a much greater fingertip force can be expected. To design a lightweight CVT with sufficient mechanical strength, we analyze the internal force of the CVT and select light plain bearings. We also analyze the stress by using the finite element method and select the materials of its links. Experimental results verify that the maximum fingertip force is more than 100 N near the singular configuration of the CVT and the maximum angular velocity is more than 550 deg/s. These motions are impossible without the CVT. We also developed a very light shape memory alloy brake of 0.56 g. The CVT with the brake can hold a fingertip force of more than 100 N not only near the singular configuration. The electric energy consumed by the brake is much less than that by the DC motor

Rigid body analysis of power grasps: bounds of the indeterminate grasp force

The equilibrium equation of a power grasp cannot determine the grasp force uniquely in general. This paper proves that the indeterminate grasp force is, within a bounded hyper-polyhedron, not necessarily convex by considering the compatibility with rigid body motion. The maximum fraction angle is found at its vertex. Thus, if all forces at the vertices are within the friction cone, then all forces in the hyper-polyhedron are within the friction cone. Based on this analysis, we present an algorithm for computing the joint torques that can balance a given set of external disturbances.