This paper presents an anthropomorphic robot hand called the Gifu hand II, which has a thumb and four fingers, all the joints of which are driven by servomotors built into the fingers and the palm. The thumb has four joints with four-degrees-of-freedom (DOF); the other fingers have four joints with 3-DOF; and two axes of the joints near the palm cross orthogonally at one point, as is the case in the human hand. The Gifu hand II can be equipped with six-axes force sensor at each fingertip and a developed distributed tactile sensor with 624 detecting points on its surface. The design concepts and the specifications of the Gifu hand II, the basic characteristics of the tactile sensor, and the pressure distributions at the time of object grasping are described and discussed herein. Our results demonstrate that the Gifu hand II has a high potential to perform dexterous object manipulations like the human hand.
Index Termsâ€Force sensor, hand, humanoid, multifinger, robot,
It is highly expected that forthcoming humanoid robot will execute various complicated tasks via communication with a human user. The humanoid robots will be equipped with anthropomorphic multifingered hands very much like the human hand. We call this a humanoid hand robot. Humanoid hand robots will eventually supplant human labor in the execution of intricate and dangerous tasks in areas such as manufacturing, space, the seabed, and so on. Further, the anthropomorphic hand will be provided as a prosthetic application for handicapped individuals. Many multifingered robot hands have been developed. These robot hands are driven by actuators that are located in a place remote from the robot hand frame and connected by tendon cables. The elasticity of the tendon cable causes inaccurate joint angle control, and the long wiring of tendon cables may obstruct the robot motion when the hand is attached to the tip of the robot arm. Moreover, these hands have been problematic commercial products, particularly in terms of maintenance, due to their mechanical complexity. To solve these problems, robot hands in which the actuators are built into the hand have been developed. However, these hands present a problem in that their movement is unlike that of the human hand because
the number of fingers and the number of joints in the fingers are insufficient. Recently, many reports on the use of the tactile sensor â€œ have been presented, all of which attempted to realize adequate object manipulation involving contact with the finger and palm. The development of the hand, which combines a 6-axial force sensor attached at the fingertip and a distributed tactile sensor mounted on the hand surface, has been slight.
The Gifu hand II has a thumb and four fingers; the thumb has four joints with four-degrees-of-freedom (DOF) and the finger has four joints with 3-DOF; and the two joint axes of the thumb and the finger near the palm is orthogonal. Moreover, a developed distributed tactile sensor with 624 sensing points can be attached to the handâ„¢s surface and it is equipped with a six-axes force sensor at each fingertip. The design concepts and the specifications of the Gifu hand II, the basic characteristics of the tactile sensor, and the pressure distributions at object grasping are described and discussed herein. Our results show that the Gifu hand II has a high potential to perform dexterous object manipulations like the human hand.
II. ROBOT HAND DESIGN
An overview of the developed anthropomorphic right and left version of the Gifu hand II is shown in Fig. 1, in which the right hand is equipped with force sensors and tactile sensors. The right and left hands are designed symmetrically and have a thumb and four fingers. The design mechanism of the thumb and finger are shown in Fig. 2(a) and (b), respectively. The thumb has four joints with 4-DOF and the fingers have four joints with
3-DOF. The main difference between the thumb and the fingers is that the fourth joint of the fingers is actuated by the third servomotor through a planar four-bar linkage mechanism. Thus, the Gifu hand II has 20 joints with 16-DOF.
A. Design Concept
The hand is designed to be compact, light-weight, and anthropomorphic in terms of geometry and size, such that it performs grasping and manipulations like the human
hand. The design concept is as follows:
1) Size: It is desirable for the robot hand to resemble the human hand in size and geometry for purposes of skillful manipulation. The robot hand was designed to be similar to a relatively large human hand, and has a thumb and four fingers.
2) Number of Joints and Number of DOF: In a human hand, both the thumb and fingers have four joints. It is difficult for humans to control the outermost two joints of the fingers independently, because the fourth joint engages with the third joint. However, humans can control the joint angles of the thumb almost independently. The thumb is more dexterous and powerful than the fingers. The independent joint needs an independent actuator in the robot hand. This makes it hard to design a light weight hand. The finger can be modeled as a link mechanism with four joints and 3-DOF, and the thumb can be modeled as a link mechanism with four joints and 4-DOF. The finger made with coupled joints, which has more links than the finger made with independent joints, can grasp and manipulate more objects of various shapes than the finger made with fewer links. This is due to the fact that the area for grasping in the finger made with coupled joints is larger than that of the grasping area of the finger made with fewer links. Therefore, coupling will augment the dexterity of the hand. The number of joints and number of DOF of the robot hand were designed to mimic those of the human hand. The thumb is actuated by four servomotors and the fingers actuated by three servomotors. The fourth joint of the fingers are driven by the third servomotor through a planar four-bar linkage m echanism. The first joint and the second joint of human finger cross almost orthogonally at one point. Hence, the hand was designed such that the first joint and the second joint of each finger cross orthogonally at one point by means of an asymmetrical differential gear. Moreover, the asymmetrical differential gear enables the second joint axis to be placed near the surface of the palm, which make an effect to resemble a finger motion of the human.
3) Opposability of the Thumb: The thumb of the human hand can move in opposition to the fingers. Dexterity of the human hand in object manipulation is caused by this opposability. The robot hand was designed such that it has an opposable thumb.
4) Built-In Servomotor: For easy attachment to the robot arm, the robot hand was designed such that all joints are driven by built-in dc servomotors with a rotary encoder. To produce a high stiff hand, the transmission system was created by using high stiff gears such as a satellite gear and a face gear instead of low stiff gears such as a harmonic drive gear, and without using tendon cable.
5) Unit Design: Easy maintenance and easy manufacture of the robot hand are very important, so each joint was designed as a module and each finger was designed as a unit. Due to the unit design of the finger, hands having from two to five fingers are easily made.
6) Force Sensor: Each finger of the robot hand was designed to be equipped with a six-axes force sensor for compliant pinching.
7) Distributed Tactile Sensor: There are many sense organs in the human hand. These permit the human hand to manipulate an object dexterously. It is expected that more tactile sensors enable more dexterous manipulations. The robot hand was designed to be mounted with a developed distributed tactile sensor with 624 detecting points.
8) Wiring: Wiring is important in robotic mechanisms. All of the wires in the motor, force sensor, and tactile sensor should not prevent object manipulation by the robot hand. We designed all of the wires to be located along the back of each finger and palm. The hard wire used in the commercialized force sensor was changed to a soft wire, so as not to cause an external force to arise due to motion in the hard wire.
B. Improvement of Hand Mechanism
The mechanism of the Gifu hand II is a dramatic improvement
over the Gifu hand I in the following ways.
1) Reduction of Backlash: The Gifu hand I employed an asymmetrical differential reduction gear consisting of bevel gears at the first and second joints of the thumb and fingers. This asymmetrical differential gear enabled the second joint to be placed near the surface of the palm and the axes of the first joint and the second joint to be orthogonal. However, a backlash occurred because the axial force of the bevel gear moved the shaft of the bevel gear. Moreover, abrasion of the shank made by the aluminum material increased the backlash. To reduce the backlash in the Gifu hand II, face gear, in which the axial force is not generated, was adopted instead of bevel gear, and the frame material was change to the titanium alloy, which shows strength and excellent performance against abrasion. By these modifications, the backlash of the first joint decreased from 8 to 1. The backlashes of the other joints also decreased by nearly the same amount.
2) Higher Output Torque and Higher Response: For higher output torque, a higher-power motor and high-reduction ratio were adopted at the first and second joints. The minimum bandwidth of the robot hand is
7.5 Hz, which exceeds the responsibility of the human finger, the bandwidth of which is, at most, 5.5 Hz. This means that the robot hand can move more quickly than the human hand.
3) Higher Stiffness: By stress analysis using a finite element analysis system, the regulation of the safety factor of the mechanism was attempted. On the condition that the largest force acting on the fingertip is 12 N. We assumed that a standard safety factor was 10. In the Gifu hand I, the safety factors of some parts were less 10 or over 50. The Gifu hand II was greatly improved in terms of the safety factor and deemed to have good balance.
III. DISTRIBUTED TACTILE SENSOR
The distributed tactile sensor for the Gifu hand II was developed with the cooperation of Nitta Corporation. The shape of the tactile sensor is shown in Fig. 6. Tactile sensors are distributed on the surface of the fingers and palm. The thumb and the fingers of Gifu hand II each consist of four links and four joints, in which the base link is mounted in the palm and other three links are equipped with the tactile sensors. The tactile sensor has a grid pattern electrode and uses conductive ink in which the electric resistance changes in proportion to the pressure on the top and bottom of a thin film. Fig. 9 shows an overview of the 6-DOF robot arm (VS6354B, made by DENSO Company) and the Gifu hand II that is equipped with the six-axes force sensor at each fingertip and the developed tactile sensor. The Gifu hand II can be easily attached to another industrial robot by changing a base plate of the hand. Fig. 10 shows computer graphics of output patterns of the tactile sensor, measured while Gifu hand II is holding a soft spherical object 95 mm in diameter and a hard cylindrical object 66 mm in diameter. The height of the pole listed in Fig. 10 refers to the output level. Some outputs may include noise because of an inadequacy of attachment between the tactile sensor and the hand frame. The extended output patterns of the tactile sensor are shown in Fig. 11. The vertical line in Fig. 11 is the output of the tactile sensor of which the unit is millinewtons. The points on the horizontal plane are the measurement points of the tactile sensor, which are opened to show the measurement data of the force. It is clear that the output pattern depends on the shape of the object. When the human hand grasps the same objects using the globe attached to the tactile sensor, the output patterns of the tactile sensor are those shown in Fig. 12. In this case, a part of the tactile sensor was cut to fit the size of the human hand. The output pattern by the human hand differs from that by the robot hand in part because the human hand is covered with soft derma. However, these results show that the Gifu hand with the distributed tactile sensor has a high potential for dexterous grasping and manipulation.
We constructed a PC-based robot hand control system as shown in Fig. 13. Four 4ch counter boards, two 8ch digital- to-analog (D/A) boards, four 16ch A/D boards, and a timer board are connected to the PCI bus. Signals of D/A are inputted to servomotors through a 16ch linear amplifier. The operating system of the PC is Windows98. A tactile sensor is connected to another PC for measurement and display, and the maximum sampling cycle of detecting the 624 points is 100 Hz.
Anthropomorphic robot hands have been actively developed during past two decades. The Gifu hand II has a much higher number of sensors and higher response than any previously developed robot hand, and can move more quickly than the human hand. We consider that the Gifu hand II is useful as a research tool for dexterous robot manipulation using force sense and tactile sense. We have presented herein the design concept and the mechanism features of the Gifu hand II, which is designed to be used as a standard anthropomorphic robot hand. The Gifu hand II is actuated by built-in servomotors, distributed tactile sensors can be attached to its surface and it is equipped with a six-axes force sensor at each fingertip. The mobile space and geometrical size of the fingers are similar to those of the fingers on the human hand. We intend to use the Gifu hand II for future study of dexterous manipulation by the robot arm.