2006 IEEE International Conference on Multisensor Fusion and Integration for Intelligent Systems September 3-6, 2006, Heidelberg, Germany
MoC01.1
Modularly designed lightweight anthropomorphic robot hand Artem Kargov, Christian Pylatiuk, Heinrich Klosek, Reinhold Oberle, Stefan Schulz, Georg Bretthauer, Member, IEEE
Abstract— In this paper, the modular design of artificial hands is presented. The modular concept shall be introduced based on the example of the artificial anthropomorphic hand prototype, which is part of the project for the development of a human assistive robot. Particular attention shall be dedicated to details of modular construction and servicing of the hand prototype. Prototype components, functional activities, technical characteristics, and the control system of the hand shall be introduced as well. Additionally, two hand prototypes were attached to a humanoid service robot and first experience gained from their operation will be presented in conclusion.
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I. INTRODUCTION
HE development of service robots has experienced a rapid growth lately. According to the world robotics survey released by the United Nations Economic Commission (UNECE) in cooperation with the International Federation of Robotics (IFR) in October 2005 [2], spin-off effects for the market of professional service robots in 20052008 are forecasted. Over 1 million household robots are reported to be in use and several millions are expected for the next few years. Nowadays, the progress achieved in the development of service robots is noticeable in various research laboratories all over the world. Many fascinating prototypes of intelligent systems for household applications were designed [3]-[5]. Some service robots can already interact with humans, but they should be able to assist people autonomously. Hence, aspects of human-robot interaction, such as multi-sensory perception, cognition, mobility, and individual acceptance may well be classified a key technology. The ability of manipulation with artificial end effectors is a significant part
of the robot-human interaction. Many artificial hands and grippers are constructed [6]-[10]. In general, artificial manipulation systems are always constructed for a number of special applications. Usually, actuators are placed in forelimb of robot arm. Researchers usually try to construct systems that comply with the concept of dependability [11]. It includes, for example, such attributes as reliability, maintainability, safety, etc. Nevertheless, design concepts for service robots with manipulation abilities should be improved from the user’s point of view. Nowadays, such aspects of man-machine interaction as locomotion and manipulation capabilities, adaptability, and the ability of sensing may be very important to service robots and particularly to manipulators [12]. This paper describes the progress achieved in the development of an artificial anthropomorphic hand. It is the part of a research project on humanoid robots [1]. How versatile should and can an artificial robotic hand be and how can this be achieved? These questions will be discussed in this article.
Manuscript received April 13th, 2006. This work was supported in part by the German Research Foundation within the Collaborative Research Center under Grant SFB588-`Humanoid Robots - Learning and Cooperating Multimodal Robots`. The authors gratefully thank for the financial support. All Authors are with the Institute for Applied Computer Science, Forschungszentrum Karlsruhe, Germany. Corresponding author: ++49-7247825757; fax: ++49-7247-825786; e-mail:
[email protected]. Fig. 1. Design of the artificial robotic hand.
1-4244-0567-X/06/$20.00 ©2006 IEEE.
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II. DESIGN OF THE HAND A. Modular Design The newly designed artificial robotic hand is depicted in Fig. 1. The prototype of the hand consists of an anthropomorphic mechanical framework, flexible fluidic actuators, 8 valves, and an electronic unit. The artificial framework forms a skeleton of the artificial hand. It has an anthropomorphic appearance, the same size as a human hand and consists of 5 fingers and an artificial metacarpus, which are made of aluminium with a high tensile strength. The construction of the thumb differs from the other fingers and allows for its opposite movement. Fingers are made of artificial bones and joints. The whole system is designed modularly. All joints and bones are constructed identically and have universal fittings. The hand skeleton can be mounted in different ways using clack elements to arrange and fix different parts and elements together without any special connections. In this way, it is possible to construct, for example, fingers with variable total lengths or lengths of the fingers’ phalanges. This allows for variety of hand sizes and hand geometries and reduction of production costs. Additionally, the number of actuators can be varied or active joints can be replaced by passive ones without actuators. Thus, the dynamics of the hand during functioning can be optimized for diverse grasping activities. The modular design of the hand provides for a compact assembly of all components. Basic interfaces allow for the use of different robotic platforms within implementations of service robotics. Direct actuation of fingers saves space, and valves and electronics can be integrated in the metacarpus of the hand. The artificial hand does not require any external components or devices, but only 6 bar air pressure and 7.5V DC power supply. It allows for a lightweight construction of the hand. A new class of fluidic actuators is developed and optimized for implementation in the artificial hand. The artificial hand will be actuated by flexible fluidic actuators. Flexible fluidic actuators are placed in the hand in comparison with other technologies and elastic energy can be stored directly in the actuator itself. The actuation principle of such kind of actuators is presented in detail in [13]. The mechanical construction of joints is miniaturised to ensure the anthropomorphic appearance of the fingers. Elastic chambers will expand under the pressure applied and torsion movements of finger joints will be achieved. Consequently, the artificial hand will only need air supply for functioning. Flexible fluidic actuators have an excellent power to weight ratio, good dynamics, compactness, and light weight. As regards various criteria, fluidic actuators have already been recognized as being suitable for applications in macrorobotics as compared to other actuator technologies [14]. The position of the
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fingers of an artificial hand during grasping is set by custom made miniature valves (Fig. 1). Eight 2/2-way microvalves are constructed and can be controlled independently from each over. Thus, valves can be controlled separately, which enables different grasping types. The electronic unit is integrated in a single multi-layer, small-sized circuit board. It consists of a programmable microcontroller PIC16F877 by Microchip (Microchip Technology Inc., USA), drivers for the valves, an analogue-digital converter, and a serial RS232 interface driver. The RS232 interface is used for diagnosis of the whole control system of the hand from a PC or to transmit special data, such as positioning or pressure sensor signals, to the service robot. Mini-connectors are implemented in the same board for direct connection of all periphery units to the electronics. B. Control System The basic control concept of the hand system is presented in Fig. 2. This system has a 3-level hierarchy. The low level is responsible for the implementation of original grip samples using position and force sensors, valves and actuators. The selection of appropriate grip samples and hand-arm
Fig. 2. Flow chart of the control system.
coordination are carried out on the high and middle levels, respectively. The controller of the low-level hand control system is connected with periphery components and uses special software for intercommunication. The software has been designed using a PCW compiler by Custom Computer Services (Custom Computer Services Inc., USA). The data between high control levels and low-level control are transmitted via an RS232 interface. Control signals for hand positioning will be transmitted from the robot to the hand and read by the controller in the loop at a frequency of 200 Hz.
C. Joint Position Sensors Sensing of joint angles is realised using contact-free 10-bit
increase the magnetic field of a magnet with a resolution of 0.35 degrees or 1024 positions per revolution. The magnetic field distribution is presented in Fig. 3. The rotary encoder has small dimensions of 5.3 mm x 6.2 mm and is integrated in the small-sized circuit board which is mounted sideways on the first half of the joint (Fig. 4). The magnet’s axis of rotation corresponds to the joint axis and is positioned above the electronics board using mechanical fittings on the second part of the joint. The flexion movement of the joint enables the rotary motion of the magnet relative to the electronics chip. Other research groups have also used Hall elements for other applications [6]. D. Force Sensors Pressure sensors (Type FSR 149 from Interlink, Camarillo, CA) were chosen and modified by using a conductive polymer dome to convert it into a force sensor. These sensors are low priced and can be integrated into finger phalanx due to their small size (thickness: 0.5 mm, diameter: 7.6 mm). The resistance of the FSR sensor is reciprocally proportional to the force, which is applied to the sensor. A voltage divider setup was used to measure even low contact forces (from 1N to 5N) and contact forces up to 100N can be measured at reasonable resolution.
Fig. 3. Typical magnetic field distribution. Adopted from [16].
programmable magnetic rotary encoders Type AS5040 by Austriamicrosystems [15]. Technical characteristics of the chip are described in [15]. Contact-free magnetic rotary encoders for angle measurements consist of a ring of Hall elements and digital signal processing electronics in a single device. The two-pole magnet is placed over the rotating centre of the chip and contact-free high-resolution angle measurement over 360 degrees is possible. The Hall elements
Axis of rotation Hall sensor
Magnet
Fig. 4. Finger joint with integrated rotary encoder.
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Fig. 5. Pressure sensor.
E. Pressure Sensors The pressure for flexible fluidic actuators is controlled by using of piezoelectric relative pressure sensor (type series 1 TAB, from Keller, see Fig.5). The design of this custom made sensor is very compact, so it can be integrated into the seat of each valve. The pressure sensor consist of a silicon measuring cell. They are linear (< 0.25 % FS), have asymptotic stability of rest position of ± 1 mV, a pressure range between 2 and 1000 bar and a temperature drift of 0,01 % / °C.
F. Performance Due to the anthropomorphic design and the thumb design, a large number of hand positions and grip types can be performed. 6 typical grip types, such as a cylindrical power grasp, lateral grasp, tripod grasp, hook grasp, and spherical grasp, are defined as standard grip types for this hand and pre-programmed for the prototype of the hand. Other grip types can be pre-programmed, if desired. For example, the index finger can be extended and other fingers can be flexed at the same time to press a key or to operate a switch. Due to the modularity of the hand construction, a large operating range of the hand motion can be achieved. An opening span of 15 cm is reached for the new prototype. 11 degrees of freedom and the inherent compliance of actuators enable the fingers of the hand to adapt to the shape of an object during grasping. Hence, low contact forces are needed for grasping, and contact forces are distributed similarly to those of natural hands. Moreover, a large contact area between a hand and an object during grasping ensures stable holding of an object.
directly in cooperation with the user in a kitchen environment to assist elderly and disabled humans. Features of this robot system are a human-like motion system and intelligent control. It will be able to communicate with the user using speech, gesture, and haptics. Self-learning and learning with the aid of a human may also be considered duties and responsibilities for this service robot. First manipulation tasks
TABLE I TECHNICAL DATA OF THE ROBOT HAND Parameters hand dynamics (grasping speed ) number of actuators (degrees of freedom) total weight holding force (hook grasp) average phalange contact force (stable holding with a power grasp) torque joint (7 bar) power supply prototype/optional pressure supply airflow rate interface closure rate (full hand closure and opening)
2 rad/s 8 to 16 0.245 kg up to 100 N from 1 N Fig. 6. Humanoid Robot ARMAR.
up to 25 Nm 10 VDC air at 7 bar 6.1 l/min RS232 / Bluetooth 2/s
were performed in a kitchen environment (Fig. 6). The experiments demonstrated that the hand is able to grasp and hold objects. Objects like a bottle, cups, drawer handles, and dishwasher handles were common objects for test grasping.
Only lightweight materials were used for the construction of a prototype and the motion of the hand is very dynamical. All fingers can be flexed within one second only. The weight of the whole hand is 245 g only and a very good power to weight ratio is achieved for the hand in comparison with hand constructions with other actuation types, such as gear motors in [8]. The technical data of the hand are given in Table I. III. APPLICATION The development of a mobile assistive robot at the University of Karlsruhe is the main objective of the Collaborative Research Centre 588 project “Humanoid Robots – Learning and Cooperating Multimodal Robots” [1]. The new mobile robot is a service robot with a humanoid design. According to the main aim of the project, this service robot should be equipped with two artificial hands and work
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IV. CONCLUSION The prototype of an artificial robotic hand represents a lightweight artificial robotic system of modular design. This construction has a number of features. The anthropomorphic design of the hand is united with a high functionality, and definite motion scenarios (grasping types) can be realised and pre-programmed. Flexible actuators allow for a high flexibility of the hand and its dexterity. The sensor-aided control and adaptability ensure a precise contact force distribution during grasping as well as a precise and safe holding and manipulation of objects. Lightweight components and intelligent control make the hand safe for the user. The modular design, easy servicing, and simple installation show how versatile an artificial hand can be and allow for the use of such kinds of hands in humanoid service robotics.
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