Elsevier

Mechatronics

Volume 11, Issue 1, 1 February 2001, Pages 27-42
Mechatronics

Optimal design and development of a five-bar finger with redundant actuation

https://doi.org/10.1016/S0957-4158(99)00089-6Get rights and content

Abstract

In order to develop a human hand mechanism, a five-bar finger with redundant actuation is designed and implemented. Each joint of the finger is driven by a compact actuator mechanism having an ultrasonic motor and a gear set with a potentiometer, and controlled by a VME bus-based control system. Optimal sets of actuator locations and link lengths for cases of a minimum actuator, one-, two-, and three-redundant actuators are obtained by employing a composite design index which simultaneously considers several performance indices, such as workspace, isotropic index, and force transmission ratio. According to the optimization result, several finger-configurations optimized for a special performance index are illustrated, and it is concluded that the case of one redundant actuator is the most effective in comparison to the cases of more redundant actuators, and that the case of two redundant actuators is the most effective in multi-fingered operation in which the force characteristic is relatively important, as compared to the kinematic isotropy and the workspace of the system.

Introduction

Robot hands have been employed for fine motion control and assembling parts. Most of the existing robot hands employ tendon-driven power transmission [1], [2], [3], [4]. However, frictions existing in the transmission line require more effort on control. In light of this fact, we propose a five-bar finger mechanism which is directly driven by an ultra-sonic motor at the joints of the mechanism. Since the five-bar finger mechanism has many potential input locations for attaching actuators, a redundant actuation mode can be achieved [6], [7]. Redundant actuation prevails in general biomechanical systems, such as the human body, and the bodies of mammals and insets [5]. Redundant actuation can also be found in many robotic applications. They include multiple arms, dual arms, multi-fingered hands, walking machines, and so on. Several load distribution criteria associated with load sharing and internal load generation have been developed by many researchers [6], [7], [8], [9], [10], [11], [12]. In the case of redundant actuation, the location and number of the actuator are very important parameters for the system performance.

Redundant actuation can be easily explained in terms of mobility. Mobility of a system is defined as the number of independent variables which must be specified in order to locate its elements relative to another. It is described byM=D(L−1)−i=1J(N−Fi),where D, L, J, and Fi denote the degree-of-freedom of the rigid body (e.g. three in planar motion, six in spatial motion), the number of links, the number of joints, and the motion degree-of-freedom of the ith joint, respectively. When M is greater than N (operational or task-space degree-of-freedom), the system is called “a kinematically redundant system”. On the other hand, when the number of actuators is greater than M (this situation usually happens in a closed-chain system), the system is called “a redundantly actuated system”. For example, the mobility of the human upper-extremity (arm) can be considered as seven, while it has 29 human actuators (i.e., muscles). Accordingly, it has 22 redundant actuators.

The purpose of this paper is the optimum design and development of a five-bar finger employing ultra-sonic motors. Optimal sets of actuator locations and link lengths for the cases of using a minimum number of actuators, one-, two-, or three-redundant actuators, are obtained by employing a composite design index which simultaneously considers several performance indices, such as workspace, isotropic index, and force transmission ratio. According to the optimization result, several finger-configurations optimized for the special performance index are illustrated, and it is concluded that the case of one redundant actuator is the most effective in comparison to the cases of more redundant actuators and that the case of two redundant actuators is the most effective in a multi-fingered operation, in which the force characteristic is relatively important in the expense of the small workspace.

Section snippets

Kinematic modeling

The modeling methodology integrates the Generalized Principle of D’Alembert with the method of kinematic influence coefficients (KIC), resulting in closed form vector expressions. The reader is referred to Freeman and Tesar [14] for a more detailed description of the following scheme. In the following, the letter G stands for a first-order KIC matrix, and superscribed quantities indicate dependent parameters with subscripts denoting independent parameters.

Optimization methodology

The given problem in this work is a nonlinear discrete optimization with constraints. To deal with this problem, three numerical methods are used. The exterior penalty function method is employed to transform the constrained optimization problem into an unconstrained optimal problem. Powell’s method is applied to obtain an optimal solution for the unconstrained problem, and the quadratic interpolation method is utilized for uni-directional minimization [15].

Kinematic design indices

Based on the effective force

Structure of the five-bar finger

Fig. 6 shows the prototype of the five-bar mechanism. According to the optimization result, four actuators are placed to 1345 joints. Each joint of the finger is driven by a compact actuator mechanism having an ultrasonic motor and a gear set with a potentiometer, and the system is controlled by a VME bus-based control system. The ultra-sonic motors have a high torque/size ratio, as compared to a DC motor with a similar size [17]. A gear transmission having about a 15:1 speed reduction ratio is

Conclusions

In this paper, we proposed employment of a redundant actuation in a finger design with the purpose of enhancing the kinematic isotropic characteristic and maximum force transmission ratio of the finger mechanism. Using the concept of a composite design index, which allows multi-purpose and multi-variable optimization, optical sets of actuator locations and link lengths for the cases of using the minimum numbers of actuators, one-, two-, and three-redundant actuators are obtained. Three design

Acknowledgements

This work has been supported by the Korea Institute of Science and Technology 2000 Human Robot program.

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