Development of the 6-axis force/moment sensor for an intelligent robot's gripper

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Abstract

This paper describes the development of the 6-axis force/moment sensor for an intelligent robot's gripper. The intelligent robot's gripper should detect the forces Fx (x-direction force), Fy and Fz in the gripping direction and in the gravitational direction to safely grasp an unknown object. Also, it should detect the moments Mx (x-direction moment), My and Mz to accurately perceive the position of the object in the grippers. Therefore, the intelligent robot's gripper should be composed of the 6-axis force/moment sensor which can measure the forces Fx, Fy and Fz, and the moments Mx, My and Mz, simultaneously. But the grippers are mostly made with a force sensor that may detect only one-direction force. In this paper, the 6-axis force/moment sensor to make the intelligent robot's gripper is newly modeled using parallel-plate beams (PPBs), designed and fabricated.

Introduction

The 6-axis force/moment sensor for an intelligent robot's gripper has widely been studied recently. Ceccarelli et al. [1] made the robot's finger with a force sensor that may only detect the force in the grasping direction, and performed the position and the force controls for gripping an unknown object. Castro et al. [2] manufactured the jaw gripper with the Fx force sensor and carried out the force control using it. Tlale et al. [3] fabricated the intelligent gripper with the contact sensor and a circuit for controlling it.

The above grippers can’t not only stably grasp an unknown object but also accurately perceive the position of the object in the grippers, because they can’t detect the forces Fx, Fy and Fz, and the moments Mx, My and Mz, simultaneously. The intelligent robot's gripper needs to detect the forces in the gripping direction and in the gravitational direction for safely grasping an unknown object, and it also needs to detect the moments to accurately perceive the position of the object in the grippers. Thus, the robot's gripper should be composed of a 6-axis force/moment sensor which may detect the forces Fx, Fy and Fz, and moments Mx, My and Mz, simultaneously. BL AUTOTEC [4] developed many kinds of the 6-axis force/torque sensor, but most they get the other resolution in each component sensor, for example, the model F/T5/50 (capacity of force sensors: 5 kgf, capacity of torque sensors: 50 kgf cm) has the resolutions of 5 gf, 5 gf, 15 gf, 30 gf, 30 gf and 30 gf in the Fx, Fy, Fz sensors and the Mx, My, Mz sensors, respectively, and the model NATO 1.2/1 also has 0.8 gf, 0.8 gf, 2.5 gf, 0.5 gf, 0.5 gf and 0.7 gf, respectively. It means that the rated outputs when the rated loads (maximum forces and moments) are applied to the 6-axis force/moment sensor are different respectively, in other words, the sensitivities of each component sensor are different, respectively. NISSO ELECTRIC WORKS CO., LTD. [5] also developed the multi-component load cell, it also has the same peculiar character as those of the 6-axis sensors of BL AUTOTEC. It is required that each component sensor of the 6-axis force/torque sensor generally get the same rated output to accurately detect forces and torques. Also, it is necessary that the structure of each sensor with the same capacity (ex, Fx = Fy = Fz = 50 N, Mx = My = Mz = 5 Nm) or different capacity (ex, Fx = Fy = 50 N, Fz = 60 N, Mx = My = 3 Nm, Mz = 5 Nm) in each force component and moment component is designed according to using for various purpose in an intelligent robot's gripper. But it is very difficult that the structure (sensing element) of the sensor is complex. Thus, that important thing in the 6-axis force/moment sensor is newly modeling having the structure of each sensor with the same and different capacity. The developed 6-axis force/moment sensors in references [6], [7], [8], [9], [10], [11] are improper as an intelligent robot's gripper in the accuracy (interference error) and in the size. So, it is necessary to develop the 6-axis force/moment sensor having the structure of each sensor with the same and different capacity and low interference error for an intelligent robot's gripper.

In this paper, the 6-axis force/moment sensor to be used as an intelligent robot's gripper for safely grasping an unknown object and accurately perceive the position of the object in the grippers are developed. The 6-axis force/moment sensor which may detect the forces Fx, Fy and Fz (each maximum capacity is 50 N), and moments Mx, My and Mz (each maximum capacity is 5 Nm) simultaneously is newly modeled using parallel-plate beams (PPBs), designed and fabricated. Finally, the characteristic test of the manufactured 6-axis force/moment sensor is performed. The structure of the 6-axis force/moment sensor newly modeled may be variously designed with the same capacity or different capacity in each force component and moment component.

Section snippets

Calculation of force and position in the robot's gripper

In order to safely grasp an unknown object and accurately perceive the position of the object in the grippers, the equations to calculate the force of the gravitational direction and the length components lx, ly and lz in x, y and z direction should be derived. Fig. 1 shows the different Cartesian coordinates of an unknown object and tow robot's grippers. The Cartesian coordinates of the grippers and the unknown object are represented in x, y, z and x′, y′, z′ frames, respectively.

The force

Modeling of the sensor

Fig. 2 shows the structure of newly modeled the 6-axis force/moment sensor which may detect the forces Fx, Fy and Fz, and the moments Mx, My and Mz for the intelligent robot's gripper system. The sensing elements of the 6-axis force/moment sensor are composed of by fixing E block part of two sensors with screws. One is a 5-axis force/moment sensor, which is composed of the Fx, Fy and the Fz sensors, and the Mx and My sensor, the other is the Mz sensor. Block A and B, and C and D are fixed to

Characteristic test and consideration

The manufactured 6-axis robot's finger force/moment sensor should be carried out the characteristic test using the 6-axis Force/Moment Calibration Machine [11] to evaluate the rated strains and the interference errors of it. Each sensor is tested three times by using the 6-axis Force/Moment Calibration Machine, and the output values from each sensor were averaged, respectively. In order to correspond the unit of the rated strain in theory and the rated output in characteristic test, the unit of

Conclusion

This paper describes the development of the 6-axis force/moment sensor to use as an intelligent robot's gripper for safely grasping an unknown object and accurately perceiving the position of the object in the grippers. The 6-axis force/moment sensor using PPBs that may measure the forces Fx, Fy and Fz (rated capacity is 50 N), and moments Mx, My and Mz (rated capacity is 5 Nm) simultaneously was designed and fabricated.

As the result of the characteristic test of the fabricated 6-axis

Gab-Soon Kim got B.S. degree in Precision Mechanical Engineering from Jeonbook National University; M.S. and Ph.D. degrees in Precision Mechanical Engineering from Hanyang University, Republic of Korea, in 1986, 1990 and 1999, respectively. He was a senior researcher at Department of Force Laboratory, Korea Research Institute of Standards and Science, Republic of Korea, from February 1990 to February 2000, and a visiting researcher at Department of Intelligent Robot Laboratory, University of

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Gab-Soon Kim got B.S. degree in Precision Mechanical Engineering from Jeonbook National University; M.S. and Ph.D. degrees in Precision Mechanical Engineering from Hanyang University, Republic of Korea, in 1986, 1990 and 1999, respectively. He was a senior researcher at Department of Force Laboratory, Korea Research Institute of Standards and Science, Republic of Korea, from February 1990 to February 2000, and a visiting researcher at Department of Intelligent Robot Laboratory, University of Tsukuba, Japan, from July 2003 to June 2004. Since 2000, he has been with Gyeongsang National University, where he is currently Assistant Professor at Department of Control & Instrumentation Engineering. His main research interests are in the areas of multi-axis force/moment sensor of intelligent robot, intelligent service robot and intelligent system.

Joong-Jo Park got B.S., M.S. and Ph.D. degrees in Electrical Engineering from Korea University, Republic of Korea, in 1981, 1983 and 1995, respectively. He was a senior researcher at Department of Control & Instrumentation Laboratory, Research Institute of Science and Technology, Republic of Korea, from October 1988 to Feb. 1991. Since 1996, he has been with Gyeongsang National University, where he is currently Associate Professor at Department of Control & Instrumentation Engineering. His main research interests are in the areas of visual measurement system and intelligent system.

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