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Research Papers

A Comparative Study on Motion Characteristics of Three Two-Degree-of-Freedom Pointing Mechanisms

[+] Author and Article Information
Jingjun Yu

Robotics Institute,
Beihang University,
Beijing 100191, China
e-mail: jjyu@buaa.edu.cn

Kang Wu

R&D Center,
Sunward Intelligent Equipment Co., Ltd.,
Changsha 410100, China;
Robotics Institute,
Beihang University,
Beijing 100191, China

Guanghua Zong

Robotics Institute,
Beihang University,
Beijing 100191, China

Xianwen Kong

School of Engineering and Physical Sciences,
Heriot-Watt University,
Edinburgh EH14 4AS, UK

1Corresponding author.

Manuscript received October 11, 2014; final manuscript received November 6, 2015; published online February 24, 2016. Assoc. Editor: Federico Thomas.

J. Mechanisms Robotics 8(2), 021027 (Feb 24, 2016) (10 pages) Paper No: JMR-14-1289; doi: 10.1115/1.4032403 History: Received October 11, 2014; Revised November 06, 2015

Two-degree-of-freedom (2DOF) pointing mechanisms have been widely used in areas such as stabilized platforms, tracking devices, etc. Besides the commonly used serial gimbal structures, another two types of parallel pointing mechanisms, i.e., spherical parallel manipulators (SPMs) and equal-diameter spherical pure rolling (ESPR) parallel manipulators, are increasingly concerned. Although all these pointing mechanisms have two rotational DOFs, they exhibit very different motion characteristics. A typical difference existing in these three pointing mechanisms can be found from their characteristics of self-motion, also called spinning motion by the authors. In this paper, the spinning motions of three pointing mechanisms are modeled and compared via the graphical approach combined with the vector composition theorem. According to our study, the spinning motion is essentially one component of the moving platform's real rotation. Furthermore, image distortions caused by three spinning motions are identified and distinguished when the pointing mechanisms are used as tracking devices. Conclusions would facilitate the design and control of the pointing devices and potentially improve the measuring accuracy for targets pointing and tracking.

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Figures

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Fig. 1

Representation of the motion of the moving platform in a 2DOF rotational manipulator: (a) two parameters and (b) three parameters

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Fig. 2

Pointing device I based on a gimbal structure. (a) Gimbal structure and (b) proof-of-concept pointing device.

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Fig. 3

Second pointing device made of a SPM. (a) 1-RR&2-RRR SPM and (b) proof-of-concept pointing device.

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Fig. 4

Third pointing device made of an ESPR manipulator. (a) 4-4R ESPR parallel manipulator and (b) proof-of-concept pointing device.

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Fig. 5

Kinematic model of a gimbal manipulator. (a) Kinematic model and (b) coordinate transformation.

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Fig. 6

Kinematic model of a FCOR pointing manipulator. (a) Kinematic model and (b) coordinate transformation.

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Fig. 7

Kinematic model of the ESPR pointing manipulator. (a) Kinematic model and (b) coordinate transformation.

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Fig. 8

Simulations of revolution and spinning motion: (a) device I, (b) device II, and (c) device III

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Fig. 9

Angular velocity composition diagrams corresponding to three pointing devices (a) device I, (b) device II, and (c) device III

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Fig. 10

The moving freedom disk patterns: (a) device I, (b) device II, and (c) device III

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Fig. 11

Illustration of image distortion principle. (a) Three-dimensional view, (b) projective image of target on the camera plane, and (c) 2D view.

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Fig. 12

Projective views about distortion of the camera coordinate axes at different attitudes: (a) device I, (b) device II, and (c) device III

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