Humanoid Eye Robot with Angle Control and Image Registration Bo-Jiun Chen, Sheng-Fan Wen, Gun-Hwa Liu, Ya-Yun Lee, Wen-Pin Shih, Ching-Liang Dai, Chi-An Dai, and Yuh-Chung Hu
Abstract—This paper presents a humanoid eye robot with five degrees of freedom, including independent horizontal rotation for each eyeball, one synchronous vertical rotation, and two vertical motions of the eyelids. Two cameras were implemented for target tracking. The color difference of the images received by the cameras was compared for precise image registration. The horizontal rotation of each eyeball was monitored by an angle gauge, and the control accuracy was investigated. Actual geometric measurement was conducted for calibrating the angle gauge. The gauge value was then compared with the image registration. The comparison result indicated that the image registration algorithm effectively promoted the horizontal angle accuracy by decreasing rotation error from more than 2 degrees to less than 0.5 degrees.
II. HUMANOID EYE ROBOT MECHANISM The eye mechanism is made with humanoid size and designed to imitate humanoid motions of eyeballs and eyelids [2]. The mechanism can be used as the visual system in 1:1 humanoid robot as the communication media to outsides [3]. There are five servo motors in the eye robot mechanism, each of which stands for a degree of freedom, as shown in Fig 1. Moreover, two miniature cameras and two angle gauges are installed underneath the eyeballs for facilitating the feedback control of the horizontal rotation, which helps us to obtain a more precise rotation angle.
I. INTRODUCTION
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owadays, intelligent robots are being intensively developed for their high potentials in conducting complex tasks. With the assistance of robot vision, intelligent robot is able to carry out harder and more dangerous jobs, such as oil well exploration, Mars Exploration Rover, fruit acquisition, target tracking, navigation, emotion expression, interaction and so on [1]. This paper focuses on the angle tracking and calibration of target object with humanoid eye robot. With the feedback control utilizing angle gauges and the image processing method, the rotation angle of the eye mechanism is calibrated and a more precise rotation angle is obtained. Furthermore, a humanoid eye robot with five degrees of freedom, two miniature cameras, and two angle gauges is designed and manufactured. The angle tracking function is tested and verified by proceeding two different methods. A model is developed to simulate the relationship between the feedback control angle obtained from the angle gauges and the angle derived from image processing. The purpose of using this model is to derive a more accurate rotation angle based on the reading of the angle gauges when the eye robot rotates. Therefore, the errors on target tracking due to the rotation inaccuracy can be significantly reduces. This accurate angle tracking is expected to be applied in diverse fields. B. J. Chen, S. F. Wen, G. H. Liu, Y. Y. Lee, and W. P. Shih are with the Department of Mechanical Engineering, National Taiwan University, Taipei, 10617, Taiwan (phone: +886-2-33664511; fax: +886-2-2363-1755; email:
[email protected]). C. L. Dai is with the Department of Mechanical Engineering, National Chung Hsing University, Taichung, 402, Taiwan. C. A. Dai is with the Department of Chemical Engineering, National Taiwan University, Taipei, 10617, Taiwan. Y. C. Hu is with the Department of Mechanical and Electro-Mechanical Engineering, National Ilan University, Ilan, 260, Taiwan.
Fig 1. Illustration of the eye robot mechanism
Fig 2. Side perspective of eye robot mechanism for vertical and horizontal motion illustration
A. Servo Motors and Transmission The eye robot with five degrees of freedom in this work is illustrated in Fig. 1. Each eyelid is connected through three linkages to a motor so that it can rotate upwards and downwards. These two eyelids possess two degrees of freedom in total. For human eyeballs, the horizontal rotation of the two eyeballs can be conducted independently while the
vertical rotation cannot. In other words, cross-eye is possible for human and the vertical rotation for the two eyes are synchronous [4]. Accordingly, the eye robot in this work controls the vertical rotation of the two eyeballs simultaneously with slide tracks and vertical bars which transmit power from servo motor to the eyeballs [5]. The horizontal rotation of the eyeballs possesses two degrees of freedom in total, and the vertical rotation possesses one only. The rotation mechanism of the eye robot is detailed in Fig 2. The shaft and disk components convert the circular motion of the vertical servo-motor into rectilinear direction. And then the vertical motion is transferred to the eyeballs through the vertical bars which are guided by the sliders. The horizontal rotation of each eyeball is driven by a servo motor through a set of gear clusters, as shown in Fig 2. The gear C is bonded with the shaft of horizontal servo motor and the meshes with the gear B. The gear A is driven by the gear B, and its geometric center coincides with the central axis of the horizontal rotation of the eyeball. Due to gear matching, backlash will degrade the angle rotation precision. Hence a angle gauge is installed for monitoring and feedback control. All the servo motors are connected to a host PC through SSC-32 interface card. The driving voltage for the SSC-32 card and the Futaba S3103 DC servo motor is 9 V and 6 V, respectively. B. Angle Gauges and Miniature Cameras Each angle gauge is fixed by a gauge holder, as shown in Fig. 2. The gauge shaft is connected and aligned to the rotation axis of the gear A. Therefore, the gear A rotates synchronously with the eyeball and the shaft of the angle gauge. The gauge holder is designed demountable and is fixed with the baseplate by screws. Hence the angle gauges can be replaced if needed. The angle gauge used in this work is MTS-360 with the low linearity error of ±0.5%. MTS-360 is produced by Piher International Corporation, and it is connected to the host PC through Arduino-Nano surface mount board integrated with an USB interface. Miniature cameras with CCD chips are attached on the eyeballs and connected to the host PC through the USB interface. The miniature camera used in this work is CM-51 from Mega System Technologies Inc. It has 1.3 mega-pixels. C. Materials and Fabrication Process The eye robot is mainly made of acrylic which has the advantages of light weight and high stiffness. The light weight is an essential merit in the overview of developing a complete humanoid robot. The acrylic components of the eye robot are manufactured using the combination of laser machining and CNC milling. Laser machining is a two-dimensional cutting method which can make simple components faster than CNC milling. Conversely, three dimensional components with relatively higher precision can
be obtained using the CNC milling. The combination of the both methods enables the manufacturing of complicated components. The assembled eye robot with the manufactured acrylic components is shown in Fig. 3. The servo motors, miniature cameras, and the controllers have been embedded in this robot.
Fig 3. Assembled eye robot mechanism and SSC-32 servo controller
Fig 4. Flow chart of angle calibration using angle gauge feedback control
III. ANGLE CALIBRATION USING ANGLE GAUGE FEEDBACK CONTROL The feedback control of the horizontal rotation is achieved by sensing the rotation angle using the angle gauge. The control flow is shown in Fig. 4. The target angle of rotation is denoted by D in the flow. Due to the error mainly caused by the backlash of the transmission mechanism, the rotation angle D is deviated from the desired value when the servo motor is firstly driven. T denotes the feedback angle measured by the angle gauge when the servo motor finishes rotation. Accordingly, ΔT=D-T represents the difference between the desired and actual angle of rotation. Because the resolution of the servo motor in this work is 1o, the achieved angle of rotation (T) is considered equal to the desired angle (D) when |ΔT| is less than 1o [6]. For |ΔT|>1o, the control loop is continued. To precede the following process, the sign of ΔT has to be determined. Positive ΔT represents that the angle has been overly rotated
while negative ΔT indicates that the actual rotation is less than the desirable angle. The backlash error will be induced for the case of ΔT>1o. Conversely, the eyeball is continuously rotated for the case of ΔT
