Robotic Gripper Design
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Li Yingying Department of Mechanical Engineering, National University of Singapore 21 Lower Kent Ridge Road, Singapore 119077 ABSTRACT A robotic gripper gripper is developed developed for universal use of gripping small objects. This robotic gripper gripper is a two-jointed two-jointed three fingered grasper. Its three fingers have synchronous mobility as driven by the sole DC motor. The upper joint of each each finger is driven driven by the lower lower joint by purely mechanism; mechanism; no extra extra actuator is required. required. This gripper is designed as driven by only one actuator at the palm. It is non-backdrivable but programmable. programmable.
INTRODUCTION A robotic gripper is an essential component of a robotic manipulator. It serves as the robot’s hand and allows the robot to manipulate objects. Recently robotic gripper is widely used for different tasks in various various fields. Variety of robotic grippers is developed in high flexibility flexibility and multi-function. Particularly, humanoid robot technology in this area attracts high attention of public. This project is aimed to study current existing robotic grippers and design a basic functional gripper to achieve simple grasping tasks.
DESIGN 1. Structure of the gripper This gripper consists of two major parts as shown in Figure 1 below: (1) Stationary base, ( Figure 1) including bottom base 1, support shafts 2, motor 3, motor attached base 4, palm 5, planetary gear 6, rods 7 and worm gear 8. The gripper palm 5 is designed in round shape. Three fingers are placed in triangle position on palm 5. Base 1 and 4 are in the same size. Shafts 2 here connect base 1 & 4, and support the total weight of the gripper. DC motor 3 is fixed onto base 4 and attached to planetary gear 6. Planetary gear 6 is fixed on base 4 as driver for the 3 fingers. Rods 7 are used to drive the worm gears 8 attached on them, so as to drive the three fingers.
(2) Three Fingers, ( Figure 3) each finger including three parts, upper part and lower part and finger holder 11. Upper part includes a pulley 12 and solid 13, shaft 14. Lower part includes a helical gear 15, a pulley 16, solid 17 and shaft 18. Shaft 14 connects upper part and lower part together. Shaft 18 connects lower part and finger holder together. Pulley 12 and pulley 16 are connected by timing belt 19. Finger holder 11 attaches finger to the stationary base.
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This project is an Undergraduate Research Opporunities Project Project (UROP) which was completed in Dec 2007.
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Finger 5
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7&8
Figure 1, Complete structure of gripper
6 2 3
Stationary Base 1
2. Driving Mechanism Each finger has two joints, lower joint and upper joint. ( Figure 2) (a) Lower Joint 13
12 14 19
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17
18
16
11
Figure 2, Lower Joint
Figure 3, Single Finger
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As shown in Figure 2, DC motor is used here as actuator to drive three fingers simultaneously. Gears are used here as major driving mechanism. The DC motor is attached to the sun gear of planetary gear to drive planet gears. Each planet gear drives worm gear rotating through driving the rod attached on it, and the worm gear drives the helical gear which is the driver of finger. The lower joint is in one degree of freedom. The joint can open from more than 180 degrees (open position) to 60 degrees (close position). This ensures the lower part of the finger has contact with the objects, no matter the size of the objects. (b) Upper Joint
As shown in Figure 3, pulleys and timing belt are used here as major driving mechanism. The lower pulley is attached to the helical gear as one piece. When helical gear rotates, the lower pulley rotates and drives the upper pulley rotating through timing belt. The upper pulley is attached onto the upper part of the finger as one piece. As the upper pulley rotates, upper part of the finger is driven rotating. The radius ratio of the lower pulley and upper pulley is 2:1. Upper pulley rotates 2 times of degree of the lower pulley. This ensures upper finger closes in enough degrees to grasp the objects.
DETAILED DESIGN 1. Motor
DC geared motor is used here. The total weight of 3kg object is affordable. Voltage 6~12V
Speed 90~180 rmp
Torque 3 kg
Weight 80g
2. Planetary Gear
In this design the planetary carrier is held stationary, and the sun gear is used as input. In this case, the planetary gears simply rotate about their own axes at a rate determined by the number of teeth in each gear. Here the gear ratio of the sun gear and planetary gear is 1:1. The speed is remained. Metal planetary gear is used here.
3. Timing belt Timing belts are used for power transmission and to interchange rotary motion. Timing belt has the advantage of avoiding back-lash problem because of the teeth on the belt. But the installation of timing belt must be very careful to ensure the exact length and position of it. 5. Reduced weight To reduce the weight of the gripper, holes are made on the supporting shafts and bottom base.
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6. Material Bases, support shafts and finger solid parts are made of Aluminum. Aluminum is in low weight and solid enough here to afford the force and weight applied. All gears and pulley are in metal, because they have to afford the high torque and can be easily damaged. The finger holde r is made in plastic, nylon. It is light weight and no much force to afford.
CONCLUSION A three fingered relatively small size robotic gripper is designed. This gripper is simply driven by one DC motor. The capability of the gripper is to grasp max 3 kg small size objects. Due to time constrain, the gripper is not fabricated. But the manufacture possibility is confirmed by one fabrication center. Function of the single finger is tested and confirmed during the design process. Further studies such as force control test and grasping test will be performed to confirm the effectiveness of the driving method. In additional, smart material (shape memory alloy) may be used to as actuator to drive upper joint of the finger to replace the timing belt driven. Further study will be performed to test this method.
ACKNOWLEDGEMENTS I would like to express my sincerely appreciation and thanks to Associate Professor Marcelo H Ang J, the project supervisor for his constant patience, guidance and understanding throughout this project. Prof. Ang helped me a lot throughout the project and never failed to guide me slowly and enlightened me on my thinking process. His understanding and encouragement throughout the project is the most motivation for me to complete this project successfully. Also my thanks go to Control Lab officers, for rendering their help to me whenever I need and enhancing my efficiency in completing the project.
REFERENCES 1. Barrett Technology Inc. http://www.barrett.com/robot/index.htm 2. Paolo Pedrazzoli, Poberto Rinaldi and Claudio R.Boer, “A rule based approach to the gripper selection issue for the assembly process”, Proc. 4th IEEE International Symposium on Assembly and Task Planning, pp. 205 &207, 2001 3. Hydraulic Four-bar Gripper , Retrieved from: http://www.sarobotics.com/ 4. RS online catalogue, http://www.rssingapore.com/cgi-bin/ 5. Planetary gears, http://mysite.du.edu/~jcalvert/tech/planet.htm
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