Autonomous Mobile Robot-I

Transcription

1 Autonomous Mobile Robot-I Sabastian, S.E and Ang, M. H. Jr. Department of Mechanical Engineering National University of Singapore 21 Lower Kent Ridge Road, Singapore ABSTRACT This report illustrates the design and realization of a robot manipulator to be incorporated into an autonomous mobile robot. The objective is to obtain the optimal configuration for the desired purpose. Various mechanical configurations and components will be considered. The robotic system consists of the mechanical structure, actuators, sensors, electronic hardware for signal processing and computer/microcontroller for processing of information. This robotic manipulator is designed for the Tech X challenge organized by Defense Science Technology Agency (DSTA). This robotic manipulator would be mounted a robot base, not covered in this report. The objectives are summarized as the purpose or the required tasks by the manipulator for the competition. INTRODUCTION Three main tasks can be formed. Firstly, it must have sufficient dexterity for surveillance with a Stereo Camera at the end-effector. Secondly, operate a 20mm x 20mm elevator button at a possible height of 1.5m with sufficient force and accuracy and, lastly, engage targets by touching them. To achieve this, considerations are noted. Mechanical Consideration In order to be competitive enough, the robot manipulator must have links that are stiff enough to prevent any significant deflection, possess sufficient dexterity to maneuver in elevator, be precise and accurate localization of end-effector and have an initial estimate weight of less than 10kg. Electrical Consideration Motors must generate enough torque that would be able to carry a pay load of two kilograms (kg) including a stereo camera at the end-effector. It must also push an elevator button and have sensors to feedback position of joints in the manipulator. DESIGNS Arm Design 1 (RRRRR) R: Revolute Joint Joint 2: R Joint 3: R Joint 1: R Joint 4: R P: Prismatic Joint Joint 5: R Fig 1(a) Fig 1(b) 1

2 In Arm Design 1, all joints indicated in Fig 1(a) are revolute joints. In this configuration, Joint 1 and Joint 4 have their rotational axis about the illustrated Y-axis while Joints 2 to 4 have the same rotational axis about the Z-axis. Fig 1(b) shows the collapsed configuration. Advantages Able to change Centre of Gravity of Robot while maintaining front vision Collapsable and compact Able to maximize vertical height while maintaining front vision Able to face perpendicularly to elevator buttons without having to move base. Disadvantages Might collide with components if arm is held too low Table 1 Arm Design 2 (RPPRR) Joint 2: P Joint 3: P Joint 4: R Joint 5: R Joint 1: R Fig 2 (a) Fig 2 (b) In Arm Design 2, as indicated in Fig 2(a), Joints 1, 4 and 5 are revolute while Joints 2 and 3 are prismatic. It is pertinent to note that although Joint 3 has two actuators actuating in the same prismatic direction as indicated by the blue arrow in Fig 2(a), it is still considered as 1 degree of freedom (DOF) but has 2 degrees of mobility. In this configuration, Joints 1 and 5 have their rotational axis about the illustrated Y-axis, Joint 4 about the Z-axis, Joint 3 in the direction of the X-axis and Joint 2 in the direction of the Y-axis. Fig 2 (b) shows the collapsed configuration. Advantages Collapsable Won t collide with components on the base. E.g Pan-Tilt-Zoom(PTZ) Cam or Sick Laser Won t take up space on robot s base Disadvantages High of Centre Of gravity Limited height vision capability Limited shifting of Centre of Gravity Table 2 2

3 Arm Design 3 (RRPRR) Joint 2: R Joint 4: R Joint 5: R Joint 3: P Joint 1: R Fig 3 (a) Fig 3 (b) In Arm Design 3, as indicated in Fig 3 (a), Joints 1, 2, 4 and 5 are revolute while Joint 3 is a prismatic. It is also pertinent to note that, together with Arm Design 2, although Joint 3 has two actuators actuating in the same prismatic direction as indicated by the blue arrow in Fig 3 (a), it is still considered as 1 degree of freedom (DOF) but 2 degrees of mobility. In this configuration, Joints 1 has its rotational axis about the illustrated Y-axis, Joint 2 about the Z-axis, Joint 3 in the direction of the X-axis, Joint 4 about the Y-axis and Joint 5 about Y-axis. Fig 3(b) shows the collapsed configuration. Advantages Able to be low and very compact Able to maximize vertical height Centre of gravity is low in compact mode Arm Design 4 (RRPRR) Disadvantages Require space on the robot s base to be compact Possible collision with components on robot s base Table 3 Joint 2: R Joint 4: R Joint 3: P Joint 5: R Joint 1: R Fig. 4 (a) Fig. 4(b) In Arm Design 4, as indicated in Fig 4(a), Joints 1, 2, 4 and 5 are revolute while Joint 3 is a prismatic. It is also pertinent to note, as the same for Arm Design 2 and 3, that although Joint 3 has two actuators actuating in the same prismatic direction as indicated by the blue arrow in Fig 3

4 4 (a), it is still considered as 1 degree of freedom (DOF) but 2 degrees of mobility. In this configuration, Joint 1 has its rotational axis about the illustrated Y-axis, Joint 2 about the Z-axis, Joint 3 in the direction of the X-axis, Joint 4 about the Y-axis and Joint 5 about Y-axis. Fig 4(b) shows the collapsed configuration. Advantages Disadvantages Able to be as compact as possible Susceptible to vibration during staircase climbing Able to maximize vertical height Higher centre of gravity Centre of gravity is low in compact mode. SELECTED DESIGN Table Fig. 5 (a) Fig. 5 (b) Fig. 5 (c) After some photography, important lessons were learnt. Part 1 of Fig 5 (a) showed a smaller surface area when looking at the elevator button at an angle and Part 2 of Fig. 5 (a) showed a button guard that would hinder the operation of the elevator button if operated at an angle too. This makes operating the elevator button perpendicularly to the button face an advantage. Also interesting to note is the necessary height elevation of the camera at the end-effector. In Fig. 5 (b), the number is not clearly visible at the digital number display at the top of the elevator. Zooming in, Fig. 5 (c), we can see that there is actually a number there which can only be read when approached closer to the display. This concludes the necessary element of height elevation m 5 1 Fig. 6 The configuration in Fig 6 was chosen based on its lesser number of disadvantages compared to other configurations. Part 1 in Fig 6 allows little occupation of space on the robots base. This allows more space for the components to be placed robot s base. Part 2 allows sufficient height clearance to prevent any collision with components on board of the robot s base. The 4

5 configuration of Part 3 and Part 4 allows Part 4 to be folded into Part 3 thus saving space and lowering its overall Centre of Gravity. Part 3 and Part 4 have lengths of 1m which is based on NUS Personal Elevator. The overall configuration allows Part 5 to have the ability to face the elevator buttons perpendicularly even if the buttons are not directly in front of it. OPTIMIZATION Our initial plan was for the robot base not to move around while in the elevator. It would just extend its manipulator and operate the elevator. This led to the manipulator s length being 1m but as illustrated in Fig. 4, it would be a hindrance during motion as the base has a maximum limit to its size. Hence it was limited to the blue line indicated in Fig. 7(a). Hence it was shortened to a length of 0.6m in Fig 7(b). Fig. 7(a) Fig 7(b) Fig 7(c) Carbon Fiber Composite Another idea thrown out to us was the use of carbon fiber composite.it is a very strong, light and expensive composite material and hence it is highly desirable. Its relatively very high cost would not be justified. Thus, we look at other cost efficient materials to optimize the weight. Plastics In order to optimize the weight, we looked to plastics to replace some housings/casings that we feel would greatly reduce the weight. To even some extent, we also look to some hard and tough plastics that would replace some structure of the manipulator. Fig 7(c) indicates the parts we wish to replace with plastics with the aim of having a significant reduction in weight. JOINTS In Fig 8, the respective joints are indicated and encased within each joint would house the motor, gearhead, brake, encoder (Motor) and encoder (Absolute). Fig 8 5

6 Fig 9(a) Fig 9 (b) Dissecting the 3D model housing, the first joint mechanism would look like Fig. 9(a) and within that joint, it would house the abovementioned components as shown in Fig 9(b). The gearhead would come in the form of a harmonic drive as labeled in Fig 9(a). The absolute encoder is located on the underside modular plate to the base. The absolute encoder is necessary to measure the exact rotation of the motor adapter rotation while the Motor encoder measures the rotation of the motor. Fig. 10(a) Fig.10(b) Dissecting the 3D model housing of the second joint mechanism would look like 10(a) and within that joint, it would house the abovementioned components as shown in Fig 10(b). The gearhead would come in the form of a harmonic drive as labeled in Fig. 10(a). The absolute encoder is located on the lateral left side of the second joint mechanism as seen in Fig. 10(b). As in the first joint mechanism, the absolute encoder is necessary to measure the exact rotation of the motor adapter rotation while the Motor encoder measures the rotation of the motor. The exact mechanism of Joint 2 is repeated for Joint 3 and 5 while the mechanism of joint 1 is repeated for joint 4. CALCULATION Motor and Harmonic Drive Selection 6

7 The end effector has a load requirement of 2kg and speed requirement of 0.5 ms -1 2m Joint 2 Link 2 Link 3 Joint 3 2kg Fig 11 In Fig 11, we note that lengths of link 2 is the same as link 3 and that Joint 2 bears a greater load than Joint 3 and will be used as reference in calculation. Torque requirement Speed Requirement Power Requirement = Force x Distance to line of action of force = 2kg x 10ms -1 x 2m = 40Nm = (Velocity at end effector)/(arc radius) = 0.5 ms -1 /2m = 0.25 rad s -1 = 2.39 rpm = 40Nm x 2.39rpm = 95W COMPONENTS SELECTION Joints 1-3 Based on the required torque and speed, we look for a motor configuration that comes with a brake and encoder. In addition, using Harmonic Drive SHD 20 that has a ratio of 160:1 together with a motor that has a No-Load Speed of 7670rpm and Stall Torque 2.29 mnm, we proceed with our verification of it attaining our requirements. We incorporate a safety factor of 2 for No- Load Speed and a safety of 4 for Stall Torque. We notice that the output from this configuration, in Fig 12, meets our torque and speed requirement. Input Motor 3790rpm ; Nm Harmonic Drive Ratio 160:1 Output Speed : 23.9rpm Torque : 46.4Nm Fig 12 Hence the selection (based on the most compact configuration) is: Maxon motor RE35 (323890), 90w, Encoder HEDL5540, 500 CTP, 3ch (110512) Brake AB 28 (223837) SHD SH Harmonic Drive (housed unit) 7

8 Encoders (Absolute) Referring to Fig 13(a), encoders are used to measure the absolute rotation of the motor adapter. This would be the same for all joints. To be space and weight efficient, we use U.S Digital MA3 Miniature Absolute Magnetic Shaft Encoder. Fig 13 (a) Fig 13 (b) Polymer Bearings It is of great importance to reduce the weight of the manipulator without affecting the overall performance. With the use of polymer bearings, we are able to achieve just that. With the use of igus iglidur bearings we are able to effectively have a lubricant-free, maintenance-free and lighter bearings. Also, they are heat and chemical resistant and are able to work under dirty and dusty conditions. In Joint 1, seen in Fig 13(b), a thrust washer bearing is employed to ensure that we receive rotational assistance and supports the vertical force that may act on the Joint 1. A flange bearing is also used to prevent any need for an interference fit between the bearing housing and the bearing. For the rest of the joints, specific bearings such as flange bearings or sleeve bearings are used for the operation. Harmonic Drive Harmonic Drive offers are very high resolution for precise movement together with its backlash-free mechanism. Its one of its kind innovation allows a high torque to be imparted while maintaining its position. There are many configurations that can be done with the harmonic drive unlike other components such as using a gearhead or a wormgear. The Harmonic Drive allows flexibility in coming up with different configurations to suit the application. Not only that, it does so with a high performance with durability over time. CONCLUSION The current project is still in progress as the rest of the team fine tunes the realization of the autonomous robot. The Tech-X challenge is in September 2008 and the manipulator will be mounted onto the robot s base. Beyond that, there is also the interfacing of the hardware and software of the whole system to ensure that the robot s operation would be smooth and achieve the necessary objectives. Not only that, the entire system must also be damped necessary to sustain vibrations or any hard shock impact to the robot. 8