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1、AxeBot Robot: The Mechanical Design for an Autonomous Omni directional Mobile Robot Tiago P. do Nascimento, Augusto Loureiro da Costa, Cristiane Correa Paim Post-graduation Program in Electrical Engineering Universidade Federal da Bahia Salvador, Bahia, Brasil tiagopnieee.org, augusto.loureiroufba.b
2、r, cpaimufba.br Abstract The AxeBot robots mechanical design, a fully autonomous mobile robot, for the RoboCup Small Size League, is presented in this paper. The AxeBot robot uses three omnidirectional wheels for movement and is equipped by a shooting device for shooting the ball in different direct
3、ions. Once the AxeBot robot is a fully autonomous mobile robot all the sensors, engines, servos, batteries, and the computer system, must be embedded on. The project can be separated in four different parts: the chassis design, the wheel design, the shooting device design and the overall assembly wh
4、ich makes a shell design possible to cover the whole robot. The AxeBot mechanical design brings up a new chassis concept for three wheels omnidirectional robot, also present a new shooting device, and finally present AxeBots prototype assembly. 1. Introduction The RoboCup Initiative is an internatio
5、nal research group whose aims are to promote the fields of Robotics and Artificial Intelligence. A standard challenge, a soccer match performed by autonomous robot teams, was proposed in 1996 1. Initially with three different leagues 2D: Robot Soccer Simulation league, Small Size Robot league, and M
6、iddle Size Robot league. Nowadays these leagues have been increased up to: Four-Legged League, Humanoid League, Middle Size League, RoboCup Junior Soccer, Small Size League, Soccer Simulation, Standard Robot League. Also, another challenge, the RoboCup Rescue was proposed in 1999 to show that the re
7、sult from the robot soccer research could be directly applied on a real world problem like a disaster rescue made by robots. Through the integration of technology and advanced computer algorithms, the goal of RoboCup is to build a team of humanoid robots that can beat the current World Cup champions
8、 by the year 2050. The AxeBot uses three omnidirectional wheels, positioned on a circle with an angle of 120o among each wheel, to move in different directions. Three Maxxon A-22 motors are used to drive the omnidirectional wheels, one motor per wheel. These motors are controlled by two Brainstem Mo
9、to 1.0 and a cascade controller made to control the robot trajectory 2 3. The AxeBot also holds a shooting device to kick the ball in different directions, a Vision System with a CMUCam Plus and GP202 Infra-red sensor 4, a embedded Computer System based on StrongArm, called StarGate Kit and a IEEE 8
10、02.11 wireless network card. This work presents the mechanical project to enclose these equipments into an fully autonomous omnidirectional robot called AxeBot. The complete AxeBot dynamics and kinematics model can be found in 5, this model was used to specify some mechanical parameter, like the whe
11、el diameter. 2. The Chassis The chassis of the robot is the frame to which all other components can be attached, directly or indirectly. Therefore the chassis must be strong enough to carry the weight of all parts when the robot is in rest o in movement. The chassis has to withstand the forces on it
12、, caused by the acceleration of the robot as well. Another important requirement of the chassis is that it fixes all components in a stiff way, so that there will be small relative displacements of the components within the robot, during acceleration and deceleration. This is particular important fo
13、r the three driving motors, which are positioned on the ground plane with an angle of 120o between each motor. The performance of the control of the robot is dependent on a precise and stiff placement of the motors 6. The chassis has to be strong enough also to withstand a collision of the robot aga
14、inst the wall or against another robot, with the highest possible impact velocity that can occur. Finally the chassis has to be built with the smallest amount of material. At first to reduce the costs, and to minimize the total weight of the robot. Less weight requires less power to accelerate. So w
15、ith the same motors, less weight gives you more acceleration. This is of course only true, when all the power generated by the motors can be transferred, via the wheels, to the ground. In other words, the wheels must have enough traction that there will be no slip between the wheels and the ground 7
16、. 2.1. Material Fiberglass was used to build the chassis. This choice is purely financial, because the material is not expensive (although it is strong) and there is no need to hire a professional constructor. The building of all the chassis (six in total) can be done by the team members themselves.
17、 Only the moulds have to be built by a professional. The upper and lower chassis can be made using one mould that can be adjusted to produce the different chassis. 2.2. Design The primary goal of the design is the fixation of the motors in the desired positions. Therefore a ground plate with 3 slots
18、 for the motors is modeled. At the front side each motor can be attached to the chassis. At the rim of the ground plate an edge is attached to give the chassis more torsion stiffness. This edge can also be used for attaching other components of the robot, like the covering shell. Also there is a cut
19、out to create space for the shooting device of the robot. In section 5 the design of this device will be discussed. However no final design will be presented and therefore we stick with this assumption that the shooting device needs these cutouts. All edges are rounded, because this will make the co
20、nstruction of the easier part. The final part, the lower chassis, is shown in the figure below. This part is modeled in Solid Edge. To get a stiffer and stronger chassis, a second chassis part, the upper chassis, is modeled. This is almost an exact copy of the first part, only now there are 3 cutout
21、s that provide more space for placing the components of the robot. These cutouts also save some material and therefore weight. The both parts are This sandwich construction gives the whole chassis more stiffness, and so the total thickness of both the chassis can probably be lower than using one cha
22、ssis part. Figure 1: Lower chassis Figure 2: Upper and lower chassis attached to each other 2.3. Chassis mould To build these parts, a mould was made. This is just a negative of the actual robot parts. In figure 3 the mould of the upper chassis. To change this mould in the mould for the lower chassi
23、s, where the ground plate does not have holes, the indicated pieces (with white stripes) and the not indicated left piece (symmetric to the most right part) should be lowered 4 mm. For the upper and lower chassis, the basis mould is exactly the same. Only piece one and two are different for the two
24、chassis, the motor piece and the shooting system piece are the same. Figure 3: Chassis mould3. Wheels The AxeBot robot is equipped with three wheels positioned on a circle with an angle of 120 among attached to each other as shown in the picture below.each wheel. These wheels have to enable the omni
25、directionality of the AxeBot robot. This means that the wheels have to be able to let the robot make two translational movements (in x and y-direction, see figure 4) without rotating the robot around its z-axis (the axis perpendicular to the y and x-axis, that is rotation in figure 4). The wheels ha
26、ve also to enable a rotation of the total robot around the z-axis. Figure 4: AxeBot wheels positions Nevertheless, the wheels have to be as small and light as possible to minimize weight and moment of inertia but still remain usable and manageable. The wheels are based on an existing design of an om
27、nidirectional wheel from the Cornell Robot 2003 8. Figure 5 shows an exploded view the final version of a wheel. The two shells are connected to each other by screws and hold every part on the right place. The hub is also attached to the shells by screws. The hub is mounted on the output axle of a m
28、otor by a screw to transfer the rotational output of a motor to the wheel. The rings of the rollers are in contact with the floor. A roller can rotate around its roller axle. As mentioned above, the wheel has to enable two translations (x and y, see figure 4) without rotating around its z-axis. The
29、whole wheel ensures one translation by rotating around the output shaft of the motors while the rollers ensure the other translation. Combining these translations on a proper way a robot can move anywhere in a plane or make a rotation. 3.1. Ring The ring is the only part of the wheel that is in cont
30、act with the floor. Note that a wheel can also be in contact with the floor by two rings. To obtain maximum grip (no slip) the Cornell Robot 2003 team first developed rollers without rings. The rollers had sharp edges to cut into the carpet of the football fieldfor maximum grip. This however ruined
31、the carpet and rubber rings were added in the design to obtain maximum grip without ruining the carpet. The rings are circular with a circular profile. Figure 5: Exploded view of the wheel Therefore the rubber rings are also used for the AxeBot 2006. Rubber rings can be bought in several sizes and s
32、ince they are highly elastic it wasnt difficult to find a ring of a right size. Since there are more than one right sizes and the geometry of the ring is that simple, no technical drawing of the ring was made. 3.2. Roller The geometry of the rollers may not restrict the rotation of the roller and sh
33、ould enable the placement of the rubber ring, without the ring falling off. This can be easily obtained (see drawings). The only problem is friction with their axles and with the shells. The geometry of the rollers can influence the friction with the shells and the friction with its roller axle. To
34、minimize the possibility of wear on the contact area between the roller and the roller axle, this contact area should be as big as possible. When the torque of the motor is transferred through a roller to the ground (driving the robot), the roller that is on the ground is pressed against the shells.
35、 It will occur often that, in this situation, the desired driving direction also requires a rotation of the roller (then the summation of the two translational movements will results in the desired driving direction). The rotating roller is, in this situation, pressed against the shells which, in so
36、me cases, can result in wear of the roller and or the shells. This depends on the material of the roller, the material of the shell, the magnitude of the force which presses the surfaces on each other (in this case it is the torque of the motor) and the geometry of both contact surfaces. Only the ma
37、terials and geometry can be chosen in the design process of the wheels. A small contact area results in low friction but possible wear of one of the two surfaces and though materials have to be used to avoid wear. A large contact area will not cause wear but will result in a large friction force. An
38、 optimum for the contact area and the materials has to be found. The geometry has to be machinable also. Problems that are mentioned above did not occur with the design of the rollers of the Cornell Robot 2003. Therefore the same geometry for the rollers is used for the AxeBot. The Cornell Robot 200
39、3 team designed a wheel with 15 rollers that worked very well. Therefore also 15 rollers are used in each wheel of the AxeBot 2006. The prototypes of the wheels of the Cornell Robot 2003 first had Delrin rollers to minimize friction and weight. Delrin is a kind of plastic which is used with moving c
40、ontact surfaces because of its low friction coefficients with other materials. After a few test with the prototype robot it became clear that the Delrin-rollers easily broke with a collision. Therefore the Cornell Robot 2003 was equipped with aluminum rollers. These were strong enough to withstand c
41、ollisions and aluminum has a low density (compared to other metals). However, during other prototype tests of the Cornell Robot 2003 team some aluminum residue built up on the steel rollers axles. The Cornell Robot 2003 did not encounter problems due to the wearing of the aluminum rollers, but to op
42、timize the design of the AxeBot wheels this problem was solved. To avoid wearing of the aluminum rollers other material for the axles can be used or the rollers can be made of a tougher material. After a few calculations it became clear that roller axles of Delrin (to reduce the friction) are strong
43、 enough (see the section about the axles), but it is not possible to produce thin bars of Delrin. Therefore steel axles are used, the same material as the roller axles of the Cornell Robot 2003. So to avoid wear of the rollers a more though material than aluminum has to be used for the rollers. Stee
44、l is more though and an easily obtainable and cheap material. A disadvantage of steel compared to aluminum is its higher density. This will increase the moment of the inertia which costs more torque of the motor. The total moment of inertia of a wheel with steel rollers is 1.39104kgm2 and the moment
45、 of inertia of a wheel with aluminum rollers is 1.30104kgm2. Using steel rollers instead of aluminum rollers would increase the moment of inertia by 7 this increase is neglectable small and steel rollers can be used. Concerning friction, using steel rollers and steel axles is also better than using
46、aluminum rollers and steel axles since the friction-coefficient between steel and steel is lower than that between steel and aluminum. A lubricant can also be used to even more reduce friction. 3.3. Roller axle As mentioned above, the use of Delrin for the roller axles was investigated since it woul
47、d reduce the wear of the aluminum rollers. In a static situation was calculated whether Delrin axles of 2.4 mm diameter would be strong enough. This was also done in a dynamical situation (dropping the robot on the floor and landing on one roller), but without using a Finite Element Method this did
48、not result in realistic results. When the total weight of 3.5 kg of one AxeBot 2006 would completely be on one roller axle this would result in a shear stress in the axle. In this situation the shear stress can be calculated by dividing the force on the axle (due to the weight of the AxeBot) by the
49、area of the shear plane. The area of the shear plane is of course the area of a circle with a radius similar to the radius of the axles. Note that the weight of the AxeBot has to be divided by two since there are two shear planes in one axle. The magnitude of the shear stress would be 3.8 MPa. Delrin starts to plastically deform in due to shear at around 44 MPa. Statically, Delrin axles would be strong enough. H