ROBOT DESIGN AND DIGITAL CONTROL

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1 Revista Mecanisme şi Manipulatoare Vol. 5, Nr. 1, 2006, pp ARoTMM - IFToMM ROBOT DESIGN AND DIGITAL CONTROL Ovidiu ANTONESCU Lecturer dr. ing., University Politehnica of Bucharest, Mechanism and Robot Dept., Spl. Independentei 313, Bucharest, Romania, oval33@hotmail.com Abstract: The paper deals with modeling, simulation and control of the industrial robots by using a CAD-CAM-CAE virtual lab. Three types of robots that have the positioning mechanism structure of type RTT, RRT and RRR have been modeled. The kinematical simulations of these robots, by assembling their components, defining the rotate and sliding joints between them, have been performed. For these robot models having 3 DOF certain ranges for rotation or translation on the moving elements, taking in consideration the geometrical constrains, are necessary to be imposed. It is presented only the technological operation for RTT robot that can be controlled step-by-step by commanding each moving part of the robot, independently, or by actuating all 3 components simultaneously. Further analysis on optimizing the dimensions of robot components in order to achieve the desired working space related to the technological process, may be performed. 1. Introduction The digital robot laboratory developing idea emerged from working on an advanced CAD-CAM-CAE (Computer Aided Design, Manufacturing and Engineering) software which performs the modeling and simulation of mobile mechanical systems. It is possible to create virtual mechatronical systems like industrial robots (IR) by using 3D (three-dimension) graphical and kinematical methods. There is a great potential to design all components from mechanical to electronic parts, with respecting of their real properties [1]. The 3D virtual assembling of the mechanical elements of robot structure is performed by a specialized module of the same soft, with which those components have been modeled. The displacement simulation of the mobile parts can be performed through defining kinematical joints that link those elements. Once the joints are established, the soft would impose certain restrictions on robot mechanism. Thus, an analysis of the moving part simulation possibilities is automatic performed. In the case of impossibility to simulate the open spatial mechanism, the user would be warned and get some information to correct the problem. The virtual simulation is essential for forecast determining of the working space (fig. 1) used by an industrial robot. Three positioning mechanisms (PsM) type RRR, RRT and RTT (R = Rotation and T = Translation or sliding) has been considered. The CAD-CAM-CAE methods allow easily some corrections on elements to be made, so that an optimum working space, related to technological process that has to be achieved by the robot, to be obtained. The parametrical designing allows any dimension of a part to be modified, that causing the automatic changing of all dimensions of mechanism parts which are in connection with, but on respecting of the imposed restrictions. 2. The 3D modeling of RTT, RRT and RRR robots Fig. 1 - Robot with PsM type RTT An open spatial kinematical chain type RTT [3] with 3 elements linked each other by rotate joints has been considered (fig. 1). In order to model in 3D this mechanism, there can be used one of the high performance software which allows, by CAD-CAM- CAE techniques, to obtain precisely virtual components regarding their dimensions, material and mass [2]. First, the assembly elements are independently modeled (fig. 2 5), starting with the 2D sketching of predominant shape of considered part, then creating the thickness (third dimension), followed by applying or removing of volume (material), and finally, proceeding some precision operations imposed by operating conditions which the industrial robot is designed for. 57

Thus, the main robot components are displayed: the base (fig. 2), the body (fig. 3), the upper arm (fig. 4) and the forearm (fig. 5) that includes the grasping mechanism (gripper). Fig.

Then the third dimension for each element, by using the 3D module, was created. For the base, that represents the fixed part (fig. 2), the rectangle and pad commands have been used.

2 Thus, the main robot components are displayed: the base (fig. 2), the body (fig. 3), the upper arm (fig. 4) and the forearm (fig. 5) that includes the grasping mechanism (gripper). Fig. 2 - Base of the RTT robot All these components of the industrial robot have been modeled by using the 2D sketcher soft module from part design workbench. Then the third dimension for each element, by using the 3D module, was created. For the base, that represents the fixed part (fig. 2), the rectangle and pad commands have been used. The hole procedure was used to obtain the rotate joint between the base and body (waist joint). In figure 3 the robot body is shown. Three pad commands, two with circle and one with rectangle, have been used. The gap in the body was obtained by pocket command in order to shape the component to allow the sliding of jointed arm (shoulder joint). Fig. 3 - Body of the RTT robot 58

The followed pocket command was used to simulate the sliding joint between upper arm and forearm (elbow joint). Fig.

3 Figure 4 shows the robot upper arm (even if it has a parallelepiped shape) that slides related to the body by external lateral sides. It was obtained by one rectangle command and of course the pad procedure. The followed pocket command was used to simulate the sliding joint between upper arm and forearm (elbow joint). Fig. 4 - Upper arm of the RTT robot The forearm shown in figure 5 slides related to the upper arm. It was obtained by one rectangle procedure and a pad command. The gripper is represented just symbolically (without the wrist joint) being achieved by an enclosed line loop that was padded, trimmed and reflected. Fig. 5 - Forearm of the RTT robot 59

After the modeling of robot components (fig. 2 5), these were assembled using a specific virtual workbench in order to achieve the whole RTT robot (fig. 6). Fig.

Fig. 7 - RRT robot The 3D rendering of the robot assembly can be achieved very easy just by computer mouse.

4 After the modeling of robot components (fig. 2 5), these were assembled using a specific virtual workbench in order to achieve the whole RTT robot (fig. 6). Fig. 6 - RTT robot It is necessary to be mentioned that before assembling the components, the accuracy of each part dimension has to be checked, so that the outcome should be a perfect combined product. Fig. 7 - RRT robot The 3D rendering of the robot assembly can be achieved very easy just by computer mouse. The robot can be rotated, moved or zoomed in/out by user in order to see the final product from any point of view. This represents a major advantage in comparing with a real scale industrial robot that is difficult to be analyzed. In order to obtain a real simulation of the robot behavior it is necessary to apply a material to each part of it. Once material is applied, the soft would calculate automatically the mass, having the element volume and material density. This type of product (fig. 6) is called multi body system (MBS) and it could consist in as many as the software and hardware capacities allowed. In addition, some finite element analyses (FEA) may be performed on any of these product s parts in order to test the strength of them in specific imposed conditions. The same procedure has been applied in the case of robot with PsM type RRT (fig. 7) and RRR (fig. 8), with the observation that the same base (fig. 2) was used for all 3 cases, and the same forearm was used in both RTT and RRT cases. 60

Fig. 8 - RRR robot The component and assembly modeling of industrial robots is easy to be achieved by that CAD-CAM- CAE soft, being possible to be obtained mechanical parts of any complexity as shape

In RTT robot case (fig. 6), this has 3 DOF (Degree Of Freedom). The rotation and sliding ranges can be imposed (fig.

5 Fig. 8 - RRR robot The component and assembly modeling of industrial robots is easy to be achieved by that CAD-CAM- CAE soft, being possible to be obtained mechanical parts of any complexity as shape and accuracy which can equip the mechatronical systems. 3. The control of RTT robot For the kinematical joint defining between robot components, the robot mobility has to be considered. In RTT robot case (fig. 6), this has 3 DOF (Degree Of Freedom). The rotation and sliding ranges can be imposed (fig. 9 or 10) so that the robot working space to be modeled, related to the necessities of robot reaching field (fig.1). The kinematical operating simulation of industrial robot can be achieved by using the command panel (fig. 9 or 10) with which the digital displacements can be controlled. Fig. 9 - RTT robot in home position Figure 9 displays RTT robot in home (basic reference) position when all three displacements (one rotation and two translations) are zero, and in figure 10 an imposed position can be observed. 61

Fig. 10 - RTT robot in a given position As it can be seen in figure 9 and 10, there are possibilities to command immediate (step-by-step) or on request (simultaneous) the three moving robot elements.

6 Fig RTT robot in a given position As it can be seen in figure 9 and 10, there are possibilities to command immediate (step-by-step) or on request (simultaneous) the three moving robot elements. In both cases it is necessary that the number of steps to be imposed. 4. Conclusions One of the paper goals is to analyze the creating of a large database of mobile mechanical systems for robots. Supposing that we do not know how a robot with RRT positioning mechanism looks like. After we would find the website called Mobile Mechanical Systems for Robots, all we have to do is to type something about the mechanism that we looking for. Then, the searching engine of the site would find in the database all the information that exists at that time about our request (e.g. fig. 7). Also, some video clips could be available to display animating operation of the requested mechanism. In situation of designing issue we would need of realistic components, not just a picture, so that we could download the whole 3D mechanism in a specific CAD file format. After that, we can easily change the parametrical dimensions, element shapes and restrictions, joint types as we wish to model our robot. More than that, some kinematical tests (e.g. fig. 9 and 10) could be performed in order to check whether the mechanism is working or not, and to measure the whole swept volume (working space) of the robot (e.g. fig.1). Also, the behavior simulation of robot means some FEA tests in order to verify the stresses and displacements into material s structure. By using this virtual lab (database) the users can learn much easier how a robot operates and interacts with his environment. The advantage of this virtual library is to be much cheaper than a real laboratory and it is possible to create infinite robot arrangements with the number of DOF as much as is needed. Moreover, those new robots could be virtually tested, and the robot databases substantially enriched. Then they could be used to equip a virtual CIM (Computer Integrated Manufacturing) cell or even a plant. These virtual robot lab-databases could be integrated in the e-learning educational process and on industrial development too. References 1. Antonescu, O., Mechanism databases for E-learning, Mechanism and Manipulator Journal, ARoTMM Press, vol. 1, nr. 1, p , 2002; 2. Antonescu, O., Graphical computed methods on modeling, simulation and control of the mechatronical systems used as industrial robots (in Romanian), Research Project, Cod CNCSIS 189, MEC, Bucharest, Romania, ; 3. Antonescu, P., Synthesis of manipulators (in Romanian), University Politehnica of Bucharest, Romania,

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