DESIGN AND DYNAMIC ANALYSIS OF GRIPPER FOR THE KUKA KR6 ROBOT

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1 DESIGN AND DYNAMIC ANALYSIS OF GRIPPER FOR THE KUKA KR6 ROBOT 12 Cristian VILAU 1, Nicolae BALC 1 and Dan LEORDEAN 1 ABSTRACT: The paper presents a new design of a clamping device for a KUKA robot, together with a complex analyses of the dynamic behaviour of the new gripper. Creo parametric software package was used both for 3D modelling of the new gripper and for static and dynamic analyses. The rack-pinion mechanism comes within the quadrangle mechanism with 4 joints, to provide the parallel movements of the jaws of the gripper. The KUKA working conditions were assumed, in order to estimate the minimum gripping force, to ensure the proper clamping, up to the maximum rotating angle of the robot arm. Also, the maximum acceleration and velocity of the part were estimated, using the Creo-Parametric software package. KEY WORDS: Gripper, dynamic analysis, PTC Creo, Kuka robot. 1 INTRODUCTION A new KUKA robot was purchased within the Department of Manufacturing Engineering (DME) from Technical University of Cluj-Napoca. New types of grippers had to be designed for new practical applications that this robot was implemented in. The type, shape and structure of the new clamping device has to be suitable both, with the type of parts to be handled by the robot and with the specific working conditions, of the robot itself. The idea was to design a specific gripper for rotational parts, with a weight of maximum 5 kg, to keep the weight of the new gripper as small as possible, but to make sure that the new gripper will be strong enough to hold the part, even at the highest velocity, when the arm is rotating with the highest acceleration, up the the maximum rotating angle of the robot, which is about 185. The Creo parametric software package was used both for 3D modeling of the new gripper and for mechanical analyses of the gripper s structure and behavior. Industrial robots are automated systems, able to perform different activities in industrial area, by having properties, such as: flexibility (being easily adapted for different manufacturing volumes), high productivity and reduced costs. The main applications of the robots are: loading & unloading the CNC machine tools, assembly operations, quality control, applications within welding field, etc. 1 Technical University of Cluj-Napoca, Departament of Manufacturing Engineering, cristian.vilau@tcm.utcluj.ro, nicolae.balc@tcm.utcluj.ro, dan.leordean@tcm.utcluj.ro A robot is called a programmable /reprogrammable manipulator, able to move work pieces, tools, materials, etc. for different manufacturing activities. The main goals of a robot are: increasing work productivity and eliminating the physical and psychical effort of a human operator Clamping devices (Grippers) The tasks of an industrial robot are: technological tasks (measurement, welding, painting, etc) and also transferring tasks (work pieces handling within manufacturing process). If the tasks of a robot are technological, than the final part of the robot is: a measurement head, a painting spray gun, a welding nozzle. If the task of the robot is to transfer something (handling), than the final part of the robot is a clamping device (gripper). This gripper fixes the work pieces to the robot. As the grippers execute the last operation, these have a very important role, such as: positioning, centering and fixing/unfixing the work pieces or the raw material while manufacturing. Clamping devices (grippers), according to the type and the dimensions of the handled object, are categorized into (Gacsadi A, 2008): - special devices for object having the same shape and dimensions, - special devices for objects having the same shape and different dimensions, - universal devices for objects having a variable shape and dimensions within a small range, - flexible devices for objects having a variable shape and dimensions within a large range. Grippers, according to the clamping system are splitted into: ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 13, ISSUE 2/2015

2 - grippers impactive the jaws or the fingers clamp the object through direct impact - grippers ingressive the clamping of the object is done by the help of the pins (used in textile industry) - grippers astrictive applying of an absorption force on the surface (through vacuum, magnetic, static) - grippers contigutive need direct contact for adhesion (such as glowing, residual stresses) The shape of the clamping jaws can be chosen according to the shape of the object that is need to be manipulated. There are several forces that occur on the object, excluding its weight, but the main force is the friction force (between the object surface and clamping jaws surface). Clamping surface must be realized from soft, material with a high friction coefficient, in order to avoid damaging the surface of clamped object. 2 DEFINE THE WORKING CONDITIONS The maximum weigh of the part was taken into account and the necessary clamping force was calculated under static conditions, in the first place: The writing style used is: Body Text Indent, Times New Roman, 11 points, normal, justify. F = µ*w*n [N] (1) F Force needed to clamp the work piece µ - friction factor n Number of clamping jaws W Weight of the object, (W=m*g). Moving direction of the object has an important role: if the movement is against gravity (upwards), the force is higher than moving the object in direction of gravity. In the above formula, one more term appears and turns into (Monkman, G. J, 2007), (Fantoni, G, 2014), [5]. F = µ*w*n*k [N] (2) k is the multiplying factor (1,2,3). If the object is moved on horizontal direction, k=2, if it is moved on vertical direction (upwards) k=3, if it is moved downwards, k=1. The clamping force must resist not only to the weight of the object, but also to the accelerations and velocities achieved after robot arm movement. Further are presented in a table 1 the values of the velocities and rotating range of the Kuka KR6 robot, having the controller KR C2 ed 05. Figure 1: Kuka KR6 robot, Dept. of Manufacturing Engineering, TUC-N Table 1: The values of the angular velocities within the rotating range. Axis Range of Speed hith rated paylod motion 6 Kg A1 +/-185º 156º/s A2 +35º/-155º 156º/s A3 +154º/-130º 156º/s A4 +/-350º 343º/s A5 +/-130º 362º/s A6 +/-350º 659º/s Source: [6], [7]. In figure 3 are presented the rotation axes of the robot (the movements of the robotic arm) and in figure 2 is presented the working space of the robot KUKA KR6. Figure 2: Working space of the robot KUKA KR6 Source: [7]. ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 13, ISSUE 2/

3 SCIENTIFIC PAPERS Figure 3: Rotation axis. Source: [8]. 3 DESIGN OF CLAMPING DEVICE The dynamic analysis had as main goal establishing the velocity of the work piece in horizontal plane, situated at maximal distance (R=1611mm) of KUKA KR6 robot arm radius (according to the figure 2). The work piece has the weight of 5 kg and is made from steel. The gripper model is presented in figure 4. It has a rack-pinion acting mechanism and the clamping is done with parallel jaws, due to the parallel-quadrilateral mechanism. b Figure 4: Gripper model; (a) 3D model, (b) principal components The model of clamping, clamping force (Fs) and the acting force (Fa) for this gripper model are shown in figure 5. This model has the jaws made from aluminum. a Figure 5: Clamping model 1- rack-pinion mechanism 2- quadrilateral mechanism 3- jaw 4- acting mechanism 14 4 SET UP THE WORKING STATIC CONDITIONS The static friction factor between aluminium and steel were taken into account, together with the type of movements and the maximum weight of the ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 13, ISSUE 2/2015

4 part to be handled. The clamping force in static conditions was calculated: W = 49.05[N] (5Kg * 9.81m/s 2 ) µ = 0.61 (static friction factor between aluminum and steel) n = 2 k = 2 Replacing the terms in relation (2), the clamping force in static conditions has the value: F s = 0.61*49.05*2*2[N] F s = [N] The clamping force on a clamping jaw has the force F s`=59.84 N After the analysis done using the Creo Simulate from PTC Creo software package, it was established the needed acting force in static conditions in order to clamp the work piece, this having the value F a =300 N. 5 DYNAMIC ANALYSES OF THE GRIPPER S BEHAVIOR The clamping force under dynamic conditions is much higher, as the static clamping force. The aim of the dynamic analyses undertaken within this research was to estimate the required clamping force, under the most stressed working conditions, when the velocity and acceleration get to their maximum values. Table 2: The values of acting force and swiveling angle Force [N] Swiveling angle [ ] , , , , , , , The maximum velocity value depends also on the rotating angle. The swivelling angle was estimated using the Creo-Parametric software, for different values considered for the clamping force. These values presented within the table 2, represent the swivelling angle, when the part tends to slip out from the gripper, due to the tough dynamic conditions. The maximum velocity value depends also on the rotating angle. The swivelling angle was estimated using the Creo-Parametric software, for different values considered for the clamping force. These values presented within the table 2, represent the swivelling angle, when the part tends to slip out from the gripper, due to the tough dynamic conditions. Figure 6: The value of dynamic clamping force Having the value of acting force in static mode, several analysis have been realized in dynamic mode, in order to establish the minimal clamping force while considering the velocity of robot arm. The simulations have been realized with several values of acting force, having the static value force and this value has been increased until the minimal acting force in dynamic conditions was reached. After analysis have resulted the values for clamping force and rotating angle of the robot arm, values for wich the work piece comes off from the gripper. These values are presented in table 2. The maximal swiveling angle of the robot arm around its axis A1 is de +/-185º according to the table 1. For a better understanding of the result from table 2, it has been done chart from figure 6. From the chart shown in figure 5, it can be observed that the value of static clamping force (300 N), swiveling angle of robot arm the work piece start coming off at, is It can be observed that the value of swiveling angles slightly increases until 760N and after the value of 765N the ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 13, ISSUE 2/

5 angle suddenly increases up its maximum value of 185. From the chart presented in figure 5 has been chosen the minimal acting force of 770N. Having this value for the clamping force, the velocity and the acceleration of the work piece from the gripper has been established, for the following conditions: the gripper is at the maximal distance related to rotation axis (A1) of the arm (see figure 2) and rotation speed of the arm has the value of 156º/s (see table 1). The velocity and acceleration values presented in figure 7, were estimated using the Creo Parametric system, under dynamic conditions, for a 770N minimum clamping force. It can be observed from the chart above that the work piece reaches the speed 4653 mm/s, within the range 1s to 1,25s, it needs an acceleration of 41800mm/s 2 within a range of 0,125s, after that decreases to medium value of 12500mm/s 2, for the entire range. Figure 7: The values of speed and acceleration 6 CONCLUSIONS Using these methods and CAD tools for design and mechanical analyses of the devices, an optimal structure of the gripper could be produced, in order to be light but strong enough in the same time, tu ensure a proper clamping of the maxim weight of the part, under the most difficult working 16 ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 13, ISSUE 2/2015

6 conditions, with the highest acceleration and velocity, considering the biggest rotation angle of the robot. The minimal acting force for clamping a work piece having the mass of 5 Kg, being situated at the maximal distance of the robot arm rotation axis (R=1611mm) and considering that the robot rotates with minimal rotating speed, has the value F=770N. The mechanical analyses methods, using the Creo Parametric CAD tools, are efficient and accurate, for different loads and work conditions. The results obtained from static analysis (forces and moments) were used further to undertake dynamic analysis, in order to estimate acceleration and maximum velocity, but also, the new results from dynamic analysis, could be further used for other static analysis of the new structure. 7 ACKNOLEDGMENT This work was partially suported by the strategic grand POSDRU/159/1.5/S/ (2014) of the European Social Fund Ivesting in People, within the Sectorial Operational Programme Human Resources Development Also, this paper was supported by the PCCA 115/2014 research project and by the European Social Fund through POSDRU Program, DMI 1.5, ID PARTING. 8 REFERENCES Gacsadi Alexandru (2008). Curs Bazele Roboticii Oradea, Blaško, M., Ridzoň, M. (2012). Conformal cooling design techniques and CAE analysis. In: Sborník konference Plastko 2012, Zlín: Univerzita Tomáše Bati ve Zlíne, 2012, s.[6]. ISBN Monkman, G. J.; Hesse, S.; Steinmann, R.; Schunk, H. (2007). Robot Grippers. Wiley- VCH. p. 62. ISBN Fantoni, G., Santochi, M., s.a., (2014), Grasping devices and methods in automated production processes, CIRP Annals - Manufacturing Technology, Volume 63, Issue 2, Pages , ISSN "Robotics Grasping and Force closure" (PDF). pdf. FU Berlin. Retrieved p129-en.html. print/kuka-kr15-kr-15-2-en.html. 9 NOTATION The following symbols are used in this paper: F s = clamping force; F a = acting force; µ = friction factor; n = Number of clamping jaws; W = Weight of the object. ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 13, ISSUE 2/