978‐1‐4799‐6002‐6/14/$31.00 ©2014 IEEE

Design and Implementation of an Educational Robot Manipulator

José Alberto Naves Cocota Júnior, Thiago D'Angelo, Paulo Marcos de Barros Monteiro, Paulo Henrique Vieira Magalhães

Departamento de Engenharia de Controle e Automação (DECAT) Escola de Minas, Universidade Federal de Ouro Preto (EM/UFOP)

Ouro Preto, Brasil cocota@em.ufop.br, thiago.d.angelo@hotmail.com, pmemop@gmail.com, phvmag@gmail.com

Abstract—Nowadays, dropping out of engineering courses

occurs in practically all universities of the world. Over the past few

years, active learning has been recognized by the educational

community as the most effective learning method. In this paper we

discuss the educational experience involving the design and

development of a low-cost robot manipulator with six degrees of

freedom, to motivate undergraduate students of control and

automation engineering and mechanical engineering, who

attended the robotics elements subject in 2012 and 2013.

Moreover, the students’ activities during the execution of the work

are presented as well as the results of their evaluation of the

methodology proposed.

Keywords— robotics; active learning; forward and inverse

kinematics; kinematic control; CAE

I. INTRODUCTION

In the last years, teaching engineering involving hands-on practice has become an essential methodology to motivate undergraduate students. However, most educational equipment applicable in teaching the undergraduate or graduate courses are very expensive or have a closed hardware and software architecture. Besides, the exclusive use of these stands suppresses the student the opportunity to develop one of the most important features of the engineer, i.e., to design, monitor, and implement a project.

One of the common areas in the formation in control and automation engineering and in mechanical engineering is robotics, that is considered a relatively new area of modern technology that crosses traditional boundaries of engineering. Understanding the complexity of the robots and their application requires knowledge in electrical engineering, mechanical engineering, industrial and systems engineering, computer science, economics and mathematics [1]. Due to the multidisciplinary nature of this field, robotics teaching is more effective when theoretical concepts are associated with tangible experiments. To make this association more effective, it is required a pragmatic way of applying the traditional robotic material to exciting laboratory exercises [2].

The use of robots as experimental platforms at universities and research institutes has become increasingly common. However, most available systems is destined for mobile robotics and there are few companies that develop manipulators for

industrial robotics courses. The Quanser has three models of robots in its catalog, two in closed kinematic chain (the Hexapod robot and the planar 2-degree of freedom - GDL) and one in open kinematic chain (Omni Bundle) [3]. In order to create a robot manipulator in an open kinematic chain with an open architecture controller, the use of the CRS Catalyst-5 robot model from Thermo Fisher Scientific Inc. with a control board from Quanser model was suggested [2]. But Quanser systems are costly for universities in developing countries. To overcome this problem it was proposed the use of LEGO MINDSTORMS due to its low cost, allowing students to gain experience in the kinematic design of fixed robot manipulators [4]. Although this had been an effort to expand access to practical experience in robotics, the robotics kit produced by LEGO is not suitable for teaching and research at university, due to its hardware limitations. More recently, it was proposed the development of a low cost robot manipulator with four degrees of freedom (DOF) to motivate undergraduate students in robotics elements subject [5]. With it, it was possible to explore the contents of robotics elements subject, e.g., the study of the forces involved; determining the forward kinematics, inverse position kinematics and the workspace of the robot; and path planning by the learning method of point-to-point movement. Subsequently, a robot manipulator with 6 DOF was developed, which allowed students to explore, beyond the mentioned topics, the open-loop control of the angular velocity of each joint, the implementation of inverse kinematics orientation, and identification of accuracy and repeatability of the robot [6].

This paper presents the improvement of the previous works [5] and [6], highlighting the improvement of the interface with the user in MATLAB ®, the sampling of the average angular velocity of each joint, and the kinematic control of position for trajectories. This paper explains how these projects have contributing to motivate students to develop their course final project, and multidisciplinary enhancements of undergraduate students detected on their reviews about this experience.

II. OBJECTIVES

The main objective of the proposed project was to motivate students through designing and implementing of a low-cost robot manipulator, which would allow to demonstrate basic concepts of robotics, such as: forward kinematics, inverse kinematics, differential kinematics, workspace, singularities,

This project had the support of Fundação Gorceix.

path planning, kinematic control, and computer-aided engineering (CAE). The other specific objectives were:

• To develop interdisciplinary works, promoting the integration of concepts from the fields of mathematics, physics, electronics, programming, mechanics and control.

• To develop transversal skills, such as critical analysis, independent learning, problem solving, teamwork, conflict management, decision making, evaluation, workflow management.

• To stimulate the theoretical study of the subject.

• To conduct a survey of the students’ difficulties to identify the areas of the course that should be improved.

• To improve students’ technical knowledge in order to contribute in their undergraduate final project.

We believe that these objectives can be achieved through a supervised undergraduate design-implementation-test experience applied to students, to encourage them develop ideas, research and skills to solve problems; all of which are essential in engineering projects.

III. METHODOLOGY

The methodology adopted was the Problem Based Learning (PBL), in which the learning is student-centered, that is no longer the passive recipient to be primarily responsible for their instruction.

The project activities and the implementation of the robot were carried out over three semesters by undergraduate students of robotics elements subject of Escola de Minas (UFOP), which started their work activities in late 2012 [6]. The mechanical engineering students carried out those activities related to the design of mechanisms that make up the robot, also testing for characterization of materials used in it, and the its static analysis when subjected to stresses. Control and Automation Engineering students’ activities were focused on electronics, programming, robot kinematic modeling, simulation and implementation of kinematic control for planar trajectories.

IV. PROJECT

In order to stimulate the students’ creativity, they were given freedom of choice of materials, components and programming language to be used in the development of the robot.

A. Mechanical Project

Over three semesters, three structures were developed for the manipulator. The first version of the robot was made with rigid PVC plates of 2 [��] thickness (Fig. 1). In the design phase, trails of tensile, compression, bending and twisting of PVC, were performed for the characterization of the material used. The data obtained in these tests supported the static analysis of the robot when subjected to a load in its last link. Figure 2 presents the CAD model of the robotic arm that was used in the CAE analysis by SolidWorks software (Fig. 3).

Fig. 1. First robot prototype, wrist and gripper made using hard PVC tube.

Fig. 2. PVC robot’s CAD model.

In the CAE analysis phase, it was observed that the elements subjected to greater efforts were the lifting arm (Fig. 4) and the body rotation axes (Fig. 5).

Nevertheless, the PVC structure was considered unsatisfactory during its use. There were often cracks in the joints of the plates that formed the links of the robot.

A second prototype was made (Fig. 6), using the Depron. This material is commonly used in airplanes models and displays the nominal density of 40 [��/��]. For a CAE analysis a new model was developed in CAD (Fig. 7). In this analysis, we also observed that the elements under greater stress were the lifting arm and the body rotation axes.

Fig. 3. PVC robot’s CAE analysis.

The structure in Depron enabled to use less torque servomotors for lifting the arm and the body rotation, compared with the previous structure of PVC. During its use, we observed rapid wear at the support points of the arm lifting, as predicted in CAE analysis.

Fig. 4. Lifting axis of the robot arm.

Fig. 5. Spin axis of the robot body.

To solve this problem, the link of the body rotation made of Depron was replaced by a link in MDF with a 5 [��] thickness (Fig. 8). Furthermore, the size and weight of the spherical wrist were reduced by replacing the previous structure in Depron and three Futaba S3003 servos, for the ServoCity SPT50 kit and three HS-5055MG servomotors. This last structure also showed improvements in the union joint of the arm link to the forearm, with the substitution of a bushing for a bearing to support the shaft of its axis. Another prominent change was the replacement of the servomotor responsible for lifting the arm from model TowerPro MG995 to HD-1501MG, which increased the torque of this joint from 13 [��/��] to 17 [��/��].

Fig. 6. Second robot prototype with wrist and gripper in Depron.

Fig. 7. CAE analysis of the robot in Depron.

Fig. 8. Third robot prototype in MDF and Depron.

B. Hardware e Softwares

For activation of the robotic joints and the reading of their respective angular positions the Arduino MEGA 2560 platform was used. Angular positions of each joint were sampled through the analog signals of the potentiometers of the servomotors. Students developed their own communication protocol, which enabled the exchange of data between the Arduino platform and PC by USB port.

The protocol allowed the user to send to Arduino platform the angular position reference and the angular velocity of each joint during a trajectory through an interface in MATLAB, as well as allowed the reading of the angular position and the average angular velocity of each joint. The identification of the average speed of each joint was necessary for the design of kinematic control of the robot.

The graphic user interface (GUI) in MATLAB allowed practicing on forward kinematics, inverse kinematics and point-to-point trajectory (Fig. 9). The GUI was developed through the MATLAB Toolbox GUIDE, and used the Robotics Toolbox [7] to plot the trajectory of the robot during the prototype’s movement. The microcomputer software architecture and its communicating interfaces to Arduino MEGA 2560 platform are illustrated in Figure 10.

For kinematic control of position practices, the control software is embedded in the Arduino platform, since it was observed that the kinematic control by MATLAB by means of a

microcomputer is not possible, since the time for serial communication between the Arduino and MATLAB was larger than the range stipulated sampling. Thus, during kinematic control practices, the angular positions were stored in the Arduino memory and transmitted to the computer after completing the trajectory. These experimental results were compared with the ones of the simulated kinematic control by feedforward.

Fig. 9. GUI in MATLAB.

Fig. 10. Software architecture.

C. Workspace

The workspace determination was necessary for the choice of trajectories to be performed by the robot. Based on the maximum and minimum limits of each joint of the robot structure of Figure 8, which are shown in Table I, and on the dimensions of the links of the robot, the students implemented a MATLAB script to plot graphs of the lateral (Figure 11) and top view (Fig. 12) workspace. They positioned the robot in different configurations, in which a joint variable was altered at a time, generating trajectories representing the total volume covered by effector according to the algorithm presented for the projection of the workspace of the planar robot of two links in [8].

Fig. 11. Slide view of the workspace.

Fig. 12. Top view of the workspace.

TABLE I. LIMITS OF ROBOT'S JOINTS

Joint Min. Angle Max. Angle

θ1 -90º +90º

θ2 0º 165º

θ3 -25º +90º

θ4 -90º +90º

θ5 -55º +90º

θ6 -90º +90º

D. Kinematic Control

Students implemented a structure of kinematic control of position by feedforward. The kinematic control of orientation was not implemented, for simplicity. The development of the program for the kinematic control was carried out in two steps: (1) in the first half of 2013 it was suggested to the students the project of kinematic control of a planar trajectory, in which only the links of the arm and forearm would move; (2) based on their acquired knowledge, in the second half of 2013 the project of kinematic control for a trajectory in space was proposed, moving, thus, the first three joints of the robot.

The kinematic control of position results in a loop of velocities control in joints level (Fig. 13), where � is the control signal of speed applied to the motor of the i-th joint. The motor, in turn, has the action of an integrator, in which given an input angular velocity, it presents in its output the angular position of

the joint associated with the motor. Where � is a definite

positive matrix, and the equilibrium point � = 0 asymptotically stable.

Fig. 13. Block diagram of the kinematic control for position.

The implementation of a kinematic control structure was possible because one can neglect the dynamics of the manipulator. This hypothesis can be performed, since the joints, which are driven by servomotors, have high reduction factors in the gears, as well as low angular velocities had been used in carrying out the trajectories.

V. RESULTS

This section contains the experimental results obtained from the robotic platform developed by the students, and also it shows the results from student reviews about the PBL methodology adopted in robotics elements subject.

A. Experimental Results

To analyze the accuracy and repeatability of the developed robotic platform, ten trials have been performed for positioning the robot to the same point and space guidance. To perform this task we used the structure of trajectory control by point-to-point. The robot of the present study showed better repeatability and accuracy (Fig. 14) compared to the previous work [6].

Fig. 14. Accurancy and repeatability testing results.

Even though the points used to compare accuracy and repeatability between the robots had not been the same, it was noted that the new version presented less dispersion. Moreover, the new version presented better accuracy: 1.09 [��] against

3.00 [��] from the former version. These results were reached after mechanical improvements from the second to the third version of the robot.

The experimental and simulated results to execute a linear and a circular trajectory in the workspace are presented in the figures 15 and 16. The trajectories were split in 80 points, and with the sampling time interval of 60 [��]. Angular velocity was not superior to 40 degrees per second in the kinematic control simulation. The experimental results mean error for a linear trajectory was 0.86 [��], and for the circular trajectory was 1.26 [��]. The � matrix used had a diagonal with gain 10.

Fig. 15. Experimental result of the kinematic control for a linear trajectory.

Fig. 16. Experimental result of the kinematic control for a circular trajectory.

B. Students Review Results about the Methodology Adopted

The main result of this project was the students’ motivation, once it required interdisciplinary to develop the robot, plus it helped student to remember theoretical content taught in classroom. To evaluate the methodology and its contributions on the robotics elements subject, a survey was conducted, which was made of 9 questions answered by 37 out of 46 students who had attended the subject from 2012 fall semester up to 2013 fall semester. The survey was spontaneously and anonymously answered.

When asked students if they had actively participated in the project, 91,9% of them answered "yes". According to the students, most difficult tasks were related to programming (40.5%), 29.7% claimed control as the hardest task in the project, whereas electronics was pointed as the hardest task for

18.9% of the students. These results are useful to point out which disciplines have to be reinforced in the undergraduate program for engineers in this area at Universidade Federal de Ouro Preto. More results are shown in Table II.

TABLE II. SURVEY ANSWERS

TABLE III. ANSWERS

A% B% C% D% E%

The project has motivated me to conclude my course

48.6 51.4 0 0 0

The project is related to the theoretical content of the subject

40.5 40.5 13.5 5.4 0

The project has motivated me to learn more about the subject.

43.2 45.9 10.8 0 0

I would recommend other students to attend this subject with this methodology adopted.

45.9 48.6 2.7 2.7 0

The period to execute the tasks was adequate.

32.4 35.1 10.8 21.6 0

The project contributed to my professional career.

62.2 32.4 5.4 0 0

Skills and knowledge earned during the project will contribute to my course final project.

21.6 24.3 18.9 35.1 0

A= Strongly agree, B=Agree, C=Neutral, D=Disagree, and E=Strongly disagree

In the sheet also had a space to comments; some students reported that the project was their first practical experience involving control. Other students suggested that active learning methodology should be adopted in other subjects as well, especially in control theory subjects.

VI. CONCLUSIONS

This paper reports the experience that the authors had by being in contact with the Problem Based Learning methodology (PBL) to develop a low cost 6-DOF anthropomorphic manipulator robot with spherical wrist in robotics elements subject. The execution of this project allowed exploring basic concepts of robotics with undergraduate students, e.g., forward kinematics, inverse kinematics, trajectory control by point-to-point, kinematic control, manipulator accuracy and repeatability. The theoretical content of the subject had been explored along the three semesters of project development. The basic concepts related to kinematic control, e.g., singularities analysis through Jacobian on planning trajectories, and the feedforward control structure, were explored by students.

The methodology had a good review from the students, who have become more motivate, according to the survey. Furthermore, most of the students (94.6%) reported that the project contributed to develop transversal skills. This result can be explained by the multidisciplinary nature of the project that, for three semesters, had pushed students to research about other subjects. Each group developed activities in determinate areas of knowledge and they needed to work together, exchanging information, to accomplish the semester goals. This scenario is quite similar to the situations that students are going to face in their professional lives. About the contribution of this project to their course final project, it was noted that many student had not started their own when they were asked; thus, this indicator was not adequate to evaluate the active learning methodology.

The authors believe that all goals have been reached and this success has to be attributed to students’ efforts. We wish these experiences can be useful to other groups of undergraduate students.

ACKNOWLEDGMENT

The authors want to thank all the students who have been dedicated to develop this project.

REFERENCES

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