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Inter-Ing 2005„INTERDISCIPLINARITY IN ENGINEERING”

SCIENTIFIC CONFERENCE WITH INTERNATIONAL PARTICIPATION, TG. MUREŞ – ROMÂNIA, 10 -11 November 2005.

ASPECTS REGARDING INTEGRATION AND INTERFACING INDESIGN OF MECHATRONIC SYSTEMS

Ciprian Lăpuşan, Radu Bălan, Sergiu Stan

Keywords: mechatronics, integration, interface, parallel robot, control, microcontroller, Delphi

Abstract: The information society development brought new standards in the way that products arebuild and the demands of the customers, traditionally mechanical products are now more and moreintegrating electronics and software systems, as an answer to this needs mechatronics emerged,bringing a new perspective in the design of products and manufacturing process, perspective definedas a synergetic combination of precision mechanical engineering, electronic control and systemthinking. Robots are one of the most representative fields in mechatronics, this article presents thedevelopment process of a mechatronic system, taking into account a 3-RRR parallel robot, pointingout the integration of the elements and showing the challenges needed to be faced for the interfacesbetween the robot modules and the GUI (Graphical User Interface).

Introduction

The information society development brought new standards in the way that productsare build and the demands of the customers, traditionally mechanical products are now moreand more integrating electronics and software system, as an answer to this needsmechatronics emerged, bringing a new perspective in the design of products andmanufacturing process, perspective defined as a synergetic combination of precisionmechanical engineering, electronic control and system thinking.

The integration of this disciplines leads to more competitive products, new technicalsolutions can be achieved from a mechatronic approach solutions like: new technicalfunctions, increased flexibility, extension of the range of parameters used for machine control,reduce size and manufacturing costs due to physical integration.

A mechatronic product is a very complex system with several components, technologiesand function all interrelated and interdependent. The technical synergy of a mechatronicssystem creates critical dependencies between involved engineering disciplines, thesedependences are demonstrated in many ways, mechanical properties may for example bestrongly linked to the control system characteristic that in turn are strongly linked to softwareproperties and the vice versa.

In the design process of the robot one important aspect that was kept in mind was themodularity of the final product, building a modular robot bring several advantages likereconfiguration, the possibility to improve just one of the module without interceding in theentire structure, the possibility to use robot modules in building other robots or otherstructures. But beside this advantage a modular product brings new challenge in buildinginterfaces between all the modules that are used for the robot.

Another aspect that was very important in the robot design was the interface betweenthe final user and the robot, this interface was realized by an application on PC, applicationthat was build using Delphi Software. When the application RoboSim was created a very

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important aspect was the accessibility and the easiness in the use of the program, in such away that the user could send commands to the robot without an elaborate preparation.

The robot structure and the equations obtained from inverse kinematics problem,equations that are used for robot control are presented in the first part of the article. Thecomplexity of the equation make them very hard to be resolved using a microcontroller, toresolve this problem an application on PC using Delphi was created, the application is alsoused for getting user commands and for simulation of robot movement, the way it was createdand can be used is presented in the second part. The link between PC and the structure ismade using a microcontroller, the third section presents the way that the microcontrollercommunicates with the computer and the way it commands the actuators and gathersinformation from the sensors.

Parallel robot 3-RRR. Structure and kinematics

Parallel robots are mechanisms that have the end-effector connected to the fixed frameby more then one kinematic chain. The 3-RRR robot is a three degree of freedom parallelmechanism which is made from 3 kinematic chains, every chain is made up from twoelements, the first element is the motor element, it is connected at one end to the motor andthe other end is connected by a passive rotation joint to the second element of the kinematicchain named the passive element, on the other end of the second element is connected theend-effector by a passive rotation joint.

a) b)Fig.1. 3–RRR planar parallel robot

a) CAD design b) experimental structure

The geometry of the robot is directly influenced by the position of the actuators and thelength of the arms, the robot discussed in this paper has all three actuators placed in the cornerof an equilateral triangle.

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Fig.2. Kinematics scheme of the 3-RRR planar parallel robot

To control the movement of the end-effector of the robot the program RoboSim uses theinverse kinematics, the user will enter the position of the end-effector and the program willcompute the angles of all three actuators. The computation of the angles is made in tosteps, in the first step is determine the position of the points A, B, and C, all threepoints are situated in the corner of an equilateral triangle, the position of the masscenter of the triangle is known Px and Py, also the angle q between the end-effectorand the Ox axe is given, L3 represent the distance from P to A.

)cos(3 βLPA xx += (1)

)sin(3 βLPA yy += (2)

Knowing the position of point A, B, C of the end-effector the problem is reducedto an inverse kinematics problem of a serial robot with 2 arms, fig. 3

Fig.3. Kinematics scheme of the 2-RR manipulator11 )(2 LxxK OB −= (3)

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12 )(2 LyyK OB −= (4)

2221

223 )()( OBOB yyxxLLK −−−−−= (5)

)],(2tan[2

)],(2tan[2

1323

22

2121

1323

22

2121

KKKKKKa

KKKKKKa

−−+−−=

−−++−=

φ

φ (6)

The angle θ have two solutions (Eq. 6), this is possible because the mechanism can havethe point B in the same position in two situations when the elbow is up and when the elbow isdown.

The control of the robot is made using a closed loop control system, the feedback isgiven by three sensor mounted on the axes that actuates each motor link.

Fig.4. Control block schemeThe system will get data from the user by the interface created in Delphi, the application

using inverse kinematics problem computes all three angles for the actuators, this data arethen compared with the data gathered from the sensor, the difference between the computeddata and the data from the sensors represents the error, this error is transformed in steps foreach actuator. These three values can be positive and negative; the sign of the value give thedirection in which the motors will turn. The error and the direction is then send from the PCto the microcontroller, the microcontroller will actuate simultaneous all three motorsaccording to the received data.

A very important influence in the system response is given by the communication speedbetween all this three modules: PC application, control board (microcontroller) and actuators.The communication between PC and microcontroller is made using RS 232 interface, thisstandard interface gives the possibility to transmit data at a very high speed, avoiding timeloses. The interface between the microcontroller and actuators is made using three drivers,each driver transform the digital command from the microcontroller to command eachactuator.

User interface

The user interface was build using Delphi, this environment allow the programmer tocreate complex application under Windows operation system, also Delphi allows access tohardware resources of the PC, this was a very important aspect because the application usesthe communication port Com Port of the PC to communicate with the microcontroller and

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also need access to the video adapter for fast hardware functions, functions used in thesimulation of the robot.

Fig.4. Graphical User Interface

The user can give commands to the robot in four types, but the most important type is“command list”, any industrial process is compose from multiple basic moves, the programallow the user to enter a list with all this basic moves and then can simulate the movement ofthe robot on the Pc or directly test the moves on the robot. The commands that is introducedin the list are similar to G Code commands, using a standard language offer the user thepossibility to easy understand the way that robot can be controlled. The other threecommand modes were mostly build for testing the robot, these modes are mouse mode,geometric mode and buttons mode.

The interface can be use to command any 3-RRR parallel robot that have the actuatorsplaced in the corners of an equilateral triangle, the user will change the dimension of thearms, the dimension of the end-effector and the distance between actuators.

Command Bloc

The command bloc have two very important roles in the robot control, first role is tocommand all three actuators and to read data from the sensor, the second role is tocommunicate the sensors values to the computer an wait the commands from the PC.

The main component of this bloc is the microcontroller AtMega 8535, beside the tworoles the microcontroller also displays on an LCD the angle of the three motors arms. In caseof any error that occurs at the system initialization or in the functioning of the robot, themicrocontroller will display an error message on the LCD according to the problem.

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Fig.5. Control board system

On the front panel of the command bloc is placed the Emergency Stop button, when thebuttons is pressed the microcontroller will stop all the three actuators and will send an errormessage to the computer and to the LCD.

Conclusions

In the paper was presented the integration and interfacing design issues of amechatronic system taking into consideration a 3-RRR parallel robot, pointing out theintegration of the elements and showing the challenge needed to be faced for the interfacesbetween robot modules and the interface between user and robot. The graphic interface can beuse to command any 3-RRR parallel robot that has the actuators placed in the corners of anequilateral triangle. The accessibility of the user interface was a very important aspectthat was kept in mind when the robot was build. Another important aspect that was keptin mind was the modularity of the final product, building a modular robot brings severaladvantages like reconfiguration, the possibility to improve just one of the modules withoutinterceding in the entire structure, the possibility to use robot modules in building other robotsor other structures.

References

[1] Bălan, R., Microcontrolere.Structura si aplicatii,Ed. Todesco[2] Bush, R., Design of an Active Acceleration Compensation robot, Georgia Institute ofTechnology, 2001[3] Ceascai, I., Microcontrolerul AT90S4433 structura şi aplicaţii, Casa cârtii de ştiinţă, ClujNapoca, 2003[4]Huang, C., Overview of modular product development, National Chi-Nan University, Puly,Taiwan, 1999[5] Lapusan, P., Lucrare Diplomă, Proiectarea si realizarea sistemului de control pentru unrobot paralel cu trei grade de libertate, Universitatea Tehnica Cluj-Napoca, 2005[6] Mătieş, V., Tehnologie şi educaţie mecatronica, Bucureşti, 2001[7]Adamsson, N., Licentiate Thesis, Mechatronic engineer – New requirements on cross-functional integration, KTH Machine Design Royal Institute of Technology, Stockholm,Sweden, 2005[8] Stan, S., Diplomarbeit, Analyse und Optimierung der strukturellen Abmessungen vonWerkzeugmaschinen mit Parallelstruktur, IWF-TU Braunschweig, 2003, Germania