Integration of Microcontroller System Design In Mechatronic Education Low Cost Solutions

Transcription

1 th IFToMM World Congress, Besançon (France), June8-, 007 Integration of Microcontroller System Design In Mechatronic Education Low Cost Solutions R. Bălan * V. Mătieş O. Hancu S.-D. Stan Technical University of Technical University of Technical University of Technical University of Cluj-Napoca, Romania Cluj-Napoca, Romania Cluj-Napoca, Romania Cluj-Napoca, Romania C. Lăpuşan S. Besoiu Technical University of Technical University of Cluj-Napoca, Romania Cluj-Napoca, Romania Abstract With the increasing use of digital control of mechanical systems, it s necessary for mechanical engineering students to have a basic knowledge of microcontrollers and microprocessors and their associated interfaces with mechanical world. As a consequence, in many universities have been introduced courses in which it is studied microcontrollers and their interface with the real world. The didactical equipments used for this aim can be expensive. On the other hand there are cases in which the existent equipments can be improved by the utilization of the microcontrollers and the PC. Taking into account the above statements, this paper presents some low cost solutions in the field of microcontroller's applications in mechatronics. Keywords: Mechatronics, Microcontroller, Robot I. Introduction With the continuing decline in cost and increase in availability, virtually every new mechanical system, from nuclear reactors to toasters, is being controlled with a microprocessor based control system []. Today the industrial world is an arena with many manufactures that produce low cost and high quality products. Creativity and time to market become a must to survive. As a result, traditional electromechanical solutions do not fit many new applications requirements. The need for an integrated approach to the design of complex engineering system involving electronic engineering, mechanical engineering and computing has led to the growth of the concept of mechatronics []. As a result, in the fields of education, appeared the necessity to educate mechanical and other non-electrical engineering students to use microcontrollers []. There is a continuous debate regarding the way in which information can be transmitted to the students [4], [5], [6]. The didactical equipments used for this aim may be relatively expensive even in the case of universities from developed countries [7]. By microprocessor is meant the general-purpose microprocessors such as Intel s x86 family or Motorola s 680x0 families. These microprocessors contain no RAM, no ROM, and no I/O ports on the chip itself. For this reason, they are commonly referred to as general-purpose microprocessors [8]. In addition, a microcontroller has a fixed amount of RAM, ROM, I/O ports, timers, ADC etc. Microcontrollers are used to reduce cost and to increase the flexibility and reliability of most new product and system design. For example, microcontrollers are now used to drive motors (they enable speed control, torque or the power on the load). They include safety features and interface to a large variety of sensors and actuators. Flexibility provided by the software enables the flexibility of the same device to control different types of actuators and equipment for a limited cost. Because of increasingly computing power needed and better control of complex processes, there are now emerging technical solution and products for satisfying those needs. For these reasons, five years ago, our department introduced a course entitled Microprocessors-Structure and Applications. The goal of the course is not to teach the students to be microcontroller engineers, but to teach them enough about electronics and software that they will able to be effective interdisciplinary team members and leaders. The course content is designed to teach the students to understand common terms such as bit, byte, memory, central unit, input/output ports, serial and parallel interfaces, timers, watchdog, PWM, analog-digital and digital-analog conversions, etc. Other courses are available to teach closed loop control theory, digital and analog electronics, and sensors and transducers. Prerequisites include a course of programming (C++). There is a continual debate as to the use of assembly language vs. some higher level language such as C. The assembly language is the language which will help the students to gain a better understanding of microcontroller's internal structure. On the other hand, it is simpler to use high language, especially for large projects. The paper presents some applications that are

2 th IFToMM World Congress, Besançon (France), June8-, 007 implementing a low cost solution using microcontrollers. II. A case study: thermal process There are situations in which the equipments cannot be connected directly to the PC or this connection is realized with some restrictions (for example the equipments that used the operating system DOS or initial version of the operating system Windows). These equipments may be corresponding to other points of view. A low cost solution means the utilization of a system realized with a microcontroller as an interface between the PC and equipments. Thus, the data taken from process with the help of existent equipments can be transmitted to application realized on PC, processed, memorized, shown etc. and the application realized on PC will transmit with the help of the microcontroller the necessary data to the process. The objective of this case study is to control the temperature of an educational HPS [9] Temperature and brightness controlled system component, by regulating the voltage supplied to a heater lamp, positioned in centre of the component. This component contains a PTC (Positive Temperature Coefficient) resistor for acquisition of the temperature actual value, a LDR (Light Dependant Resistor) for acquisition of the brightness actual value and a lamp. Possible disturbance variables are variable ventilation in the temperature controlled system and external brightness sources in the brightness controlled system. Some data sheets: PTC operating voltage-max. 0V; PTC resistance- 80 Ω; lamp rated voltage- V, lamp power-5w; dimensions- 7x56x5 mm; weight-65g. Temperature control systems are found in a host of commercial products and in many environments: buildings, ovens and refrigerators, cars, office equipment etc. For example, in our cars, we find temperature control mechanisms that control the temperature of our engine, which help to preserve the integrity of the lubrication and combustion processes [0]. From many points of view, as well as for the theoretical study and for the practical study of the control algorithms, the thermal processes are advantageous because are present in everyday life, thus can be easily understood. They are complex enough, it can be chosen convenient thermal processes from the view point of time constants (are convenient the processes where the sampling time are around seconds). It can be realized applications that are cheaper. The study of control algorithms (simulations and experiments), requires important hardware and software resources. The practical experiments permit to validate or invalidate a control algorithm, and can lead to a reappraisal of some theoretical aspects, than the simulations must be resumed. But on the other hand, the simulation permits a simple modifying of process parameters; in practical experiments, only some parameters and conditions can be easily modified. Another problem is the duration of the experiment. So, it is a good choice for applications to permit both simulations and practical experiments. Usually, for the study of control applications are used some programming environments like: Matlab-Simulink, LabView etc., or didactical equipments designed for this aim [9]. Many times, the practical implementation can be really expensive. Taking into account that in the case of the training of the students from Mechatronics specialization are presented courses of Microcontrollers and Programming a low-cost solution means using a system with microcontroller as interface between the process and PC, or in some cases utilization of the parallel interface of PC for controlling direct the process. It can be used usual control algorithms like bi-positional, three-positional, PID etc., but also evolved control algorithms like the adaptive algorithms that imply using the on-line identification. HPS SystemTechnik line contains modules for training in the field of: electrical engineering, electronics, mechatronics, control (PID Board, Motor Board, Servo Board, Temperature and brightness controlled system, PC Control Board-PCI) etc [9]. The application includes three HPS board (PID board, motor board and Temperature and brightness controlled system ), a microcontroller board (IMC 500, with 80C55 microcontroller) and a PC (Fig. ). PID board is used only for signal conversion and Motor board is mother board for Temperature and brightness controlled system. Fig.. The schematics of the application For identification it is used the on-line least square algorithm; for control it is used an adaptive-predictive algorithm [] based on the on-line simulation of the system behaviour in the future and using a set of rules for calculus of the control signal. There are two possibilities to implement the algorithms. One of them is to use the microcontroller only for signals conversion (ADC-0bits, PWM-8bits) and communication with PC. The second possibility is to implement algorithms directly on microcontroller (in C language and assembly). In this case the computer is used only for signals visualisation and to program the setpoint. Here it is considered the first case because it is simpler to change the type of algorithm, parameters, setpoint etc. To increase the resolution of ADC, it is used multiple

3 th IFToMM World Congress, Besançon (France), June8-, 007 measurements. First, the extremes are eliminated and than it are made the average of the rest of samples. These operations lead to a better behavior of identification algorithm especially in transitory regime. In this application, the sample period is 6s. As heat supply it is used the lamp of the HPS Temperature and brightness controlled system. The algorithm is implemented in Delphi-Pascal. It was used a linear model with three poles and two zeros. In the next figures, the control and output signals are represented using non-dimensional units. The conditions of the experiment are difficult: the system is not thermal isolated and there is not used a ventilator. Initially, the system has high order system behaviour (Fig. ). As a result, in the first part of the experiment the control system has not a good behaviour due to a poor identification; this thing is corrected in the second part of the experiment (Fig., 4). In Fig.,, 4 it can be remarked also the obtained predictions at every sample period using four candidate sequences. These on-line simulations are used for control signal calculus. The algorithm choose the range in which can take the control signal values (Fig. 4, curves u minst, u maxst ). Fig. 4. The control signal and process gain In Fig. 5 it is represented the positions of poles and zeros, estimation of gain factor k_est (which is represented in Fig. 4 as a time function) and model error after step 90. For the on-line calculus of the position of poles and zeroes it is used a procedure that permits the calculus of the complex roots of a polynomial [] by using the Bairstow method. By solving such problems, students are encouraged to use the existent Internet resources. Fig. 5. Poles-zeros positions, gain and model error A view of the application is presented in Fig. 6. Fig.. The setpoint, output and predictions Fig.. The setpoint, output and predictions Fig. 6. A view of the application

4 th IFToMM World Congress, Besançon (France), June8-, 007 III. Application: Mobile Platform One of the new products available on the market for controlling and processing information is Atmel's AVR microcontrollers witch have a RISC core running single cycle instructions and a well-defined I/O structure that limits the number of external components to very few. Internal oscillators, timers, UART, SPI, pull-up resistors, pulse width modulation (PWM), analog-digital converter (ADC), analog comparator and watch-dog timers are some of the features one will find in AVR devices. AVR instructions are tuned to decrease the size of the program whether the code is written in C or Assembly. With on-chip in-system programmable Flash and EEPROM, the AVR is a perfect choice in order to optimize cost and get product to the market quickly []. It s a good choice to use a high level language (if it s possible). There are two reasons at least: first, the application can be completed and implemented more fast, and second, everyone, with minimum knowledge about computing and microcontrollers, can developed their own applications. BASCOM AVR is a BASIC language compiler for AVR family, which provides a simulator, a programmer and other useful resources [4]. It is very easy to connect usual devices (for example a large variety of LCD, digital temperature sensors like DS80, optical sensors SFH506, SFH50, etc.) to microcontroller (for this, the user can use a function with parameters). Also it is possible to use directly some useful function, for example PID function. A typical application in mechatronics is the control of a autonomous mobile agent. Sending and receiving information to the actuators and from sensors is used to create a feedback to drive the mobile robot onto a given track. This mobile platform has an infrared proximity sensor capable of detecting a dark track on a light surface. The microcontroller processes the information from the sensor and decides whether to take a left or a right (Fig. 7). IM Also this mobile platform needs to communicate with other devices. For this purpose it s used a serial data link over infrared (Fig. 8). The communication interface is composed by a dedicated circuit for receiving the data and an infrared emitting diode for data transmission. Fig. 8. Block diagram of the electronic part The data sent and received is modulated at 6kHz by the microcontroller and transmitted via a communication protocol developed by Philips for Hi-Fi systems (RC5 protocol). The range of emitting/reception is about 0 meters in direct visual contact. The emitter IR-led is connected to the OC pin of the AT90s. The modulation (6 KHz) is generated using an internal timer (timer). Both the emission and reception routines are included as commands in Basic language compiler (e.g. RC5send, GetRC5). A block diagram of the main program is shown in Fig. 9. Fig. 9. Main program diagram IV. Application: Interfacing of a -RRR Parallel Robot Fig. 7. Mobile platform based on AT90S microcontroller Parallel robots are mechanisms that have the endeffector connected to the fixed frame by more then one kinematic chain. The -RRR robot (Fig. 0 a, b) is a three degree of freedom parallel mechanism which is made from kinematic chains. The positions of the actuators directly influence the geometry and the length of the arms. The mechatronic approach in robot designing differs much from the traditional approach. In this case, the mechanical architecture is conceived in parallel with a more sophisticated control strategy. 4

5 th IFToMM World Congress, Besançon (France), June8-, 007 Knowing the position of points A, B, C of the endeffector the problem is reduced to an inverse kinematics problem of a serial robot with arms. K K L () ( xd xo ) ( yd yo ) L (4) a) b) Fig. 0. a) CAD design of -RRR planar parallel robot, b) Experimental structure of -RRR planar parallel robot Another important aspect in mechatronic design is the easy usage of the product by the final user (human friendly mechatronics). Creating an easy and intuitive interface between the user and the robot was a very important aspect in the robot design. An application for interfacing the user with robot was build using Delphi Software. Also the computer offers the computation power needed for solving the equation of the inverse kinematics problem. These equations are used in the program to control the movement of the end-effector of the robot. The user will enter the position of the endeffector and the program will compute the joint angles of all three actuators. K L L ( xd xo ) ( yd yo ) q q (5) [ a tan ( K K K K, K K)] (6) [ a tan ( K K K K, K K )] Control of the DOF parallel minirobot is divided in two levels: low level control, implemented using an AtMega855 microcontroller produced by Atmel (Fig. 5), and high level control, implemented in the software created on the PC. Fig. 5. Control board system Fig.. Kinematics scheme of the -RRR planar parallel robot The computation of the joint angles is made in two steps; in the first step it s determined the position of the points A, B, and C. All three points are situated in the corner of an equilateral triangle. The position of the mass center of the triangle is known P x and P y and also the angle q between the end-effector and the Ox axis it s given. Length L represents the distance from P to A. Ax Px L cos( q 90) () Ay Py L sin( q 90) () The microcontroller controls the position (angle) of each actuator using a closed loop control system. In order to determine the angle of each actuator, three potentiometer sensors mounted on the axes that actuate each motor link are used. Actuation of the robot was made using three stepper motors with a three steps gear box. Microcontroller also displays on a LCD the resulted angle of the three motors arms (Fig 5). In case of any error that occurs at the system initialization or in the functioning of the robot, the microcontroller will display an error message on the LCD according to the problem. The communication between microcontroller and PC is made using RS interface [5]. The application created on the PC controls the overall dynamic of the robot. Using the RS interface the application will send to the microcontroller the reference angle for each actuator. Joint angles are computed using data introduced by the user through IKP equations. 5

6 th IFToMM World Congress, Besançon (France), June8-, 007 In this paper were presented some mechatronics applications, pointing out the integration of the elements and showing the challenge needed to be faced for the interfaces between modules and the user interface. The user interface was build using Delphi, this environment allow the programmer to create complex application under Windows operation system. The accessibility of the user interface was a very important aspect that was kept in mind when an application was designed. In this way a perfect medium for the teaching and training of professional and technician mechanical engineers was emphasized in a mechatronics approach. Fig. 6. Graphical User Interface for a DOF parallel mini robot User can give commands to the robot in four ways, but the most important way is command list. Any industrial process is composed from multiple basic motions. User can introduce a list with all this basic motions and then can simulate the movement of the robot on the PC or directly test the motions on the robot. Using a standard language, it offers to user the possibility to understand easily the way that a robot can be controlled. The other three commands modes were mostly build for testing the robot. These modes are mouse mode, geometric mode and buttons mode. The created interface can be used to command any - RRR type parallel robot, which has the actuators placed in the corners of an equilateral triangle. The user can change the dimension of the arms, the dimension of the end-effector and the distance between actuators. V. Conclusion Mechatronics is a design domain that is best taught with a strong laboratory component. Using a microcontroller many applications can be completed fast and simple, thus leading to an ideal choice in mechatronics education. The microcontroller makes possible the design of integrated and flexible controls for a low cost. The applications require few resources and are more accessible than other complex and expensive solutions and can be easy implemented. The software can be modularly developed to simplify the different control algorithm implementation. Using a PC application, it is possible to control the process via serial port and a microcontroller board, or via parallel port. The process (inputs, outputs, poles, zeros, predictions, errors, model parameters etc.) can be easily represented on display; setpoint and some parameters of identification and control can be modified in real time. In addition, because the PID control can be implemented too, it is possibly to compare adaptive predictive algorithm, PID algorithm or other type of control algorithms. Acknowledgment This work was supported by the Ministry of Education and Research grant CEEX INFOSOC program. Grant s title: Simulation, Control and Testing Platform with applications in Mechatronics. References [] J. E. Carryer, The design of laboratory experiments and projects for mechatronics courses, Mechatronics Vol. 5, No. 7 pp , 995. [] D. A. Bradley, N.C. Burd, D. Dawson, A.J. Loader, Mechatronics. Electronics in products and processes, Chapmann and Hall, 99. [] S. McNeill, J. Helm, A required mechanical engineering course in microprocessors, Mechatronics, Vol. 5, No. 7 pp , 995. [4] M. Grimheden, M. Hanson, Teaching fast prototype design of Mechatronic System- From idea to prototype in 4 hours, Conference REM 005, France. [5] M. Grimheden, M. Hanson, What is mechatronics? Proposing a didactical approach to Mechatronics, First Baltic Sea Workshop on Eduction in Mechatronics, Kiel Germany, 00. [6] V. Giurgiutiu, J.Lyons, D.Rocheleau, W.Liu, Mechatronics/ microcontroller education for mechanical engineering students at the University of South Carolina, Mechatronics, Volume 5, Issue 9, November 005, Pages [7] Hosni I. Abu-Mulaweh, Portable experimental apparatus for demonstrating thermodynamics principles, International Journal of Mechanical Engineering Education /, pp. -, July 004 [8] M. Mazidi, The 805 Microcontroller and Embedded Systems, Prentice Hall Inc. New York, 000. [9] [0] K. Craig, Mechatronic System Case Study: Thermal Closed-Loop Control System, Internet [] Balan R., V. Maties, S. Stan, Motion Control Using On-Line Simulation and Rule Based Control, IEEE ISIE 005, June 0-, 005, Dubrovnik, Croatia, pp. 67-7, ISBN [] [] [4] [5] Balan R., V. Maties, S. Stan, C. Lapusan, On the Control of a - RRR Planar Parallel Minirobot, Journal Mecatronica, Journal of Romanian Society of Mechatronics, no. 4/005, ISSN , pp. 0-, December, 005. * radubalan@yahoo.com matiesvistrian@yahoo.com ohancu@netscape.net sergiustan@hotmail.com lapusanciprian@yahoo.com sorinbesoiu@yahoo.com 6