Stepper motor modeling and control design for a 1.5 ...

WAC 2012 1569535911

Stepper motor modeling and control design for a 1.5 square meters heliostat prototype Victor Benitez

Nun Pitalua-Diaz

Departamento de Ingeniería Industrial Universidad de Sonora Hermosillo, México [email protected]

Departamento de Ingeniería Industrial Universidad de Sonora Hermosillo, México [email protected]

Jesus Pacheco-Ramirez Departamento de Ingeniería Industrial Universidad de Sonora Hermosillo, México [email protected]

disadvantages in the mechanical and control design. The heliostat control design has been investigated using the well known control strategies: open and closed loop [6, 7, 10].

Abstract²This paper presents the modeling, closed loop control and simulation of a stepper motor actuator, which is used to control an arrangement of miniheliostats in a solar tower plant located outside of Hermosillo, Sonora desert. The real time results show that the proposed control strategy is able to track the solar sun position. The controller is implemented in real time via LabView computational environment and applied in the solar tower plant facility.

For the open loop control stepper motors were habitually used for simple point-to point positioning tasks. Thus, the stepper motors were driven by a pulse train with a predetermined time interval between successive pulses applied to the power driver, and no information on the motor shaft position or speed. The open-loop control scheme suffers from low-performance capability and lack of adaptability to load variations and system variations. In fact, without feedback, there is no way of knowing if the motor has missed a pulse or if the speed response is oscillatory.

Keywords-heliostat, central tower technology, stepper motor modeling.

I.

INTRODUCTION

The heliostat is an important element in a thermal-electrical conversion system of a central tower. This device is an electromechanical dispositive which redirects the solar rays to a specific point by a mirror. For an efficient thermal energy concentration, it is required many heliostats, i.e. heliostat field. Through the years it has been proposed different strategies on heliostat design [11] and heliostat field design [5] to reduce the costs and optimize the thermal energy capitation. Figure 1 shows the conceptual idea of a central tower, where •& is the ray from the sun, ”& is the ray of light reflected to the focus of the tower and I ,,& is the direction of the mirror. We can observe that the heliostat has to move in 2 different positions, azimuth and elevation. The heliostat control is designed using 2 stepper motors to achieve the above positions. The stepper motor is a device used to convert electrical pulses into discrete mechanical rotational movements. The heliostat design tendency is on the increment of the reflection area of the mirror, that is for a major solar concentration which results in a reduction of the number of heliostats [5]. However, in the last years the tendency has change to a new paradigm of having a great amount of small heliostats [3]. Both paradigms offer advantages and

Figure 1. Rectangular coordinates system.

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VI.

CONCLUSIONS

The stepper motor dynamical model presented in this work is designed according with the electromechanical properties equations. A closed loop controller was designed and implemented in real time in the CPH. The performance of the controller is able to track solar sun position with good performance and accuracy in simulation. More research is required to implement a robust control action capable of reject disturbances such as wind and mechanical backlash. The nine mini heliostat plant has been used as a bench mark system for solar tracking control in a solar tower plant with good mechanical performance.

Figure 8. Steps sequence position

V.

This demonstrate that the nine mi heliostat plant can be used to test out other control strategies for the community interested in solar tracking research.

REAL TIME RESULTS

The controller structure of figure 6 is implemented using LabView. We tested out the controller scheme in the CPH facility in real time in order to verify the sun tracking capabilities of our design. We arranged the nine heliostats in three clusters with one heliostat as master and two slaves for each cluster. The controller hardware can be seen in Figure 9. The Figure 10 shows the real time tracking testing in the CPH, where vector r is redirected to a lambertian surface mounted on top of tower.

As far as we know, this is the first time that a closed loop controller structure using stepper motors applied to control mini heliostats in a solar tower plant, is reported in the literature.

It is important to mention that the controller is implemented using a regular laptop in the CPH. This is not the best practice as we verify in real time. There are several adverse environmental conditions in the CPH such as wind, dust, dry climate, temperature over 50°C in summer and many others that generate hostile conditions to a PC based controller. A microcontroller based controller is currently being developed to deal with weather conditions of such desert environment. For the sake of completeness we refer to [2] where we the solar tracking of nine miniheliostats performance is shown.

Figure 10. Real time solar tracking Figure 9. Hardware implementation

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ACKNOWLEDGMENT The authors thanks to Centro de Investigacion en Energia ± UNAM for the financial support.

REFERENCES [1]

V. H. Benitez, C. A. Eredias y H. M. Ramírez, "Síntesis de señales de control para seguimiento solar y su aplicación a un prototipo de heliostato" VIII Congreso Internacional sobre Innovación y Desarrollo Tecnológico, pp. 1-6, Cuernavaca, Morelos, 2010. [2] 9 + %HQLWH] ³&DPSR GH 3UXHEDV GH KHOLRVWDWRV VLVWHPD GH  PLQLKHOLRVWDWRV PHFDWURQLFD´ $YDLODEOH 2QOLQH http://www.youtube.com/watch?v=tezCn_94s2c, October 2011. [3] eSolar, Available Online: http://www.esolar.com/, May 2010. [4] 6 $ .DODJLURX ³'HVLJQ DQG FRQWUXFWLRQ RI D RQH-axis sun-tracking V\VWHP´6RO(QHUJ\-469. [5] G. J. Kolb et al., "Heliostats cost reduction study," SAND2007-3293, Sandia National Laboratory, June 2007. [6] C. Y. Lee, P. C. Chou, C. M. Chiang and C. F. Lin, Sun Tracking Systems: A Review, Sensors, pp. 3875-3890, May 2009. [7] R. Marino, S. Peresada and P. Tomei," Nonlinear adaptive control of permanent magnet step motors," Automatica Vol. 31, No.11, 1995. [8] D. Marroquin, Sistema de Control para un Conjunto de Heliostatos (in spanish). Master Dissertation. Centro de Investigación en Energía. UNAM. Temixco, Morelos, México, 2011. [9] A. Morar, ³6WHSSHU 0RWRU 0RGHO IRU '\QDPLF 6LPXODWLRQ´ 7HFKQLFDO University of CLUJ-NAPOCA, Romania-ACTA ELECTROTEHNICA, Volume 44, Number 2, 2003, pp. 117-122. [10] S.F. Rezeka, N.M. Elsodany, N.A. Maharem ³Fuzzy Gain Scheduling Control of a Stepper Motor Driving a Flexible Rotor´European Journal of Scientific Research, Vol.39 No.1, 2010, pp.50-63. [11] ;:HLHWDO³$QHZPHWKRGIRUWKHGHVLJQRIWKHKHOLRVWDWILHOGOD\RXW IRUVRODUWRZHUSODQW´5HQHZDEOH(QHUJ\YROQRVHSWSS 1970-1975.

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