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Microprocessor based automatic sun trackerA. Konar, MPhilA.K. Mandal, PhD

Indexing term s: Solar power, Servomotors, M icroprocessors, Control systems

Abstract: A microprocessor-based automaticposition control scheme has been designed forcontrolling the azimuth angle of an optimallytilted photovolta ic flat type solar panel or a cylin-drical parabolic reflector to get the illuminatingsurface appropriately positioned for the collectionof maximum sola r irradiance. The scheme presented results in considerable saving in power and isindependent of the geographical location of thesite of erection or temporal variations in environ-mental parameters.

1 IntroductionEficient collection ofmaximum solar irradiance (MSI) ona flat plate type photovoltaic solar panel [i ] or a cylin-drical parabolic solar reflector [2] requires adjustmentsof two parameters of the energy collecting surface namelythe angle of azimuth, ((I, and the angle of tilt, a, of thesurface to be illuminated as shown in Fig. 1.As the ele-

Fig. 1 Solar geometryvation angle of the sun remains almost invariant in amonth [3] and varies little (latitude lOq [3] in a year,there is no need for automatic adjustment of the tiltangle. A single axis position control scheme is sufficientfor the collection of MS I on a solar energy collector. Adescription of such a scheme for the energy collectorsmentioned above is presented.

Rumala [4] has developed a tracking system whichtracks the sun both in the azimuth as well as in the ele-vation plane. Hession and Bonwick [2] have devised atracking system for placing a cylindrical parabolic reflec-Paper 7979A (Sl), first received 18th May 1989 and in revised form t4thJanuary 1991A. Konar and A.K. M andal are with the Department of Electronics andTelecommunication Engineering, Jadavpur University, Calcutta 700032,West Bengal, IndiaIEE PRO CEED IN G S -A, Vol. 138, N o . 4, J U L Y 1991

tor appropriately in the direction of MSI. Both thesetracking systems are continuous types [SI, i.e., controlcommand is continuously generated for the trackingsystem. Batlas et al. [5] have made a comparative studybetween the effects of continuous tracking and step track-ing. They have shown that as much as 99.7% of the con-tinuously trackable solar insolation can be received if aplate type photovoltaic array is rotated by 7.5" everyhour. The step tracking, where the motor in the trackingsystem would remain idle for most of the time [SI, is thuspreferable to continuous tracking.

Batlas et al. [SI have also reported that a two steptracking system which needs to be operated twice a day,once at solar noon and once at night to turn east, cancapture as much as 95 % of the total continuously track-able solar insolation. Two-step tracking is thus preferablefor a small photovoltaic system which can be manuallypositioned. But for a large panel an n-step (n > 2) track-ing is preferred to two-step tracking since the energyrequired by the stepper motor in n-step tracking wouldnot differ from two-step tracking as the total angular dis-placement of the motor shaft in both cases is 360". Addi-tional energy would be required for the generation ofcontrol signals. This energy becomes negligibly smallwhen a microprocessor is used.

A common question arises which is how to fix thevalue of n. This can be experimentally determined bysatisfying the following criteria of energy gain. The tr ack-ing system should continue n-step tracking if the extraenergy collected at the nth step exceeds the energy con-sumed by the tracking device for the nth positioning.Thus the value of n can be determined through a set oftrials by the user.One way of implementing the n-step tracking withoutmuch power consumption is to use a pseudo-trackerinstead of using the solar panel itself. In this arrange-ment, a pseudo-tracker which consumes negligible powermay be used for determining the direction of MS I andwhen the change in the direction of MSI is adequate (saythree motor step angles, i.e. 5.4") then a control commandis generated for the stepper motor for positioning thesolar panel.

Temporal variations in the atmospheric refractiveindex caused by rain, cloud, fog, etc., a t a dis tance fromthe location where the solar panel is mounted may giverise to an erroneous detection in the direction of MSI.This may lead to the wrong positioning of the solar panelbetween two successive tracking cycles which may belong. The problem can be avoided if the tracking is per-formed using sensors in the red to infrared wavelengthsince the light at this wavelength has minimum path devi-ation. The detector should be highly directive (+So) fordetermining the direction of MSI. An infrared photo-transistor detector that incorporates the above features

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has been used in the pseudo-tracking system for findingthe direction of MSI.The hardware aspects of the pseudo-tracker aredescribed. The algorithm used for tracking is thendescribed. Finally, the merits and demerits of the trackingsystem presented are highlighted.

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2The pseudo-tracker has been designed and developed inthe Control Engineering Laboratory at Jadavpur Uni-versity, India. The laboratory model with contro l in oneaxis is shown in Fig. 2. The block diagram of the trackeris given in Fig. 3.An infrared detector has been mountedon the shaft of the motor. The axis of the motor is keptvertical with the plane of the table where the motor isplaced. An infrared source is kept a few inches from theaxis of the motor. The infrared source-detector pair are atthe same height from the plane of the table.

The detector is basically a phototransistor whoseemitter drop varies linearly with the input irradiance atthe base terminal. The detector output is first digitisedand then transferred to an input port of a microcomputerboard. The SDK-85 microcomputer kit board has beenused in the system as the controller. Another output portof the microcomputer is used to transfer four controlpulses to the stepper motor through a current driver

Hardware of the pseudo tracker

Vc c V i E F V i E F GN D CLPA 0

7nfraredsource LED_'-"II I !

infraredtransistordetector

_ - - 0 .5v

/ I 8155 I I

Fig.2 Photograph ofpseudo tracker

card. The current driver card increases the currentstrength of the output pulses generated by the micro-computer. A description of some of the major blocks ofFig. 3are given in the following.2.1 Microcomputer systemThe SDK-85 microcomputer system used as the control-ler of the position control scheme offers 2 kbytes ofmonitor ROM, 512 bytes of RAM and eight 1/ 0 ports

I

PA PB PC8155 I II II ' SD K 85 microcomputer system I

Fig.3238

Schemntic representation of pseudo trackerIEE PRO CEED IN G S -A, Vol. 138, No . 4 , JULY 1991

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[SI. Out of eight 1/0 ports six are 1 byte wide and theremaining two are of 6 bits. 8155 chips [7] which are partof the microcomputer system have 256 bytes of RAM,two 1/ 0 ports (port A and B) of 1 byte, one 1/ 0 port of 6bits width and a 14 bit timer. The ports of the 8155 areconnected to the ADC and to the driver card for thestepper motor as shown in Fig. 4.

IN4007bit

Fig. 4 Stepper motor driver card2.2 Analogue to digital convertorAn ADC LM 0809 chip was used in the system for digi-tising the detector output. The ADC [8] has a built-ineight-input analogue multiplexer. The zeroth channel ofthe ADC was tied to the emitter of the phototransistordetector. The three bit channel select lines weregrounded, indicating that the channel zero has to beactive all the time. Address-latch-enable (ALE) and startconversion signals are communicated to the ADC fromthe microcomputer. The microcomputer then polls for alow to high edge of the end of conversion (EOC) messageproduced by the ADC. A data byte can then be trans-ferred from the ADC to the microcomputer through aport. The dynamic range of the input signal was chosento be 0 V to 5.12 V and the Kef- and vel+erminals ofthe ADC were aflixed with those values, respectively. A100KHz clock, generated from a 555 timer drives theADC.2.3 Stepper motorThe stepper motor requires a + 12 V supply and fourcontrol pulses at its terminals. The pulses are generatedfrom the microcomputer in sequence so as to rotate themotor the required number of steps (one step angle of themotor is equal to 1.8") n the forward or the reverse direc-tion [SI.2.4 Driver cardThe driver card consists of four Darlington pairs of tran-sistors. The inputs to this card are four pulse trains gen-erated by the microcomputer. The outputs of the card areconnected to four coils of the stepper motor with therequired current drive. Details of this card are shown inFig. 4.

3The software was developed in assembly and 8085A com-patible machine level language and tested on the micro-computer board. The software consists of a main moduleand a few subroutines. The software was written in ahighly structured manner.IEE PROCEEDINGS-A, Vol . 1-38. N o . 4 , J U L Y 1 99 1

Software for proposedtracking system

3.1 Main moduleThe main module of the software comprises a lockingcycle followed by tracking cycles. The locking cycle isused to determine the direction of MSI in the wholeazimuth angle of 360" and is executed once daily afterpower-on of the tracking system. This cycle is executedfor both the pseudo-tracker and the solar panel driven bya stepper motor. At the end of the cycle, the circuit boardcontaining the phototransistor and the solar panel arepositioned perpendicular to the direction of MSI. Afterthe execution of the locking cycle, tracking cycles arerepeated n times in a solar day to keep track of the posi-tion of the sun in the azimuth.

3.2 Locking cycleThe locking cycle is described in Fig. 5. Initialisation ofports is performed at the beginning of this cycle. A vari-able count is initialised to 200 (360"/1.8"= 20 0 steps) and

start

Icount = 200(dectmal),0 - 11 cal l moveforward

call adcon

store digitised detectoroutput i n memory location

count:count -1

backcountzaddress of200 th memory location-address o f memory

I / call movebackwardt1 1 call delayback 11:ockcount = backcount -1

exi tFig. 5 Flowchart fo r locking cycle

each time the 'moveforward' and 'adcon' routines arecalled, the count is decremented by one. The movefor-ward routine generates the pulses required to rotate the239

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-1 ca ll movebac-]I

co l I moveforward

exitFig. 6 Flowchartfor tracking cycle

1record digitis ed detector

location Moutput in memory

keep the flowchart simple. In the actual system, the direc-tion of MSI obtained in the present tracking cycle has tobe compared with that of the last cycle. If they differ byat least by three steps of motor rotation, then the corre-sponding control command has to be communicated tothe stepper motor that drives the solar panel. Otherwise,the pseudo-tracker will return back to its last position.240

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Another point that needs special mention regardingthe reliability of the tracking system will be described. Ifthe detector is temporarily shadowed by clouds or air-craft during the locking cycle, then the true maximummay be missed which will make the subsequent trackingcycles ineffective. This problem can easily be avoided byallowing bidirectional searching during the trackingcycles, at the cost of additional energy and computa-tional time. To describe the bidirectional tracking algo-rithm, let F and V + l be the detector outputs at the ithand (i + 1)th instants between which motor movement bya step takes place. The tracking algorithm for bidirection-al search comprises of the following steps:(1) Move the motor clockwise by one step.(2) If F + l3 F, epeat step 1 until it fails.(3) Move the motor anticlockwise by one(4)If &+ 2 v , repeat step 3 until it fails.(5) Move the motor clockwise by one step.(6)Exit.3.4 Adcon routineThe Adcon routine was developed from the timingdiagram [6] of the ADC LM 0809. The routine isexplained clearly in the flowchart representation shownin Fig. 7.\ enter adcon /

keep ALE high.make start conversion

receive dlg i ta lda ta byte

program moduleFig. 7 Flowchartfor Adcon routine

3.5 Moveforward routineThe moveforward routine behaves like a software counterwhich counts 05, 09, OA, 06 in sequence and outputsthem through the port. Between two successive counts anequal time delay may be introduced by another routineto control the speed of the stepper motor.

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3.6 Movebackward routineThe movebackward routine counts the reverse sequence,i.e. 06,OA, 09, 05 in sequence. A transition from one stateto another moves the motor by one step. A very smalluniform delay may be provided between each pair of suc-cessive count ing sequences to contro l the speed of themotor in the backward direction.3.7 Maxevaluate routineThe maxevaluate routine is called from the main moduleto determine the maximum value of 200 measurements ofthe infrared detector outputs. A linear search algorithmhas been used to determine the maximum value.3.8 Delayforldelayback routinesThe delayfor and delayback routines are called from themain module between two successive counts of thecounting sequence during forward and reverse move-ments of the motor, respectively. The routines basicallyinitialise the DE register pair with a value and decre-ments them until it reaches zero. The control exits fromthe routines when the content of the DE register pairbecomes zero.4 ConclusionsThe sun-tracking system presented has the followingadvantages: The proposed tracking system results in aconsiderable saving in energy, as a pseudo-trackerinstead of the solar panel itself has been used for detect-ing the direction of MSI. Use of the step-tracking schemeinstead of continuous tracking keeps the motors idle formost of the time which also helps to save energy. Thetracking system is not constrained by the geographicallocation of installation of the solar panel since it isdesigned for searching the MSI in the whole azimuthangle of 360 during the locking cycle. Temporal varia-tions in environmental parameters caused by fog, rainetc., at a distance from the location where panel ismounted, do not affect proper direction finding. This isbecause sensing for tracking has been performed in the

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infrared wavelength region which has minimum pathdeviation when there is a change in the atmosphericrefractive index.Relative change in the elevation angle of the sun hasbeen neglected. This does not create problems within theperiod of a month, but adjustment of the tilt angle isnecessary every month. A complete automation scheme

using a two-dimensional search strategy could easily bedeveloped following the present scheme at the cost ofadditional energy.

5 AcknowledgmentThe authors appreciate the effort of their students whohave assembled the subsystems and verified the algo-rithms as part of their project assignments. The authorsare also grateful for the assistance provided by theAppropriate Automation Promotion ProgrammeProject, sponsored by the Department of Electronics,Government of India.

6 References1 PISCIMA NIS, D., NOTARIDOU , V., and LALAS, P.D.: Estimat-ing direct, diffuse and global solar radiation on an arbitrarily inclinedplane in Greece,Sol. Energy, 1987 ,39, (3), pp. 529-5382 HESSIAN, J.P., and BONWICK, J.W.: Experience with a sun

tracker system,Sol. Energy, 1984.32, (I), pp. 3 1 13 ELSAYED, M.M.: Optimum orientation of absorber plates, Sol.Energy, 1989.42, (Z), pp. 89-1024 RUMALA, N.S.: A shadow method for automatic tracking, Sol.Energy, 1986 ,37, (3). pp. 245-2475 BATLAS, P., TORTORELI, M., and RUSSEL , E.P.: Evaluation ofpower output Cor fixed and step tracking photovoltaic arrays, Sol.Energy, 1986,37, (2), pp. 147-1636 Intel Corporation: SDK-85 sers manual (Santa Clara, 1978), pp.5-1-5-27 Intel Corporation: Microprocessorand peripheral handbook (SantaClara, 1983), pp. 2-30-2-328 National Semiconductor: Linear databook (Santa Clara, 1982). pp.8-60-8-70, 9-33 9-379 H ALL, V. D.: Microprocessor and digital systems (McGraw -Hill,1983). p. 315

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