Project Sun TrackerThomas Maier

Department of Electrical Engineering, Indian Institute of Technology, Mumbai (IITB)Course EE617 Sensors in Instrumentation taught by Mr. Siddharth Tallur

400076 Mumbai (Powai), Maharashtra, IndiaEmail: thomas@blue-l.de

Abstract—The goal of this project will be to track the move-ment of the sun over the horizon in two dimension. Both azimuthand altitude of the sun will be sensed by using two photoresistorsfor both angles. A mechanical setup will be built that willallow the position sensing to be possible to the accuracy of1◦ using stepper motors and a custom made gearbox. Acrylicsheet material will be used to build the mechanics, externallybought bearings will be used for the gearbox and to realizethe rotation of the two axes. The photosensors will be placedimmediately next to a black wall that will produce a shade onone of the photosensors when not properly aligned with the sun.Two operational amplifier will be used as a window detectioncircuit to introduce a necessary hystheresis when adjusting theangle of the wall. The position of the sensors and the shape of thewall is of big importance whe aiming for a positioning accuracyof 1◦. In the project report special emphasis will be put on howthis is achieved. Furthermore, hallsensors will be used to resetthe position of the stepper motors and a compass sensor willbe implemented to determine the absolute angles that the sun ispositioned in. Again, the accuracy of the whole system will bequestioned with the use of the tolerances that the different sensorshave by applying the principle of propagation of uncertainty. Thedataprocessing and calculation of the final angles will happen ona Raspberry Pi 3B+. If there is time at the end of the project, auserinterface for the Raspberry can also be implemented.

Index Terms—Sun Tracking, Light Source Detection, WindowComparator, 2D Mechanical Angle Measurement

I. BASIC APPROACH

A. Idea and previous project

The idea for choosing this project for the Sensors inInstrumentation class originated from an observation I madeduring my first few days here in Germany. From back home Iwas used to see the sun rising in the east and then climbing afew degrees over the horizon before setting again. Especiallyin winter the sun is almost always blinding you when yougo for a walk outside since it is staying that low. Here inIndia I once discovered while walking around on campus inJuly that my body is almost not projecting any shadow on theground beneath me. This very counterintuitive sensation forme to see the sun directly above my head rose awareness inme for the different angles that the sun can be seen in whenbeing in different places on the planet. When looking throughthe previous projects of this course a few days later, I noticedthat another group previously attempted to build a device thatis tracking the position of the sun to power a solar cell andcharge a battery. [1].

Their setup can be seen in figure 1. As can be seen fromthe picture, the previous group implemented one rotational axis

Fig. 1. Elevation tracking solar cell from previous project.

to track the elevation of the sun above the horizon. BecauseI was subconsciously remembering the sunpaths that can beobserved from Europe, my first impression was that this is anincomplete project since it is only tracking one direction ofthe sun’s path. But then I remembered my experience aboutme not projecting a shadown on the ground and I realized thatduring summer the sun is almost perfectly moving from east towest in India. This can also be seen in the so called sun chartof Mumbai shown in figure 2. This chart shows the positionof the sun over the course of a year by using the two anglescalled elevation and azimuth. The elevation angle on the y axisrepresents the angle between the sun and the horizon, whereasthe azimuth on the x axis is the angle between the sun andthe cardinal direction north. The different blue lines in figure2 represent the path the sun takes in the sky at different timesof the year. The red intersecting lines mark the sun positionsat the same time of a day throughout the year where 12 PM isdefined to be the exact zenith of the sun ignoring timezones.Now when looking at the blue line plotting the path of the sunon the longest day of the year (Jun 21st), the sun rises sometime before 6 AM at an azimuth angle of 65◦. The azimuth

Fig. 2. Sun chart of Mumbai (19◦ North). [2]

Fig. 3. Sun chart of my hometown Erlangen (49.5◦ North). [2]

does not really change until 11 AM, when the sun is passingclose to 90◦ elevation. It quickly jumps from 65◦ to 360◦ -65◦ = 295◦ at 1 PM before setting again at the angle of 295◦.So because of this the previous project is not really incompletebecause of the low latitude that Mumbai is located in and thevery steep path that the sun takes during the summer time inMumbai.

However, one example where this system is not very effec-tive is my hometown Erlangen in Germany. When looking atthe sun chart of Erlangen in figure 3, the path the sun takes inthe sky is much less exciting than in Mumbai because of itshigher latitude of 49◦ North. Over the whole year, the azimuthangle of the suns position is much more important than inMumbai and therefore a sun tracking system only adjusting theelevation angle will not yield a power production even close tothe maximum. This is most notable in European winter, whenthe sun just barely reaches over the horizon and the alreadysets at 4 PM (Terrible time of the year, trust me!)

So because of the incompleteness of the previous projectI chose to improve the existing idea by introducing a secondangle of rotation and also having a closer look at the accuracy

of the positioning system. My focus will not be on producingpower with a solar cell but instead providing a very precisemeasurement of the suns position only. Positioning a solar cellin the respective angles can be a further module that could bethe focus of another project.

B. Goals for the design

As mentioned in the previous subsection, implementing thesecond rotational axis will be the biggest improvement to thedesign. Aside from that, I chose to set myself an accuracy goalof 0.5◦ for the azimuth and elevation angle of the sun. Thiscan only be achieved by using stepper motors and appropriatemechanic gears to divide down the angle increment taken atevery step. Stepper motors will also need some method to resetto a known angle.

Aside from these mechanical goals, the tracker should alsobe able to express the found angles of the suns positionin proper azimuth and elevation angles by using a compasssensor. Uneven surfaces could also lead to the calculation ofwrong sun positions. Therefore it should be possible to takeinto account if the tracker is positioned on a slope. Theserequirements will be achieved by the measurement procedurepresented in subsection I-D.

C. Sun detection device

Before going on to the complete setup plan, the central ideaused for detecting the sun position will be explained here.The previous project used common light dependent resistors(LDRs) and analog digital converters (ADCs) to measure theanalog voltage that the LDRs are providing at a specific lightintensity. Then a microcontroller adjusted the angle of thesolar cell and the LDRs until they approximately measured thesame brightness value. Now while this solution is very easy,it provides some errors in the detection of the angle. First,the LDRs were not mounted properly in a fixed angle. Thisintroduces an error because the LDRs will provide differentoutputs if the sunlight is shining perpendicular to the LDRssurface or in a lower angle since the illuminance reachingthe surface in both cases will be different. Figure 4 showshow much the resistance of the LDR PDV-P8103 varies overthe illuminance. While this might not necessarily be the sameLDR that was used by the project [1], the behaviour willbe very similiar. This indicates that an improper mountingangle will lead to errors. Furthermore, the LDRs themselvehave some manufacturing tolerances and might have differentresistances even if the illuminance is the same. By directlysampling the voltages over the LDRs, this will also lead toimproper angle detection.

To increase accuracy and eliminate effects of manufacturingtolerances, the setup in figure 5 is proposed. In this it canbe seen, that two photodiodes in a 5mm LED package aremounted on a flat surface on different sides of a seperatingwall. When the sun is shining on the setup in an angle thatis not directly perpendicular to the surface, the wall will casta shadow on one of the sides of the wall and also on oneof the photodiodes. By connecting these two photodiodes in

1

10

100

1000

1 10 100Illuminance in lux

Res

ista

nce

in K

oh

ms

Fig. 4. PDV-P8103 LDR Resistance vs. Illuminance plot

series and adding a potentiometer, manufacturing errors canbe counteracted. Also by using the wall for the shadow gener-ation, there will always be a significant difference between theirradiance on both sides. This difference will be much easier todetect than the slight change in resistance that was used for themeasurement in the previous project. Further details about theelectronics will be given in chapter IV-A, the requirementsfor the mechanics will be mentioned in chapter II-A. Afterdetermining which side of the wall is currently receiving morelight, the stepper motors will move the axis until light is almostperpendicular to the surface. Now we have pin-pointed theposition of the sun and it will automatically be tracked whenthe sun will have moved enough to again cast a shadow onone of the photodiodes.

Fig. 5. Sun detection setup.

D. Complete measurement procedure

In order to ensure a proper operation of the sun tracker thatis going to be designed, it is helpful to have a quick overviewabout the different steps it has to go through to measure theposition of the sun. Figure 6 is a draft about all functionalitiesand their relative position to each other. It starts with turningon the power of the solar tracker. After that a Raspberry PiZero is booting, which will be used to control all incomingdata variables and also log the sun’s position. It is chosenbecause of its versatile usability for many measuring casesand the personal experience that is available in handling.

Fig. 6. Measurement procedure for the sun tracker

Next, both axes will calibrate using the mentioned Hallsensors and magnets. The detecting mechanism explained inthe previous subsection will start working afterwards anddetermine whether or not the angle of the sun has changed.I will not pass any measurement values unless they changedrelative to the last value. When they are passed, the valuesfrom the equipped compass sensor and accelerometer will beused to calculate the actual azimuth and elevation angle fromthe measured angles. This will counteract any slope that thedevice is standing on and also remove the need for previousaccurate positioning by the user. Lastly the data is saved witha timestamp on the Raspberry Pi Zero, where it can be readfrom afterwards. Now that the measurement principle waspresented, the next chapter will have a look at the mechanicsof the setup.

II. MECHANICS

A. Requirements for accuracy

While looking around in the Wadhwani Electronics Lab fora useful stepper motor, the stepper motor PM35L-048 from thecompany Minebea shown in figure 7 was found. It seemedreasonably small and useful for the purpose, so it is nowused for driving the two axes. Steppers are useful in this casebecause they have a fixed increment in rotational angle thatcan be controlled easily, therefore making them ideal for thisproject. According to the datasheet [4], their incremental angleafter one step is 7.5◦. This is 15 times more than the desiredangular precision of 0.5◦ which means that this stepper can’tbe used directly to step the axis. Insted, one or two sets ofgears are needed with a transmission ratio of at least 1:15. Asuiting two stage gearbox was designed using the software [5],since a single set of gears would have been far too big. Therealized transmission rate is 1:20, oversizing the requirementa bit on purpose.

Fig. 7. PM35L-048 Stepper Motor from Minebea [4]

Another topic that has to be mentioned is the minimumheight of the wall in figure 5 needed to ensure that at anangular displacement of 0.5◦ the shadow casted will cover

one of the photodiodes. This can easily be estimated by usingthe following forumla:

tanϕ =∆x

h(1)

where ϕ is the desired detection angle (0.5◦ in this case),∆x is the length of the shadow casted on the mounting plateof the photodiodes and h is the height of the wall. The 5mmdiodes will be mounted in an O-ring with an outer diameterof 6.1 mm. Also it won’t be possible to place the LED exactlynext to the wall because of constructive reasons. So it isassumed that the outermost edge of the LED will be 7 mmaway from the edge of the separating wall. It is not necessaryto cover the whole LED in a shadow to trigger a detection sowe assume a shadow length ∆x of around 3 mm should beenough for casting a shadow that is long enough. With this,the minimum heigt of the wall can be calculated as:

h =3 mm

tan 0.5◦= 34.3 cm (2)

This is a relatively high wall which will be difficult to realizemechanically. However, this is set as the goal and realized inthe next chapter, where the constructed upper rotational stageis presented.

B. Upper rotational stage

Following the ideas of the project advisors, the upperrotational stage was realized by laser cutting the pieces witha lasercutter from acrylic sheet material. Figure 8 showsthe constructed upper rotational stage without any additionalpieces. The left closed compartment contains the two stagegear system and ensures that it is not possible to accidentiallytouch the rotating gears and squeeze in fingers or tools. Thesegears rotate a 12 mm steel rod, on which the sun detectingsystem will be mounted. In the ground plate another circularcutting is visible which will later be used to implement thesecond rotational axis. The overall dimensions of the upperstage became a bit bigger as expected. Mainly because theminimum wall length turned out to be quite large.

III. NEXT STEPS

IV. ELECTRONICS

A. Photodiode

V. CONCLUSION AND FUTURE WORK

Due to time limits this report could not be finished. Instead,priority was give to finishing the complete model. Both axesof rotation are now realized and the position of the sun canbe detected accurately. A picture of the final setup is shownin figure 9. The accelerometer and the magnetometer are notbeing read yet. This could be the focus of the next personworking on this topic.

Fig. 8. Upper rotational stage of the sun tracker

A WORD OF THANKS

REFERENCES

[1] LT. COL. SANDHU ET AL.: Project Report: Auto Adjusting Efficient SolarCharging Station, WEL Projects Server, Accessible over Mr. SiddharthTallur.

[2] UNIVERSITY OF OREGON, SOLAR RADIATION MONITORING LAB-ORATORY: Sun patch chart program, Website: http://solardat.uoregon.edu/SunChartProgram.html (Last accessed: September 2019), Used forgenerating the sun charts.

[3] LUNA OPTOELECTRONICS: PDV-P8103 Datasheet, Luna Optoelectron-ics, 1240 Avenida Acaso, Camarillo CA 93012

[4] MINEBEA MOTOR MANUFACTURING CORPORATION: PM35L-048 Stepper Motor Datasheet, Website: https://www.eminebea.com/en/product/rotary/steppingmotor/pm/standard/pm35l-048.shtml (Lastaccessed: September 2019)

[5] HARRINGTON, B. ET AL., Inkscape, Website: http://www.inkscape.org/,(Last accessed: September 2019)

Fig. 9. Finished Sun Tracker just after glueing together final pieces