{"id":256,"date":"2019-01-13T08:33:59","date_gmt":"2019-01-13T08:33:59","guid":{"rendered":"http:\/\/fenneclabs.net\/?p=256"},"modified":"2019-01-14T22:23:47","modified_gmt":"2019-01-14T22:23:47","slug":"solar-tracking","status":"publish","type":"post","link":"https:\/\/fenneclabs.net\/index.php\/2019\/01\/13\/solar-tracking\/","title":{"rendered":"Solar Tracking"},"content":{"rendered":"

This post is a follow-up to the former blog post about the solar cooker. The mirror array can now track the sun and rotate toward it. Here I will give a full description of the design and building process, alongside with explanation about the tracking algorithm.<\/p>\n

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Focusing lens array<\/figcaption><\/figure>\n

The tracking device is quite easy to build (though challenging to design), it is made out of three stainless steel rods, two stepper motors, a solar sensing device, a RAMPS\/MKS controller, ball bearings and 3D printed parts for the frame and torque transmission.<\/p>\n

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Overview of the solar tracking cooker<\/figcaption><\/figure>\n

Skeleton<\/h1>\n

The base of the device is 3D printed and shaped like a cone, it has small ears that are meant to be fastened to a wide wooden base with countersink self-tapping screws. Two bearings (6000Z) are pressure inserted to the base, providing low friction for azimuth rotation of the shaft (10X415mm stainless steel) and stiffness against bending moments on the other axis of rotation.<\/p>\n

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The base of the solar cooker<\/figcaption><\/figure>\n

The base also mounts a stepper motor and hides a larger gear attached to the azimuth shaft. The larger gear is glued with Loctite to the azimuth shaft and is driven by a smaller gear mounted on the stepper motor, providing the motor with additional torque.<\/p>\n

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Gear transmission is based inside the base. The two bearings can also be seen.<\/figcaption><\/figure>\n

 <\/p>\n

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The zenith base<\/figcaption><\/figure>\n

The zenith shaft is mounted on the zenith base<\/span>, which is a 3D printed part with two functions:\u00a0 first, it mounts the zenith shaft (8x90mm stainless\/aluminum) on two bearings which provide rigidity and freedom of rotation, and second, it provides additional torque for the stepper motor by a pulley gear transmission.<\/p>\n

The zenith base is composed of two parts – a body, and a cover. The zenith shaft is fitted inside of the two bearings, one at the cover and the other on the body, where it is also glued with Loctite (though superglue is just as appropriate). A transmission mechanism is hidden inside the zenith base, and is implemented using two pulley gears and a rubber pulley.<\/p>\n

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Exploded view of the zenith base<\/figcaption><\/figure>\n

I have used a pulley transmission instead of gear transmission because zenith rotation requires some extra torque in comparing with the azimuth axis, as it needs to overcome small torque imbalance of the mirror array and the counterweight. The explanation is such that small pulley can be 3D printed much smaller than a spur gear, providing much higher torque for the zenith rotation.<\/p>\n

The pulley is 3D printed using TPU filament by Creozole (widely sold on Amazon and Aliexpress), and it is both functional and very durable. In fact, I was very surprised that I couldn’t find anyone else who has printed their own custom TPU pulley. It saves the trouble of loop closing stock pulley, which requires A LOT of technique.<\/p>\n

Even with the transmission, the stepper motor is still too weak to overcome the torque of the weight of the mirror array. This was solved by adding a\u00a0 \u00a0 \u00a0 900 gr counter-weight, strapped to a slider with zip ties. The slider is clamped to the balancing shaft (8×350 mm stainless) in a position where it balances the torque.<\/p>\n

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The balancing mechanism<\/figcaption><\/figure>\n

Solar Tracking sensor<\/h1>\n

To track the sun I’ve used stepper motor and feedback from a solar sensor. Any diversion from the main source of light in the azimuth or zenith angle would result in a correction signal from the sensor for the corresponding motor.<\/p>\n

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On the left an image of the solar sensor, it\u00a0 is made of 4 LDRs (Light Dependent Resistors), and a cross that shadows them indiscriminately. On the right, the solar sensor is depicted from above, where light is projected from the left side to the right side. The cross makes a shadow to the right, and the LDRs are differentially lighten.<\/figcaption><\/figure>\n

The LDRs are pulled up with ~600 Ohms, providing good sensitivity under direct sunlight (the resistor value should be about the same as the LDR under sunlight). When an LDR is shadowed or even partly shadowed, the voltage drops drastically. by comparing the LDR signal on two opposite sides we can get a very strong correction signal.<\/p>\n

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The sensor is aligned so that 2,3 and 4,1 are displaced along the zenith rotation, and 1,4 and 3,4 are displaced along the azimuth rotation. Using this calibration we can write:<\/p>\n

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P.S – I don’t know the proper model name for the LDR I used, but you can try buying one that has the same slithering geometry as seen in the images above. These should probably produce similar results, as most sensors use the same semi-conductors materials.<\/p>\n

Motors<\/h1>\n

The motors used in this project are 12V stepper motors with an internal gear (28BYJ-48<\/a>). These motors are very easy to find in Aliexpress and are very cheap. Although they are originally designed as a unipolar stepper motor, incompatible with the RAMPS style stepper modules, they can be modified to a bipolar stepper motor very easily, as I will show in the assembly section. For those who are curious about stepper motors, the difference between unipolar and bipolar motors, and the implications of the modification,\u00a0 check out this<\/a> article.<\/p>\n

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Controller<\/h1>\n

The solar tracking cooker is controlled with an ATMEGA2560 with a RAMPS 1.4 shield (or MKS gen 1.4 which is fully compatible).\u00a0 It makes connecting the stepper drivers and motor very standard and easy, and requires soldering a small pull-up board for the 4 LDRs of the solar sensor.<\/p>\n

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ATMEGA2560 with a RAMPS 1.4 on top of it<\/figcaption><\/figure>\n

Alternatively, you can use an Arduino nano, only needing to change the pin names in the Arduino code. Also, I recommend buying stepper driver modules so you can save yourself the trouble of soldering or wiring breadboards (been there done that, its a waste of time).<\/p>\n

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Driver Module<\/figcaption><\/figure>\n

Fabrication and Assembly<\/h1>\n

I really do hope that you find this project interesting enough that you would actually want to build it, or at least use the azimuth-zenith axis system, because it is quite cheap and easy to build and can be used for other uses (such as a radio telescope).<\/p>\n

This is why I’ve uploaded the MCAD design and the code files to GrabCAD<\/a>. The project folder is separated into three subfolders:<\/p>\n