View source for Kit experimental de polarização da luz com múltiplos polarizadores

=Descrição do kit experimental=

The setup for the construction of the multiple polarizers twin experiment is composed of three main components: (i) the supporting 3D printed plastic parts whose schematics are available here, (ii) a raspberry Pi running the control software over the internet and performing the video streaming and (iii) the low-level slave controller electronics comprising the sensing and the experiment motorisation.

The following drawings document the construction of the plastic parts:
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descrição dos desenhos explodidos 
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|[[File:PendulumPivot.jpg|thumb|Pendulum holder showing the pivot support structure for the 50mmx9mm triangular pyramid.]]
|[[File:Stringsuport.png|thumb|Pendulum string support to avoid elongation errors. The cable is fixed by soldering it into an M4 brass screw 40mm long or an intravenous needle.]]
|[[File:Fulcro.jpg|thumb|Pendulum string fulcrum detail showing the screw to hold the wire. Remanium is very ductile and easily wound around the screw.]]
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|[[File:Fulcro&Apoio.jpg|thumb|Fulcrum support structure to be held on top of the pyramidal pendulum support. Strong M6 or M8 screws are used.]]
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=Montagem Mecânica=

{|
|[[File:Imagem_Experiência_1.jpg|thumb|x250px|Top|Top view of the experiment]]
|[[File:Imagem_Experiência_2.jpg|thumb|x250px|Top|Front view of the experiment]]
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Nesta parte, a montagem mecânica do kit experimental é explicada em detalhe para poder ser corretamente montada.

==Ordem de Montagem==

1. Verificar se todas as peças necessárias à montagem estão disponíveis.
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|[[File:Parts_Needed.png|thumb|x400px|Top|Peças necessárias para a montagem]]
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2. Retirar os suportes de impressão das polias com a ajuda de pinças ou de um X-Acto.
{|
|[[File:peeled_support_1.jpg|thumb|x250px|Top|Retirar os suportes]]
|[[File:peeled_support_2.jpg|thumb|x250px|Top|Retirar os suportes]]
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3. Colocar as correias nas polias.
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|[[File:belt_on_pulley.jpg|thumb|x250px|Top|Correia na polia]]
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4. Ligar as polias aos suportes dos polarizadores. Ouvir um “click” para saber que estão bem ligados. Adicionalmente, colocar os polarizadores nos suportes. (Não esquecer de retirar as películas protetoras dos polarizadores, caso seja necessário)
{|
|[[File:pulley_polarizer.jpg|thumb|x250px|Top|Posição de ligação das polias com os polarizadores]]
|[[File:pulley_polarizer_connected.jpg|thumb|x250px|Top|Polia e polarizador ligados]]
|}

5. Repetir os passos 2, 3 e 4 até ter a corrente de polias e polarizadores completa. (Não esquecer de ter as correias nas polias, que não estão representadas na figura.)
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|[[File:pulley_polarizer_chain.jpg|thumb|x250px|Top|Corrente de polias e polarizadores]]
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6. Cortar as camadas finas de suporte que tapam os furos das placas principais da estrutura.
{|
|[[File:thin_layer_cutting_process.jpg|thumb|x250px|Top|Corte das finas camadas de suporte]]
|[[File:thin_layer_cut.jpg|thumb|x300px|Top|Placas principais depois de cortadas]]
|}

7. Juntar dois dos quatro pilares e colocar as porcas nos buracos específicos no topo dos pilares.
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|[[File:nuts_on_pillars.jpg|thumb|x250px|Top|Porcas colocadas nos pilares]]
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8. Ligar os dois pilares colocando os parafusos nos furos específicos.
{|
|[[File:bolts_on_pillars.jpg|thumb|x250px|Top|Parafusos colocados]]
|[[File:pillars_bolted_together.jpg|thumb|x250px|Top|Pilares ligados]]
|}

9. Juntar as placas principais nesta posição.
{|
|[[File:junction_placement.jpg|thumb|x250px|Top|Posicionamento das placas principais (igual ao posicionamento da imagem do passo 1]]
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10. Colocar os pilares ligados no canto da junção das placas principais. 
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|[[File:main_plates_placement.jpg|thumb|x250px|Top|Posicionamento dos pilares ligados na junção]]
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11. Colocar os suportes da corrente no outro lado das placas principais, de modo a que fiquem em lados opostos. Verificar se o suporte da corrente está colocado em cima dos furos hexagonais.
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|[[File:chain_support_opposite_to_pillars.jpg|thumb|x250px|Top|Posicionamento do suporte da corrente]]
|[[File:chain_support_in_position.jpg|thumb|x250px|Top|Posicionamento do suporte da corrente]]
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12. Colocar as porcas nas "caixas" mais próximas da "parede" do suporte da corrente.
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|[[File:nuts_placement.png|thumb|x250px|Top|Posicionamento das porcas]]
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13. Colocar os parafusos nos pilares ligados para aparafusar os pilares às placas e ao suporte da corrente.
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|[[File:bolts_placement.png|thumb|x250px|Top|Colocar os parafusos nos furos destacados]]
|}

14. Colocar as porcas no resto das "caixas" do suporte da corrente.

15. Colocar os parafusos através das placas principais para aparafusar o suporte da corrente às placas.
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|[[File:bolt_chain_support.jpg|thumb|x250px|Top|Aparafusar os pilares às placas principais e ao suporte da corrente]]
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16. Repetir os passos 7 e 8.

17. Go to the opposite side of the main plates and place the bolted pillars under the circular holes. 

18. Place the nuts inside the top holes of the bolted pillars.

19. Insert the bolt through the main plates and bolt them together with the pillars.
{|
|[[File:bolt_the_other_pillars.jpg|thumb|x250px|Top|Bolt the other pillars]]
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20. Connect the chain with the bolted chain support and with the loose one, as well.
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|[[File:chain_in_place.jpg|thumb|x250px|Top|Chain structure placement]]
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21. Place the nuts inside the specific “boxes” of the loose chain support.

22. Insert the bolts through the holes in the main plates to connect the loose chain support to the main plates. 
{|
|[[File:fully_bolted_chain.jpg|thumb|x250px|Top|Bolted chain structure]]
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23. Pick one of the pillars and place the nut inside the middle “box”.
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|[[File:nut_middle_box.jpg|thumb|x250px|Top|Nut inside the middle "box"]]
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24. Place it beneath the main plates in one of the corners.

25. Insert the bolt through the main plates to bolt them to the pillar.
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|[[File:corner_placement.jpg|thumb|x250px|Top|Corner placement]]
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26. Repeat steps 23, 24 and 25 until the four corners of the structure are supported.

27. Remove the small pillars on the surface facing downwards of the main plate to allow nuts to be inserted into those “boxes.”
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|[[File:remove_small_pillars_1.jpg|thumb|x250px|Top|Small pillars removal]]
|[[File:remove_small_pillars_2.jpg|thumb|x250px|Top|Small pillars removal]]
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28. Insert the nuts inside those “boxes”.
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|[[File:nuts_on_main_plate_1.jpg|thumb|x250px|Top|Nuts placement on the main plate]]
|[[File:nuts_on_main_plate_2.jpg|thumb|x250px|Top|Nuts placement on the main plate]]
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29. Place the stepper holder above the holes.

30. Insert the bolts through the holes of the stepper holder in order to connect it to the main plates.
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|[[File:stepper_holder_placement.jpg|thumb|x250px|Top|Stepper holder placement on the main plate]]
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31. Repeat steps 28, 29 and 30 for the other four stepper holders.

32. Place the stepper on the stepper holder by first putting the wires through the top and bottom holes. Then, hear a click to ensure the stepper is well fixed.
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|[[File:wires_placement.jpg|thumb|x250px|Top|Wires entering position]]
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33. Repeat step 32 for the other 4 steppers.
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|[[File:stepper_placement.jpg|thumb|x250px|Top|Stepper placement]]
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34. Place the belt in the thread of the stepper engine.

35. Connect the stepper engine (with the belt) to the stepper. 
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|[[File:motor_placement.jpg|thumb|x250px|Top|Motor placement with the belt on]]
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36. Tighten the stepper engine.
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|[[File:motor_tightened.jpg|thumb|x250px|Top|Tightening of the motor]]
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37. Adjust the stepper holder position to ensure the belt is not loose.
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|[[File:adjust_stepper_holder_position.jpg|thumb|x250px|Top|Stepper holder too close to the chain (Belt is loose)]]
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38. Tighten the bolts of the stepper holder to fix it.
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|[[File:stepper_holder_position_adjusted.jpg|thumb|x250px|Top|Stepper holder in the correct position]]
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39. Repetir os passos 34, 35, 36, 37 e 38 para os outros quatro suportes dos steppers.

40. Montagem Completa.

=Circuito Eletrónico=

The experiment has two main electronic parts, (1) the drivers for the step-motors and the (ii) light source and detection.

==Step-motor drivers==

==Light source and detection==
[[File:NPolarizersElectronicCircuit.png|thumb|Schematic for the LED PWM connection to the A4 pin of the controller board and the filter for the photodiode detection circuit.]]

The red LED is feed by a PWM output pin (A4) from the main controller board, which allows for a variable light intensity. The default PWM from the board has a 490Hz modulation in steps of 1/256, giving a resolution of less than 0.5%.

This frequency has to be filtered by a low-pass first order RC-filter. As the time constant is ~1s, is necessary to delay the first acquisition for the setting of the circuit voltages. Then, as the signal varies smoothly and slowly due to the polarizer rotation, and oversampling is in place, a much lower settling time is needed.   

=Optical path=
incluir esquema da optica

==Optical path alignment==
The main body of the device has the light propagating in parallel rays through the cascade of polarizers. Those rays later are focused in the sensor (photo-diode). It is crucial for a good signal-to-noise reading to have the system perfectly aligned. For that end, the linear position of the emitting LED and the photo-diode receiver can be adjusted according to the following procedure:

#Firstly assemble the system lens and the light source (LED);
#Energize the LED and follow the emerging circular image from the output, eg. projecting it in a wall a couple of meters apart;
#Move the LED position in order to have an output image the closer to the size of the exit circle (~30mm);
#Install the structure for the cascade of polarizers without any lens or hard film in it;
#Put in place the second collimating lens in order to focus the light in the photo-diode;
#Using a voltmeter for reading the collected light intensity to the photo-diode terminals, move back and forward the photo-diode position in order to maximize the signal;
#Firmly glue the light source and photo-diode positions in their final position.

==Optical path calibration==

Once the support structure is in place, is necessary to calibrate the absolute position of each polarizer; effectively all the polarizers will have a small offset giving a systematic error. It is important to note these angular value that maximizes the transmissivity.

The first polarizer is fixed and shall be positioned with a couple of degrees in order to avoid starting the experiment from a maximum, allowing for an easily observation of such maxima. Consider to have it around ~15º to 30º and well secured, eventually with glue.
Then start the calibration procedure by inserting the second polarizer and rotating it till the maximums are detected and measured (usually two). Take note of their value and leave the second polarizer at rest in one of such that position. Now insert the third polarizer and repeat the procedure for the maximums detection and do this for the rest of them.
Every time a hard film or lens is installed it has to be firmly fixed or glued. If glue is used it ''must not damage the polarizers film''.

You will end up with a table of maximum transmission angles, leading to the reference value of maximum intensity in the cascade of polarizers.

Later, when performing the experiments this values of offsets must be consider in order to eliminate the systematic error of the system.

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=Links=

*[[Multiple polarizers experimental apparatus | Versão Inglesa (English Version)]]
*[https://elab.vps.tecnico.ulisboa.pt:8000/execution/create/33/14 Link direto à sala de controlo]
*[[Light Polarization with multiple polarizers | Referência teórica]]