Difference between revisions of "Multiple polarizers experimental apparatus"

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The following drawings document the construction of the plastic parts:
 
The following drawings document the construction of the plastic parts:
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|+ World Pendulum standard manufacturing drawings
 
|+ World Pendulum standard manufacturing drawings
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|[[File:pendulo_fulcro.png|thumb]]
 
|[[File:pendulo_fulcro.png|thumb]]
 
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descrição dos desenhos explodidos  
 
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:PendulumPivot.jpg|thumb|Pendulum holder showing the pivot support structure for the 50mmx9mm triangular pyramid.]]
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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.]]
 
|[[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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=Mechanical Assembly=
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 +
{|
 +
|[[File:Imagem_Experiência_1.jpg|x250px|Top|Top view of the experiment]]
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|[[File:Imagem_Experiência_2.jpg|x250px|Top|Front view of the experiment]]
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|}
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 +
In this section, the mechanical assembly of the experiment is explained in detail so that it can be used correctly.
 +
 +
==Order of assembly==
 +
 +
1. Check if all the parts needed to assemble the mechanical structure of the experiment are available.
 +
{|
 +
|[[File:Parts_Needed.jpg|x250px|Top|Parts needed for the mechanical assembly]]
 +
|}
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2. Peel the supports of the pulleys using pliers or an X-Acto knife.
 +
{|
 +
|[[File:peeled_support_1.jpg|x250px|Top|Peeling the support]]
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|[[File:peeled_support_2.jpg|x250px|Top|Peeling the support]]
 +
|}
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3. Put the belt on the peeled pulleys.
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{|
 +
|[[File:belt_on_pulley.jpg|x250px|Top|Belt on pulley]]
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|}
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4. Connect the pulleys with the polarizers. Make sure to hear a “click”.
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{|
 +
|[[File:pulley_polarizer.jpg|x250px|Top|Pulley and polarizer connection position]]
 +
|[[File:pulley_polarizer_connected.jpg|x250px|Top|Pulley and polarizer connected]]
 +
|}
 +
5. Repeat steps 2, 3 and 4 until a complete chain is achieved. (Don't forget to put the belts on, as they are not represented in the example picture.)
 +
{|
 +
|[[File:pulley_polarizer_chain.jpg|x250px|Top|Chain of connected pulleys and polarizers]]
 +
|}
 +
6. Cut the thin layers covering the holes of the main plates of the structure.
 +
{|
 +
|[[File:thin_layer_cutting_process.jpg|x250px|Top|Main plates thin layers cutting process]]
 +
|[[File:thin_layer_cut.jpg|x250px|Top|Main plates thin layers cut]]
 +
|}
 +
7. Place two of the four pillars together and put the nuts in the specific holes on top of one of the pillars.
 +
{|
 +
|[[File:nuts_on_pillars.jpg|x250px|Top|Nuts placed on the pillar]]
 +
|}
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8. Insert the bolts through the holes and bolt the two pillars together.
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{|
 +
|[[File:bolts_on_pillars.jpg|x250px|Top|Bolts placed on the pillar]]
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|[[File:pillars_bolted_together.jpg|x250px|Top|Pillars bolted together]]
 +
|}
 +
9. Place the main plates next to each other
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10. Place the bolted pillars on the side of the junction of the two plates.
 +
{|
 +
|[[File:junction_placement.jpg|x250px|Top|Placement of the pillars]]
 +
|}
 +
11. Place the chain support on the other side of the main plates, so that they are in opposite positions. Check if the chain support is placed on top of the hexagonal holes.
 +
{|
 +
|[[File:chain_support_opposite_to_pillars.jpg|x250px|Top|Placement of the chain support]]
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|[[File:chain_support_in_position.jpg|x250px|Top|Placement of the chain support]]
 +
|}
 +
12. Place the nuts on the chain support inside the “boxes” closest to the chain support “wall”.
 +
13. Insert the bolts through the holes on the bolted pillars and bolt the pillars, the main plates and the chain support together.
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14. Insert the nuts inside the other holes of the chain support.
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15. Insert the bolts through the main plates and fully bolt the chain support to the main plates.
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{|
 +
|[[File:bolt_chain_support.jpg|x250px|Top|Bolt the chain support to the main plates and the pillars]]
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|}
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16. Repeat steps 7 and 8.
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17. Go to the opposite side of the main plates and place the pillars under the circular holes.
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18. Place the nuts inside the top holes of the pillars.
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19. Insert the bolt through the main plates and bolt them together with the pillars.
 +
{|
 +
|[[File:bolt_the_other_pillars.jpg|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.
 +
{|
 +
|[[File:chain_in_place.jpg|x250px|Top|Chain structure placement]]
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|}
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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.
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{|
 +
|[[File:fully_bolted_chain.jpg|x250px|Top|Bolted chain structure]]
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|}
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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|x250px|Top|Nut inside the middle "box"]]
 +
|}
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24. Place it beneath the main plates in one of the corners.
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25. Insert the bolt through the main plates to bolt them to the pillar.
 +
{|
 +
|[[File:corner_placement.jpg|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|x250px|Top|Small pillars removal]]
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|[[File:remove_small_pillars_2.jpg|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|x250px|Top|Nuts placement on the main plate]]
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|[[File:nuts_on_main_plate_2.jpg|x250px|Top|Nuts placement on the main plate]]
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|}
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29. Place the stepper holder above the holes.
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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|x250px|Top|Stepper holder placement on the main plate]]
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|}
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31. Repeat steps 28, 29 and 30 for the other four stepper holders.
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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|x250px|Top|Wires entering position]]
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|}
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33. Repeat step 32 for the other 4 steppers.
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{|
 +
|[[File:stepper_placement.jpg|x250px|Top|Stepper placement]]
 +
|}
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34. Place the belt in the thread of the stepper engine.
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35. Connect the stepper engine (with the belt) to the stepper.
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{|
 +
|[[File:motor_placement.jpg|x250px|Top|Motor placement with the belt on]]
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|}
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36. Tighten the stepper engine.
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{|
 +
|[[File:motor_tightened.jpg|x250px|Top|Tightening of the motor]]
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|}
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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|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|x250px|Top|Stepper holder in the correct position]]
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|}
 +
39. Repeat steps 34, 35, 36, 37 and 38 for the other four stepper holders.
 +
40. Assembly completed.
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 +
=Electronic circuit=
 +
 +
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=
 
=Optical path=
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==Optical path alignment==
 
==Optical path alignment==
−
The main body of the apparatus have the light traveling in parallel rays through the cascade of polarizers later being 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:
+
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:
  
−
#Assemble only the first lens in the system and the light source;
+
#Firstly assemble the system lens and the light source (LED);
−
#Energize the LED and follow the emerging image from the exit, eg. projecting it in a wall a couple of meters apart;
+
#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);
 
#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;
 
#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;
 
#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.
+
#Firmly glue the light source and photo-diode positions in their final position.
  
 
==Optical path calibration==
 
==Optical path calibration==
  
−
Once the support structure is in place, we start to calibrate the absolute position of each polarizer; effectively all the polarizers will have a small offset systematic error and is important to note their value.
+
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 fixed polarizer shall be positioned with a couple of degrees in order to avoid starting the experiment from a maximum, allowing for a easily observation of such maxima. Consider to have it around ~15º and well secured, eventually with glue.
+
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 first maximums are detected (usually two). Take note of their value and leave the second polarizer at rest in that position. Now insert the third polarizer and repeat the procedure for the maximums and do this for the rest of them.
+
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.
 
You will end up with a table of maximum transmission angles, leading to the reference value of maximum intensity in the cascade of polarizers.
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Later, when performing the experiments this values of offsets must be consider in order to eliminate the systematic error of the system.
 
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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[[Remote Experiment Controller | Previous Page (Remote Experiment Controller)]]
 
[[Remote Experiment Controller | Previous Page (Remote Experiment Controller)]]
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[[Precision Pendulum Assembly: Mechanical assembly| Next Page (Mechanical assembly)]]
 
[[Precision Pendulum Assembly: Mechanical assembly| Next Page (Mechanical assembly)]]
 
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=Links=
 
=Links=
  
−
*[[Montagem do Pêndulo de Precisão: Descrição do aparato | Portuguese Version (Versão em português)]]
+
*[https://elab.vps.tecnico.ulisboa.pt:8000/execution/create/33/14 Direct link for the control room]
−
*[[Conjunto de péndulo de precisión: descripción del aparato | Spanish Version (Versión en español)]]
+
*[[Light Polarization with multiple polarizers | Reference lesson]]

Latest revision as of 14:22, 12 May 2025

Apparatus description

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:


descrição dos desenhos explodidos

Mechanical Assembly

Top view of the experiment Front view of the experiment

In this section, the mechanical assembly of the experiment is explained in detail so that it can be used correctly.

Order of assembly

1. Check if all the parts needed to assemble the mechanical structure of the experiment are available.

Parts needed for the mechanical assembly

2. Peel the supports of the pulleys using pliers or an X-Acto knife.

Peeling the support Peeling the support

3. Put the belt on the peeled pulleys.

Belt on pulley

4. Connect the pulleys with the polarizers. Make sure to hear a “click”.

Pulley and polarizer connection position Pulley and polarizer connected

5. Repeat steps 2, 3 and 4 until a complete chain is achieved. (Don't forget to put the belts on, as they are not represented in the example picture.)

Chain of connected pulleys and polarizers

6. Cut the thin layers covering the holes of the main plates of the structure.

Main plates thin layers cutting process Main plates thin layers cut

7. Place two of the four pillars together and put the nuts in the specific holes on top of one of the pillars.

Nuts placed on the pillar

8. Insert the bolts through the holes and bolt the two pillars together.

Bolts placed on the pillar Pillars bolted together

9. Place the main plates next to each other 10. Place the bolted pillars on the side of the junction of the two plates.

Placement of the pillars

11. Place the chain support on the other side of the main plates, so that they are in opposite positions. Check if the chain support is placed on top of the hexagonal holes.

Placement of the chain support Placement of the chain support

12. Place the nuts on the chain support inside the “boxes” closest to the chain support “wall”. 13. Insert the bolts through the holes on the bolted pillars and bolt the pillars, the main plates and the chain support together. 14. Insert the nuts inside the other holes of the chain support. 15. Insert the bolts through the main plates and fully bolt the chain support to the main plates.

Bolt the chain support to the main plates and the pillars

16. Repeat steps 7 and 8. 17. Go to the opposite side of the main plates and place the pillars under the circular holes. 18. Place the nuts inside the top holes of the pillars. 19. Insert the bolt through the main plates and bolt them together with the pillars.

Bolt the other pillars

20. Connect the chain with the bolted chain support and with the loose one, as well.

Chain structure placement

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.

Bolted chain structure

23. Pick one of the pillars and place the nut inside the middle “box”.

Nut inside the middle "box"

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.

Corner placement

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.”

Small pillars removal Small pillars removal

28. Insert the nuts inside those “boxes”.

Nuts placement on the main plate Nuts placement on the main plate

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.

Stepper holder placement on the main plate

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.

Wires entering position

33. Repeat step 32 for the other 4 steppers.

Stepper placement

34. Place the belt in the thread of the stepper engine. 35. Connect the stepper engine (with the belt) to the stepper.

Motor placement with the belt on

36. Tighten the stepper engine.

Tightening of the motor

37. Adjust the stepper holder position to ensure the belt is not loose.

Stepper holder too close to the chain (Belt is loose)

38. Tighten the bolts of the stepper holder to fix it.

Stepper holder in the correct position

39. Repeat steps 34, 35, 36, 37 and 38 for the other four stepper holders. 40. Assembly completed.

Electronic circuit

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

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:

  1. Firstly assemble the system lens and the light source (LED);
  2. Energize the LED and follow the emerging circular image from the output, eg. projecting it in a wall a couple of meters apart;
  3. Move the LED position in order to have an output image the closer to the size of the exit circle (~30mm);
  4. Install the structure for the cascade of polarizers without any lens or hard film in it;
  5. Put in place the second collimating lens in order to focus the light in the photo-diode;
  6. 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;
  7. 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.

Links

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