Handheld Scatterometer

=Problem Definition= We aim to design a self-contained, portable instrument for measuring and quantifying the turbidity of a solution over time by measuring the light scattered from the solution.

=Background=

Optical Scatterometry
Optical scatterometry is a method of determining the concentration of a contaminant— i.e. bacteria, proteins, or particulates— in a solution by measuring the scattering or absorption of a laser through a sample. There are two primary variations of optical scatterometry. Their usage and methods of measurement are determined largely by the type of contaminant and it’s concentration. Nephelometry is the detection of light energy scattered or reflected towards a detector that is not in the direct path of the transmitted light. The detector in this scenario is typically placed between 30° and 120° from the direct path. Diluted colloidal suspensions scatter light such that the intensity of the scattered light is directly proportional to the concentration. This is typically used for low-concentration measurements, such as protein and bacterial samples, potable water tests, or other medical uses. Turbidimetry uses a single detector directly in the path of transmitted light to measure the intensity of incident light. The intensity of this measured value is decreased by scattering, reflectance, and absorbance in the sample. Such measurements are used for high concentration or semi-opaque samples. That is, samples that are visibly clouded. Such measurements are used for water quality testing, such as grab samples from a stream or other water source in the field.

Existing Technology
Existing nephelometers and turbidimeters tend towards bulky countertop machines or prohibitively expensive handheld models. This limits access for smaller facilities, discourages field use, and results in a reliance on central labs.



Specifications

 * Handheld
 * Touch Screen
 * Wireless Charging Battery
 * Durable
 * Production model Less than $200

Applications
Medical Clinics Fish and Wildlife
 * Point of Care: Negates need for central lab
 * Affodable/Portable: Use in small clinics or developing countries
 * On-site measurements: Stream/groundwater contaminant check
 * Ease of use: Minimal training required

=Design Considerations=

Initial Choices
For initial testing multiple PCB's are being used, one for each photodiode and for the laser diode, however, there are size constraints to consider and the three PCB's may be consolidated to one if it will save space.
 * Single PCB vs. Multiple PCB's

A laser module would be easier to install, because it has the lens built in, however the laser diode and external lens will have a greater degree of accuracy when installing.
 * Laser Module vs. Laser Diode and Lens

Final Decisions
Due to how small the PCB's are, the final design will not require the PCB's to be consolidated together as it does not save much space. Focusing a laser diode with a lens proved to be quite difficult, with not much benefit. For this reason the laser module was chosen for ease of use and compact design.
 * Multiple PCB's
 * Laser Module

=Project Learning=

PhotoDiode Amplification Circuit
In order to amplify the current provided by the photodiode, a logarithmic amplifier will be used in order to normalize the values given by the photodiode. The chosen Logarithmic amplifier is the Analog Devices ADL5303. Using the shown schematic a PCB was created. The photodiode that will be placed 90 degrees to the laser will have a higher linear amplification at the output of the logarithmic amplifier than the photodiode placed 180 degrees to the laser. This decision was made because of the little amount of light that it will be detecting. The final range of voltages for each photodiode range from 0.5V-2.36V for the 180 degree photodiode, and 1.4V-7V for the 90 degree photodiode.

Laser Diode Circuit
The laser diode needs an amplification circuit and protection circuit in order to drive the laser at a specified light and to protect the laser diode from being damaged due to over voltage or over current. The IC-WJ laser driver chip has been chosen due to its watchdog protection circuitry.

After much consideration, the team decided to go with a pre-built laser module with a built in lens. This would limit the amount of PCB's we would need to incorporate to the final design, as well as eliminate the need of calibrating a laser diode to a lens.

=Casing=



Fabrication
All aspects of fabrication were completed with University of Idaho facilities and tools. The motivations behind this decision include: constantly accessible materials, free of charge, and minimal wait time. The Making, Innovating, and Learning Laboratory (MILL) was utilized to 3D print all prototypes on a LulzBot Mini and/or a Taz 1 with High Impact Polystyrene (HIPS) filament. This resource could be used once a week with semi-accurate results, and prints could be dropped off and picked up within the same day. Unpreventable distortion did occur occasionally, but benefits far outweighed this con. University of Idaho tools such as a dremel, drill press, soldering iron, etc. were also used throughout the entirety of the project.

3D Design Software
TinkerCad, a 3D modeling program supported by Autodesk, was chosen as the software to create all prototypes for the project. Since the tools are very basic and easy to apply, it was the most ideal program for a beginner. Tinkercad was also chosen because the project only required very basic geometric structures, such as squares, rectangles, and circles. In addition, files could be easily exported as both .stl and .obj for the users convenience.

Cuvette Holder
The project required a module to encompass a cuvette laser, temperature sensor, and multiple photodiodes. However, the very first prototype was only made for a laser module with large windows, to visually see the cuvette, for the very first Snapshot Review. Numerous revisions were made throughout both semesters to account for the deformities from the 3D printer at the MILL. In the beginning of the project, threads for a lens to focus a laser diode were designed but failed to print out. Self-tapping the hole could have fixed the issue, but it would have required purchasing a metric tap. Therefore, a unanimous decision was made by the team to abandon the laser diode and pursue a laser module. After making this change, perfect circular holes, proved to be another major issue for the University of Idaho printers. Therefore, a dremel was used to perfect the inaccuracies. An additional hole above the laser module was added to the design to account for a temperature sensor. The final design required the photodiode PCBs to be attached directly to the module, so everything could be mounted properly within the case. The laser module and temperature sensor were added through press-fitting, which secured them effectively.

Final Case
The case was designed into two seperate sections, one to attach the MCU development board and another to mount the final revision of the cuvette holder to. The entire design of the case surrounded around minimizing the entire space used to make the device as handheld and portable as possible. The lid of the case was designed first to properly fit the dev. board screen and gain access to the SD card slot and USB Mini port. The mini port allows for both external power and programming and the SD card for file exportation. The lid was also created to remove the cuvette top with ease. 5mm square corners were added in future revisions to account for screw holes. The bottom half of the case also had the square holes to attach brass inserts, so the two pieces could be linked together. To ensure the wireless QI receiver worked properly a 1mm high pocket was added to the bottom half of the case. Numerous revisions were also done on both parts of the casing, due to inaccuracies from the printer. The DC/DC boost converter PCB and Protoboard were mounted to the base of the case with mounting pillars, like the ones used with the cuvette holder. The QI receiver was simply super glued directly to the plastic, and the battery was tied down to the top of it with elastic material. The final was both handheld and portable and fit all components in an easy to access manner.

Thermal Sensing
=Final Design=

Team Information
=Additional Documentation=

Project Planning

[Team Contract] [Product Requirements] [Project Schedule] [Budget]

Meeting Minutes

[September 11, 2018] [September 20, 2018] [September 27, 2018] [October 4, 2018] [October 11, 2018] [October 18, 2018] [October 25, 2018] [November 1, 2018] [November 8, 2018] [November 29, 2018] [January 17, 2019] [January 24, 2019] [February 7, 2019] [February 21, 2019] [February 28, 2019] [March 7, 2019]

Presentations

[Design Review] [Engineering Release Review]

Client Interview

[Interview September 27, 2018]