MAPE Extension Design

[HannahSi]

The purpose of MAPE turbidity sensor extension is to hold the non-reflective material in front of the camera lens without compromising the diffusion of particles in front of the lens. Our initial plan was to hold the material at the end of three prongs attached to the endoscope by a shaft collar, but for sturdiness, appearance, and ease of fabrication we have changed to a design involving two plates separated by standoffs. See next comments for details.

[HannahSi]

Below is the design with prongs we started to implement. We chose prongs made of tempered steel for durability but did not include the shaft collar yet, because we were uncertain of how to securely attach the prongs to a metal collar. (A soldered connection would not be too weak. We considered welding but preferred this to be a last resort because of the skills and experience required.) See the "drawings" folder in our repository for a CAD rendering of this design.

[HannahSi]

Sorry I never finished posting this last week. Below is the design with two plates separated by standoffs. The plate in front of the endoscope lens serves the hold the nonreflective material, while the other plate serves to secure the entire contraption to the endoscope. A slot was cut in the latter plate so that the cable can pass through and the plate can be glued to flush the base of the endoscope. (not yet glued, and nonreflective material not yet attached)

[HannahSi]

Fabrication notes on the standoff design:

• This design requires at least 4 standoffs, depending on their length, each with a female and male end. Two need to be cut about 1 cm from the ends to obtain just the female and male ends, and the other two are placed in between the two plates (they are the main standoffs). The latter are secured by screwing into or being screwed into the shortened female and male ends. (Alternatively, the former two can be replaced with screws and bolts if you can find the right size.)
• Next time use PVC or other plastic instead of metal for the two plates. It would be cheaper, and perhaps the plates would not become so hot when cutting.
• The standoffs we used had male ends of 5/32".
• Instead of using an endmill to cut a slot for the endoscope cable, we simply drilled a hole, then cut out the slot with a bandsaw. The diameter of the endoscope cable is 11/64".

[HannahSi]

To calculate the dimensions of the plate, we first determined the optimal distance of the endoscope from the non-reflective material. This distance should minimize the area of material required (thus minimizing the overall diameter of the sensor) as well as the light reflected from the material. (A greater distance decreases light reflected but increases viewing area.) For our endoscope and with matte rubber as the non-reflective material, we estimated 1.5 cm.

We then used this distance and the FOV (field of view) angle of the endoscope, 60º, to calculate the width of the viewing area: 1.5 / √3 * 2 = 1.732 cm.

Adding extra length on two sides for the standoffs and a few millimeters of buffer space, we chose to cut a plate of 3.5 x 2 cm to hold the non-reflective material. The plate attached to the base of the endoscope has the same dimensions for symmetry.

[HannahSi]

We're currently trying to figure out the best way to attach the base of the endoscope to the metal plate. We waiting on a silicone sealant we used to cure (harden), but it's white and thick and looks quite messy. Our research advisor suggested applying a ring of hot glue around the metal plate as a secure and more attractive sealing method. More to come.

[HannahSi]

I think any way of adhering the endoscope base to the metal as shown above (with the base sitting on the metal) is prone to detachment. Right now, the metal is hot glued around the body of the endoscope, which is probably a more secure method. This required first drilling a hole large enough for the whole endoscope to slide through.

[HannahSi]

The MAPE extension has been finished, but it can be improved so that the endoscope is held more perpendicular to the non-reflective material. Right now, it is slightly angled because the hole in top metal plate was 1/16" too large, and the hot glue is not completely secure/rigid, so the endoscope is able to tilt. The problem can be solved with a smaller hole, perhaps thicker metal/PVC plates, and/or shorter distance between the two metal plates (so the tilt is less perceptible).