Final Design

Parallel Approach of Two Sensing Mechanisms: Light & Sound

Overall Design Outline

• Similar framework in appearance to the traditional exophthalmometers

• Smooth bridges to be applied on the orbital bone as the reference point

• Linear slider assembly with the mounted sensor of choice - either infrared or ultrasonic

• After acquiring the required measurement using the chosen sensing mechanism, the value is to be projected on a LED display

Design 1. Infrared Sensor

The basic mechanism of infrared sensor used for the design is to place an emitter and a receiver at a given angle as shown in the figure below. When the reflected intensity is read on the receiver, its maximum value can be precalculated given a specific distance between the device and the surface of interest. Utilizing this mechanism, the final design using an infrared sensor holds two degrees of freedom. The sensing mechanism requires to be moved towards the surface of eye as well as the horizontal movement in order to scan for the tip of cornea. Using a linear slider with motor mount as well as an encoder, these movements are closely monitored and recorded by an Arduino software. 

Angled Sensing Mechanism of Infrared Emitter and Receiver

Design 2. Ultrasonic Sensor

Unlike the previous design with infrared method, the prototype of ultrasonic mechanism only has one degree of freedom. The linear slider mount of ultrasonic sensor is to be moved horizontally in order to record the distances around the expected tips of cornea. The minimum distance read by the ultrasonic sensor is interpreted as the measurement for the cornea, and the value will be converted into the actual distance between the orbital bone and the cornea based on the reference frame. The beam collimator attached to the end of the sensor helps narrowing down the width of the output beam in order to only target a small region on the tip of the cornea (approximately 3 mm of sound wave sonic cone width).

Baumer Ultrasonic Sensor of Choice - with a beam collimator ($450)

& Test Curve of a Sample Semi-Sphere of the Sensor Above

How We Arrived to the Final Solution: 

Risk Reduction & Proofs of Principle Process

1. Laser Diode Test

• Objective: Initial risk reduction in order to identify how an optical light source will react when blocked by a solid acrylic

• Conclusion:

• Optical light source can determine the end of solid acrylic with a sudden voltage drop

• Phototransistor or line sensor will NOT meet the size constraints around the eye region

2. Infrared Pig Eye Experiment

• Objective: Second risk reduction test to confirm the usage of an infrared sensor on opaque and curved eye surface

• Conclusion:

• Each surface of pig eye slightly differs in its reflectivity, but the pattern is similar for both samples

• Ping pong ball and opaque pig eyes follow a linear fit that could be used to relate the intensity and distance of reflecting object

3. Ultrasonic Sensor Practice

• Objective: New risk reduction test to observe how sound waves react on curved surfaces

• Conclusion:

• Similar behavior from both flat and curved surfaces

• Accurate at every distance within sensor range

4. Ultrasonic Sensor Calibration

• Objective: Proof of principle in order to compare and contrast the sensing ability on flat and curved surfaces with the new ultrasonic sensor

• Conclusion:

• Both calibration curves produced the same slope and fit, with the sensing range for curved surfaces being about 40 mm as expected.

• The sensor was able to accurately map the shape of a curved target that is placed in front and moved perpendicularly