Overview of Status of Analysis, Design, Fabrication, Tests, etc.
Tests
K value test
Found through empirical testing that best K value was 0.29 kg/m or lower
Static spring force analysis was performed
Most springs that are readily available had a k value of 0.29 kg/m, anything smaller took longer processing and had an expensive per-unit price
Cantilever beam test
Used to determine the sensitivity of using a longer moment arm on the strain gauge using different lengths and different materials
Delrin (5.75 inches) and aluminum alloy (## inches) moment arms
Gain selection
Met with Steve Roberts to get suggestions on instrumentation amplification and gain. We developed a circuit using a Texas Instruments INA114 Instrumentation amplifier.
Gain played a major factor. Optimal gain through physics on the spec sheet, then testing compared to gains above and below mathematically optimal gain.
Determined an optimal gain of 750 for a 3-V voltage span.
Accomplishments from Previous Week
Decided to move forward with doctor-applied force instead of motor-applied force
Avoids vibration issues, adding unnecessary weight and complexity, power and current restrictions
Agreed of implementation of a spring and tested loads against lightweight static masses
Will help reduce noise
Amplification and ideal gain
Determined an optimal gain of 750 for a 3-V voltage span
Goals for Next Week (list names after each item). Use specific and measurable objectives.
Design proposal presentation
Prototype of internal workings with spring and load cell setup
Order metal rods/probing device
First must design mounting and work on integration with load cell setup
Design for the chassis system
Prototype with acrylic, but ideally with aluminum
Order LCD shield
2-pin max on the Arduino, order by Friday
Determine lower limit requirement for sensation detection
2-gram minimum
Sponsor Comments from Last Meeting and Actions Taken to Address these Comments (indicate date of comments and if via email or in person
Meeting, 4/9/2013
Force application needs to be addressed
After demonstrating with the force gauge, Dr. Hofflich was able to see some issues involving sensitivity
Addressed concern for time needed to perform test
The range could be simplified if needed and this will reduce the amount of time needed to complete the test
Research the tonometer (eye pressure sensing machine)
Not very accurate in general based on research on more than one device
Instructor Comments from Last Meeting and Actions Taken to Address these Comments (indicate date of comments and if via email or in person)
4/4/2013
Key decisions
Force application
Doctor-applied or doctor-controlled force
What we will use to measure force
Pre-made load cell with spring
Instrumentation amplification
What kind of op-amp and gain
Texas Instruments INA114 with gain of 750 based on 2 V reference voltage
Elongated length
Tests with two materials extending from the load with different lengths
Delrin and aluminum
Not appreciable increase for the output received, at least for the load cell used
Risk reduction tests with springs
Tested multiple springs, found optimal k value
increasing the difference between the outer and inner diameters reduces the k value (depends on material and thickness of material)
How to determine an optimal diameter situation and should we cater to commercially available springs?
Risks and Areas of Concern
Force application
Implementing a spring reduces the impact noise and allows for more control, but it is not enough. It is good, but not good enough.
We are still planning to incorporate the spring into the design, but a rack and pinion is an option.
Time requirement to perform tests
One of the requirements is that the test still take roughly 1 to 2 minutes
Sensor range
We need to determine if we are able to find a sensor with a smaller range that is delicate enough so that we can use the Arduino or other microprocessor
Potentially use reference voltages to dissolve the range, currently we are able to get 1.66 gram/unit with the Phigetz kitchen scale gauge
Schedule
Prototyping and building
Budget (list amount spent and amount remaining)
Sets of springs total: $17.10
Total cost of supplies; $85.59
Budget remaining: $1414.41
Progress on Report and Webpage
Uploaded individual component analysis