User:Medelen8/ENES100/Grip Strength Measurement

=Problem= In college anatomy and physiology courses (e.g. BIOL-203 at Howard Community College), students learn about the stimulation, contraction, relaxation and fatigue of muscle tissue. Specifically, students learn the physiological relationship between muscle fiber length and tensile force. The relationship can be demonstrated experimentally by measuring biomechanical forces at various muscle lengths. In particular, the hand's grip force can be measured at different lengths of the forearm muscles, showing that grip strength is maximized at a specific optimal muscle length. Biology faculty could demonstrate this phenomenon with the aid of a simple grip strength measurement device, to be used by students during an in-class activity.

=Conceive= Project Goal: Develop a grip strength measurement device for use by college faculty and students in a physiology course.

Background Research
Physiology of muscle contraction
 * Muscle fibers only generate tensile forces (i.e., they can only pull, not push).
 * The force generated is related to the muscle's length, according to this relationship. The curve of muscle tension versus length shows an optimal length, with decreased force for longer or shorter fibers:

Data on grip strength - A wealth of data is available on grip strength measurement from the fields of ergonomics and biomechanics.
 * Human strength data, used for ergonomic design.
 * Human performance data, published by NASA based on testing of military personnel and industrial workers (for grip data, see Figures 4.9.3).

Benchmarking
The following products exist for measuring grip strength: The existing products typically measure grip force up to approximately 200 lb. This maximum should not be necessary for the educational product, as it will not be used by professional athletes. Of the products reviewed, options 2 and 3 are feasible solutions in terms of cost. It is unclear how/if option 2 would work at different wrist orientations. All of these devices feature quantitative output, instead of the preferred qualitative output. Most appear easy to use, but some do require calibration, adjustment for different hand sizes, and/or understanding of the electronic user interface (buttons, screen, etc).
 * 1) Hydraulic hand dynamometer used by physicians and physical therapists.
 * 2) *Not designed for educational use.
 * 3) *Most expensive option, at $210.
 * 4) *Features a "standard" grip diameter of 2.5 inches.
 * 5) *Force cannot be observed by person holding the device (until after the test is complete).
 * 6) Electronic Hand Grip Dynamometer used primarily by athletes to measure and record maximum grip strength.
 * 7) *Low cost ($25) compared to some other options
 * 8) *Digital display of grip force (quantitative output)
 * 9) *Ergonomics unclear for different wrist orientations
 * 10) *Display not visible for certain wrist orientations
 * 11) Squeeze Bulb Dynamometer, measuring grip strength using air pressure (psi)
 * 12) *Moderate cost of $60
 * 13) *Quantitative output, but in psi, not a unit of force
 * 14) Smedley Hand Dynamometer, purely mechanical operation (springs, etc)
 * 15) *Costly option at $160
 * 16) *Measures highest max force (220 lb) of all options

Requirements
The grip strength measurement device must:
 * Cost less than $50.
 * Fit all hand sizes (from 10th percentile female to 90th percentile male)
 * Be simple to use, requiring minimal verbal instructions (no written instructions).
 * Be self-contained, requiring no wired or wireless connections for power, data, etc.
 * Measure grip forces ranging from 0 to 100 lbs.
 * Display the grip force (qualitative display preferred) with sufficient accuracy to show differences at different hand orientations.
 * Display the grip force, such that it is visible at any hand orientation.
 * Be durable enough to survive a drop from table/desk height (4 ft).

=Design= This design includes mechanical, electrical, and software engineering. Mechanical design is required for the physical housing, including the grip design and electronics enclosure. Electrical design is required to develop circuitry to connect sensor(s), indicator(s), the microcontroller, and power (battery). Software is required to produce meaningful output, based on the sensor data.

Theory of Operation
The device has been designed to work in the following manner. The user will grip a pair of parallel beam-like extrusions, which will be flexible enough to bend. The two beams will meet at/near the end of the beams, compressing a thin force sensor between the beams. The sensor reading is sent to a microcontroller a displayed visually to the user via an LED array or other method (e.g. LCD screen).

Mechanical Design
The physical dimensions of each component drive the enclosure design. The housing/enclosure must hold all components, permit simple assembly of the device, and provide access for battery replacement. Part of the device is a split cylinder, designed to fit ergonomically into a clenched fist. The force sensor is located at the end of the cylinder to minimize loading on the sensor. The sensor is only rated for loads up to 100 N (22.4 lbf), which could be exceeded with a typical grip force. The bending stiffness of the half-cylinder should be increased such that at least 10 lbf of gripping is required to make contact with the sensor.
 * 9V battery: 1.9x1.0x0.68 inches
 * force sensor: 2.35x0.73 inches with 0.5 inch diameter sensing surface. Thickness is negligible (paper thin).
 * Pro Micro: 1.3x0.7 inches
 * 2 12-LED bargraphs mounted on circuit board
 * single bargraph: 0.4x1.2 inches
 * board: 0.5x4 inches

Successive iterations of the housing design are shown below. The first version of the design was a single piece, inspired by some of the benchmarked products (see above). After printing that version, it became clear that the display window would not be visible for all wrist orientations. Version 2 consists of two pieces: a grip cylinder and angled enclosure. This version resolved the main problem of first design, and provided a more comfortable grip. Version 2 was used to create the first operational prototype of the device.

Electrical Design
The circuit (shown below) makes electrical connections between the force sensor, LED display board, battery, and microcontroller (Pro Micro). Power is supplied by a 9V battery to the raw power input pin of the Pro Micro board.

The force sensor is a two-pin device, including a 5V power connection to the Pro Micro and an output voltage pin. The force sensor is simply a variable resistor, with a resistance that decreases with increase applied pressure. A voltage divider circuit is used, with a 10,000 Ohm resistor, to send the force data (as a voltage) to pin A0 of the Pro Micro.

The LED output is controlled by the LED board, which receives data from the Pro Micro via two connections (SDL to Pin 3 and SCA to Pin 2). The LED board also requires 5V and ground connections.

Software Design
The following code was generated using the Arduino IDE and uploaded to the Pro Micro microcontroller. Note that two non-standard Arduino libraries must be installed in order for the code to compile: Adafruit_LEDBackpack and Adafruit_GFX.h. Instructions for installing these libraries is found here. In addition, drivers must be installed for the Pro Micro. Setup instructions for the Pro Micro are found here.

Parts List

 * Enclosure/housing subassembly, $5 (estimated cost of PLA filament)
 * Force Sensitive Resistor, $8
 * Pro Micro Microcontroller, $20
 * 24-LED bargraph with I2C backpack kit, $10
 * 9V battery, $1
 * Snap connector for 9V battery, $1
 * Assorted wires and heatshrink tubing

Prototype Testing
=Implement=

3D Printing the Housing
The Makerbot Replicator 2 (5th Generation) was used to print various iterations of the housing design. All versions were printed with 2 shells, 10% infill, rafts, and no supports. Future design may require supports, depending on the complexity and print orientation.

Classroom Pilot and User Feedback
The first operational prototype of this device was tested in an Anatomy & Physiology class during the fall 2014 semester. A single device was used by the entire class (passed from person to person) to demonstrate the relationship between muscle length and force.

Students, and the biology professor, noted that the device was easy to use and successfully demonstrated the intended physiological concept. There were two suggestions for improvement:
 * 1) The device could be more "sensitive" to changes in muscle length.  For most users, the difference is small (only 2 to 4 LEDs) from strongest orientation to weakest.  This difference should be increased, such that small differences in grip force correspond to large changes in the display output.
 * 2) The device needs to be more durable.  It was accidentally dropped, which broke the case and weakened soldered connections.  The device should be able to withstand a 4 ft drop with only cosmetic damage (e.g. scuff mark).

=Operate= Discuss considerations of long-term reliability, battery usage and replacement, etc.

=Demo=
 * video of prototype device in use

=Next Steps=
 * Add on/off switch to eliminate frequent opening of housing. -Marcelo,Andy
 * Add mechanical feature (e.g. snap-fit cover) to access battery for replacement. - Tyler
 * Modify housing to allow assembly of all components without clearance/fit problems. -Tyler
 * Modify code to show green/yellow/red zones in bargraph, instead of all red. -Marcelo
 * Adjust sensitivity of LED display, if needed.
 * Incorporate feedback from students and/or faculty.
 * Improve documentation by adding flowchart for code.
 * Test circuit on breadboard. Then solder circuit. - Marcelo,Andy