Select Product Concept(s)
Step 1. Clarify The Problem
System concept overview:
Areas of interest on the human body for the placement of these sensors, in this project, are:
1. Spine: 3 possible locations-Thoracic, Upper Lumbar, Lower Lumbar.
2. Arm: Shoulder-Elbow-Wrist
3. Leg: Hip-Knee-Ankle
Customer requirements for sensors:
1: The device should obtain and measure the angles formed by a person's lower back and limbs with at least one or three degrees of freedom.
2: The accuracy of angles should ideally be +/-1 degree.
3. A portable motion tracking system.
4. Sensor readings are not affected by external factors (skin, bumps, etc)
5. The sensor may not interfere with any electromagnetic devices, such as, pacemakers, implanted defibrillators, etc.
We explored, in depth, those sensors which meet our initial requirements: measures angles with one, three, six degrees of freedom accurately, be light, and small.
This section goes into the details for each sensor we chose from analysis of many different products and technologies. Some of the many possibilities we are considering are accelerometers, gyroscopes, magnetometers, potentiometers (linear, rotary, etc.), and also any available digital options. We rule out magnetometers due to above mentioned requirement about electromagnetic interference.
For the feasibility analysis, we are compiling the specifications for each sensor provided by the datasheet/ website/ vendor/ company and compare them to the ideal values given from the customer in the document of Engineering Specs. Additional notes about the pros and challenges of the sensor, and how it will be possibly interfaced with the MCU (this will tie into the MCU and interface feasibility) will be given.
Sensor Concept Selection Research
DE-ACCM3D2 Buffered +-2g Tri-axis Accelerometer
- Triple axis +-2g sense range
- Up to 720mV/g sensitivity
- Operating voltage 3.5V to 15V (onboard regulator)
- 3.3V regulator can power external microcontroller
- Output short protected
- Standard DIP-16 form factor
- Draws 0.9mA
- Cost: $36.50
ADXR5614 Gyro and ADXL203 Tilt Sensor IMU Board
- Low Voltage Operation: 5V Regulated
- Highly Accurate/High Sensitity
- Rated Analog Output
- Self Test For Both Sensors
- 25mm x 20mm x 2mm
- Cost: $125.00
LIS302DL Smart Digital Output "Piccolo" Accelerometer
- 2.16 V to 3.6 V supply voltage
- <1 mW power consumption
- +-2g/+-8g dynamically selectable full-scale
- I2C/SPI digital output interface
- Embedded high pass filter
- 10000g high shock survivability
- Cost: $9.30
SEN-08606 Resistive Response Flex Sensor
The flex sensor works using the concept of a voltage divider. As the flex sensor is bent in one direction the resistance gradually increases. When the sensor is bent in the other direction its resistance will gradually decrease. We have equations to be able to convert the raw data into angle that correspond to it. The image below is how the circuit will be assembled. The interface circuitry will most likely be incorporated near the MCU. This is already used in flex-gloves, robotics, bio-mechanics, fitness, medical, and gaming devices for measuring movement and angles. This will be a perfect match for one degree of freedom measurement- elbow, knew, even other joint if the need only calls for one degree of motion information. For the spine, we can possibly use three flex sensors that form a triangle around the region, since this is a one degree of freedom sensor.
- 1 DOF (Elbow/Knee Joint)
- Variable Lengths Available
- Light Weight
- Robust Design (> 1 Million Life Cycles)
- Easy To Correlate to Relative Angle
- Very Inexpensive
- Cost: $11.95
Atomic IMU - 6 Degrees of Freedom
- 6 DOF
- 10 Bit ADC on Board
- Operating voltage 3.4V to 10V (Onboard regulator)
- Uses MMA7260Q accelerometer
- Draws 24mA
- Cost: $124.95
6 Degrees of Freedom Razor - Ultra-Thin IMU
- 6 DOF
- All On-board Filtering for Analog Outputs
- Operating voltage 3.3V (MCU Rail)
- Uses ADXL335 accelerometer
- Low Power Consumption
- Ultra-Thin(1.5cm X 3cm)
- Cost: $89.95
Arduino Mega Microcontroller
- $50 board + $28 microSD shield + $10 (est) USB A to USB B cable = $88
- Atmel ATmega1280 MCU
- Completely free and open source (software and hardware)
- 2x 8-channel, 10-bit ADC (0-5V) [total of 16 ADC inputs]
- 54 digital I/O
- 4 UART
- 16 MHz clock
- 5 VDC power, MCU uses 500 uA
- Dimensions: 4" x 2.1" x 0.75"
- 2500 mAh and 1000 mAh Battery Packs!