Wearable Sensor Systems Flashcards
(38 cards)
Wearable technology
Any technology you can wear
Wearable sensors
- Often synonymous with the term wearable technology
- Measuring some physical quantity
Examples of measurable physical quantities
Movement, force, light, temperature, chemicals
Examples of Sensors
Inertial sensors, force sensors, light sensors, thermocouples, chemical sensors
Measuring Movement
- Inertial Sensors - Utilize the principle of inertia
- Variety of applications - drones and vehicles, smart phones, electronics, industry measurements, human movement
Application of inertial sensors for human movement
- Activity trackers
- Gait analysis
- Balance assessment
- Fall detection
- sleep analysis
- sport performance
What are movement constructs
Physical activity
Mobility
Gait
Metrics
Things that measure movement constructs
PA: step counts, activity tracking, sedentary time, activity intensity
Mobility: Timing functional tasks, turning velocity, balance
Gait: gait speed, stride time, impact accelerations, knee flexion angle
Primary types of inertial sensors
Accelerometer (linear acceleration) and gyroscope (rotational movement)
- Integrated together as an inertial measurement unit (imu)
- may also integrate magnetometer or microcontroller unit
Newton’s laws of motion
- First Law: Object remains at rest unless acted on by a force
- Second Law: F=ma
- Third Law: Equal and opposite reaction
How do accelerometers work
- Case containing proof mass (known)
- Force exerted by proof mass movement during acceleration is measured
- m and F used to calculate a
- at rest measured as 9.81
- during free fall measured as 0
- Aligned orthogonally to measure acceleration in 3D
How do you calculate the resultant acceleration
SQRT (ax^2+ay^2+az^2)
How do you convert voltage to gs
- Know resting data is 1g and upside down -1g
- change in g over change in mV = conversion factor
Range
Min/max values that can be accurately measured
Sensitivity
How much the output changes in relation to the input
- How well you can measure changes in movement
How are range and sensitivity related
Inversely related
Linearity
The output does not follow a straight line with input
- Change in voltage is not linearly proportional to change in acceleration
- Usually more common near end-range
- Try to stay with away from end-ranges
Tips when selecting an accelerometer
Select a sensor that offers the greatest sensitivity within a safe range for your movement/placement
GENERAL RECOMMENDATIONS
- +/-2g lower back during walking or ADL
- +/-8g lower limbs during walking or ADL
- +/-16-32g lower limbs during running
- +/- 100-200g high impact activities
Magnetometers
An electrical compass
-Work based on the hall effect (voltage difference produced across an electrical conductor due to a perpendicular magnetic field
- Uses earth’s magnetic field to measure heading
- Outputs can be significantly distorted by other magnetic fields, electrical signals, or devices
What can linear acceleration and angular velocity be used for
- Assessing overall amount of movement or type of activity
- determining timing of gait events
- segment motion, pelvic stability or between limb asymmetry
- Impacts magnitudes
- Can be integrated to determine location or position and segment or joint angles
Using accelerometers as tilt sensors
- Uses gravity and simple trigonometry
- Words very well when there is little to no movement
Sampling Frequency
- The number of samples taken in 1 second
- Balance amount of data collected with enough to not miss events and get full pattern view
What are sensor reactions to dynamic changes in input tied to?
- The type of input
- Step, ramp, impulse, sinusoidal - The type of mechanical system
- Zero, first, second order
-n+1 coefficients characterized the system
Input types
Changes in the input, where we can measure the time response of the system
1. Step input: Sudden change in input that is held constant
2. Ramp input: linear increase in input
3. Impulse input: sudden spike in input
4. Sinusoidal input: sine wave input
