A Resource Guide to Wearable Device Sensors

This is Part 4 of our series, “The Future of Medical Technology: Wearable Devices.” Read Part 3 here.

Last week’s blog covered the many wireless communication options available for wearable devices.  With these wireless options available, one must be able to provide useful data for transmission.  This blog will cover an array of sensors commonly used in wearable devices for both commercial and medical purposes.  These sensors are split into two main categories, motion-tracking sensors and bodily function sensors.

Motion-Tracking Sensors

Microelectromechanical Systems (MEMS) Devices

MEMS devices utilize modern semiconductor fabrication processes to build devices that measure real-world forces.  These devices contain structures on the scale of micrometers that are created so they can move within the device.  Taking advantage of Newton’s three laws of motion, these moving structures can be used to detect the direction and magnitude of the device’s acceleration.  Additionally, using specific materials to produce these structures can make them very sensitive to magnetic fields, which allow the device to provide measurements that indicate its orientation.

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Accelerometers – The most common MEMS sensor, accelerometers are capable of sensing gravity as well as linear accelerations.  MEMS devices and can be used for a variety of wearables that measure motion, ranging from walking (a modern-day pedometer) to monitoring sleep patterns to detecting seizures.  Accelerometers used in wearable devices are generally specified by the maximum acceleration the device can measure. Common values for this maximum acceleration range from 2g to 16g.

Gyro – In a similar way that an accelerometer can measure linear accelerations, a gyro can measure rotational accelerations.  The rotational measurements alone are generally not as useful as the measurements obtained from an accelerometer, but when used in conjunction with an accelerometer, each device can correct minor errors in the other.  With these corrections, a more precise description of the user or patient’s movements can be determined.  Accelerometers used in wearable devices are typically specified by the maximum rotational acceleration the device can measure. Common values for this maximum acceleration range from 250 deg/s to 2000 deg/s.

Magnetometer – A magnetometer measures magnetic fields, primarily the magnetic field of the earth.  In other words, a magnetometer is the 21st-century version of the compass.  While accelerometers and gyros sense movement in 3D, these measurements are generally in relation to an unknown starting point.  A magnetometer can be used to fix these relative movements to the coordinate system of the earth, helpful in detecting the absolute orientation of a user or patient to monitor movements such as when an elderly person has fallen.

These MEMS sensors can be as small as 2mm x 2mm x 1mm individually or all three can be integrated into a single package as small as 3mm x 3mm x 1mm.  In addition, the power consumption of these devices varies depending on the data acquisition speed but can be as low as only a few micro-amps.  These specifications make the sensors very suitable for including in small wearable devices in which weight and power consumption are a high priority.

Global Positioning System (GPS)

GPS is a satellite navigation system that provides location and time information in any location where the receiver has an unobstructed line of sight of four or more GPS satellites. The receiver gathers signals from different satellites, calculates the distance to these satellites, and uses this information to deduce its location. GPS sensors in phones are commonly used for navigation. GPS sensors in fitness devices can track the distance an athlete runs. GPS sensors in devices worn by patients can provide their location in an emergency. The only concern that needs to be addressed while using a GPS receiver in a wearable device is the power consumed because battery power conservation is critical in devices that monitor safety and detect medical disorders.

Bodily Function Sensors

Heart Rate Sensors

Tracking a user’s heart rate is an essential feature in most of the fitness-related wearable devices.An effective heart rate-sensing technology that can be used in wearable devices measures the electrical activity of the heart. Bio potential signals such as ECG can be measured using a capacitive sensing method in which the skin and the electrode attached to it form the two layers of a capacitor. The major challenge in using electrodes is the motion artifact introduced under movement. This can be prevented by improving the conductivity between the skin and electrodes. Chest strap heart rate monitors make use of skin electrodes to record the heart rate and transmit it to a receiver. Chest straps have given way to biometric shirts and jackets, which have electrodes woven inside the fabric.

Pulse oximetry is another non-invasive method used in wearable devices to measure a user’s O2 saturation, heart rate, and blood pressure. It involves a light source, usually LEDs, emitting light into the tissue and a photo detector to collect light reflected or transmitted from the skin. The amount of light absorbed by the hemoglobin in the blood allows the device to detect the O2 saturation levels, while the blood volume changes in the blood vessel reflect the heart rate and blood pressure. These optical sensors are used in smart watches, fitness bracelets, and earphones to monitor the heart rate of a user.

Temperature Sensors

Another sensor that can be easy integrated into many wearable devices is a temperature sensor.  Used in a variety of situations, temperature sensors can provide early warning for heat-stroke in athletes.  A second interesting application is the use a woman’s basal body temperature, or the lowest temperature the body reaches during sleep, as an indication of fertility. A wearable device capable of routinely tracking body temperature is much more accurate than previously used manual tracking methods.

Most wearable devices do not require all of these sensors in a single product.  Need help selecting the right sensors for your application? Contact Anuva! We have years of expertise integrating a variety of sensors into products and can help you get the best performance your device.

By Anuva
Anuva