How sensors work in smartwatches for fitness tracking?

Smartwatches are popular wearable devices that are divided into two broad categories: phone companions and fitness trackers.

The phone companion acts as an extension or mini clone of the wearer’s smartphone, staying connected via Bluetooth. Common features include email notifications, calling, messaging, social media integration, calendar management, music control, mobile payments, and voice assistant functionality. While certain fitness tracking features might be present, they’re not the primary focus.

The fitness trackers are for health and fitness monitoring. Key features include heart-rate monitoring, step counting, sleep tracking, calorie counting, workout tracking, and GPS navigation. While some models may also offer phone companion attributes, they’re secondary features.

Fitness trackers rely on many sensors for their core functioning. These sensors are integrated into the watch and consistently exchange the user’s health and fitness data within an Internet-of-Things (IoT) architecture.

In this article, we’ll review various smartwatch sensors and discuss some of the more popular ones used in fitness watches.

The sensors

A fitness tracker is designed to monitor a user’s physical activity and overall fitness, providing insights into their progress. Typical features include step counting, distance tracking, sleep analysis, calorie tracking, heart rate monitoring, blood oxygen, and body temperature monitoring. These functionalities are made possible by various sensors integrated within the device. Multiple sensors work in tandem to deliver accurate and comprehensive fitness data.

Here’s an overview of the esential sensors typically found in fitness trackers:

Accelerometer
Gyroscope
Heart rate monitor
GPS
Altimeter
Compass
Ambient light sensor
SpO2 sensor
Body temperature sensor
GSR sensor

Accelerometer

The accelerometer is a sensor that measures acceleration, detecting motion in one, two, or three dimensions. In fitness trackers, it’s used to monitor speed, direction, and orientation changes. Its primary function is step counting by analyzing acceleration and deceleration patterns associated with walking or running. It can also estimate the distance traveled by multiplying the number of steps by an average step length.

Additionally, the accelerometer can predict the type of activity the user is engaged in, such as walking, running, or cycling, based on movement patterns. Beyond activity tracking, the accelerometer is instrumental in monitoring sleep duration and quality by detecting changes in movement while the user sleeps.

Popular accelerometer sensors used in fitness trackers include:

STMicroelectronics LIS3DH, a low-power, high-sensitivity accelerometer
Bosch BMA250, a widely used accelerometer known for performance and reliability
InvenSense MPU6050 combines a three-axis accelerometer with a three-axis gyroscope, making it ideal for accurate orientation and motion tracking
Analog Devices ADXL362 stands out for its high resolution and low power consumption

Gyroscope

A gyroscope measures angular velocity, detecting how fast an object rotates or changes orientation. They’re essential in fitness trackers for determining a user’s orientation during activities such as cycling or swimming. They can also detect subtle movements that accelerometers might miss, like arm swings or wrist rotations. The gyroscope complements the accelerometer in determining the type of activity, whether walking, running, cycling, or swimming. It also stabilizes the fitness tracker’s display and can adjust the camera’s orientation.

Popular gyroscope sensors used in fitness trackers include:

InvenSense MPU6050 combines an accelerometer and gyroscope, providing a comprehensive motion detection and activity monitoring solution
Bosch BMA250, an accelerometer with a built-in gyroscope and known for its versatility
STMicroelectronics L3G420D offers high sensitivity and low noise, making it a popular choice for fitness trackers that rely on precise orientation data
Analog Devices ADXL355 offers high resolution and low power consumption

Heart rate monitor

A heart rate monitor measures a user’s pulse, and fitness trackers typically use wrist-based sensors. These sensors track cardiovascular health, providing insight into workout intensity by measuring fluctuations in heart rate. They also monitor recovery by assessing how quickly the heart rate normalizes after exercise. Heart rate monitors can also help detect irregular heart rate patterns, potentially indicating underlying health concerns. While the data from fitness trackers is not medical-grade, it can still serve as a helpful reference for diagnosing potential issues.

Three primary types of heart rate sensors are used in fitness trackers: ECG, optical heart rate, and ballistocardiography (BCG). ECG sensors are more expensive and detect heart rate by measuring the heart’s electrical activity. Optical heart rate sensors are more common, and green light is used to track blood flow changes. BCG sensors measure the mechanical vibrations of the heart, allowing heart rate detection even without physical contact.

Popular heart rate sensors in fitness trackers include the Texas Instruments AFE4400, Maxim Integrated MAX30100, and Valencell OPT3001 — all of which are optical sensors.

GPS

The GPS receiver in a fitness tracker detects and tracks a user’s location. It measures the distance covered during activities like walking, running, cycling, and swimming. GPS also aids in mapping routes and visually representing the path taken. It can be used for navigation, location-based challenges, and social sharing of activities.

Additionally, GPS helps calculate pace and speed during workouts. Some commonly used GPS receivers in fitness trackers include the MediaTek MT3339, U-Blox NEO-M8P, and Sony GNSS Solutions. The Qualcomm Snapdragon W5 wearable platform also has an integrated GPS receiver.

Altimeter

An altimeter measures changes in altitude or elevation. In fitness trackers, altimeters are for hiking, climbing, or running activities to track elevation changes, total ascent, and descent. Some devices provide altitude alerts or emergency assistance features. Altimeters can also help monitor how elevation impacts heart rate and body temperature. Two main types of altimeters are used in fitness trackers: GPS-based altimeters and barometric pressure sensors. GPS-based altimeters rely on location data to estimate altitude but may be less accurate when GPS signals are weak. Barometric pressure sensors, which measure atmospheric pressure, are more precise. Popular altimeters include the Bosch BMP280 and STMicroelectronics LPS25HB, both of which are barometric pressure sensors.

Compass

The compass, or magnetometer, determines the direction of fitness watches and other wearables. It becomes advantageous when navigating unfamiliar terrain, mainly with no visible landmarks. The compass helps orient the user and assists in route planning by detecting which direction they are facing. This is particularly helpful for outdoor activities such as hiking and trekking.

A fitness smartwatch may include a magnetometer or an electronic compass. The more commonly used magnetometer aligns with the earth’s magnetic field to determine direction relative to the north. An electronic compass, which combines a magnetometer with additional sensors, provides more accurate directionality.

Popular compass sensors include the AKM AK8963, Honeywell HMC5883, and STMicroelectronics LSM303DLHC. The AKM AK8963 and Honeywell HMC5883 are magnetometers, while the STMicroelectronics LSM303DLHC is a digital compass that integrates a magnetometer, accelerometer, and gyroscope for enhanced motion detection and directional accuracy.

Ambient light sensor

An ambient light sensor detects the intensity of light in the surrounding environment. In fitness trackers, this sensor automatically adjusts the display’s brightness based on the ambient light conditions, improving both battery efficiency and screen visibility. It also plays a role in preventing accidental screen activation by locking the display when the tracker is not in use.

This type of sensor can also distinguish whether a user is indoors or outdoors. Some commonly used ambient light sensors in fitness trackers include the Maxim Integrated MAX44000, Vishay BH1750, Texas Instruments TPS62571, and Rohm BD34301GS.

SpO2 sensor

The SpO2 sensor, a pulse oximeter, measures blood oxygen saturation, indicating the percentage of red blood cells carrying oxygen. In smartwatches, SpO2 monitoring can help detect early signs of health issues such as respiratory problems, sleep apnea, or heart disease. When paired with an altimeter, the SpO2 sensor tracks how the body responds to changes in altitude, which is particularly useful during high-altitude activities. It also helps monitor sleep quality, as low oxygen levels during sleep may signal fatigue or restless sleep.

Common pulse oximeter sensors in fitness trackers include the Valencell OPT3001, Maxim Integrated MAX30100, Texas Instruments AFE4400, and Honeywell HMC5883.

Body temperature sensor

The temperature sensor in a fitness watch monitors a user’s body temperature, providing insights into potential illnesses or infections. Elevated body temperature can indicate fever, and continuous temperature tracking can help monitor changes related to different sleep stages. The sensor can also identify stress, as physical or emotional stress increases body temperature.

Popular temperature sensors used in fitness trackers include the Analog Devices AD8232, Texas Instruments TMP007, and Bosch BME680. The TMP007 is a high-precision infrared thermometer, while the AD8232 measures high-resolution temperature. The Bosch BME680 is a versatile sensor that combines gas, humidity, and temperature sensing, useful for body temperature monitoring and environmental tracking.

GSR sensor

The galvanic skin response (GSR) sensor, also known as a skin conductance sensor, measures the skin’s electrical conductance. In fitness trackers, this sensor detects changes in skin conductivity, which are often associated with stress levels. By monitoring these variations, it helps users identify and manage stress.

The GSR sensor also plays a role in assessing sleep quality, as skin conductance changes during different sleep phases. Additionally, it can monitor the user’s arousal level during physical activities, optimizing performance by gauging the body’s physiological response.

Popular GSR sensors in fitness trackers include the Maxim Integrated MAX30100, Analog Devices AD8232, and Texas Instruments ADS1015. While the MAX30100 is primarily used for heart rate and SpO2 monitoring, its built-in red LED can also measure skin conductance. The AD8232 is known for its low-noise, high-sensitivity performance, while the ADS1015 is a 16-bit ADC commonly paired with GSR sensors to measure output.

NFC chip

NFC (Near Field Communication) chips in fitness trackers are primarily used for contactless payments, allowing users to make quick transactions directly from their wrists. These chips also enable data transfer between the fitness tracker and other devices, such as smartphones or computers.

In addition to payments and data sharing, NFC chips pair devices and control access to secure areas. Some widely used NFC chips in fitness trackers include the STMicroelectronics ST33H715B, NXP NXP7150, Broadcom BCM20795, and Infineon SLE6600. The choice of NFC chip depends on several factors, including cost, security features, performance, and design requirements.

Conclusion

Fitness trackers represent a signficant category of smartwatches, distinct from phone companion devices by their sensor-driven functionality. Unlike phone-based smartwatches, which act as miniature versions of smartphones, fitness trackers are focused on integrating a range of sensors. These include accelerometers, gyroscopes, heart rate monitors, pulse oximeters, altimeters, body temperature sensors, GSR sensors, ambient light sensors, GPS, and compasses. Each sensor plays a unique role in enabling various fitness tracking features, and in many cases, they work together or share data to generate more meaningful health and fitness insights.