Perhaps ever since the introduction of the Dick Tracy Two-Way Wrist Radio many decades ago, the fascination with and utility of wearable devices has steadily increased (1). But today's wearables are a far cry from the creative inventions of Hollywood copywriters from an age gone by. IoT wearable devices today are powerful tools that can sense, process, store, and communicate significant information. The great leap forward in wearable devices is not only the result of its innovative technology, but also the applications they now can provide such as patient monitoring, wellness/sports/fitness, entertainment, and other forms of computing. But all wearable devices today have one thing in common: they all use sensors. And there are all kinds of IoT wearable device sensors available today, including temperature, UV, proximity, heart rate, motion and many others. This learning module is an introduction to some of the common types of sensors used in IoT wearable devices today.
(1) Of course, this statement is the subjective inclination of the author of this learning module. Perhaps the reader may find his/her fascination with wearable devices from The Jetsons' Promotional Wrist Watch or the Star Trek Communicator or some other très chic device. If you are so inclined to evangelize about your preferred technological inspiration, please leave your comments below.
The objective of this learning module is to provide you with basic knowledge of sensors used in IoT wearable devices. You will first review some of the main concepts of sensor technology and then get an overview of the approaches to human body sensing. In the last section, you will learn about the main types and characteristics of sensors for wearable IoT devices.
Upon completion of this learning module, you will be able to:
Review sensor essentials covered in Sensors I
3. ReviewBack to Top
In the first Essentials Sensors learning module, the definition of a sensor was presented, as well as the classifications and characteristics of IC sensors. Let's revisit some of the important terms from Sensors I that are applicable to this learning module:
Categories: There are two main categories of sensors: simple and complex. Simplex sensors typically have a sensing function only, while complex sensors can have both transduction and sensing functions due to the integration of signal conditioning, A-to-D conversion and other circuitry within the sensor's integrated circuit package.
Classifications: Sensors can be classified in a variety of ways. Passive/Active, Absolute/Relative and Digital/Analog are the most common classifications. There are also other ways to classify sensors, but, for the most part, these are for special situations. These special situations include: characteristics, material, applications, and type of stimulus.
Characteristics: Sensor characteristics describe the capabilities and parameters of specific sensors. The common characteristics include: Accuracy, Dead band, Drift, Hysteresis, Linearity, Nonlinearity, Offset, Precision, Range, Repeatability, Resolution, Response Time, Saturation, Sensitivity, and Stability. Sensor characteristics are normally found in a datasheet, user guide or other documentation. These documents provide specific information that's essential to understanding not only how to select a sensor, but also on how to use it in a specific application.
4. Approaches to Human Body Sensing Back to Top
While there are many types of physical conditions that IoT devices are capable of sensing – acoustic, electric, magnetic, mechanical, optical and thermal – wearable devices primarily sense biological (or biochemical) conditions and the body's movement. Gaining an understanding of these conditions with respect to human body sensing is a necessary prerequisite to understand the applications of sensors in IoT wearable devices.
To begin, the physical condition of the human body can be sensed in three different ways: the skin, body fluids and movement. Let's discover in this section of the learning module how these components can be used in a wearable device sensing design.
- 4.1 The Skin
While we may discount the importance of the human skin (excluding perhaps a nice tan at your favorite beach in the summer) or even forget that the skin itself is a body organ, the fact is that the skin is a superb “natural” sensor. It senses both internal and external conditions. And it responds to heat, cold, fear, pressure, pleasure and pain. As a medium for determining the overall condition of the human body, the skin can be leveraged to gather data on body temperature, blood pressure, heart rate, peripheral capillary oxygen saturation (SpO2) and more.
- 4.2 Body Fluids
Body fluids also tell us a lot about the condition of the human body. Blood has long been used as a medium for sensing the body's medical condition; however, it requires an invasive sensing technique that is not always desirable to use. Therefore, a lot of new and non-invasive techniques are being developed utilizing sweat, tears, saliva and interstitial fluids. In general, body fluids can be used by wearable device sensors because they contain a lot of chemical and biochemical information about the state of the body's functions. What follows is an overview of the information body fluids can provide:
Sweat contains a lot of biological substances such as sodium, chloride, potassium, calcium, ammonia, glucose, and lactate. For fitness activities, sweat can tell a lot about the body's hydration level and electrolyte balance. Since it is readily accessible by a wearable device, it is the easiest fluid to leverage as a source of information about the condition of the body.
Saliva contains an incredible amount of biological information. It includes ions of sodium, potassium, chloride bicarbonate, nitrates, urea, uric acid, creatinine, and hundreds of types of proteins. The downside of saliva as a sensing stimulus is that it also possesses, in varying degrees, mucus, food debris and blood, all of which can impede the operation of a sensor.
Tears are another body fluid that can be used by a wearable device to sense the condition of the body. They contain proteins, electrolytes and sugars like glucose that can be leveraged in diabetes monitoring.
Interstitial fluids – fluids that surround tissue cells – contain sugars, salts, fatty acids, amino acids, coenzymes, hormones, and more. These fluids tell a lot about the condition of the body and would be typically used in wearable medical devices such as diabetes monitors.
- 4.3 Body Movement
The movement of the body can be utilized in monitoring the motor activities of a human being. The human body's motor activities are useful in patient monitoring, especially for movement disorders such as Parkinson's Disease or diseases related to Parkinson's such as bradykinesia. Motion sensors such as accelerometers, gyroscopes or magnetometers can be placed in wearable devices or in garments to obtain movement data.
5. Introduction to Photoplethysmographic Technology Back to Top
For many years, heart rate monitoring has been recognized as a useful parameter in both diagnosing diseases (e.g., autonomous neuropathy, cardiac arrhythmia or infarction, etc.) as well as in optimizing the physical regimen of an athlete. In general, heart rate monitoring has been accomplished using a variety of technologies, with the most common ones, being:
Despite the above time-tested technologies, photoplethysmographic technology (PPG) has found new interest by researchers and designers in the area of heart rate monitoring because of it offers a compact, low cost, simple and low power technology that's a good fit for the growing wearable market of fitness and medical devices.
In its most basic form, PPG technology utilizes an LED and photo-detector as well as associated circuitry to make up a pulse oximeter, which offers a way to determine the heart rate by assessing the arterial pulsability of tiny networks of blood vessels in the tissue of the skin. As an optical sensor, PPG illuminates living tissues with a light source, gathers a portion of the light that propagates through the tissue, and then analyzes the resulting attenuated light. LEDs are typically used as the light source and detector for PPG-based heart rate monitors.
One of the challenges of using PPG technology in this application is that in some areas of the body (e.g., forehead, ankle, and torso) the emitted light is fully absorbed by the body. In these cases, the PPG optical sensor can be operated in an alternative “reflectance” mode where the light source is placed next to the detector to collect the propagated light by means of the light scattering effect. The reflectance mode allows the PPG-based heart rate monitor to be used on many different parts of the body such as the wrist, forearm and ankle – all ideal for use in wearable devices such as smart watches, and fitness or arm bands.
6. Types of Sensors Back to Top
Since the field of wearable IoT devices is expanding so rapidly, it would be difficult to cover every type of sensor that IoT wearable devices would utilize. Electronic textiles, micro needle arrays, wearable colorimetric sensors, body-conformable electronics, one-time/re-usable sensors, invasive/non-invasive sensors, and implantable devices are all part of this exciting yet burgeoning field of technology. Since this is an essentials learning module, we will only focus on the most common types of wearable sensors that feature the following characteristics: low-power, lightweight, compact form factor, and multi-functional.
7. Sensor Evaluation BoardsBack to Top
Sensor evaluation boards make it easy to learn, test, and develop sensor applications. Here are some of the currently available sensor evaluation boards for the sensors described in this learning module:
The SENSOR-PUCK is a demo platform for the Silicon Labs' Si114x Series Optical Sensors and Si701x/2x Series Relative Humidity and Temperature Sensors. Powered by a coin-cell battery, it is controlled by an EFM32™ MCU. A Bluetooth Low Energy (BLE) module is used to broadcast sensor data to iOS or Android smart phones with the downloadable SENSOR-PUCK app. Placing your finger tip over the Si1147 sensor allows you to measure heart rate. Environmental sensing of UV Index, ambient light, relative humidity, and temperature are also provided. For power management, the board features a Touchstone TS3310 boost DC/DC converter.
The is an evaluation board for the CPT112S TouchXpress Capacitive Sensor Controller. The board serves as a user input peripheral for application development. It can be configured for different touch sense capabilities and also contains breakout pads and other peripherals for user feedback. It has 8-Capacitive Sense touch pads a 4-Channel Capacitive Sense slider. A Buzzer and a 20-pin expansion header is available for connection to a Silicon Labs Starter Kit (EFM8 or EFM32).
The SENSOR-EXP-EVB is a development board for Silicon Labs' Si701x/2x Series Relative Humidity and Temperature Sensors and Si114x UV Index, Ambient Light, Proximity and 3D Gesture Sensors. The card plugs into the expansion header of the EFM32™ Zero Gecko Starter Kit and is supported with example software and source code in the Simplicity Studio.
The Biometric-EXP is an evaluation board for the biometric applications of the Si7013 Humidity and Temperature Sensor and the Si1146 Proximity/UV/Ambient Light Sensor, which is capable of monitoring pulse rate and peripheral capillary oxygen saturation (SpO2). A Biometric-EXP Software Demo is available for download to an EFM32 Wonder Gecko STK through the Simplicity Studio.
*Trademark. Silicon Labs® is a trademark of Silicon Laboratories, Inc. Other logos, product and/or company names may be trademarks of their respective owners.
Test Your KnowledgeBack to Top
Are you ready to demonstrate your knowledge of sensors for IoT wearable devices? Then take a quick 15-question multiple choice quiz to see how much you've learned from this Essentials Sensors 2 module.
To earn the Sensors 2 badge, read through the module to learn all about sensors for IoT wearable devices, attain 100% in the quiz, leave us some feedback in the comments section, and give this page a star rating.
1) What characteristics would be most appropriate for a sensor that would be used in a wearable device such as an activity-tracking arm band?
2) Choose the best possible answer from the choices below. Which of the following is considered to be the technological inspiration of IoT wearable devices today?
3) What type of sensor(s) would be used to provide data on a patient movement disorder such as Parkinson’s disease?
4) Temperature sensing by means of thermistors often lack accuracy and possess higher power consumption. How does the Series Si705x Series digital temperature sensor ensure stable temperature accuracy over its entire operating voltage and temperature ranges without the need for end-of-line calibration?
5) Choose the best possible answer from the choices provided below: Generally speaking, conventional UV sensors combine UV-sensitive photodiodes with an external microcontroller (MCU), analog-to-digital converter (ADC) and signal processing firmware. Why would this design pose a problem for a fitness wrist band?
6) True or False: The digital UV index is a weighted scale that provides a standardized measure of the skin's response to different sunlight wavelengths including UVB and UVA.
7) Why is saliva more difficult to use as a medium to sense the condition of a human body?
8) True or False: The Silicon Labs’ Sensor Puck broadcasts sensor data wirelessly via NFC.
9) What type of sensor is used to measure the heart rate of a person wearing a fitness band?
10) Which of the following body fluids can be sensed to indicate the condition of a human body?
11) If you were designing a device with a photoplethysmographic optical sensor, what type of body condition would you be sensing?
12) What part of the human body can sense heat, cold, fear, pressure, pleasure and pain?
13) True or False: Relative humidity is sensed in the Si 7005 by using a polymer dielectric film.
14) True or False: According to this learning module, simple sensors have both transduction and sensing functions due to the integration of signal conditioning, A-to-D conversion and other circuitry within the IC sensor’s package.