Humidity and Temperature Sensors Widen Potential for Wearable Devices
Author: Pascal Gerner, Head of Product Management
Wearables are attracting increasing attention in the market, miniature computers that are worn as clothes or accessories, and what make them smart are the integrated sensors. With environmental sensing, these devices can also be made aware of their environment. Sensors are able to measure relative humidity and temperature (RH/T) to enable devices and their users to understand their ambient condition and what is going on around them. The main advantages of these sensors are that they are very energy efficient, do not require lots of computation power and are becoming smaller and smaller. Hence, they are ideal for wearables and offer a broad range of potentially valuable applications.
Personalized Ambient Conditions
The integration of temperature and humidity sensors into wearable devices allows measurement not only of the ambient conditions but also the user's physiological information, such as skin temperature or sweating rate. This can give a better understanding and interpretation of achieved performance (e.g. disturbed sleep in hot weather, slower running in high humidity) and is the basis for a variety of potential new applications. This information can also be used in a connected home (smart home) to automatically control the indoor climate. If skin temperature and sweat rate is shared with a climate system, the room ambience can be optimized to personal preferences without the need of human intervention. This is particularly relevant if a user is sleeping and is not aware of unhealthy or uncomfortable conditions. In addition to increased comfort, energy is consumed only when needed, leading to cost savings.
Tracking ambient conditions is also useful for other applications. Depending on the temperature and dryness of the environment, a wearable device could provide useful skincare tips. Our skin is highly sensitive and an understanding about exposure could be used in cosmetics to recommend the right product for the right skin and exposure. But it's not only the beauty industry that can enhance its products to satisfy customers - the healthcare market would also benefit. People with respiratory diseases need a climate adjusted to their condition. A bad indoor climate can increase the risk of illness - asthma, mites and mold infestation are just a few of RH/T-dependent health risk factors. By tracking the ambient condition with a wearable device, distinct patterns will emerge and abnormal or risky conditions will trigger the adjustment of heating, ventilation, air-conditioning or humidifiers.
In the near future, spectacles, watches, articles of clothing and other items will have the ability to sense temperature and humidity, making it possible to integrate the measurement of our environment into every facet of our lives. This will help us to get a better understanding of our environment and the ambient condition in the space we live in. As a result, processes in daily lives could be optimized, energy consumptions minimized, money saved and comfort and health could be improved.
Integration and Sensor Fusion
Integrating ambient sensors into wearable devices like Smartwatches is a non-trivial undertaking. This is especially true for ambient temperature sensors and all measures dependent on ambient temperature such as humidity. Temperature sensors built in a wearable or mobile device face three major challenges.
First, the electronic components in the tightly packed device generate heat and influence the sensor reading. This effect gets even worse as the heat dissipation of the different components is highly load dependent and therefore is changing all the time. Second, the sensor readings are influenced by the heat of the skin. Third, the device has a certain thermal mass which results in a slow thermal response. Similar to the fact that a hot cup of coffee needs about 30 minutes to cool down to room temperature a smart watch or phone needs about 30 minutes to adopt to temperature changes.
There are multiple measures to mitigate or even eliminate this obstacle. One of the most crucial parts is the placement of the sensors. It is important that they are very well decoupled from the main device-internal heat sources and the human skin. The placement of the sensor is highly device specific and has to be well considered for every product independently. An ideal sensor placement, however, is not enough as a complete decoupling is most likely not feasible. To compensate for the remaining influences, the influence factors have to be monitored and their impact on the temperature reading has to be determined. For example to compensate for the body heat, an additional sensor can be placed closely to the skin. A heat-propagation model can then be applied to estimate how big the temperature gain of the body heat on the sensor is. This information allows for compensation. This environmental sensor fusion software called Sensirion Engine is already actively used in several smart phones enabling accurate ambient temperature and humidity sensing. Further the Sensirion Engine allows to speed up the response of the temperature and humidity signal to ambient changes far beyond the physical limits. Which is necessary as no user wants to wait up to 30 minutes to get accurate readings. Combining all these approaches allow wearable devices an accurate measurement of ambient temperature and humidity by providing an experience users expect from wearable devices.
Sensor Packaging Innovation
Implementation of such a system is possible only with innovative technology in both hardware and software. The hardware in question is currently one of the smallest humidity and temperature sensors available. It has been developed by the Swiss high-tech company Sensirion specifically for devices where space is limited, and is optimized for the requirements of the consumer electronics industry. This innovative product combines minimal size with maximum performance to define the latest generation of humidity and temperature sensors. The SHTW2 is Sensirion's 2nd generation WLCSP (wafer-level chip-scale package) humidity sensor, offering a complete digital humidity and temperature sensor system in a package so tiny that it fits into virtually any application. The SHTW2 comes in a flip chip package, which is an established technology and represents one of the simplest and smallest possible ways of packaging for a semiconductor chip. The supply voltage of 1.8 volts and the low power consumption of just 2 µW at 1 measurement per second provide an optimal base for use of the sensor in small, wearable devices. The company provides not only the sensor but also the accompanying software, ensuring a significant time reduction in obtaining accurate readings after a change in ambient conditions. The tiny SHTW2 sensor extends the range of possible applications; for example, as a basis for physiological signals, such as skin temperature and sweat rate, where tailored algorithms are required. Sensirion has created the space for new ideas - now it is up to wearable providers and app developers to take the next step.