Building wearables that sense, think, and communicate, part 2
It may be tempting to write your own custom proprietary protocol so that the design can achieve the optimal power consumption according the requirements of the specific product. However, today’s devices are expected to communicate with other devices in an ecosystem over a standard protocol. Thus, interoperability is key. This is one of the most compelling requirements motivating designers to choose standard protocols. The adoption of a standard communication protocol also helps developers write better applications that are energy efficient, highly responsive, and easy to adopt for wearable devices.
Wi-Fi is a popular choice of wireless protocol in general because it offers very high data throughput, is medium to long range, and has very sophisticated security options. However, Wi-Fi comes with the cost of consuming higher power than other protocols and so it is often not the best choice for a wearable device. Also, while the high data rate of Wi-Fi may be a great advantage for applications such as Internet access, such throughput may not be required for wearable devices, which typically only transmit small amounts of data.
The other common choice is ZigBee, which offers low power, a light stack, and good market mindshare. However, ZigBee has not yet been universally adopted in PCs or smartphones. The best alternate is Bluetooth classic, which was developed as a way to exchange data over a short range. Bluetooth classic is a legacy standard for personal area networks made popular by audio streaming to cell phone headsets. Bluetooth classic gives better power efficiency compared to Wi-Fi. However, for extreme low power consumption, Bluetooth Low Energy (Bluetooth Smart/BLE) is the often a better choice as it was designed short-range, low-power wireless applications that communicate state or control information.
There are a couple of reasons why BLE is best suited for wearable devices. BLE technology is ideal for applications requiring episodic or periodic transfer of small amounts of data, which aligns with a typical wearable device data transfer requirement. Also, the key feature of BLE is its low power consumption that makes it possible to power a small device with a tiny coin cell battery, making it a clear choice for wearable devices. For these reasons, BLE is often the best choice for wearable devices.
Field upgrade capabilities (OTA)
Users have come to expect to use wearables in a continuing diversity of ways in environments of all types. To keep relevant to users, wearable devices deployed in the field need to be able to adapt to new use cases. Over-the-air (OTA) firmware upgrades enable developers to field program wearable devices. This allows reprogramming of control parameters, upgrading firmware to add new features, and the ability to fix bugs. OTA firmware upgrade is a bootloader mechanism over a wireless link (e.g. BLE) to update the firmware on a target device. This is similar to upgrading over a conventional wired link like UART, I2C, or SPI. Figure 1 shows the block diagram of a bootloader using a wireless link.
Figure 1 Over-the-air (OTA) bootloader system
Connectivity is the key. Objects will connect to each other either directly or through other devices using many technologies. Connectivity implies OTA updates. Suppose a series of embedded devices in the home required firmware upgrades. If they don’t follow an OTA strategy, users would have to run a cable to each one to change their basic configuration.
It is typically not practical to power wearable devices from a wired source. The immediate alternative is to use a battery as a power source. Batteries, however, introduce various limitations and concerns, including the need for periodic recharging, limited operating life, and environmental disposal concerns. Energy harvesting technology allows wearable devices to power themselves. Energy harvesting is a process by which energy is derived from sources such as solar energy (light), thermal energy (heat), and/or kinetic energy (vibration). Energy derived from these sources can be stored to self-power the wearable device. Because energy harvesting offers a consistent and reliable source of energy, device battery size can be reduced while extending effective operating life.
Since energy harvesting devices (EHDs) provide a very small amount of power for low-energy electronics, it is critical to catch and use every joule of energy generated. There are various types of EHDs, and the best to use depends upon where the source of the energy resides. For example, solar modules are most popular among EHDs to derive the energy from light because of their ready availability, easy of use, and low cost. In general, the power generated by a solar module is directly proportional to the size of the module and varies with brightness.
The other common EHD is a thermoelectric generator (TEG), which can generate power with high current from a heat source. The limitation of TEGs is that they have a constant temperature gradient to provide a usable power output.
For energy harvesting from kinetic energy sources such as vibration, there are two common EHDs: piezoelectric and electromagnetic that generate AC power. Similar to TEGs, the limitation here is to provide a vibration source with constant predictable frequency.
An energy harvesting power management IC (PMIC) is part of an energy harvesting system (EHS) that includes the EHD and an energy storage device. The energy harvesting PMIC device takes the low energy from the EHD and converts into a stable energy output ready to be stored in the energy storage device (ESD). The ESD–typically a battery, conventional capacitor, or supercapacitor–is then connected to the rest of the system, providing the power needed to operate the load (for example a wearable device). Figure 2 shows a block diagram of an EHS.
Figure 2 Energy harvesting system