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    Batteries Not Included: Wireless Sensor Solutions

    Published January 2022

    According to the forecasts from Bosch, Texas Instruments, and Hewlett-Packard, the number of sensors installed annually between 2017 and 2025 is expected to total between 1 and 10 trillion. With this rapid growth – combined with requirements on data storage, processing, and power transmission – sustainability is an unavoidable factor.

    And so, when it comes to operating sensors, any kind of energy harvesting solution is highly desirable - if not absolutely necessary.

    Modern low-power sensors enable precision control across a wide variety of applications, either locally, remotely, or autonomously. They are key components, sold globally for a huge range of applications. Energy harvesting technology means that these wireless sensors can work end-to-end without a mains connection or the need to replace batteries. The flowchart in Figure 1 shows the energy harvesting element as an energy source for a typical sensor with very low power consumption.

    Figure 1

    Flowchart of a typical sensor with very low power consumption, powered by energy harvesting.

    Figure 1

    An autonomous sensor includes the following system elements:

    • Energy harvester
    • Sensor(s)
    • Energy storage device
    • Power management device
    • Microcontroller
    • Wireless connectivity element

    One of the crucial challenges in designing this kind of sensor system is keeping within the energy budget. This includes the operating voltage, as well as the peak, quiescent and average operating currents.

    Energy Harvesting Sources

    The main types of power normally provided by nature or the environment are relatively easy to harvest. Energy harvesting techniques and sources include:

    • Energy from light – sunlight and solar energy, as well as indoor and outdoor lighting, can be converted into electricity through solar cells. To work efficiently, these cells should be optimised for the typical distribution of incident light
    • Mechanical energy – objects that vibrate or move with kinetic energy can generate electricity. They provide significant voltage when generated with piezoelectric materials and can easily provide hundreds of volts. However, this type of energy source has a very high internal impedance. It's so high that a piezoelectric energy harvesting device cannot generate much power. Additionally, the polarity of voltage and current – depending on the direction of the vibration change – is reversed
    Energy Harvester from ZF.

    The mechanical energy of pressing or moving an object like a switch can generate a current when the action changes the flow of a magnetic core located in a coil.One example is an energy harvester (Figure 2).

    Figure 2 (right): Energy Harvester from ZF.

    • Thermoelectric energy – if the temperature at one point on the surface of an object differs from that of a nearby point, this temperature difference can be converted directly into an electric current by way of the Seebeck effect
    • Electromagnetic waves – radio emissions are used in common RFID solutions, e.g. in bank cards, both for data transfer and for power

    Table 1 shows the performance of the energy sources.

    Performance of the energy sources

    Table 1: Parameters of each energy source.

    Energy Harvesting Sensors

    There are many different types and manufacturers of mini sensors for measuring physical quantities. Suppliers of MEMS chips include STMicroelectronics, Bosch Sensortec, Texas Instruments, NXP, Analog Devices, Seiko Epson, Infineon Technologies, Murata, Sensata and Melexis.

    Energy Storage

    The energy storage system serves as a buffer between the load and the energy harvester. It supplies power to the electronic devices when the harvester cannot provide enough energy. Equipment such as rechargeable batteries, super-capacitors, and solid-state batteries can be used to store electrical energy in sensor systems with very low power consumption.

    • Rechargeable batteries – come in many different chemical compositions and far-ranging shapes, sizes, and capacities. Specific features and specifications – which are not standardised – can have a significant impact on their performance
    Supercapacitor
    • Supercaps – supercapacitors are similar to normal capacitors, except they offer higher capacities. They come in cylindrical and prismatic (rectangular) forms. A thumb-sized supercap has a capacitance of 1 farad at 2.5V. Supercaps have relatively high leakage currents. If they are used with an energy harvester, the latter must provide sufficiently high performance. The capacitance of supercaps drops at higher temperatures. Vishay's 235 EDLC-HVR ENYCAP series super-capacitors (Figure 3) are the world's first of their kind with a lifespan of 2,000 hours at +85°C, according to the manufacturer. They are used at 85°C and 85% r.F, tested under voltage for 1,500 hours. The capacitors are available in fifteen package sizes from 10mm x 20mm to 18mm x 40mm, capacitance range: 5F to 60F

    Figure 3 (right): Supercapacitor from Vishay's 235 EDLC-HVR ENYCAP series.

    • Solid-state batteries – the development of solid-state batteries gives developers another energy storage option for designs with very low electrical power consumption. Solid-state batteries are characterised by low self-discharge (4% per year) and can be charged 1,000 times – even more often if they are not deeply discharged. The use of a ceramic solid eliminates the risk of fire, explosion, or leakage of electrolyte liquid
    CeraCharge

    CeraCharge (Figure 4) from TDK is one such solid-state battery used in SMD technology. Available in size EIA 1812 (4.5mm x 3.2mm x 1.1mm), this battery offers a capacity of 100μAh at a rated voltage of 1.4V. In the short term, currents can be harvested in the range of a few mA. The operating temperature range is between -20°C and +80°C. To increase capacity and voltage, individual CeraCharge chips can be connected in series or in parallel as desired.

    Figure 4 (right): CeraCharge from TDK.

    Power Management

    Wireless sensor solutions with very low power consumption require a power management device due to their variable voltage and current states. This component converts the delivered unregulated voltages and currents into regulated electrical energy that can be stored. It can also pass energy to the system load at the right voltage. Typically, a power management chip will contain circuits to protect both the load and the energy storage:

    • Undervoltage protection – this protection circuit interrupts the power supply of the load and switches to the standby load of the energy storage when the output voltage of the energy harvester gets too low. This capability is important to protect the battery from excessive discharge, as it can be permanently damaged or lose part of its storage capacity
    • Surge protection – this function is for monitoring the charging voltage. If the voltage gets too high, the power management chip either directs the excess charge to earth or electronically inserts a high impedance between the harvester and the energy storage. Whichever method is used, the aim is to protect energy storage
    • Overcurrent protection – this protection circuit has a similar function to a household fuse. If the load draws too much power, the overflow circuit isolates it from the battery and makes sure it does not empty. When this feature is enabled, there is usually an error state in the system

    An example of a power management chip is the MAX20361 from Maxim Integrated. This is a fully integrated control chip for generating energy from single or multi-cell solar sources. The device requires a quiescent current of 360nA and has a boost converter that typically starts from 225mV. To make the most of the power delivered by the source, the MAX20361 features Maximum Power Point Tracking (MPPT) technology, owned by Maxim Integrated, which efficiently processes from 15μW to over 300mW of available input power. The MAX20361 also has an integrated charging function with a protection circuit, which is optimised for Li-ion batteries. The chip can also be used to charge supercapacitors, thin-film batteries, and conventional capacitors. The charging circuit includes a programmable charging switch-off function. A description of the MAX20361 Evaluation Kit (Figure 5) can be found on the RS DesignSpark website.

    Figure 5

    Figure 5

    Figure 5: MAX20361 Evaluation Kit. This is a fully integrated control chip for generating energy from single or multi-cell solar sources.

    Choosing the Microcontroller

    The choice of MCU (microcontroller unit) is crucial for embedded systems with very low power consumption. Ideally, it will have the following features as a minimum:

    • Multiple power-down, sleep, or power-off modes to maximise battery life
    • Good performance for fast and efficient processing
    • Very fast wake-up times from power-down modes

    The latter is important because the circuits of the sensor system are designed to last as long as possible in a low consumption state before moving to an operating mode that consumes more power. The Renesas RX111 MCU, for example, offers these features.

    Renesas RX111 MCU

    Three power-driven running modes of the RX111 (high-speed, middle-speed, and low-speed) and three low-power modes (sleep, deep sleep, and software standby) can be programmed to operate various combinations of on-chip functions. For wireless sensing applications, a common requirement might be waking up the system when an event has occurred, for example, or booting it up at regular intervals using the built-in real-time clock (RTC).

    Figure 6 (right): MCU Analysis Board, RX111.

    The supply voltage requirements of the MCU are not affected by the power-controlled run modes. Operation is always permitted over the component's entire range of 1.8V to 3.6V. However, the clock frequencies that can be used in high, middle, and low-speed modes depend on the supply voltage.

    Various on-chip functions are stopped or shut down in the MCU's energy-saving sleep, deep sleep, and software standby modes:

    • Sleep mode – the CPU is stopped and the data is saved. This reduces the dynamic power consumption of the CPU, which contributes significantly to the total operating current of the MCU. The CPU wakes up again in 0.21μs at a 32-MHz clock rate
    • Deep sleep mode – CPU, RAM, and flash memory are stopped and the data is saved. Running at 32MHz with multiple active peripherals, the typical operating current is 4.6mA. The chip requires 2.24μs for the CPU to wake up from deep sleep mode and switch to run mode
    • Software standby mode – the PLL and all oscillators except Sub-Clock and IWDT (Independent Watchdog Timer) are stopped. Almost all modules of the RX111 CPU, SRAM, Flash, DTC (Data Transfer Controller) and peripheral modules are stopped and the data saved. The power-on-reset circuit keeps running. The IWDT, RTC, and LVD (Low Voltage Detection) modules can also be operated if required. The power consumption in this mode is 350nA to 790nA, depending on whether the LVD and RTC functions are used. When waking up in 4 MHz run mode, CPU operation begins after a delay of 4.8μs. When waking up in superfast 32-MHz mode, the waiting time is extended to 40μs

    Although the sleep, deep sleep, and software standby modes of the RX111 MCU are very helpful in reducing power consumption, other techniques can achieve further output reductions. For example, different clock signal-frequency division ratios can be set individually. Each peripheral module in the RX111 also has a separate stop control bit.

    Wireless Communication Solutions

    Most of the wireless telecommunications solutions used today work in the 2.4 GHz ISM band under the ZigBee, Z-Wave, or Bluetooth LE protocols. ZigBee and Z-Wave are widely used in building technology, while Bluetooth is used for home automation applications as well as wearable devices such as health and fitness monitors. Since all current smartphones support Bluetooth, the number of sensor solutions that use this protocol is very high.

    onsemi RSL10 Solar Cell Multi-Sensor Board

    Figure 7 (right): The onsemi RSL10 Solar Cell Multi-Sensor Board with BLE connectivity.

    The Solar Cell Multi-Sensor Board RSL10 is a development platform for self-powered systems (battery-less) and IoT applications including smart building, smart home, and Industry 4.0. It is based on the Bluetooth Low Energy RF module RSL10 from onsemi. This board has a 3-axis accelerometer, a smart environmental sensor, and a temperature sensor. It also includes a 47F memory capacitor, a solar cell, and a programming and debugging interface.

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