Ultra-low-power sensor nodes and sensor hubs play a key role in the ever-expanding “Internet of Things.” Projections are that by 2020 over 20 billion “things” will be connected to the Internet. The challenge would then be “Who is going to change all those batteries?” Realizing this, many developers are looking at “Energy Harvesting (EH)” as the ideal solution to extend battery lifetime and possibly even become the primary power source for these ultra-low-power systems.
EH involves collecting ambient energy from solar, thermal, vibration, RF or other sources. EH technology allows small, standalone sensors to function continuously for extended periods of time — decades even — without power-line connections or battery replacements. This technology greatly enhances the problem-solving capability of low-power sensors and its use is growing rapidly. For that reason, an EH function is shown as the power-source element in the block diagram of a typical very-low-power sensor product shown in Figure 1.
Energy Storage Devices
There are many component choices for storing the energy needed to power a standalone sensor product (see Figure 2). Conventional batteries are inexpensive, readily available, well understood and relatively easy to incorporate into a design. They would be the first choice if they meet a product’s target energy budget.
If conventional primary cells can’t be used, however, the alternatives are either rechargeable batteries, supercapacitors, solid-state batteries, or a combination thereof. Solid-state batteries are rechargeable, but because they are so new, we have categorized them separately in this article.
Free Energy Available in the Environment
The major types of energy normally wasted in the environment that can readily be captured are described below:
Energy from light — Sunlight and indoor and outdoor lighting can be converted into electricity by photoelectric energy cells; i.e., solar cells. To work efficiently, those cells have to be optimized for the characteristic spectra of the incident illumination. Despite the fact that indoor lighting is a good power source for wristwatches and handheld calculators, it doesn’t provide enough energy to be useful for most harvesting applications.
Mechanical energy — Objects that vibrate or move can be made to produce electricity. Vibrations generate considerable voltage when they are applied to piezoelectric materials. Also, the mechanical energy of pressing or moving an object such as a switch can generate a current if the action changes the flux of a magnetic core situated within an internal coil.
Thermoelectric energy — If the temperature at one point of the surface of an object is different than what it is at a nearby point, that temperature difference can be converted directly into an electric current via a physical phenomenon called the Seebeck effect. Certain semiconductors and metals with high Seebeck coefficients transform temperature differentials into useful electric energy.
Electromagnetic waves — Radio emissions are pervasive everywhere today, and only a small fraction of the energy in those transmissions is consumed by the intended receivers. Radio receivers with antennas can convert some of that wasted RF energy into electricity.
EH is still in its infancy. Implementations are evolving rapidly as harvesters are improved, better power management chips are introduced, and engineers acquire application experience with the technology.
What circumstances favor the use of EH technology? Power budgets, packaging requirements and operating lifetimes are among the determining factors.
This image provides a quick reference for determining whether a primary battery will suffice or whether EH is needed to obtain the necessary lifetime. The horizontal axis of this graph shows days of operation required over the lifetime of the product, while the vertical axis shows the average load current of the electronics on a logarithmic scale.
The microcontroller is the digital brain of any embedded system. Choosing the right MCU for an EH-based sensor product is a critically important design decision. Ideally, the optimum MCU for battery and remote applications such as security sensors will offer the following features, among others:
- A very-low-power architecture providing multiple power-down modes for maximizing battery life
- Good performance for fast, efficient processing
- Very fast wake-up times from power-down modes to ensure that the system spends the greatest possible amount of time in a low-power state, yet responds quickly to deliver essential system operational capabilities
- A hardware Digital Signal Processor (DSP) for rapidly and conditioning raw signals, filtering sensor outputs, determining a signal’s spectral content, and eliminating false signals
True Low Power™ capability of the RX111
The 32-bit Renesas RX111 MCU is an ideal design choice for sensor products that apply EH technology. This inexpensive chip combines a breakthrough power-control technology — True Low Power™ capability — with exceptional features such as ultra-fast wake-up times, zero-wait-state flash memory and enhanced DSP capability. It also provides multiple safety functions and a host of advanced peripherals, including USB 2.0 support, LCD Drive capability, Real Time Clock (RTC) and a Capacitive-Touch Sensing Unit (available in the RX113).
Design Features of the RX111 MCU
Key features and characteristics of the RX111 MCU include the following:
- Exceptional run-mode power efficiency: 100μA/MHz
- Sleep-mode power consumption as low as 310nA
- Ultra-fast wake-up time: 4.8μs
- Superior architecture: 3.08 CoreMarks/MHz performance
- Six operating modes, plus numerous other design options for saving power
- Standard and advanced on-chip peripherals:
ADC, LVD, RTC, USB, and more.
|Run Mode||Flash (KB)||RAM (KB)||Package||Suggested Starter Kit||Suggested Demonstration Kit|
Ambient energy is energy available from the environment. Light, heat, vibration, fluid flow, gas flow, RF, force, and rotation are just a few of the possible sources of energy. Harvesting energy from the environment doesn’t cost anything and, as long as the harvesting subsystem remains active, the product can operate many years without the need for battery changes. EH has the following important advantages:
- Environmentally friendly (no replacement batteries into landfill)
- Product powered for life
- For commercial applications, avoid costly maintenance calls to replace batteries
- For infrastructure applications, set it and forget it
An ultra-low-power 32-bit MCU with very fast wake-up and multiple power-down modes, the Renesas RX111 MCU is ideally configured for EH applications.