White Paper – RX100 Microcontroller Family Energy Harvesting for Low-Power Sensor Systems By: David Squires, Squires Consulting; Forrest Huff, Renesas Electronics America Inc. Feb. 2015 Abstract This white paper details important design issues associated with designing the very-low-power remote sensors that play important roles in the feedback control loops of embedded electronic systems. Using a basic configuration for a standalone sensor system as a reference, the discussion covers sensor/transducer types, power budgets, power sources (especially devices that harvest ambient energy) and energy-storage solutions. It also summarizes microcontroller requirements and highlights the latest power-management chips and wireless ICs. Special emphasis is placed on the concept and reality of energy harvesting as a viable method for powering standalone embedded systems for extended periods of time. An example security alarm design is described and explained to provide engineering context and perspective. Index I. Introduction – III. Additional Insights on Very-Low-Power Sensor Products 2 Sensor Components 7 II. Basic System Design Issues 2 • Sensors 7 • Sensors 3 • Energy Storage Devices 7 • Power Budgets 3 • Energy Harvesting Solutions 12 • Energy Storage Devices 3 • MCUs 15 • Energy Harvesting Solutions 4 • Power Management Devices 18 • Microcontrollers (MCUs) 6 • Wireless Connectivity Modules 20 • Power Management Devices 6 IV. Example Design: Glass Break Sensor 22 • Wireless Connectivity Modules 6 V. Summary 28 VI. Appendix 28 White Paper – Energy Harvesting for Low-Power Sensor Systems Page 1 of 29 I. Introduction – Very-Low-Power Sensor Products Modern low-power sensors enable precise local, remote or autonomous control of a vast range of products. Their use is rapidly proliferating in vehicles, appliances, HVAC systems, hospital intensive care suites, oil refineries, and military and security systems. They are key components for a vast range of applications with global markets. Importantly, energy harvesting (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 energy-harvesting function is shown as the power-source element in the block diagram of a typical very-low-power sensor product shown below in Figure 1. Events Ambient Energy Sensor MCU RF Power Energy Energy Mgmt. Storage Harvest Optional elements including actuators, displays, keys, etc. Figure 1: Components in a Typical Very-Low-Power Sensor Product. II. Basic System Design Issues Electronic engineers developing standalone sensor systems must address design issues associated with the following system elements, among others: • Sensors (transducer type, performance characteristics, operating requirements, etc.) • Power budget (operating voltage; peak, quiescent and average operating currents) • Energy storage devices (capacity, leakage, temperature performance, etc.) • Energy harvesting chips (capability, requirements, limitations, etc.) • Microcontroller (processing performance, power efficiency, DSP, I/O, low-power modes, etc.) • Power management devices (features, performance, etc.) • Wireless connectivity device (peak power, range, frequency, protocols, etc.) Other technical issues may include size limitations, challenges of operational environments, life- time costs, long-term reliability, safety, and more. Aspects of the main design areas listed above are presented in the sections that follow. White Paper – Energy Harvesting for Low-Power Sensor Systems Page 2 of 29 Sensors Eight physical characteristics are most commonly monitored in embedded system applications. They are listed below with the types of sensors and transducers typically used for measuring them: • Motion Ultrasound position detectors • Temperature Silicon diodes or thermistors • Presence of people Passive IR detectors • Pressure Micro Electro-Mechanical (MEMs) chips • Acceleration Micro Electro-Mechanical (MEMs) chips • Picture LCD cameras (like those used in cellphones) • Location Geomagnetic sensors (like those used in cellphones) • Proximity Sensors similar to cellphone components Power Budgets For standalone sensor designs that cannot connect to the AC power mains—especially those that must operate for long periods of time between battery changes or cannot be accessed for main- tenance—the amount of power consumed by every component in the system must be carefully identified and minimized. If the data reveals that a design can be powered by a reasonable-sized battery, or a battery + energy-harvester solution, that’s great! If it doesn’t, then it may be neces- sary to implement a top-down design approach. The top-down approach starts with the embedded system’s total power requirement. A primary battery must be sized to fit in the available space, and that size and the battery type will determine the maximum power-source capability, usually measured in mAh. The Energy Storage Devices sections of this white paper provide technical data on different types of batteries. For discussion purposes here, however, let’s assume that a hypothetical design has enough space to accommodate two AA-size batteries occupying a volume of 16cm3. To function properly, the electronics in this design requires a supply voltage of 1.8V, so two 1.5V batteries must be connected in series. The total battery capacity is 16cm3/1000cm3/l*450Wh/l = 7.2Wh. To convert this to mAh, we know that the nominal voltage is 3V, therefore 7.2/3 = 2.4Ah = 2400mAh, which is consistent with pub- lished numbers for AA batteries. This hypothetical sensor product mandates a continuous battery-operating lifetime of 5 years. Thus, the average current consumed by the sensor’s circuits must not exceed 2400mAh/(5yr x 24hr/day x 365days/yr) = 2400mAh/43800h = 55µA. In practice, this value should be derated some- what—perhaps by 10% to 20% or more over a 5-year timeframe, depending on the self-leakage of the particular battery. In this hypothetical design, that would impose an upper limit on the average current of about 45µA. Although the 45µA average operating current limit might seem to be very low, it’s more than sufficient for designs built with very-low-power MCUs. That’s the case for the example design described later in this paper, which applies the Renesas RX111 MCU. 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 prod- uct’s target energy budget. White Paper – Energy Harvesting for Low-Power Sensor Systems Page 3 of 29 If conventional primary cells can’t be used, however, the alternatives are either a rechargeable batteries, supercapacitors, solid-state batteries, or a combination thereof. Solid-state batteries are rechargeable, but they are so new that for the purposes of this document it was deemed best to put them into a separate category. Conventional Batteries Supercapacitors Solid State Batteries + High discharge current + Peak power delivery + Moderate energy density + High energy density + Long life + Near zero leakage + Inexpensive + Inexpensive + Long life / Permanent – Limited life – High leakage + Low cost of ownership – Replacement labor cost – Very low energy density + Form factor – Unsafe, polluting – High temperature + Safe / Eco-friendly – Form factor degradation + Broader temp. range – Form factor Figure 2: Comparison of types of energy storage devices. More application information about batteries is contained in Section III. Energy Harvesting Solutions Energy harvesting technology is exciting, beneficial and—thanks to developments in ultra-low- power semiconductors—meets the needs of and facilitates exciting classes of new embedded system applications. Free Energy Available in the Environment The major types of energy normally wasted in the environment 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 semiconduc- tors and metals with high Seebeck coefficients transform temperature differentials into useful electric energy. White Paper – Energy Harvesting for Low-Power Sensor Systems Page 4 of 29 • Electromagnetic waves—Radio emissions are pervasive everywhere