Read the energy harvesting technology of embedded devices

The development of energy harvesting has two focuses: on the one hand, it is necessary to focus on energy conversion itself (the technology is not yet mature, but a large number of applications will emerge soon); on the other hand, the industry is studying ultra-low-power sensor node devices, nA-level Power consumption has minimal impact on battery life.

If your IoT project is not a robot or machine tool, then it may be (or include) a remote sensor node. It will use a miniaturized battery to power itself. Ideally, energy harvesting techniques are used in IoT projects to completely remove the battery. More likely, the energy collected is used to supplement the battery output, resulting in longer battery life. Therefore, the development of energy harvesting has two focuses: on the one hand, we must focus on energy conversion itself (the technology is not yet mature, but a large number of applications will emerge in the near future); on the other hand, the industry is studying ultra-low-power sensor node devices, nA The power consumption of the stage has minimal impact on battery life.

Ironically, some remote sensor nodes are referred to as "energy harvesters" (referred to as "EH" in some product literature). They use devices that operate with very little current—such as μA/MHz power-consuming microprocessors. The development theme of the EH development kit is not a technology that converts ambient energy into a usable DC voltage, but an ultra-low-power system device such as a sensor, signal conditioning IC, micropower controller, and communication port, making the battery appear dispensable. .

In fact, despite several evolving technologies (for example, large-scale energy conversion using photovoltaic panels), energy harvesting technology is actually in its infancy. Currently, rooftop solar panels can convert enough energy to power a home or office. But large-scale energy storage is still a challenge. Capacitive storage systems (some as large as dump trucks) provide long-term usable energy storage, but for embedded mobile systems, such systems are largely non-portable.

The size of the energy harvesting transducer remains the key to energy harvesting technology. High-power systems rely on larger energy converters: solar cells can deliver 100mW/cm2 of power – 1cm2 is enough to power a pocket calculator. However, the energy collected by other energy harvesting technologies does not reach this target. The thermal gradient energy harvester provides 10 mW/cm2; the vibrating (piezoelectric) energy harvester provides 100 μW/cm2. The radio frequency (RF) energy harvester seems to be very popular because it has too much energy to collect, but it produces 0.1μW (100pW)/cm2.

Energy harvesting technology

Although the efficiency of the ambient energy converter is not particularly high, it is still subject to its working environment. As a ridiculous example, although dye sublimation technology (DSSC) can be used to adjust the sensitivity of a solar converter to indoor illumination wavelengths (eg, about 600 nm emitted by a fluorescent tube), you won't get solar cells locked in an indoor closet. A lot of energy is collected.

Piezoelectric devices (hetero-metal "sandwich") can generate usable voltages by mechanical deformation, but the collected electrical energy is still very small compared to the area and deformation of the sensor. You can charge your phone's battery with the energy generated by the piezoelectric devices in your shoe, but it takes a week.

Thermal gradients are a different type of "sandwich" that is claimed to have high conversion efficiency and high output. It utilizes the Seebeck effect and the semiconductor is sandwiched between the hot plate and the cold plate. Although proponents claim to have high conversion efficiencies, the amount of electricity collected is a function of the hot plate (their size and the temperature difference between the hot and cold metals). The greater the temperature difference, the greater the available electrical energy. However, this energy conversion method is only effective when the temperature difference is large (like a hot plate in the Canadian Arctic).

Applications for piezoelectric energy harvesters include motion devices and vibration monitors. Wireless HVAC sensors and mobile asset tracking have been identified as viable sensors in various manufacturers' product literature; piezoelectric devices appear to be more suitable for detecting mechanical forces and deformations than gaseous conditions such as temperature and humidity.

Intelligent building application

Intelligent building energy harvesters are mainly HVAC sensors for monitoring room occupancy (infrared function) as well as air temperature, humidity and CO2 content. Other intelligent building sensors are used to monitor lighting (including window lights, room lights and shade controls). Safety sensors are used to check for illegal occupancy and intrusion in the room. Power companies monitor the implementation of meter reading and power usage peak-to-peak control. The EH system provides "Platform as a Service" (PaaS) cloud service interaction for Bluetooth and other network communications.

Mailbox features such as device service tags are used with the microcontroller. For data logging applications, the EH module supports cold chain time and temperature monitoring for frozen food shipments. Medical applications include smart patches where sensors are used to monitor blood glucose, body temperature, humidity, pH, and oxygen levels. (The Texas Instruments website has a gas detector reference design)

A cross-application of a piezoelectric device may be an automotive tire pressure sensor that reports the mechanical forces generated by the gas. An interesting new trend in the application of piezoelectric motion detectors is the integration of them into fabrics that support wearable technology.

Figure 1: An interesting new trend in piezoelectric motion detector applications is the integration into fabrics that support wearable technology. (Source: University of Bolton)

Keep small

In technology development, analog semiconductor manufacturers focus their research and development efforts on ultra-low-power semiconductors rather than solar cells or specific tuned vibration sensors. Wearable devices, remote sensor nodes (including mesh networks), motion sensors (such as gas detectors), and motion detectors require small (or even miniature) energy converters rather than using energy harvesters as large as railway locomotives.

Figure 2: A zero-power sensor can collect energy from virtually any environment. Energy includes light, vibration, flow, motion, pressure, magnetic fields, and RF. (Source: Cymbet)

As a result, announcements from various semiconductor manufacturers in their data sheets and white papers emphasize ultra-low power consumption. Signal conditioning ICs such as Analog Devices LTC3588, Maxim's MAX17710, or Texas Instruments bq25504 emphasize ultra-low current consumption even under a variety of mixed loads. For example, the LTC3588 data shows that although its high-impedance input can be used for a variety of energy sources, it is optimized for piezoelectric input. The LTC3588 is essentially a low power AC-DC converter with a quiescent current of 450nA. The input range is 2.7V to 20V, the output can be as low as 1.8V, and the dropout voltage does not exceed 400mV.

Maxim claims that its MAX17710 can manage a poorly regulated input source with an output power range of 1μW to 100mW. The device can deliver more than 20mA from a variety of energy conversion sources. TI's BQ25504 is also essentially an ultra-low power, high efficiency DC-DC that provides continuous energy harvesting from low input sources such as 80mV. Its quiescent current is less than 330nA.

EH processor

The specification of MCU power consumption is similar to that of ultra low power applications. Technically, power consumption is the product of the current consumption (μA or nA) and the generated voltage (usually mV). At the receiving end of the energy harvester, this number may be μV.

A common design goal for IoT is to minimize the size and current consumption of the device used to start the node. System modules (in addition to power management devices) include sensors, sensor signal conditioning circuits, microcontrollers (μA level current consumption), and communication devices (such as Bluetooth Low Energy, BLE) to notify data differences.

Each version of the ARM Cortex M boasts its seemingly minimal power consumption, such as Atmel's 32-bit ARM Cortex M0+, which consumes 35μA/MHz in active mode. The processor consumes a total of 200 nA in sleep mode. "New ARM-powered chip aims for battery life measured in decades" on the US technology blog ARS Technica points out that if we talk about long-lasting battery life, then for this low-power microcontroller, it’s not just a few Years, but decades.

As another example, the Cypress Semiconductor Wireless Sensor Energy Harvest PMIC features a long-interval interrupt timer mode that extends battery life by utilizing long-term interval standby. It works with low-power microcontrollers such as Cypress's PSoC.

The MSP432 is the Texas Instruments version of the ARM Cortex M with 95μA/MHz operating current and 850nA standby current.

Of course, TI made a recommendation in a microcontroller power consumption tutorial: the actual processor power reflects the sum of several different operating conditions. The work situation includes active mode and sleep mode. In embedded system applications, the microcontroller may sleep most of the time. Therefore, sleep mode power consumption may be a more useful indicator than the response at a few MHz clocks.

Therefore, extending battery life seems to be completely related to the ratio of time spent sleeping and active. TI recommends that the power consumption of the microcontroller be "not a number."