The Internet of Things (IoT) is ubiquitous. Although it is powerful, it also brings some challenges. Each sensor element is defined as an electronic device, and the common feature of all electronic devices is that power supply is needed to work.

It is relatively easy to supply power for sensors whether it is wired connection or even button batteries that are replaced from time to time. However, due to the wide deployment of sensors in the Internet of Things, passive sensors must be used in many dangerous situations, such as power supply, long-term monitoring, battery replacement or flammable and explosive applications.

In the application of wireless sensor networks, battery replacement is difficult to solve because of the large number and distribution of nodes. Therefore, the passive sensor with self-powered has a wide application prospect, and is also a research hotspot at home and abroad.

Miniature passive sensor
Passive sensors can not directly convert energy forms, but they can control the input energy from another input or excite the sensor to undertake the task of converting the specific characteristics of an object or process into quantities.

Its "object" can be solid, liquid or gas, and their state can be static or dynamic (i.e. process). Object characteristics can be detected in a variety of ways after being converted and quantized. The properties of objects can be physical or chemical. According to its working principle, the sensor converts the object characteristics or state parameters into measurable electrical quantities, then separates the electrical signals and sends them to the sensor system for evaluation or labeling.

Signal flow of active (a) and passive (b) sensors

Advances in low-power large-scale integrated circuits (VLSI) design and the application of advanced power management technology can control the power consumption of micro-sensors and low-power digital signal processors below 1 mW. Such low power consumption makes it possible to collect ambient energy to power micro-sensors and other electronic devices (i.e., self-powered technology).

Light energy, electromagnetic radiation, temperature variation (temperature difference), human motion energy, vibration source and so on are potential energy sources. The commonly used self-powered energy can be divided into three categories: kinetic energy, radiation energy and thermal energy.

1. Kinetic energy

Kinetic energy is one of the widely existing and easily accessible energy sources. Usually, a mass block that resonates with the main frequency of the ambient vibration is used to collect the vibration energy. Researchers simplified the vibration model and obtained the influence of various vibration parameters on the output power.

(1) The output power is proportional to the square of the amplitude of the vibration source and the mass of the mass block.

(2) Under given excitation conditions, the output power is inversely proportional to the vibration frequency.

There are three typical methods of electromechanical energy conversion: electromagnetic induction method, electrostatic (capacitive) conversion and piezoelectric conversion. Electromagnetic induction electromechanical conversion device, when the coil is perpendicular to the constant magnetic field movement, the coil produces voltage output; variable capacitance conversion has two different ways: voltage constraints and charge constraints. According to the voltage formula between capacitors, when the charge quantity Q or voltage V on the plate remains unchanged, reducing or increasing the distance between the plates or the length and width of the plates can increase the output voltage and obtain energy. When the external harmonic excitation occurs, the harmonic open circuit voltage can be obtained.

Under the existing technical conditions, the maximum open-circuit voltage on the coil is about 15-30 mV, and a transformer with a conversion ratio of 10 is needed to be used as a power supply. Electrostatic conversion can directly generate 2V to several volts of voltage, which can be easily combined with micro-electromechanical system. However, electrostatic conversion requires an independent voltage source to initialize the conversion process. The piezoelectric conversion is not needed, and the voltage is high, and no transformer is needed. The only disadvantage of piezoelectric conversion is that it is difficult to realize miniaturization and integration with microelectronics.

2. Radiation energy
Radiant energy (such as sunlight and electromagnetic waves) is ubiquitous. At noon, solar cells can get about 100 mWcm-energy density under direct solar irradiation on the surface of the earth, but solar cells in cloudy days and indoors can get lower energy density. Solar energy is the most mature technology at present. Its voltage is stable and it can directly supply power for micro-sensors.

In some special applications, by transmitting electromagnetic energy, the receiving end is coupled in a specific way and used, such as radio frequency identification (RF ID) system which is widely used. Secondly, the voltage and power of magnetoelectronic conversion can be improved by designing new transducers or applying new materials. For example, the composite of giant magnetostrictive material and piezoelectric material can obtain higher magnetoelectronic conversion coefficient, which may be used for electromagnetic energy collection.

3. Thermal energy

There are various heat sources in nature. Usually, the temperature difference between the existing and occurring heat sources is used to collect heat energy. According to the principle of Carnot cycle, the temperature difference T limits the Carnot efficiency, that is, the maximum conversion efficiency of thermoelectric. It has been reported that a micro thermocouple device can generate 20 uW power from a temperature difference of 20 C. But within a small range, a large temperature difference is rare.

Technology Development of Passive Sensors

Passive sensors can effectively solve various problems caused by the use of batteries and power sources, but they also face some technical problems that need to be improved and optimized urgently.

For the future research of passive sensors, we should strengthen the design, fabrication and networking technology of the core components of multi-functional, long-life, high sensitivity and long-distance passive sensors, break through the key technology of the development and application of new high-performance intelligent sensor materials for passive sensors, and realize multi-functional integration, high precision, high sensitivity and long-life passivity. Independent research and development of self-energy technology, self-energy technology will make progress in the following directions:

(1) The development of low-power integrated electronic technology and advanced power management will greatly reduce the power consumption of micro-sensors.

(2) Energy conversion efficiency and power will be further improved through the design of new transducers or the application of new materials.

(3) Combination of several collection methods will improve the ability to collect energy under different conditions. It can be predicted that passive sensors with various functions will play an important role in various fields in the era of the Internet of Things.