Design of power supply for wireless sensor nodes based on environmental electromagnetic waves2

In order to enable sensor nodes to work in areas with less environmental electromagnetic wave energy and further reduce the power consumption requirements of sensor nodes, the sleep/wake-up mechanism was studied and a power management circuit with a timed wake-up function was designed. The power management circuit controls the working and sleep states of the node. The power management circuit consists of a storage capacitor and a voltage detection circuit, as shown in Figure 5.

The energy storage capacitor is composed of a 1000μF tantalum capacitor, and the voltage detection circuit is composed of the AD and MOS tube of the MCU. When S1 is closed, the energy collection system starts to charge the energy storage capacitor. S2 is first turned on the bottom to limit the charging voltage. When the LED is on, the voltage across the energy storage capacitor is about 3.4 V. At this time, S2 can be turned on to make the node enter the timed wake-up working state.

Then, when the charging current is greater than the static current consumption of the node, the capacitor can be charged. The AD of the MCU detects the voltage across the energy storage capacitor every 5 s. When the voltage is <3 V threshold, the MCU and the radio frequency unit (RF) are in sleep mode to reduce power consumption. When the voltage reaches the 3 V threshold, the MCU is awakened, and the temperature data is collected using its internal temperature sensing device and returned to the PC through the radio frequency unit. In the sleep state, the static current consumption of each part is shown in Table 1. In the sleep state, the total static current consumption is <2μA, which reduces unnecessary energy consumption while meeting the regular operation of the node. Low-power RF wake-up wireless sensor network nodes have lower power consumption than nodes using traditional sleep/wake-up mechanisms.

3 Experimental test and result analysis

3.1 AM radio wave energy stability test

In order to evaluate the stability of AM radio wave energy, a 7-day test was conducted. During the test, a 5-level voltage multiplier was used and the output voltage of the antenna was measured every 10 minutes. Since the medium wave transmission tower works around the clock and the medium wave is mainly ground wave propagation, it is basically not affected by climatic conditions. The actual test results are shown in Figure 6. There was a period of time from Wednesday to Thursday when the voltage dropped sharply. After investigation, it was found that it was caused by the weekly shutdown and maintenance of the medium wave transmission station. During the rest of the time, the output voltage fluctuated by about 7 V, and the amplitude deviation did not exceed 30%, basically showing a periodic change of 24 hours.

Qualitative tests were conducted on the interaction between the energy harvesting antenna and the AM radio. Comparing the effect of listening to the radio station with an ordinary AM radio within 1 m of the antenna and 50 m away from the antenna, it was found that the sound quality and volume of the two were basically the same. At the same time, the antenna output voltage was also relatively stable in both cases.

3.2 Analysis of node working stability

In order to detect the correctness and feasibility of the energy harvesting scheme, a sensor node was designed to detect the ambient temperature and perform timed transmission communication through a radio frequency unit.

In order to ensure that the sensor node works normally and stably in the long term when collecting temperature data and timing communication, the MCU must also work within the allowable voltage range: 1.9~3.6 V. This requirement can be met by selecting a suitable energy storage capacitor value.

In general, the time required for nRF2402 to transmit data is 3.5 ms, and the average current is 11 mA. Each time the sensor node collects temperature, it consumes the energy stored in the energy storage capacitor, resulting in a voltage drop across the energy storage capacitor. The calculation of the voltage drop VD is shown in formula (1).

In the formula, CS is the capacitance of the energy storage capacitor; IW and TW are the average current and time required for the RF unit to transmit once. The calculated VD is about 0.04 V, that is, after the RF unit works once, the voltage across the energy storage capacitor is ≥2.9 V, which is greater than the lower limit of the MCU operating voltage of 1.9 V. The voltage across the energy storage capacitor will not exceed the 3 V threshold, so choosing a 1 000 μF energy storage capacitor can make the MCU work within the allowable voltage range.

3.3 Calculation of effective working range

The cut-off operating voltage of the wireless sensor node is 1.9 V, and the cut-off operating current is 3 μA. When powered only by AM radio waves, the sensor node can only be driven to work when the output power of the energy receiving antenna is >5.7 μW. According to the receiving efficiency of the L-shaped antenna with a length of 10 m and a height of 2 m from the ground used in this study, the spatial field strength needs to be >44 mV/m for the sensor node to work. By calculating the electromagnetic radiation field strength distribution of the medium wave transmitter in space, the effective working range can be calculated.

For a single-tower medium-wave antenna, in the far field, as the distance increases, the radiation field strength decreases and can be calculated using the field strength calculation formula (2).

Where: r is the distance between the measured location and the medium wave transmitter, in kw; P is the nominal power of the transmitter, in kw; G is the antenna gain relative to the basic oscillator; A is the ground wave attenuation factor. In urban areas, when the transmitter 100 m high transmits 810 kHz radio waves, A=1.39. Calculated by formula (2), within 30 km from the transmitter, the field strength can reach 44mV/m. Therefore, wireless sensor nodes powered by AM radio waves can work within 30 km from the medium wave transmitter.

4 Conclusion

The key technologies for obtaining environmental electromagnetic wave energy for wireless sensor nodes were studied, and a feasible power supply scheme was designed. First, the energy distribution of the electromagnetic wave frequency band in the environment was measured and analyzed to provide a basis for the design of energy collection circuits. Reasonable antennas and resonant circuits were designed to convert, store and reasonably amplify signal energy. A power management circuit with a timed wake-up mechanism was designed to enable the node to work in areas with less electromagnetic wave energy. Experimental tests show that the scheme is correct and feasible, and can provide working energy for low-power sensor nodes to complete the designed data collection and communication tasks. The energy obtained is stable and can work for a long time around the clock. In addition, through effective power management technology, the node can work in a weak electromagnetic field environment. Using different forms of antennas can adapt to the application requirements of different occasions. By improving the receiving efficiency of the antenna and combining multiple power supply methods, the sensor node can work farther away from the medium-wave transmitter.