2.1. Establishing Sensing Elements
The IoT architecture of this experiment is mainly divided into the perception layer, network layer, and application layer. In terms of the perception layer, this study uses a microcontroller to present the data of the temperature and humidity sensors. In the network layer, the packet is transmitted to the receiving module through LoRa transmission technology, and the receiving module is additionally loaded with an Ethernet interface to transmit the data to the local server using the ethernet network. In the application layer, the returned data is stored in the database, and finally the content need to be displayed is displayed on the monitoring webpage. Because the research site of this study was an actual campus, this study used temperature and humidity sensors that met relevant demands such as the temperature range, adaptability, and the moderate precision and resolution of the research subjects were considered. The research site was divided into three categories: above ground and underground of the permeable pavement, shade of the solar panels, and greening rooftop. The measurement parameters were the temperature and humidity felt by the human body.
SHT10 is a patch package sensor type that consists of two parts: the sensing element and the signal processing circuit. The designs are located on a microcircuit board. The output is a designated digital signal. The sensor employs the patented CMOS technology to ensure that it has extremely high reliability and stability. The sensor consists of a capacitive polymer humidity measuring element and a temperature measuring element, a 14-bit analog to digital converter, and a serial interface. The sensor leverages the fact that dielectrics can absorb or release water molecules to maintain a positive correlation with the relative environmental humidity. The change in the capacitance capacity can be measured using an electronic circuit, in turn enabling the measurement of the relative humidity in the air. The CMOS technology involves a layer of protective polymer covering a finger electrode system, thereby enabling the capacitance function. The technology improves the characteristics of a sensor and shields the sensor from external influence. Before the temperature and humidity sensors were buried in the soil to measure soil temperature and humidity, they were covered in casings. The ideal temperature measurement range was between 0 °C and 40 °C, and the temperature range in which the humidity had the smallest bias was between 20 °C and 80 °C.
Table 1 presents detailed information on the parameters.
In the aforementioned description, relative humidity refers to the ratio of absolute humidity to the maximum humidity, representing the water vapor content in the environment. When relative humidity reaches 50%, the air contains half of the water vapor of the air saturation point (100% relative humidity) at the same temperature (25 °C). When relative humidity exceeds 100%, water vapor condenses. As the temperature increases, the amount of water the air can contain increases. Therefore, temperature changes alter relative humidity. Thus, to calculate humidity in a study, temperature must be included as a parameter. Using humidity and temperature, dew point temperature can be calculated.
where
is absolute humidity, or the amount of water vapor contained in a unit volume,
is the amount of water vapor,
is volume,
is relative humidity, or the ratio of absolute humidity to the maximum humidity, and
is the maximum humidity, which is proportional to the air temperature.
Dew point temperature refers to the temperature required for water in the gaseous state in the air to reach saturation and condense into liquid state water under a fixed barometric pressure [
18]. At this temperature, the condensed water floating in the air is called fog, and its attachment to the surface of a solid object is called dew, resulting in the name “dew point temperature.” By measuring humidity and temperature, a precise measurement of the dew point can be obtained. In this study, we adopted the Pearson product-moment correlation formula to measure the test-retest reliability of the sensor. The Pearson product-moment correlation formula is as follows:
where
= reliability indicator
= measured temperature
= mean of the measured temperatures
= test temperature
= mean of the test temperatures
Relevant statistics of the sensor are listed in
Table 2, and the data are presented in a trend graph in
Figure 1. The temperature measured by the sensor differed by approximately 1 °C from that measured by the electric thermometer. The test–retest reliability method was subsequently adopted.
99.95%, where ① =
, ② =
, and ③ =
.
2.2. LoRa Communication Module Setting
The promotion of Industry 4.0 has led to exponential growth in the IoT. However, the IoT, which has been utilized by various fields, is mainly based on wireless transmission technology, and its increased application causes wireless transmission technology such as Bluetooth, Wi-Fi, Thread, Zigbee, and 4G to receive increasing attention from research teams and the public [
19]. Each of these wireless transmission protocol has distinct advantages and disadvantages. Thus, the specific considerations of the implementation sites determine which technology is applied. For example, Bluetooth transmission uses digital modulation technology to increase its interference resistance. It also features coding encryption for security protection. Bluetooth data transmission speeds can reach 1 Mbit/s. Because of its limited transmission distance, typically less than 10 m, this technology is suitable for small areas such as smart homes. Zigbee can transfer data over distances up to 100 m. By increasing its transmission power, the distance can reach several hundred meters or even nearly 1 km. The data transmission speed of Zigbee is 250 kbit/s [
20]. Currently, it has a large market share in wireless illumination. Zigbee is most effective for transmitting small amounts of data over short distances. Thread is a technology similar to Zigbee. It is also used in short-range transmission, such as in smart homes, and it can integrate multiple devices. Wi-Fi, used for the transmission of large amounts of data, is convenient for use in portable devices. Its disadvantage is that it consumes more energy than other wireless transmission technologies [
21]. Moreover, use of a mobile network necessitates the use of a SIM card, which is not suitable for networks with many devices. Therefore, for situations where low power consumption and long-distance transmission are required, such as for long-term biotracking in an environment, LoRa technology was developed. The name LoRa is derived from the combination of “long range.” The major advantage of LoRa is long-distance transmission [
22]. With a single signal receiving and transmission base station, an area of several kilometers is covered. In terms of technology, its wireless assembly is focused on satisfying long-distance communication. Numerous conventional wireless systems are modulated using physical layer frequency-shift keying because it is a high-efficiency and low power consumption approach. Bluetooth, another wireless transmission protocol, has a transmission distance of approximately 60 feet (or 20 m). By contrast, that of Wi-Fi is 200 feet (or 60 m). To increase the packet transmission range as well as transmission speed of these technologies, power consumption must be increased, consequently reducing battery life. Therefore, a key feature for products such as LoRa is low power consumption along with long-distance data transmission.
This study selected the module designed using the SEMTECH SX1278 chip. Because the experimental site was in an urban campus full of buildings, the module with 1 watt transmission power was selected to ensure the effective return of sensing data. The working band was set between 410 and 441 MHz, which could be adjusted and assessed. The detailed specifications are presented in
Table 3.
The LoRa transmission module selected in this study had four work modes, namely general mode, wakeup mode, power-saving mode, and sleep mode. M0 and M1 were used to determine work modes.
General mode: When M0 = 0 and M1 = 0, the module receives serial data. The user can enter 512 bytes of data; however, the length of the wireless packet transmitted by the module is 58 bytes. When the data in the buffer register reaches 58 bytes, the module automatically transmits a packet. At this point, the register can continue adding to the data length from the previous data entry. When the packet to be transmitted is smaller than 58 bytes, the module automatically waits for an interval sufficient for entering 3 bytes. If no packet data are entered, then it determines that the data have ended, and it transmits all packets wirelessly. Packets transmitted under the general mode can only be received by the receiving module under the general mode or the wakeup mode. When receiving, the module’s wireless receiving function is open.
Wakeup mode: When M0 = 0 and M1 = 1, the packet transmission condition initiated by the module and the AUX function are identical to that of the general mode. The only difference is that the module automatically adds a wakeup code in each packet. The length of the wakeup code is determined by the wakeup time set in the parameter in the module setting. The purpose of the wakeup code is to wake up the receiving module that is in power-saving mode. Therefore, when the module is working in wakeup mode, the data transmitted can be received by modules in the general, wakeup, and power-saving modes.
Power-saving mode: When M0 = 0 and M1 = 1, the module is in power-saving mode; that is, it enters a sleep state (which differs from the sleep mode.) The serial sequence is closed and cannot receive serial sequence data from the microcontroller. The mode regularly monitors for the wakeup code. Once it receives a valid wakeup code, the module remains in a receiving state until all valid packets are received. In this situation, the AUX transmits in a low frequency. After a 5 ms delay, it opens the serial sequence wireless data and transmits through TXD. Subsequently, it uses AUX to transmit in a high frequency. Next, the receiving module enters the sleep–monitoring state (polling). According to the wakeup time set, the module exhibits a different receive response delay (up to 2 s) and mean receiving current (minimum of 30 uA;
Table 4).
Sleep mode: When M0 = 1 and M1 = 1, sleep mode is active. During sleep mode, packets cannot be transmitted or received. This mode is for setting parameters. The module parameters are entered according to the series sequence (
Figure 2). Module parameter is as shown in
Table 5. Module parameters can be set directly or by using computer software. They are set according to the research purposes. When a short response time is required, then wakeup code parameters are set. When low power consumption is required, then transmission power parameters are set. When data packet accuracy is demanded, then the transmission rate is adjusted.