This project is a persistent monitoring system with the capability of monitoring parameters like:
• the presence of toxic gasses (methane, liquid gas, and Carbon monoxide)
• temperature and humidity
• helmet wearing detection
• has the damage to the head scale.
The assessed values are transferred into the processor, and the processing operation is performed on the data. The processor compares the input data with the threshold limits (according to the present standards) and, by occurring unexpected unnatural conditions, activates the alarm protocol, including informing the control room operator and activating the alarm section installed on the helmet. This system includes safety helmets, data transmission, and monitoring sections. In the present set, the helmet plays the role of evaluating and transferring data to the data transmission section. The data transmission section provides the communication link between the safety helmet and the control room. In what follows, each section is discussed in more detail.
3.1. Safety Helmet Section
The structure of the safety helmet is demonstrated in figure 1. This section is generally responsible for increasing safety (that is obvious), evaluation, processing, and transferring of data. The smart safety helmet has three subsections of evaluations, data transmission, and alarm. The responsibility and performance of these subsections are discussed in the following.
3.1.1 Evaluation Subsection
This subsection has the responsibility of monitoring and includes DHT11, CNY70, MQ9, LM35, and ADXL345 sensors that are temperature and humidity, infra-red, gas detection, temperature, and accelerometer sensors, respectively. The type and method of operation by these sensors are described in what follows:
A) Gas measuring:
In underground mines, air pollution is caused by methane gas, sulfur dioxide, nitrogen oxides, carbon monoxide, and also hovering particles. When a person is exposed to toxic gases for a long time, it could cause physical damages. MQ9 sensors are utilized to measure liquid gases in this project. MQ9 gas sensor has a high sensitivity to carbon monoxide, methane, and LPG. This sensor is fabricated from a ceramic AL2O3 micro-pipe and a sensitive layer of tin dioxide (SnO2) using a plastic grid and anti-corrosion steel (Kumar and Hancke, 2014). The detection performance of this sensor is through temperature and humidity variations, and the rising density of the gas increases the conductivity of the sensor. These sensors detect carbon monoxide gas while the temperature is falling. As the temperature is rising, this sensor detects gasses of methane, propane, and other flammable gasses (Kumar and Hancke, 2014). As mentioned before, the sensor above has different responses to the density of gasses in various temperatures and humidities. Therefore, some temperature and humidity sensors must be applied at the proximity of these sensors.
Figure 2 represents the index of sensitivity of the MQ9 sensor in 20 degrees Celsius, 65% humidity, and oxygen density of 21%. As it is evident from the figure, the sensor’s resistance has different responses for three kinds of gasses, including carbon monoxide, methane, and liquid gas. It is worth mentioning that these details are programmed by coding in the processor of the helmet and the developed software.
B) Helmet wearing detection
IR sensors are utilized to recognize whether a worker is wearing a helmet or not. The mentioned sensor is composed of a transmitter and a receiver. The IR transmitter is a light-emitting diode radiating IR rays. Although IR lamps look like a regular Light Emitting Diode (LED), the radiated wave is invisible to human eyes. IR receivers are renowned as IR sensors as well because they detect radiation from an IR transmitter such that an IR wave is illuminated on the head while wearing a helmet, and the receiver of this ray does not acquire any data. Otherwise, it is realized that the helmet is not worn after propagation of the wave and receiver’s acquisition (Hermanus, 2007).In addition, to increase the assurance of helmet wearing, IR sensors and accelerometers are implemented simultaneously. It could be deduced that the human head has a particular acceleration range by doing numerous experiments in various modes such as stationary, moving, etc. As a result, in case of not wearing the helmet, if the measured acceleration by accelerometer remains unchanged, wearing or not wearing the helmet would be detected. One of the other anticipated applications of these sensors is reporting the personnel's working hours and attendance performance.
C) Blow to head detection
According to the Federal Motor Vehicle Safety Standard 208 (FMVSS 208), a scale is defined with the symbol of HIC to detect the blow to the head, which should have values not exceeding 1000. This value is calculated using equation (1) (Chou, Song and Lim, 1997), (forooshani, Bashir, Michelso and Noghanian, 2013), (Laksari et al, 2020), (Davis et al, 2000), (Saboori, Mansoor-Baghaei and Sadegh, 2013), (Thombare, 2019) and (Hutchinson, Kaiser and Lankarani, 1998).
(1)
The damage scale is known as a measuring scale. The severity of the damage could be evaluated using the Abbreviated Injury Scale (AIS) that is varied from AIS(0), meaning no damage up to AIS(6), meaning unavoidable damage (forooshani, Bashir, Michelso and Noghanian (2013). In order to validate and calibrate the relation between the injury scale and the damage to head scale, some experiments should be performed under certain circumstances. HIC is a damage scale related to a skull fracture (AIS ≥ 2) and brain injury (AIS ≥ 4). The conversion from HIC into damage scale could be achieved by referring to the data plotted on figures 3 and 4 that are represented according to the experiments by Mertz, Prasad and Irwin (1997) to detect the presence or absence of skull fracture and intracranial hemorrhage. Furthermore, it is noted that the accelerometer produces wrong measurements. To overcome this design issue and compensate for it, the accelerometer is placed in the helmet rather than its plastic tape, which holds the head (Newman, 1975).
D) Temperature and humidity detection
The lack of appropriate ventilation in underground mines leads to temperature and humidity rise in these spaces that could cause reduced efficiency of the production process. Therefore, real-time and continuous monitoring of the temperature and humidity of the workplace is an unavoidable issue. On the other hand, to measure the density of the gasses precisely, the need for temperature and humidity sensors is obvious.
The implemented temperature sensor in this project is DHT11, one of the accurate temperature and humidity sensors with an analog output with a linear dependence on the environment’s temperature in degrees Celsius. One of the advantages of this sensor is measuring the temperature on a Celsius scale, while most sensors work on the Kelvin scale. In addition, the humidity measured and represented by this sensor is in the form of relative humidity. This sensor does not need further calibrations (Ahalya, Babu and Rao, 2013).
3.1.2 Data Transmission Subsection
After processing data acquired from the evaluation subsection, the processor must send these data to the control room for monitoring and decision making. So, in order to transmit mentioned data, we decided to use a radio module. The processor is the core of the project because it processes the evaluated data from sensors and communicates with the data transmission subsection according to its predefined responsibilities (Fisher, Ledwaba, Hancke and Kruger, 2015). The ZigBee module is a wireless chip signals of which are capable of penetrating the walls and conforming to underground mine circumstances. The standard ZigBee module has a digital interface that allows every processor or microprocessor to rapidly use this protocol's services. ZigBee's performance is conformed to the IEEE 802.15.4 standard (Hongjiang and Shuangyou, 2008). ZigBee systems could be programmed to work in various network topologies like the star, light mesh, mesh, and clustered tree, four of the most applicable structures. Temporal networking structures are used where a node should send the information to all other nodes.
3.1.3. Alarm subsection
Alerting the mineworkers is a tricky process due to working conditions. There is not enough light in the space of underground mines, so the workers use safety helmets with replaceable mineral lamps. The utilized equipment generates too much noise and vibrations due to space limitations. The alarm subsection is composed of vibration, alarm buzzer, and LED units to overcome the mentioned problems. The considered protocol for alarm subsection is capable of responding in various conditions.
3.2. Monitoring Subsection
After acquiring the processed data, all of this information will be transferred into a software through a wireless network platform. The provided software has the duties of on-time detection of the occurred dangers and activation of the relief and rescue protocols. Figure 5 shows the developed software.