# Smart Helmet-Based Personnel Proximity Warning System for Improving Underground Mine Safety
## 1 Introduction  
In underground mines, worker safety accidents frequently occur, owing to collisions between equipment and pedestrians or other equipment. The U.S. Bureau of Labor Statistics stated that there were 45 fatalities due to equipment collisions in underground mines in the United States between 2011 and 2019 [1]. Accidents were mainly caused by being caught in running machinery, struck by powered vehicles, or compressed by equipment. According to a disaster report published by the government of Western Australia, there have been a total of 34 equipment collisions involving haulage trucks and charge-up trucks in underground mines in Western Australia since 2015 [2]. Proximity warning systems (PWSs) have been developed to prevent equipment collision accidents in underground mines. PWSs provide visual and/or audible proximity alerts to equipment operators of pedestrians or other equipment approaching within a certain distance [3]. The National Institute for Occupational Safety and Health (NIOSH) in the U.S. has developed PWSs using radio-frequency identification (RFID) and electromagnetic signals. [PERSON] and [PERSON] [4] developed an RFID-based collision warning system to provide a proximity warning to equipment operators when a pedestrian approaches a front-end loader. Active tags were attached to the belt or cap of a pedestrian worker, and an RFID reader was installed on the front-end loader to recognize the unique ID of the active tag. This system sets the progressive sensing distance (near, middle, and far) and provides visual and audible alerts to the equipment operator through lamps and buzzers. [PERSON] [5] suggested the hazardous area signaling and ranging device to provide a proximity warning to the equipment operator approaching a continuous miner. Thewire loop antenna was installed on a continuous miner to generate a magnetic field. The receiver measured the magnetic field strength and provided visual, audible, and vibration alerts to the equipment operator when the magnetic field strength exceeded a certain threshold. Various commercial products based on PWSs developed by the NIOSH have been released [6; 7; 8; 9; 10]. Bluetooth low energy (BLE) technology has been actively applied in the development of PWSs in underground mines. BLE is a Bluetooth 4.0 communication technology in the 2.4 GHz band that was launched in 2010 [11]. Compared to the existing Bluetooth classic technology, this enables high-speed data transmission with low power consumption [12]. [PERSON] and [PERSON] [13] proposed a Bluetooth beacon-based PWS that provides proximity alerts to equipment operators using a smartphone. A Bluetooth beacon was attached to the pedestrian worker's helmet, haulage equipment, and dangerous areas. In addition, a smartphone was mounted on the equipment. The smartphone measured the received signal strength indicator (RSSI) of the BLE signal and provided visual, audible, and vibration alerts when the RSSI exceeded a certain threshold. [PERSON] and [PERSON] [14] implemented a system in an underground limestone mine and successfully demonstrated the system utility. However, existing PWSs provide only one-way proximity warnings to equipment operators through in-cab screens, such as smartphones and tablets installed inside the equipment. Consequently, it is difficult for pedestrian workers to be aware of the collision risks in underground mines. Various wearable PWSs have been proposed to provide proximity warnings to pedestrian workers. In contrast to conventional PWSs, signal transmitters were installed on the equipment, and receivers were attached to pedestrian workers. Wearable PWSs can be classified into worn-type personal alarm devices (PADs), smart vests, and smart glasses. PADs can be worn on the worker's belt, pocket, and cap, and mainly provide audible or vibrating alerts. Examples of commercialized PAD products are ELOsheld by ELOCKON [15], Smartzone proximity system by Komatsu & JoyGlobal [16; 17], HazardAvert(r) by STRATA [18], and more [19; 20; 21; 22].