# Smart Helmet-Based Personnel Proximity Warning System for Improving Underground Mine Safety
## 3 Experiment of the Proximity Warning System Based on Bluetooth Beacon and Smart Helmet
### Subjective Workload Assessment of Smart Helmet-Based Personal PWS  
Workload is a quantitative measure of the amount of mental stress a person experiences while performing tasks within a particular system [51]. Workload is affected by psychological (focus on work and anxiety), physical (physical difficulties and difficulty in controlling machines), temporal (deadlines), and environmental (noise and relationships with colleagues) factors [52]. If the workload is not properly adjusted when designing the system, overload can occur, and the work efficiency can be reduced. Therefore, it is necessary to improve the work efficiency by designing and operating a system with minimal workload. Subjective workload evaluation can be performed using a questionnaire. It is frequently used in human-machine system development [53]. Representative subjective workload evaluation methods include the NASA-TLX [45], subjective workload assessment techniques [54], and workload profile techniques [55]. In this study, subjective workload was evaluated using the NASA-TLX method. The psychological, physical, and temporal effects on workers using the personal PWS while wearing smart helmets and working at the mining site were evaluated. The NASA-TLX is a multidimensional grading procedure that estimates the overall workload score based on a weighted average of six factors [56]: mental, physical, temporal, overall performance, effort, and frustration. These workload parameters are defined as follows:  
* Mental demand: how many mental and cognitive skills are needed to accomplish this task? * Physical demand: how much physical ability do you need to perform this task? * Temporal demand: how much duress did you feel due to the rate or pace at which you performed multiple tasks? * Overall Performance: how successfully do you think you have achieved the goals of this task? * Effort: how much mental and physical effort was required to achieve your work goals? * Frustration Level: how much discomfort have you felt while working on this task? The response of the worker to these the six workload parameters was evaluated. All the parameters except for \"Overall Performance\" (scoring from good to bad) were graded from low to high with values between 0 and 100 (in increments of 5). The weights of the six parameters were calculated using pairwise comparisons, and the overall workload score was calculated by averaging the product of each factors score and weight. Figure 7: Overview of the performance test of the smart-helmet based PWS. (**a**) Experimental model of BLE signal detection distance measurement performed by considering the Tx power of the Bluetooth beacon; (**b**) BLE Signal detection distance measurement model according to the perception angle between the Bluetooth beacon and the smart helmet. Figure 8 is a schematic showing the experiment conducted. Three equivalent experiments were performed under the same experimental conditions to compare the effect on the subjective workload. In this study, the subjective workload evaluation was performed on 10 experimental subjects aged 24 to 26 years old (average age was 24.9 years) at the same location where individual PWS performance was evaluated. More than half (60%) of the test subjects said they had knowledge of smart glasses, and the majority (80%) said they had no knowledge of smart helmets. The test subjects used (a) a smartphone-based personal PWS (driver's position), (b) a smart glass-based personal PWS (worker's position), and (c) a smart helmet-based personal PWS (worker and driver's position). For this experiment, we used the smartphone-based personal PWS by [PERSON] and [PERSON] [13], the smart glass-based personal PWS by [PERSON] and [PERSON] [25] and the smart helmet-based personal PWS developed in this study. In the experiments, the test subject stood at the center of the transport route and examined the condition of the transport route (worker's position) or boarded a truck or loader (driver's position) to approach the subject.