34 March 2019 Mınıng engıneerıng www.miningengineeringmagazine.com

Safety

Intelligent monitoring system for improved

worker safety during plant operation and maintenance

by D. Parks, M. McNinch, R. Jacksha, H. Nickerson and A. Miller

D. Parks, M. McNinch, R. Jacksha and A. Miller are mechnical 

engineer, mechanical engineer, 

electronics engineer and senior 

scientist, respectively, CDC NIOSH, 

Spokane, WA and H. Nickerson is 

batch plant lead, Central Pre-Mix, 

a CRH Co., Spokane Valley, email 

nij4@cdc.gov.

Each year hundreds of mine workers are involved in machinery-related accidents. 

Many of these accidents involve inadequate 

or improper use of lockout/tagout (LOTO) 

procedures. To mitigate the occurrence of these 

accidents, new safety methods are needed. 

These methods should include the monitoring 

of access to hazardous areas around operating 

machinery, improving documentation/monitoring 

of maintenance activities which require 

shutdown of the machinery, and preventing of 

unexpected startup or movement during machine 

maintenance activities.”

Of the hundreds of machinery-related 

accidents occurring every year in the mining 

industry, incidents involving stationary 

machinery at surface mines continue to be 

among the most frequent (MSHA, 2017). A U.S. 

National Institute for Occupational Safety and 

Health (NIOSH) study showed that the majority 

of fatal accidents involving stationary machinery 

at surface mines occurred at sand and gravel 

(38 percent) and stone (26 percent) operations. 

Of these accidents, entanglement in conveyor 

components were the most common cause of 

fatal accidents (48 percent) (Ruff et al., 2011). 

The same study stated that one-third of these 

accidents involved improper lockout/tagout 

(LOTO) procedures as a contributing factor. 

The U.S. Mine Safety and Health Administration 

(MSHA) acknowledged this problem, stating in 

a recent request for information (MSHA, 2018): 

“Since 2007, there have been 17 fatalities related 

to working near or around belt conveyors, 

of which 76 percent were related to miners 

becoming entangled in belt drives, belt rollers 

and discharge points. Factors that contribute to 

entanglement hazards include inadequate or 

missing guards, inadequate or an insufficient 

number of crossovers in strategic locations; and/

or inappropriate lock out/tag out procedures. 

Systems that can sense a miner’s presence in 

hazardous locations; ensure 

that machine guards are 

properly secured in place; 

and/or ensure machines 

are properly locked out 

and tagged out during 

maintenance would reduce 

fatalities” (MSHA, 2018).

In response to this 

problem, NIOSH’s Spokane 

Mining Research Division 

(SMRD) is exploring the potential application of 

Internet of Things (IoT) technologies to provide 

cost-effective intelligent machine monitoring 

systems for improved worker safety (Zhou et 

al., 2017; Reyes et al., 2014). SMRD partnered 

with Central Pre-Mix CRH Co. (Central PreMix) to develop and install a proof-of-concept 

wireless IoT solution to monitor machinery and 

conveyors during operation and maintenance. 

The primary goals of the system were to provide 

real-time monitoring of access points and 

facilitate the planning and execution of LOTO 

procedures. 

This article describes the design and field 

deployment of this system. Field demonstration 

is a first step toward widespread adoption of 

intelligent safety monitoring systems and holds 

promise to reduce accidents related to machine 

guarding and improper implementation of 

LOTO procedures.

Monitoring of LOTO and confined-space 

requirements

Machinery monitoring requirements for 

this study were driven by Central Pre-Mix’s 

concrete batch plant daily operational and 

maintenance practices. High-priority needs 

included monitoring access to the mixing area, 

measurement of concrete batch temperatures, 

measurement of the temperature of the concrete 

mixing drum main support bearings and, end-ofshift maintenance of the concrete mixer drum.

Access to mixing area. While access to the 

mixing area is not restricted, monitoring the 

access door allows batch-plant operators to be 

aware of any workers entering or leaving the 

vicinity. 

Concrete batch temperature. Knowing the 

concrete batch temperature is critical to ensure 

quality and long-term integrity of the concrete. 

The batch temperature could be determined 

by a worker using a handheld infrared (IR) 

thermometer. However, this is hazardous as the 

worker could be entangled in the mixer drum 

while taking measurements.

Temperature of mixer drum main support 

bearings. The temperature of the concrete mixing 

drum main support bearings is checked regularly 

as a predictor of necessary maintenance or 

potential failure. A sudden bearing failure 
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Safety

could be hazardous to workers and incur costly 

repairs, but this can be mitigated through routine 

monitoring for elevated bearing temperatures.

LOTO/confined space. Daily maintenance of 

the mixer drum requires a worker to crawl inside 

the drum and remove accumulations of hardened 

concrete using a pneumatic rotary hammer 

(mixer drum chipping), as shown in Fig. 1. Access 

to the mixer drum entry is through a gate labeled 

with a confined space warning sign. Prior to 

this, electrical power is isolated from the drum, 

charge belt, mixer feed conveyor and hydraulic 

pump motors using four disconnect switches. The 

daily cleaning procedure thus entails both LOTO 

and confined space protocols. 

During daily mixer maintenance, the four 

disconnects are locked in the OFF position 

(lockout) using a long bar held in place by 

a single padlock (Fig. 2). A tag belonging to 

the worker(s) involved is placed on the lock 

(tagout), then verification of electrical isolation is 

performed by attempting to start each motor (try 

out), at which point the LOTO is complete and 

the worker may enter the drum.

Before entry, a confined space permit must 

be filled out. Both the worker who is performing 

the chipping and an attendant who will remain 

outside the drum sign the form. Once the mixer 

drum chipping is complete, the confined space 

entry permit is indicated as complete by writing 

the word “canceled” in big letters across the 

form. The last step is filing the completed permit 

onsite.

System design considerations and 

hardware selection

The system was designed as proof-of-concept 

and thus is exploratory in nature. The central aim 

was to determine the viability of IoT in providing 

intelligent machine monitoring and assisted 

LOTO. System design considerations included 

wired versus wireless technologies, sensor types, 

sensor node power requirements, sensor node 

network topology and data transport method. 

 Wired sensors were considered impractical 

for this work, since NIOSH’s eventual goal 

is to demonstrate the technology for mine 

machinery and conveyor systems in a large 

surface operation such as an SSG facility. 

A wired solution would severely increase 

installation and maintenance costs, along with 

slowing deployment. Further, some sites would 

have limited access to ac power. It was with these

factors in mind that battery-powered sensor 

nodes were selected. This requires that batteries 

be replaced on a yearly basis.

The mixer area presented a challenging 

environment for a wireless network since it 

contains large metal objects such as the mixer 

drum and its support structure, conveyors, 

conduits, and electrical panels, all of which will 

create signal interference. Also, only three of 

the proposed sensor node locations had a lineof-sight to the IoT node coordinator (gateway). 

Given the difficult environment, it was decided 

that a self-configuring, self-healing mesh network 

topology would offer the best chance for robust 

wireless links (Akyildiz and Wang, 2005; Bruno 

et al., 2005).

Figure 1

Entering mixer drum to perform maintenance.

Figure 2

Locked out disconnect panel.

In order to process data independently 

of Central Pre-mix’s information technology 
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Safety

network, a cellular data transport (backhaul) was 

determined to be the best choice to transport 

sensor data to cloud storage (Zheng et al., 

2012). This does not place a burden on the 

site’s network infrastructure, and it avoids any 

concerns regarding confidential data.

Figure 3

A SmartSwarm cellular gateway (left), industrial-grade Wzzard node 

(middle) and commercial-grade Wzzard node (right).

With the system design criteria defined, a 

review of various wireless IoT technologies 

and manufacturers was conducted, seeking 

products that were commensurate with the 

requirements. It was found that the Wzzard 

platform by Advantech (Ottawa, IL) provides 

such a solution in the form of rugged IP67 rated 

nodes. Additionally, these nodes come configured 

with a variety of analog and digital sensors and 

create a self-forming and self-healing mesh 

network topology. This solution is scalable to 

hundreds of nodes and has available a cellular 

gateway for ease of data transport to cloud 

storage. Figure 3 shows a photo of a SmartSwarm 

cellular gateway (by Advantech) on the left and 

two Wzzard sensor nodes on the right. These 

sensors utilize the message queuing telemetry 

transport (MQTT) protocol, which is a publish/

subscribe protocol requiring a publisher (the 

sensor), a broker (the gateway), and a subscriber 

(the webpages). This type of network is time 

synchronized, which helps to ensure low power 

by reducing the duty cycle. However, this also 

means that data is received, at most, every 10 

seconds.

Cloud data storage

The sensor data is sent to the cloud from 

the local mesh network using a cellular gateway, 

creating an additional network parallel to the 

site’s infrastructure. This provides additional 

security by segregating the collected data from 

sensitive material on the site’s existing network. 

Additionally, the use of a cellular backhaul 

provides a quick installation, which can be easily 

upgraded, replaced or relocated at the site. 

The monitoring system uses the cloud for 

data storage, which has become common for 

enterprise applications, as it offers many benefits 

over traditional local storage. Foremost, the 

monitoring data can be easily viewed remotely, 

providing personnel with real-time data offsite via a PC or mobile device. A safety officer, 

management or foreman will receive alerts 

concerning LOTO violations or impending 

equipment failure and can readily check current 

conditions in the plant. Cloud storage also offers 

scalability. Should the monitoring system expand, 

the local network and storage would be placed 

under a heavier load and would require upgrades 

and maintenance. Cloud storage removes this 

problem while keeping costs low. 

One concern about cloud storage is security, 

namely data leaks. To help alleviate these 

concerns the data should encrypted during 

transport to the cloud and while it is stored on 

the cloud. Further, users of cloud services are 

able to effectively own their data by controlling 

their own encryption keys (Wall, 2016). These 

practices along with two-factor authentication 

will do a great deal for improving data security. 

It should be noted that many of the recent 

infamous data breached systems, for example 

Sony and Target, were not cloud based but rather 

internal data systems. 

Installation, configuration and commissioning 

The first step in field installation was to 

install sensors and connect them to the wireless 

nodes. For the access door and entry gate, 

magnetically activated reed switch sensors were 

selected. With this method, the opening of the 

gate/door is detected while vibrations do not 

cause false readings (Reyes et al., 2014). For 

temperature measurements on the bearings, 

contact thermocouples were installed under 

mounting bolts with thermal grease. Finally, the 

batch temperature was measured using a selfpowered IR thermocouple.

Once sensors were in place, each was 

connected to its dedicated node. The roller 

temperature nodes were set to a publish rate of 

two minutes, whereas the door, gate and safety 

disconnects were set to publish every 10 seconds. 

Publishing every 10 seconds will greatly impact 

battery life but was deemed necessary to provide 

timely worker location data. 

After installation, all of the nodes were 

configured to communicate with a gateway on 

which a scripting software (Node-RED) was 

used to parse the data and create local webpages 

(independent of Internet connectivity for 

redundancy), displaying the sensor data on a 

web browser. Additionally, the gateway collects 
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Safety

the data from sensors and sends the data in the 

form of encrypted MQTT messages to the cloud 

where scripting software parses the data and 

relays it to webpages for remote viewing.

The redundant local viewing (store and 

forward configuration) provides a safeguard 

in the (yet to be encountered) case where the 

cellular network fails. The local display consisted 

of a 48-cm (19-in.) touchscreen and a Raspberry 

Pi, essentially acting as a thin client, modified 

to operate in kiosk mode. This does not ensure 

remote viewing capability in the event of 

cellular network failure but does provide those 

on site with accurate up-to-date safety-related 

information. Remote or local viewing is possible 

on any device with a current web browser, as 

shown in Figs. 4 and 5. 

 In addition to viewing data, the prototype 

system allows the worker to use a tablet or cell 

phone to populate forms that are required for 

LOTO or confined space procedures (typically 

done using paper and pencil). While this 

currently does not supplant paper forms, the 

digital submission and archiving of such forms 

provides useful data that can easily be referenced 

later. It is however hoped that in the future 

easy-to-use mobile forms or strategically located 

LOTO kiosks will replace the current paper and 

pencil method.

Currently the supplemental form data are 

compared with sensor data thereby confirming 

that the LOTO process is performed according 

to policy and providing alerts in the event that 

it is circumvented. For example, if there has not 

been a form submitted to plan the chipping of 

the mixer and the gate is opened, an alarm is sent 

via text message and email. Alarms are also sent 

in the event that the chipping of the mixer takes 

inordinately long.

Two key challenges to the implementation 

of the system were battery life of the nodes and 

configuration of the network to ensure all nodes 

had a robust connection. These two issues are 

closely related in that one of the main causes 

of reduced battery life occurs when nodes are 

repeatedly seeking and failing to connect with 

each other. This can be aided by using better 

antennas (e.g. larger, externally mounted) or 

by installing repeater nodes in cases where 

neighboring nodes are failing to connect 

properly. 

To address these issues and troubleshoot 

connectivity problems, one may utilize the 

“network health reports.” Using these reports, 

nodes that are having to repeatedly rejoin the 

network can be detected, and the signal strength 

among all of the nodes is detailed. 

Figure 4

Web browser view of disconnect switches and gate/door statuses.

Figure 5

Web browser view of online confined space entry form.
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Safety

Summary

A wireless IoT system was utilized to 

monitor a LOTO process and shows promise 

for both making LOTO less burdensome and 

providing additional accountability for following 

required safety practices. NIOSH researchers 

are currently expanding this system in terms 

of scope and functionality. The final system is 

expected to include predictive failure analysis 

as well as additional sensors to monitor worker 

proximity to hazards. Significant research will 

be conducted in the field of human factors in 

hopes of ensuring that the system is easy to use 

and effective, thereby encouraging widespread 

adoption throughout the mining industry.

The proliferation of IoT devices in the 

business and consumer spheres illustrates 

that the public is ready to adopt these new 

solutions. The mining sector in particular will 

benefit enormously from the ability to remotely 

view safety-related data in real time and to 

receive alarms when safeguards are potentially 

failing. There is a preponderance of evidence 

that conveyor operation and maintenance is 

hazardous to workers, and NIOSH will continue 

working to reduce these accidents through 

leveraging emerging technologies such as IoT.

Future opportunities 

In the future, it is hoped that LOTO can be 

performed seamlessly without having to leave 

the machine and return to the office to fill out 

paperwork. This will first require mobile or 

strategically placed devices for planning the 

LOTO, and secondly, appropriately placed locks, 

which ideally will be networked as well. Such a 

lock will discern not only if it is open or closed, 

but also that it is being used on the appropriate 

piece of equipment, i.e. the piece of equipment 

for which maintenance has been planning to use 

the electronic forms.n

Acknowledgments

The authors wish to acknowledge the 

management and staff of Central Pre-mix, 

a CRH Company, for its cooperation and 

assistance in this work. Additionally, the 

authors would like to express their gratitude 

to Greg Bierie, Managing Director of Safe 

Reliable Systems, for partnering with NIOSH 

in an effort to bring this solution to wide-scale 

implementation. 

Disclaimer

The findings and conclusions in this paper 

are those of the authors and do not necessarily 

represent the official position of the National 

Institute for Occupational Safety and Health, 

Centers for Disease Control and Prevention. 

Mention of any company or product does not 

constitute endorsement of NIOSH.

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