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A MINIATURE DATA ACQUISITION SYSTEM WITH LED WARNING LIGHTS 

C. Sunderman, S. Signer, and J. Johnson 

ABSTRACT 

 Researchers at the National Institute for Occupational Safety and Health have developed a miniature data acquisition 

system (MIDAS) that can measure resistance changes and temperature, store data for later retrieval, and illuminate LED 

lights. This instrument can be used with strain-gaged bolts, cable bolts, CSIRO stress-measuring gages, string pots, and 

any other instrument that uses single-ended, resistance-type gages. MIDAS has the capability to measure up to 16 gages 

at preset time intervals and store more than 2000 scans, yet is small enough to fit in a 35-mm hole. Because the MIDAS 

consumes little power, a 9-V alkaline battery will operate the system for 6 months. Data from the MIDAS can be sent to 

a PC via an RS232/RS485/IRdA port. 

 A software program was written to set operation parameters, graph data, and store all information directly to a 

spreadsheet. Three LED lights change from green to yellow then red based on reading levels and/or rates of change. This 

feature can be used with different types of gages to warn miners of possible rock instabilities. The MIDAS has received 

MSHA experimental permissibility approval and is currently being tested at several mines. 

1. INTRODUCTION 

 Roof falls are a leading cause of injury and fatality in underground mines in the United States. Mine Safety and 

Health Administration (MSHA) statistics from 1997 to 2001 show that fall of rock from a mine roof was the leading 

cause of injury to all underground mine workers. Of 40,004 reported accidents, 10,438, or 26.1%, were caused by roof 

falls. 

 Although studies conducted by researchers from the U.S. Bureau of Mines and the National Institute for 

Occupational Safety and Health (NIOSH) have demonstrated that geotechnical instruments can be effective in 

identifying and monitoring ground control hazards, modern rock mechanics instruments are rarely used by the mining 

industry in the United States. Instead, data are collected by either taking readings with hand-held instruments or 

installing electronic data acquisition systems. Hand readings are very labor intensive and tend to be collected 

infrequently. Typical continuous electronic data acquisition systems are large, difficult to use, and expensive, and long 

lead wires are required to connect sensors to the instruments. In addition, many mines, especially smaller operations, do 

not have the expertise nor the resources to design an effective instrumentation plan, properly install and monitor a variety 

of instruments, and analyze and interpret electronic readings obtained from the instruments. 

 Another problem is that all electronic data acquisition systems must have MSHA approval if they are to be used 

anywhere but in fresh air in mines that release methane as a by-product. Two companies that used to manufacture 

general-purpose, MSHA-approved systems no longer do so, which has severely limited the use of such systems. 

 NIOSH developed a miniature data acquisition system (MIDAS1

) that addresses these problems. This system has the 

potential to make the use of geotechnical instruments in underground mines more feasible. The MIDAS can be used with 

any instrument that uses resistance-type sensors. Several different instruments were developed by the U.S. Bureau of 

Mines (fully grouted, cable, and friction bolts with strain gages attached; bolt load cells) and NIOSH (rock strain strips) 

to monitor rock movement (Marshall 2000) and loads on rock bolts (Martin 2001; Signer 2000). The MIDAS is one of 

the newest instruments developed to measure rock movement and has been given an experimental permit by MSHA. 

 

1 The MIDAS has a patent pending, and information regarding licensing or use of this circuit should be directed to the 

authors. 
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2. MINIATURE DATA ACQUISITION SYSTEM (MIDAS) 

1. Hardware 

Recent advances in analog integrated circuits enabled the MIDAS to be small, accurate, power efficient and

relatively inexpensive, yet rugged and watertight (figure 1). The heart of the MIDAS is a sigma-delta analog-to-digital

converter (figure 2). The specific converter chosen for this design is more sensitive and accurate than other parts of the

circuit and can produce nearly 24 bits of useful data at a sampling rate of 2 Hz. When coupled with the rest of the input

circuitry, it can resolve approximately 20 bits. 

At the point of input to the converter is an excitation source

and multiplexer (figure 3). The excitation source is fixed at 4.5 V 

and is buffered so that it can power 120-ohm strain gauges with 

negligible voltage offset. Sixteen input channels connect through 

multiplexer integrated circuits to the inverting input of the 

converter. The circuit cycles through the channels in a preset 

sequence, exciting one channel for each sample to produce a 

“scan.” The excitation and multiplexer circuitry are arranged in a 

standard wheatstone bridge configuration and are specifically 

designed for two-wire quarter-bridge or half-bridge sensors.

Completion resistors for the quarter-bridge sensors are included on

the circuit board. The reference voltage divider connected to the

noninverting input of the converter is also included on board 

(figure 2).

Figure 1.─MIDAS demonstration model 

The converter can be configured for a sampling rate of from 2

to 650 Hz. Maximum resolution is obtained at the 2-Hz update rate. 

In addition, the converter includes a programmable amplifier 

having a range from 1 to 64. We have found that the most accuracy

is obtained with a gain setting of 8 at a 2-Hz sampling rate. For 

general use, a gain setting of 2 at a 15-Hz sampling rate also gives 

excellent results. This setting will reject nearly all ac (60 Hz) 

interference and still provide nearly 19 bits of useful data. Figure 2.─Block diagram of MIDAS 

Figure 3.─MIDAS multiplexer circuit 
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The digital side of the MIDAS is composed of data storage and

communication integrated circuits. In addition, an LED warning light

feature (figure 4) can be activated. The MIDAS is configured with a

memory capacity of 128 kbyte, which is enough memory to hold

approximately 2200 16-channel scans and provides enough storage for the 

long-term, low-scan, rate-testing programs currently in use at several mine

sites. The scans are stored in nonvolatile FLASH memory, so data loss 

resulting from a removed or low-voltage battery is not a concern.

When stored in memory, each scan is composed of several fields. The

first field consists of diagnostic information, such as condition of the 

battery. The second field is a date and time stamp, and the third field is

temperature of the circuit board. The remaining fields contain samples from

the sensors. Knowledge of circuit board temperature can be useful when

post-processing data to reduce temperature-induced errors from the sensors. 

The MIDAS is configured and data are recovered though one of three 

serial interfaces. RS232 is the primary interface, and the one we have used

most often in our testing programs. RS232 allows connection of a MIDAS

directly to a personal computer (PC). An RS485 interface, a long-haul, differential signaling interface, is also 

incorporated into the circuit board. This interface allows communication between a PC and a MIDAS over 

approximately 1000 ft of cable at 57.6 kbaud. An external signal converter is required at the PC end of the link to convert

the RS485 signal into an RS232 signal that can be read by the PC. An IRdA port is also included on the circuit board. 

This interface allows communication between the MIDAS and computers equipped with IRdA ports through the use of 

infrared light. 

Figure 4.─MIDAS LED lights and 

serial connector

Incorporated in the firmware is a feature that will flash an LED light periodically depending on the data returned

from a scan. At the end of each scan, the firmware calculates the difference between reference values and the values of

the current samples. A green LED light flashes if any of these differences are below a set threshold, a yellow light flashes 

if values exceed a warning threshold, and a red light flashes if any samples exceed a danger threshold. The user can set

the threshold values, reference values, time between flashes, and flash duration. 

Power requirements for the MIDAS are relatively low. Between scans, the circuit draws approximately 18

microamps from the battery. When actively scanning 350-ohm sensors, the circuit draws 28 milliamps. In long-term test 

programs, a 9-V alkaline battery can power a MIDAS up to 6 months. The low power usage enables the MIDAS to 

comply with MSHA permissibility requirements that state that the unit (1) cannot generate an electronic spark that could

initiate an explosion and/or (2) reduces operating temperatures to below spark-inducing levels. Furthermore, all 

electronic equipment that operates downwind of the working face must comply with MSHA standards, requiring that 

batteries must always be changed in fresh air. The MIDAS meets these requirements and has been given permissibility

approval by MSHA. 

A successor to the MIDAS is currently in development. This circuit differs from the MIDAS in the following 

respects. First, the memory has been increased to 2 Mbytes from 128 kbytes. Second, power consumption has been

decreased to allow longer tests between battery changes. Third, the gain has been fixed at 1 Hz and the sampling rate has 

been fixed at 15 Hz since this produces acceptable results for most of the tests. Fourth, the size of the device has been

reduced while making the unit easier to manufacture and assemble. 

2. Software 

The MIDAS communicates with an ASCII communication protocol so that all the functionality of the MIDAS can

be accessed with a terminal emulator program running on a PC. This approach, however, can be difficult, especially for 

casual users. Therefore, a Microsoft Windows2

-based software program was written to communicate with the MIDAS 

 

2

 Mention of specific products and manufacturers does not imply endorsement by the National Institute for 

Occupational Safety and Health. 
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forset-up, monitoring, and data download. Simplicity 

of use was a primary consideration so untrained users

could easily operate the system. 

The software is organized into four pages:

startup, download, graph, and setup. When the 

software loads, it automatically opens an RS232 

serial port to communicate with the MIDAS, and the 

startup page is displayed (figure 5). The status of the

serial port connection is reflected in the color of an

LED light. If a problem occurs during communication, the LED light turns red, requesting that the user

change the communication parameters of the serial 

port. If the MIDAS unit is not functioning or is not

connected to the computer, then an error box will 

appear, and the status light will reflect the state of the

connection. The “connect” button will attempt to 

reestablish communications. Also, on the startup 

page, the user can set MIDAS’s clock. 

Figure 5.─Start-up window The primary function of the download page

(figure 6) is to download data from the MIDAS to the

computer. The data are written to an Excel

spreadsheet file when the “download” button is

pressed. This page also shows the current status of the 

LED lights, battery, and temperature. All data from

the MIDAS is in hexadecimal format. The software

converts the hexadecimal data into both millivolts

and microstrain when it is written to the files. The 

user also starts and stops the MIDAS from this page.

Data can also be monitored or viewed on the

ap

The setup page (figure 8) is used to set all the 

operating properties except the clock. The channels that measure strain

Figure 6.─Download window

gr h page (figure 7) of the software. The user has

the option of viewing the data in microstrain (the

default), millivolts, or raw analog-to-digital converter 

counts. The graph can be printed and/or copied to the

Windows clipboard and pasted in another document.

The graph allows full control of titles, axis properties, 

and graph types. Already-downloaded data can be

viewed on the plot by pressing the “open file” button.

This currently only allows viewing of strain data. 

 are selected with check boxes. The scan rate can

be set using increments of seconds, minutes, hours, or days. The excitation time, filter cutoff, and gain are set with pulldown menus or edit boxes. The data can be collected in either relative or absolute mode. Relative mode is the default and

records changes with respect to when the re-zero button was last pressed. Resetting the MIDAS will erase all data. The

“constants” button allows the user to set the threshold values for the LED lights and the strain gage factor for each of the

attached sensors. Blink interval and duration can be set to either increase or decrease the amount of light from the LED,

which would affect power drain on the batteries. A user can select whether to continue scanning when a low-battery

condition isreached or when memory isfull. This becomes useful when running the MIDAS connected to a PC, because 

the PC software can be configured to store scans regardless of remaining memory capacity on the MIDAS. 
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The functions of the MIDAS are activated by 

command codes and parameters. These codes are sent 

to the MIDAS when the user interacts with the

Windows program. Some actions are initiated with

e click of a button, while others may require 

elec

previous command g unit. To solve thi

command buffer would be purged, and fresh status information co

a different communication scheme so this problem will be resolv

interval were short and the scan itself long, 

ommunication could be difficult. A solution will be addressed in the next revision of the MIDAS hardware. 

ve been running

ontinuously for over a year and over 100,000 hours of underground operation have been logged. 

th

s tion from pull-down menus, radio boxes, etc.

After the MIDAS receives the command code, it 

performs the function and echoes the command and

parameters. 

The MIDAS can only execute one command at a

time. Windows users may execute commands faster 

than the MIDAS can process them. The solution to 

this problem was to make a queue that could store

many commands and send each one after the

preceding command was completed. The problem

with this approach was that if the MIDAS became 

disconnected from the system, the commands would

wait. Then, if another MIDAS were attached, the 

s problem, a time-out function was incorporated that

. The user would reconnect on the startup page, the

uld be obtained. The successor to the MIDAS will use 

ed. 

s would be sent to the wron

disconnected the MIDAS after a short period with no response

Figure 7.─Graph window

Figure 8.─Setup window

3. VERIFICATION TESTS

Initial verification tests were done in the 

laboratory by comparing the response of the MIDAS

with other data acquisition systems. These tests 

consisted of applying two sets of strain gages to the 

same sample, loading the sample, and comparing

strain readings. Material with known strain yield

points were used, and the samples were loaded past

the elastic yield to ultimate failure of the material.

The tests showed that the MIDAS could measure to 

0.1 microstrain over a pan of 170,000 microstrain at a

gain setting of 4. 

MIDAS systems were tested for over a year to 

evaluate any potential problems or malfunctions in 

both the hardware and the software. During this

evaluation, the only problem was that the MIDAS 

could not communicate during a scan. If the scan

c

To be an effective tool in an underground mine, the MIDAS will have to hold up under extreme temperatures, 

humidity, electrical interference, mineralized water, and dirt. Tests were set up in three different types of mines: coal, 

silver, and platinum-palladium. The silver mine is a hot humid environment, while the coal mine undergoes subfreezing 

temperatures. Five different types of instruments were connected (fully grouted, strain-gaged bolts; strain-gaged cable 

bolts; strain-gaged friction bolts; rock strain strips; and linear extensometers). Some MIDAS units ha

c
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4. SUMMARY 

 The MIDAS has proven to be a very dependable unit and can make the use of geotechnical instruments easier and 

cheaper. It can be used in areas that have been very difficult or impossible to monitor, such as at the working face where 

large equipment or blasting prevents running electrical wires. MSHA permissibility approval allows monitoring 

instruments to be used in return air. Software written for operation of the MIDAS makes set-up, data collection, and 

processing easy. Raw data are converted to engineering units and saved to Excel spreadsheets. When used in 

combination with strain-gaged bolts, rock strain strips, and/or electrical extensometers, bolt loads or rock movements can 

be measured and the results made available through LED lights to underground workers so that miners can be warned of 

hazardous conditions. 

REFERENCES 

Marshall, T.E.: Roof Monitoring in Limestone Mines─Experience with the Roof Monitoring Safety System (RMSS). In 

S.S. Peng (ed.), Proceedings,19th International Conference on Ground Control in Mining, 2000, pp. 185-191. 

Martin, L.: Evaluation of Instrumented Cable and Rebar Bolts as Ground Support at a Trona Mine. In: S.S. Peng (ed.), 

Proceedings, 20th International Conference on Ground Control in Mining, 2001, pp. 309-316. 

Signer, S.P.: Load Behavior of Grouted Bolts in Sedimentary Rock. In: C. Mark, D. R. Dolinar, R. J. Tuchman, T. M. 

Barzak, S. P. Signer, and P. F. Wopat (eds), Proceedings: New Technology for Coal Mine Roof Support. NIOSH 

Information Circular 9453, 2000, pp. 73-80. 
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A MINIATURE DATA ACQUISITION SYSTEM WITH LED WARNING LIGHTS 

*C. Sunderman, Spokane Research Laboratory, National Institute for Occupational Safety and Health, 

E. 315 Montgomery, Spokane, WA (509) 354-8049 csunderman@cdc.gov 

S. Signer, Spokane Research Laboratory, National Institute for Occupational Safety and Health, 

E. 315 Montgomery, Spokane, WA (509) 354-8013 ssigner@cdc.gov 

J. Johnson, Spokane Research Laboratory, National Institute for Occupational Safety and Health, 

E. 315 Montgomery, Spokane, WA (509) 354-8087 jjohnson@cdc.gov 

*To whom inquiries should be addressed.