Title of Invention

ROOF STABILITY TESTER

Abstract

A roof stability tester for testing the stability of the roof the mine comprising means for generating the acoustic vibration in the outer layer of the roof of the mines being subjected for testing, sensing means for collecting the said vibration signals and converting said acoustic vibration signals into electric signals, amplifying means to amplify the said electrical signal to a desired level, means for suppressing the noise signals energized by a common power source, analog to digital converter having its input connected to the said suppression means having its output connected to a Central processing unit having a empirical relation directly proportional to the status of the said rock, a display device connected to the output of said central processing units displaying the required analysed signal data exhibiting the status of the said rock.

Full Text

In underground coal mines, unexpected roof falls are common, causing serious accidents. The main reason for the sudden roof failure is presence of hidden flaws and other anomalies that cause separation of the rock strata, Till date, the approach for evaluating the roof stability is based on listening to the drumminess of roof with a soimding bar. This method can detect weak planes upto about 30 cm above the roof level and cannot provide sufficient information about the hidden weak planes inside the roof, to take precautionary measures for prevention of roof fall accidents. Stability evaluation of the coal mine roof at working places in underground mines is of foremost importance for a mining engineer for ensuring safety of men and equipment. With the application for mechanized mining techniques for increasing the production, the need for safety of the working region becomes still more essential. Though some improvements such as roof bolting have been introduced for improving the safety, there is no scientific instrument as yet to provide instantaneous information about the roof condition and stability so that precautionary measures can be taken. Many developments have taken place in recent years for monitoring the roof condition with displacement sensors, convergence meters, load cells, extensometers and the like., connected with advanced data loggers for roof fall monitoring at working places. However the information provided by them is general in nature and inadequate for taking a reliable decision regarding the roof stability well in advance. The is more so in case of local roof falls which take place due to hidden flaws or other discontinuities in the roof, because none of the above instruments are capable of identifying such minute problematic places in the roofs. Therefore there is a need for the development of a scientific tool to identify hidden flaws and weakness planes in the coal mine roof to have a reliable measure of the rock mass condition in the roof. However, even today mining engineers are adopting the good old practice of tapping the roof with a dressing rod or with the walking stick fixed with a metallic cap and listen to the quality of sound produced therefrom for judging the roof condition.Although this method to testing was proved useful over the years, it has got its limitations. It is not free from human bias and if fails to assess the roof quality whenever there are hidden flaws beyond 30 cm in the roof The statistics of roof fall accidents indicate that amongst all mining hazards, an abrupt fall near to the working area and upto 0.5m thick layers contribute to major fatalities and serious injury accidents. Till date there is no method or tool to evaluate the in-situ condition of coal mine roof The objective of this project is to develop an approach for scientific evaluation of the roof quality and ultimately develop a portable instrument which can evaluate the roof stability especially in the presence of hidden flaws. The instrument should be handy and easy - to - use by a lay man so that qviality of roof can be assessed on day to day basis. It should be able to overcome and improvise the limitation of existing method of testing the condition of the roof for evaluating its stability. The present approach is a step forward in scientific analysis of the drumminess of roof through an instrument. This facilitates evaluation of the roof condition with respect of Rock Mass Rating (RMR), the accepted rock mass classification index. Since presently no portable instrument is available to detect such hidden flaws.Therefore,an instrument known as Roof Stability Tester (RST) has been developed to detect such hidden slips inside the mine roof. This instrument would help improve safety in coal mines are due to roof falls.This instrument also displays the nature of roof strata,quantifyinfying the rock mass quality from 0 to 100 RMR (rock mass rating).Thus, this instrument will serve to characterise the roof so that suitable support systems can be adopted. In order to acquire the real noise-free data from different coal mine roofs with varying conditions, 180 centimeters long dressing bar, a round headed small size hammer weighing about 250 grams and Schmidt hammer were used to impact the roof An accelerometer data recorder and a telescopic stand to couple the accelerometer firmly to the roof, were used. In order to ensure a firm contact of sensor with the roof and to prevent the loss of signal, the position of roof in contact with the sensor was dressed and neatly cleaned; a high viscosity silica grease was applied in between the sensor and roof The accelerometer output is sent through a shielded co-axial cable to the preamplifier module which has a sharp cut off high pass filter followed by a low pass filter. The amplifier module has an adjustable gain facility and an LED indication for saturated signal. The filter is used to eliminate the cultural noise and surface waves (that are produced due to impact) fi-om the signal output. The output of the amplifier and filter section is connected to a single channel magnetic data recorder through buffering arrangement. After the instrumentation set up which takes two to three minutes, the roof is tapped with a small hammer at four to five places around the sensor at a distance of 0.6 to 1.5 m from the sensor location at each place 10 to 15 impacts with different intensities were given and the data were recorded on a magnetic tape recorder. These data pertain to different mining conditions like wet roof, dusty areas, noisy locations, water dripping areas, freshly exposed working faces and old workings with and without roof supports, The data (signals) were collected firom coal, shale and sandstone roofs. Time Domain Analysis Mcthod:- In the time domain analyses method,the rock vibrational signal,directly proportional to the acceleration of the roof in vertical direction,is directed by a transducer called accelerometer. The accelerometer is coupled to the root firmly v^dth the help of a telescopic stand and using a coupling agent(a high viscosity fluid)between the roof and the transducer. The accelerometer output signals serves as the input to buffer amplifier where output is fed to adjustable gain amplifier and in turn to the triggering system, tester, wherein the said amplifying means are instrumentation amplifier. The amplified signal passes through a filter module comprising high cut off frequency and low cut off frequency to eliminate the local noise and spurious signals.The filter module output as is fed to three preset Band Pass Filters simultanously for suppressing the noise energised by a common power source. The filters output is scaled and given to a squarer circuit. This output is integrated over the duration of the signal in the three fi-equency bands so as to get the power contained in three bands.A triggering circuit produces a control pulse corresponding to the occurrence of event and the duration of the signal to be processed. This control signal is given to the energy processing circuit,the sample and hold circuit and to the digital logic unit.The output of the three energy processing circuits is passed to independent peak detectors,and to the sample and hold circuits to compute the peak energy content in the respective frequency bands i.e. peak values of EL,EM,and EH These peak values of energy signals are held for a duration of predetermined time to enable the logic circuit complete its operation.This stability evalution logic module sends a resultant signal to light up any one of the five different LEDs of the display unit corresponding to the condition of the rock.This system power ratings are within the limits of intrinsically safe standards of DGMS for use in underground mines. Frequency Domain Analysis Method: The present instrument is based on the frequency domain analysis method, the output signal from the transducer is fed to the buffer amplifier and then to an adjustable gain amplifier. The output is passsed through a band pass filter of cut-off frequencies FL=500Hz and FH=5KHz.This filter output signal is passed to a scaling amplifier and to an ADC module,where the signal is digitized with the help of the a microprocessor at a sampling frequency of.The digitized data is stored in the memory(RAM) for further processing. The data acquisition and FFT processing software loaded in the ROM module controls the data acquisition and other computational operations.. The data acquisition software keeps track of ADC output and examines the amplitude of the signal,whenever the signal crosses a preset threshold level for triggering.The digitized ADC output of 50msec data is transferred to the RAM for data analysis operation. The data stored in the RAM is subjected to the FFT analysis ,starting from the triggering position,to get the power spectrum and to compute the values of EL,EM and EH(the total amoiint of energy present in the pre-designated three frequency bands). Once the values of the EL,EM and EH are computed,the processing unit uses these values as basic parameters for the logic computation. In an another embodiment the fourier spectrum is computed by a sophisticated instrmentation,using the microprocessor.Entire stages of this system including the data acquisition,data analysis,logic operation and display of results are fully software controlled. The results are displayed ona LCD module. Here the total energy in that particular frequency band is taken into account instead of considering peak values.This method of computing and analysing the epectral characteristics provides a better result than its earlier coimterpart. The present novel instrument also uses the frequency domain analysis technique,.The data acquisition and all other computations are done by a micro controller instead of a processor to make it more compact and operatable at low very low voltage and current,and can be fitted to the belt of miner's cap lamp. The Roof Stability Tester(RST)is a portable and battery operated instrument and can also be used as a scientific tool to take decision regarding support system for the roof.The performance and fimctioning of the equipment has been certified by mining experts. The basics of this instrument is based on the empirical relation for analyzing and computing power specfra and values EL,EM and EH,(the total enrgy content in low middle frequency band and band respectively), The earlier instrument was based on the time domain analysis method to extract the peak energy values of EL,EM,andEH by using the electronic hardware. The logic processing too is designed by using simple digital IC's and the results displayed on coded LEDs,which indicate the rock quality in the five categories based on their RMR ranges. RMPTRTCAT. RELATTON; The three important parameters that are required to establish the relation between different rock conditions are EL,EM and EH. EL,EM, and EH stand for low band energy,middle band energy and high band energy, respectively. EL denotes the total energy content in the frequecy range of 600-1200 Hz. EM denotes the total energy content in the frequency range of 1800-2400 Hz, EH denotes the total energy content in the frequency range of 3000-3600 Hz. DEVELOPMENT OF EMPIRICAL RELATION :- The empirical relation of the said central processing unit is a three basic parameter indicating the energy level of the said electrical signal of the said sensing means. The three important parameters that are required to establish the relation different rock conditions the five conditions of roof are EL,EM and EH.EL, EM and EH stand for low band energy, middle band energy and high band energy, respectively, as explained earlier. EL denotes the total energy content in the frequency range of 600 -1200 Hz. EM denotes the total energy content in the frequency range of 1800 - 2400 Hz. EH denotes the total energy content in the frequency range of 3000 - 3600 Hz. The following logic has been derived after analysing the vast database of signals from various mine roof conditions. The logic holds good for coal, coal-shale and sandstone (coarse, medium and fine grained)In case of shale roofs the roof sounded good in most of the places but its power spectral density (PSD) was concentrated only within 1.2 KHz range. From these tables (distribution of energy vs. frequency), it can be seen that the above empirical relation is valid, and that a reliable judgement of rock stability can be evaluated by using vibrational frequency spectrum. FILTER BANDS SELECTIQNr- Based on the analysis of over 10000 signals of vibrational frequency bands were selected according to typical frequency characteristics of the different roof conditions as given below. 1. HIGH FREQUENCY BAND(E3 = EM): This band ranges from 3KHzto 3.6 KHz . The power spectral density of the recorded signal dominate in this band only for signals from Very Good roof 2. MIDDLE FREQUENCY BANDS ( E2 = EM) : This frequency band is spread from 1.8 KHz to 2.4 KHz, and the power spectral density of signal dominates in this band for signals from Good to Fair roof conditions. 3. LOW FREQUENCY BAND (El = EL) : This frequency band is spread from 50 Hz to 1100 Hz and the power spectral density of signal is predominant in this band for signals from Very Poor to Poor roof conditions. SIGNAL CHARACTERISTICS:- On further observation of the FFT spectrum of the rock it was felt that the middle frequency band has a predominant role in explaining the behaviour of rock mass stability. The rock mass attains a stable state in between good condition to poor roof condition( while shifting the energy from EH nd EL). This intermediate stable state has a dominent energy content EM,which lies between EH and EL. After an intensive analysis of data for the specfral characteristics of the rock mass of previously known roof conditions,the following important point emerges: 1. Unstable or drummy rock slabs vibrate at lower and at higher amplitude in comparison with good rock mass. Spectral power density of drummy rock slabs is concentrated between 600 Hz to 1.2 Khz only, and the power spectral density above 1.2 Khz is almost negligible. 2. The power spectral density value of stable rock slab is present only above 3 Khz, and at low and middle frequency band it is almost negligible. 3. As quality of rock decreases, it is found that the power spectral density peak at 3 Khz and above is reduced, and it begins rising in the 2 Khz band proportionally. 4. Further reduction in the quality of the rock results in the energy in high frequency band getting shifted to 2 Khz (middle frequency band). 5. If the rock slab is partially detached, the power spectral density will not appear in the high frequency band. In the middle frequency band the signal content starts reducing, with a corresponding increase in the low frequency band. The study of spectral characteristic of the impacted roof rock reveals that they have clearly distinguishable frequency characteristics, categorizing them from competent to unstable rock mass. We can describe this difference in characteristics as the different states of rock behavior in the process of increasing instability. In this process, we are able to distinguish another three states in between stable (Very Good) and unstable (Very Poor) rock conditions as Poor, Fair and Good roof conditions similar to the five categories of roof conventionally described in RMR classification. According to the present invention Roof Stability Tester for testing the stability of the roof the mine comprising means for generating the acoustic vibration in the outer layer of the roof of the mines being subjected for testing, sensing means for collecting the said vibration signals and converting said acoustic vibration signals into electric signals, amplifying means to amplify the said electrical signal to a desired level, means for suppressing the noise signals energised by a common power source, analog to digital converter having its input connected to the said suppression means having its output connected to a Central processing unit having a empirical relation directly proportional to the status of the said rock, a display device coimected to the output of said central processing units displaying the required analysed signal data exripiting the status of the said rock. In the present embodiment the sensing means are transducer or preceisely a Peizo Electric Accelerometer,converting the vibration energy into the electric signal,which are fed to instrumentation amplifier,the output of which passes through a band pass filter,connected in series to a ADC module.The output is passed through a band pass filter of cut -off fi-equencies FL=500Hz and FH=5KHz.This filter output signal is passed to a scaling amplifier and to an ADC module,where the signal is digitized with the help of the a microprocessor at a sampling firequency of.The digitized data is stored in the memory(RAM) for further processing. The data acquisition and FFTprocessing software loaded in the ROM module controls the data acquisition and other computational operations.. The data acquisition software keeps track of ADC output and examines the amplitude of the signal,whenever the signal crosses a preset threshold level for triggering.The digitized ADC output of 50msec data is transferred to the RAM for data analysis operation. i The data stored in the RAM is subjected to the FFT analysis ,starting from the triggering position,to get the power spectrum and to compute the values of EL,EM and EH(the total amount of energy present in the pre-designated three fi-equency bands). Once the values of the EL,EM and EH are computed,the processing unit uses these values as basic parameters for the logic computation and gives a result. This result is calibrated in terms of RMR and is displayed on an ADC module. BRIEF DESCRIPTION OF THE DRAWINGS: The present invention will be described with reference to the accompanying drawings wherein:- Figure 1 relates to the Instrumentation setup for data acquisition Figure 2 relates to the Block diagram of Roof Stability Tester Figure 3 relates to the Functional Block diagram of Roof stability tester (Time domain) Figure 4 relates to the Block diagram of Roof Stability Tester(Frequency domain) DETAILED DESCRTPTTON OF THE DRAWTNCS. As shown in figure 1, A long dressing bar, a round headed small size hammer weighing about 250 grams is used to impact the roof An accelerometer data recorder and a telescopic stand to couple the accelerometer firmly to the roof, is used. In order to ensure a firm contact of sensor with the roof and to prevent the loss of signal, the position of roof in contact with the sensor was dressed and neatly cleaned; a high viscosity silica grease was applied in between the sensor and roof The accelerometer output is sent through a shielded co-axial cable to the preamplifier module which has a sharp cut off high pass filter called Band Pass filter. The output of the amplifier and filter section is connected to a single channel magnetic data recorder through buffering arrangement. The accelerometer (s) output signals serves as the input to buffer amplifier(l) where output is fed to adjustable gain amplifier and in turn to the triggering system, tester, wherein the said amplifying means are instrumentation amplifier(l). The amplified signal passes through a filter module comprising high cut off fi-equency and low cut off frequency to eliminate the local noise and spurious signals.The filter module output (2) as shown in figure 2 is fed to three preset Band Pass Filters (2) simultanously for suppressing the noise energised by a common power source(7). The filters output is scaled and given to a sqviarer circuit,where the signal A(t) is converted to A2(t) as shown in figure 3.This output is integrated over the duration of the signal in the three fi-equency bands so as to get the power contained in three bands.A triggering circuit produces a control pulse corresponding to the occurrence of event and the duration of the signal to be processed. This control signal is given to the energy processing circuit,the sample and hold circuit and to the digital logic unit.The output of the three energy processing circuits is passed to independent peak detectors,and to the sample and hold circuits to compute the peak energy content in the respective frequency bands i.e. peak values of EL,EM,and EH For the frequency domain the output signal from the fransducer is fed to the bufferamplifier(l) and then to an adjustable gain amplifier.The output is passsed through a band pass filter(2).This filter output signal is passed to a scaling amplifier and to an ADC module(3),where the signal is digitized with the help of the a microprocessor (3)at a sampling frequency.The digitized data is stored in the memory(RAM) for fiuther processing. The data acquisition and FFT processing software loaded in the ROM module controls the data acquisition and other computational operations.. The data acquisition software keeps track of ADC output and examines the amplitude of the signal,whenever the signal crosses a preset threshold level for triggering.The digitized ADC (3)output of 50msec data is transferred to the RAM(4)for data analysis operation.The data stored in the RAM (4)is subjected to the FFT analysis. The results are displayed on a LCD (5). WE CLAIM:- 1. A roof stability tester for testing the stability of the roof the mine comprising means for generating the acoustic vibration in the outer layer of the roof of the mines being subjected for testing, sensing means for collecting the said vibration signals and converting said acoustic vibration signals into electric signals, amplifying means to amplify the said electrical signal to a desired level, means for suppressing the noise signals energised by a common power source, analog to digital converter having its input connected to the said suppression means having its output connected to a Central processing unit having a empirical relation directly proportional to the status of the said rock, a display device connected to the output of said central processing units displaying the required analysed signal data exhibiting the status of the said rock. 2. A roof stability tester, as claimed in claim 1, wherein the said means for generating vibration is a hammer or crow-bar. 3. A roof stability tester, as claimed in claim 1, wherein the said sensing means is a transducer or preceisely a Peizo Electric Accelerometer fitted on a telescopic stand. 4. A roof stability tester, as claimed in claim 1, wherein the said amplifying means are buffer amplifier coupled to a gain amplifier. 5. A roof stability tester, as claimed in claim 1, wherein the said means for suppressing the noise energised by a common power source is a band-pass filter. 6. A roof stability tester, as claimed in claim 1, wherein the said display device is a alpha numerical display device as herein described. 7. A roof stability tester, substantially as hereinbefore described with reference to the accompanying drawings.

Documents:

0985-mas-1995 abstract.pdf

0985-mas-1995 claims.pdf

0985-mas-1995 correspondence others.pdf

0985-mas-1995 correspondence po.pdf

0985-mas-1995 description (complete).pdf

0985-mas-1995 description (provisional).pdf

0985-mas-1995 drawings.pdf

0985-mas-1995 form-1.pdf

0985-mas-1995 form-26.pdf

0985-mas-1995 form-4.pdf

0985-mas-1995 form-5.pdf

0985-mas-1995 form-6.pdf

0985-mas-1995 form-9.pdf