AU2012100177A4 - Rock stress measurement apparatus and method - Google Patents

Abstract

An apparatus for measuring rock stress, said apparatus comprising: a plurality of transducers (34) mounted on a body and each producing at an output a signal (A, B, C, 5 D, E, F, G, H, I) indicative of at least one physical parameter; a signal convertor (40) co-located with said body and coupled to the respective outputs to convert each of the signals from the plurality of transducers into digital format; and a data transmission cable (38) coupled to the signal converter for serial transmission of the digitized signals (sig 1, sig 2) to a remote location for monitoring and/or recording. A method for stress 10 measurement is also disclosed. (FIG. 5) 3133921_1 (GHManers)P87712.AU.1 16/02112 -- - --- - --- -- - - - rN I c~co

Description

GM CRFFITH Am SPECIFICATION OF PATENT APPLICATION COUNTRY AUSTRALIA TYPE Innovation Patent TITLE ROCK STRESS MEASUREMENT APPARATUS AND METHOD APPLICANT(S) Environmental Systems & Services Pty Ltd AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION Innovation Patent Applicant(s): Environmental Systems & Services Pty Ltd Invention Title: ROCK STRESS MEASUREMENT APPARATUS AND METHOD The following statement is a full description of this invention, including the best method for performing it known to me/us: -2 ROCK STRESS MEASUREMENT APPARATUS AND METHOD TECHNICAL FIELD An apparatus and method are disclosed for stress measurements, especially 5 although not exclusively, in rock formations for mining, construction and other geotechnical purposes. The apparatus may take the form of a stress sensing system for rock walls in mines or tunnels. However, the disclosure is to be broadly interpreted, in that the apparatus may find other applications in measuring stress fields and the method may find application in remote sensing applications for physical parameters other than 10 stress. BACKGROUND ART Measuring internal rock stress fields is important for determining the strength and strain to which rock walls are subjected during mining operations and tunnel construction. A pre-existing sensor known as a "HI-Cell" is used to measure stress 15 inside rocks in walls of mines or tunnels. The HI-Cell generally contains a number of strain gauges (for example 12) mounted on a cylindrical substrate in a specific pattern which converts physical distortions due to stress into varying electrical resistances. A photograph of a HI-Cell sensor is shown in Figure 1. The strain gauges are traditionally measured using long wires from each strain 20 gauge to a dedicated multi-channel logger, as shown schematically in Figure 2. The use of long wires from each strain gauge in the HI-Cell sensor causes two serious problems: (i) it introduces electrical noise because of the long lengths of the wires and (ii) also reduces in the accuracy since the small signals are measured in an analogue fashion at a significant distance from the sensor. In addition, using many wires bundled into a thick 25 cable (see Figure 3A) makes the HI-Cell and its associated wiring harness relatively difficult to handle and awkward to operate. These difficulties are magnified since the HI-Cell sensor is commonly used in confined spaces, typically underground such as inside mines and tunnels. The above references to the background art do not constitute an admission that 30 the art forms a part of the common general knowledge of a person of ordinary skill in the art, in Australia or any other country. The above references are also not intended to limit the application of the apparatus and method as disclosed herein. 3133921_1 (GHMaters) P87712 AU 16/02112 -3 SUMMARY OF THE DISCLOSURE In a first aspect there is disclosed an apparatus for measuring rock stress, said apparatus comprising: 5 a plurality of transducers mounted on a body and each producing at an output a signal indicative of at least one physical parameter; a signal convertor co-located with said body and coupled to the respective outputs to convert each of said signals from the plurality of transducers into digital format; and io a data transmission cable coupled to the signal converter for serial transmission of the digitized signals to a remote location for monitoring and/or recording. In an embodiment the plurality of transducers may comprise an array of strain gauges for producing stress signals indicative of magnitude and orientation of the stress at a measurement site. In a further embodiment, the plurality transducers may also is include a temperature sensor. Suitably the signal converter comprises an analogue-to-digital convertor, preferably of low-power consumption and high precision. Preferably a serial output of the signal convertor is coupled to a communications adapter configured to transmit the digitized signals in a serial stream to the remote 20 location. The apparatus can provide ease of handling, low noise and improved accuracy of signal recording indicative of the physical parameter, such as stress. In an alternative embodiment the apparatus may comprise a wireless data transmission link in substitution for the data transmission cable, the cable being 25 optionally replaced by a tether for lowering and subsequent recovery of the transducer and signal converter assembly. In a second aspect, there is disclosed a method for measuring stress, including the steps of: 30 sensing conditions at a site to produce a plurality of signals indicative of at least one physical parameter; 3133921_1 (GHMatters) P87712 AU 16/02/12 -4 converting at the sensing site each of said plurality of signals into a digital format; serially transmitting the digitised signals to a remote location for monitoring and/or recording. 5 The method suitably includes the precedent step of fixing a sensor assembly, preferably comprising a plurality of transducers, at the site. Suitably the steps of the method are undertaken periodically to conserve power and reduce undesirable heating effects, which may otherwise follow from continuous application of power to the sensor. 10 In an embodiment the plurality of signals are indicative of stress in rock at the site, suitably including the magnitude and orientation of the stress. In another embodiment of the method, the digitized signals are carried by a pair of signal conductors. Alternatively, the method may comprise the digitized signals being carried on a wireless communications link between the site and the remote 15 location. BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms which may fall within the scope of the apparatus and method as set forth in the summary, specific embodiments will now be 20 described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a photographic representation of a rock stress sensor of the prior art; Figure 2 shows a schematic diagram of a stress measurement apparatus incorporating the prior art sensor of Figure 1 together with a multi-channel data logger; 25 Figure 3A shows a photographic representation of the stress sensor apparatus of Figure 2 omitting the multi-channel data logger; Figure 3B shows a comparative photographic representation of a stress sensor apparatus of an embodiment; Figure 4 shows a schematic diagram of a stress measurement apparatus 30 incorporating the stress sensor of Figure 3B, including a single channel data logger; and Figure 5 shows a functional block diagram of the stress sensor of the embodiment. 3133921_1 (GHMatters) P87712.AU 16/02/12 -5 DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Referring firstly to Figure 3B, a first embodiment of apparatus for rock stress measurement 30 is shown in the form of a stress sensor having a body 32 upon which a plurality of transducers 34 are mounted, together with a case 36 containing electronics s (not shown) for converting signals produced by each of the transducers. The electronics case 36 is coaxially located adjacent to the body 32 at one end and a flexible cable 38 containing four (4) wires extends from at an opposite end of the case 36. In the embodiment the 4 wires or cores of the cable 38 include power (PWR), ground (GND), a first signal conductor (sig 1) and a second signal conductor (sig 2). 10 A stress sensor 20 may be glued into a bore hole inside a rock face in one application and includes a plurality of transducers as apparent from Figure 1, typically from 12 to 15 strain gauges 22. If required, a temperature sensor may also be included as a further transducer. A body of the sensor may be formed by a tubular body of known elastic modulus. Stress impinging on the body of the sensor carrying the is transducers distorts its shape, and these distortions are converted to varying resistance by careful spatial arrangement of the strain gauges. In a comparative prior art arrangement, as depicted in Figures 2 and 3A, each strain gauge is coupled directly to a multi-channel logger 26 by respective conductors 24 a long complex multi-core cable for measurement at a remote location by a separate channel in a multichannel logger. 20 Referring now to Figure 4, where like reference numerals are used to denote similar or like parts, a simple four (4) wire or core flexible cable 38 of about 4mm in diameter connects the signal converter electronics 40, which include a multi-channel low power analogue-to-digital converter, to a single channel recorder 60, such as an RS 232 data logger or personal computer (PC). The arrangement and function of the 25 electronics 40 comprising the stress measurement apparatus will be described further in relation to the block diagram of Figure 5. Turning now to Figure 5, a functional block diagram of the electronics in an embodiment of the apparatus for stress measurement is shown. Each of the strain gauges 34 mounted on the body 32 of stress sensor 30 is allocated to a selectable 30 channel (A - I) of the multi-channel A/D convertor 44 via interface impedances 42 that are directly coupled to the individual strain gauges. The A/D convertor is controlled by a micro-processor 46 according to associated operating software, which A/D converter 3133921_1 (GHMaters) P87712 AU 16/02/12 -6 periodically converts the analogue signals produced by the strain gauges into digital data. The processor 46 is operative to format the data obtained by conversion to digital form in the convertor 44 for transmission to a remote location. The processor 46 is in turn coupled to a communications interface adapter 48 which sends the formatted data s over an external two (2) wire communications cable (not shown) in a serial data stream, such as according to the RS 232 standard. The flexible communications cable 38 (see Figure 3B) of the embodiment includes two signal conductors for connection to signal terminals (sigl, sig2) and two supply conductors (PWR, GND) for powering the electronics 40. The electronics are io protected from spurious voltages and signals by protection circuits 50 provided on the PWR conductor and across the communications signal terminals (sigl, sig2). The internal power bus 52 is also provided with a pair of voltage regulators 54 to ensure stable operation of the electronics 40. Protection provided on the input power supply and the communications is implemented to make the measurement apparatus robust in 15 view of the harsh operating conditions generally seen in environments where this device will be used. Monitoring of strain may be possible in applications where the communications cable is carried in the hollow core of drilling rods and via a suitable interconnection swivel. The ability to record stress data during over-coring, for example, provides 20 valuable information, including detection of conditions such as the onset of core cracking, cell be-bonding or inelastic response of the rock being cored. A retrieved over-cored section of rock containing the cell may be restressed later to derive actual rock properties. The stress measurement apparatus and method described here is an advance on 25 the state of the art since it converts all the low level HI-Cell strain gauge resistances to electrical voltage at the site of the HI-Cell sensor itself. This eliminates the requirement for a bundle of long wires, and thereby significantly reduces the problems described above. A further advantage of the apparatus and method is that no specialized data logging equipment is required, as is the case with the currently used HI-Cell type stress 30 sensor. Multi-channel data-loggers are typically custom built, bulky, relatively expensive and sometimes require manual switching amongst measurement channels. Instead, a simple single channel logger or common laptop computer with a suitable 3133921_1 (GHMatters)P87712.AU 16102/12 -7 signal interface can be used with the apparatus of the disclosure, instead of a dedicated multi-channel data logger. The apparatus of the embodiment is also suited to long term stress change monitoring, including compressive or tensile stress changes, and may be employed in 5 both isotropic and anisotropic rock formations. Whilst a number of specific apparatus and method embodiments have been described, it should be appreciated that the apparatus and method may be embodied in many other forms. For example, the communications cable may be replaced by a wireless communications link and the electronics package may be powered by a battery 10 pack of suitable service life. A tether may be provided for lowering and subsequent recovery of the transducer and signal converter assembly of the stress measurement apparatus from a measurement site. In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word 15 "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein. 3133921_1 (GHMatters)P87712AU 16/02/12

Claims (11)

1. An apparatus for measuring rock stress, said apparatus comprising: a plurality of transducers mounted on a body and each producing at an output a signal indicative of at least one physical parameter; s a signal convertor co-located with said body and coupled to the respective outputs to convert each of the signals from the plurality of transducers into digital format; and a data transmission cable coupled to the signal converter for serial transmission of the digitized signals to a remote location for monitoring and/or recording. 10

2. The apparatus as claimed in claim I wherein the plurality of transducers comprises an array of strain gauges for producing stress signals indicative of magnitude and orientation of stress at a measurement site. 15

3. The apparatus as claimed in claim I or claim 2 wherein the signal converter comprises an analogue-to-digital convertor for periodically converting the transducer output signals.

4. The apparatus as claimed in any one of the preceding claims further 20 comprising a communications interface coupled to a serial output of the signal convertor to transmit the digitized signals in a serial stream to the remote location.

5. Apparatus for measuring rock stress, substantially as herein described and/or reference to any one or more of figures 3B, 4 and 5 of the accompanying drawings. 25

6. A method for measuring stress, including the steps of: sensing conditions at a measurement site to produce a plurality of signals indicative of at least one physical parameter; converting at the sensing site each of said plurality of signals into a digital 30 format; serially transmitting the digitised signals to a remote location for monitoring and/or recording. 3133921_1 (GHMatters) P87712.AU 16/02/12

7. The method of claim 6 including the precedent step of fixing a sensor assembly having a plurality of transducers at the measurement site. 5

8. The method of either claim 5 or claim 6 wherein the steps are undertaken periodically to conserve power and reduce heating effects.

9. The method of any one of claims 5 to 8 wherein the plurality of signals are indicative of stress at the site. 10

10. The method of claim 9 wherein the plurality of transducers are arranged in order to sense both the magnitude and orientation of the stress.

11. A method for measuring stress, substantially as herein described with 15 reference to figures 3B, 4 and 5 of the accompanying drawings. 3133921_1 (GHMatters) P87712 AU 16/02/12