GB2296323A - Monitoring vertical movement of structures - Google Patents

Abstract

Vertical movement of buildings or civil engineering structures (25) is monitored by scanning a laser (11) in the horizontal plane. Retro-reflecting prisms (24) fixed at various locations on the structure reflect the laser beam back onto a vertical photodetector array such as a CCD strip (12) positioned adjacent to the laser. Signals from the array indicate any vertical displacement of a prism relative to the laser. The laser is positioned at a datum point (22) and a drive motor (14) rotates the laser to perform the scan. An angular position memory is used to relate a signal to a particular prism. <IMAGE>

Description

LEVEL MONITORS This invention relates to level monitors.

It is necessary in the construction of buildings, earth works and civil engineering structures such as bridges, tunnels, dams, motorways, railways, embankments, cuttings and dockyards to establish a datum level at a point on the site and then to mark and monitor horizontals and verticals over the site and structure by accurate reference back to the datum point.

Building sites are crowded with heavy moving equipment and with materials, and thus sensitive and accurate measuring equipment is prone to physical damage.

It is known to use a surveyor's theodolite to set up and monitor such levels, but these instruments require fairly skilled personnel, are time consuming and would normally only be used to take isolated measurements at extended time intervals. The whole instrument is carried from position to position. It is therefore of little use in monitoring movement of a building or civil engineering structure, i.e. to detect subsidence, heave or tilt which may take place on a time scale of just days or even hours.

Accurate sensing and monitoring of sub-millimetric absolute displacement from a pre-set level can give advance warning of problems, for example when driving tunnels between or closely adjacent to the foundations of existing buildings. A consistent trend of day-to-day accumulating displacement should be noticed promptly rather than after the problem has occurred. For example the collapse of an entire building might be averted if displacement data could be obtained and monitored substantially continuously and in real time.

It is also known to use arrays of electrolevels. An electrolevel is effectively a spirit level with electrical signal read-out. The electrolevels are secured to preselected points around the site and the signals fed by cable to a central data logger and monitoring station.

This system can be useful in monitoring angle movements of as little as one second of arc. Where a plurality of electrolevels are secured in a linear array, effectively end-to-end, it will be appreciated that the detected angular changes at each electrolevel, together with knowledge of their lengths and positions, can effectively provide data relating to absolute displacement of those parts of the building or civil engineering structure away from the desired positions relative to the datum level.

However, electrolevels are primarily intended for local angle sensing, and thus absolute displacement is deduced indirectly and relative to the other electrolevels in the array rather than being an absolute direct detection of displacement. An electrolevel array can also be complex and difficult to set up and monitor in a useful manner.

An object of the present invention is to make it possible to provide improved level monitoring equipment mitigating the above-described disadvantages. Such improved equipment would have utility both in the normal construction of a building or civil engineering structure, and also to provide monitoring of absolute displacements for advanced warning of undesired movements such as subsidence, heave or tilt.

According to the present invention there is provided level monitoring equipment comprising a laser positioned in use at or in relation to a datum point, means causing a light beam generated by said laser to scan repeatedly in a horizontal plane, a plurality of retro-reflector devices initially positioned in use in said horizontal plane in line of sight of said laser scan and each secured to a portion of a building or civil engineering structure whose level is to be monitored, a vertically extending array of photosensitive elements positioned adjacent the laser to receive a reflected beam and to generate a signal representative thereof, and means responsive to said signals to provide an output signal representative of any vertical displacement of each retro-reflector device relative to said laser.

The retro-reflector devices are preferably passive devices, for example retro-reflector prisms or corner cubes. Such devices can be provided in relatively robust housings with mounting brackets for adjustable securing in precise desired positions on a building structure, and require no power supply or input or output signal cabling.

The laser is conveniently provided with a selflevelling mounting table for accurate positioning at the datum point. The datum point may be provided by a vertical datum rod driven down to bed rock or otherwise secured in relation to a relatively massive adjacent structure.

The scanning means can comprise a drive motor for rotating or oscillating the laser housing to cause the laser light beam to rotate or oscillate through an angle in the horizontal plane. The laser beam thereby repeatedly sweeps across each retro-reflector device.

The array of photosensitive elements is conveniently provided as a CCD (charge coupled device) sensor strip in vertical alignment immediately adjacent or below the laser. Where the retro-reflector devices are retroreflective prisms, it will be appreciated that they are mounted to the building structure with their apex horizontal so that the reflected beam returns on a path parallel to and spaced beneath the incident beam. The CCD sensor array is then positioned beneath the laser to receive the reflected beam. If that prism is displaced vertically, for example due to vertical movement of the portion of the building structure to which it is secured, then the reflected beam shifts vertically by twice the extent of the absolute displacement of the prism. This doubles the sensitivity of the CCD sensor array.For example, an absolute vertical displacement of 5 mm at the prism causes the reflected beam to move 10 mm up or down the CCD sensor strip.

CCD sensor arrays are available commercially in various sensitivities defined by the number of photo sensitive elements per unit length. They are readily available to a sensitivity of 0.1 mm in displacement. For a building structure level monitor, a CCD array of 50 mm in length would normally be sufficient.

The laser and scanning means are preferably provided with a control means for relating each reflected beam during a scan to a particular prism. The control means may include an angular position memory adapted during initial calibration to record the angular position of each target prism, whereby to permit the relation of each eventual output signal to a particular prism. This simplifies the initial setting up of the equipment.

Moveover, if the prism array is changed, or prisms are added or taken away, or one fails to function or is obstructed, then the equipment continues to function.

The equipment may be used in cooperation with an electrolevel array. For example, one or more of the electrolevels in an electrolevel array may be itself provided with a retro-reflector prism. This then permits the establishment of absolute vertical displacement signals in combination with tilt angle signals for each point on a building structure. The site contractor then has available a continuous read-out of absolute levels and angles both for normal construction of the building and for monitoring undesired absolute or angular movements.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic diagram of level monitoring equipment according to the invention.

The drawing shows a housing 10 for a laser 11, photosensitive array 12, control means 13, and scanning drive means 14. The equipment housing 10 is rotatably mounted on a mounting table 15 by means of an axle and bearing race shown schematically at 16 and 17. Power supply and signal leads 18 extend to the equipment by a slip ring 19. The mounting table 15 is automatically or manually adjustable to a precise horizontal position, for example by means of an electrolevel during initial setting up of the equipment. Manual level adjustment supports are shown schematically at 20. The supports bear on a fixed structure 21 which may be a portion of a building or civil engineering structure whose levels are to be established and monitored, or an adjacent building or fixed structure.

A datum rod 22 extends below the laser mounting position to a datum anchor 23, for example bedrock, grouting into deep clay, or other relatively fixed formation or structure such as a portion of an adjacent established building. It will be appreciated that all levels are essentially relative, but there will usually be some relatively fixed and massive structure in the vicinity of new works from which a suitably constant datum may be taken. The datum rod 22 bears against the underside of the laser equipment mounting table 15 to establish a datum level for the laser 11.

A plurality of passive retro-reflector devices in the form of retro-reflecting prisms 24 are secured to portions of the building or civil engineering structure 25 whose levels are to be monitored. Each prism 24 is housed in a robust mounting device 26 provided with adjustable mounting flanges 27 to enable the retro-reflector to be secured in a desired position to the building structure 25. The drawing shows a single prism 24, but it will be appreciated that in use there will be a plurality of prisms initially positioned in use in the horizontal plane defined by the scanning line of sight of the laser 11.

Reference numeral 28 indicates a light beam from the laser incident on the prism 24, and numeral 29 shows the parallel reflected beam. Due to the double internal reflection within the prism 24, it will be appreciated that a vertical displacement of the building structure 25 and prism 24 would cause a doubled vertical displacement between the incident and reflected light beams 28, 29.

In operation, the scanning drive 14 oscillates or rotates the equipment 10 so that the laser light beam 28 scans in the horizontal plane to be incident successively on each of the plurality of prisms 24. Each reflected beam 29 is in turn received by a particular photo sensitive element on the photo sensitive array 12, thereby generating a signal representative of the illuminated cell. The succession of signals are passed to the control means to relate each reflected beam during a scan to a particular prism, and to provide an output signal representative of any vertical displacement of each prism 24 relative to the laser 11. The output signal may be taken by signal lead 18 to a remote monitoring station, or the levels may be monitored by a display or memory provided at the control means 13.

The control means 13 also includes an angular position memory adapted for initial calibration during setting up of the equipment. Once the laser equipment is mounted at the datum point, and the prisms 24 are secured in their desired positions, the scanning drive is activated to sweep the laser beam 28 once across all the prisms in succession. The first succession of reflected beams 29 is then stored against angle of rotation in a look-up table in the control means 13. This subsequently permits relation of each eventual output signal to a particular prism, and enables the equipment to continue to function or to be readily re-set in the event of a change in the prism array.

The equipment 10 may be set up either within or outside the site where levels are to be monitored, provided the prisms 24 can be secured in line of sight of the laser. The datum point may effectively be provided by any adjacent established building or structure that is believed unlikely to be affected by the new works. If none is suitable, then the datum can be provided by a datum rod as described above in relation to the drawing.

The equipment 10 can itself be mounted directly at the datum point, for example by being secured directly to the adjacent established structure, or by means of a table 15 on a datum rod as described above.

Alternatively, the equipment 10 can be mounted on any convenient structure which is believed to be reasonably fixed on or outside the site and spaced apart from the datum point itself. At least one of the prisms 24 is secured in position at the datum point to provide the necessary reference back to the equipment 10. On a larger or complex site it may be desired to use both an independent datum rod and one or more reference prisms on adjacent fixed structures to provide a cross-check. An advantage of the invention lies in this adaptability to various arrangements according to the nature of the site.

Claims (9)

CLAIMS:

1. Level monitoring equipment comprising a laser positioned in use at or in relation to a datum point, means causing a light beam generated by said laser to scan repeatedly in a horizontal plane, a plurality of retroreflector devices initially positioned in use in said horizontal plane in line of sight of said laser scan and each secured to a portion of a building or civil engineering structure whose level is to be monitored, a vertically extending array of photosensitive elements positioned adjacent the laser to receive a reflected beam and to generate a signal representative thereof, and means responsive to said signals to provide an output signal representative of any vertical displacement of each retroreflector device relative to said laser.

2. Equipment according to Claim 1 wherein the retroreflector devices are passive devices.

3. Equipment according to Claim 2 wherein the passive retro-reflector devices are retro-reflector prisms each mounted with a horizontal apex so that the reflected beam returns on a path parallel to and spaced beneath the incident beam.

4. Equipment according to any one of Claims 1 to 3 wherein said scanning means comprises a drive motor operable to rotate or to oscillate a housing carrying said laser whereby the laser beam repeatedly sweeps across each said retro-reflector device.

5. Equipment according to any one of Claims 1 to 4 wherein said array of photosensitive elements comprises a CCD (Charge Coupled Device) sensor strip disposed in vertical alignment adjacent the laser.

6. Equipment according to Claim 5 wherein said sensor strip has a displacement sensitivity of substantially O.lmm and a length of substantially 50mm.

7. Equipment according to any one of Claims 1 to 6 including control means associated with the laser to relate each reflected beam during a scan to a particular said prism.

8. Equipment according to Claim 7 wherein said control means includes an angular position memory adapted during initial calibration to record the angular position of each target prism.

9. Level monitoring equipment substantially as described herein with reference to the accompanying drawing.