WO1989006783A1

WO1989006783A1 - Position measurement system

Info

Publication number: WO1989006783A1
Application number: PCT/GB1989/000043
Authority: WO
Inventors: John Perry
Original assignee: John Perry
Priority date: 1988-01-18
Filing date: 1989-01-17
Publication date: 1989-07-27
Also published as: GB8800988D0; GB2217454A

Abstract

A position measurement system comprises an angle measuring apparatus (11) which can operate unattended, and an attended target device (10) mounted on a support (22). The target device has a sight (20) for locating the apparatus and means for approximately measuring the angle (A) between the direction (S) of the apparatus and a reference direction (G). The angle is transmitted by a data link to the angle measuring apparatus, and used to appropriately angle an alignment device (12) on the apparatus which is then rotated to locate the target (19) within the field of view of the alignment device. The angle measuring apparatus then makes an accurate measurement of the position of the device, and the results of the measurement may be transmitted by a data link to the target device for use by the operator, or be otherwise logged.

Description

POSITION MEASUREMENT SYSTEM

This invention relates to a position measurement system, and is particularly applicable to land surveying but other applications are possible, for example in setting up large machinery.

Position measurement systems often include an angle measur¬ ing apparatus consisting of an alignment device mounted on two axes of rotation with angular scales for each of these axes of rotation, and a remote target. The surveyor's theodolite is such an apparatus, having a sighting telescope which is manually adjusted to bring a target such as a calibrated staff into the centre of the field of view. To achieve good accuracy, the alignment device requires good resolution, but this is often assoc- iated with a small field of view which can lead to diffi¬ culty in initially bringing the target within the field of view:- For this reason, a secondary sight 'having a wide field of view but relatively poor resolution is often attached to a theodolite telescope. Such an arrange- ment requires a human operator at each end of the system.

It is an object of the present invention to provide a position measurement system which uses an automated method for bringing a target within the field of view of an alignment device, and can thus be operated by one person.

According to the present invention, there is provided a position measurement system, comprising an angle measur¬ ing apparatus capable of unattended operation and having an alignment device, a target device for use attended by an operator at a position remote from the angle measur- ing apparatus and having sight means for establishing the line of direction of said angle measuring apparatus, means for determining the angle between said line of direction and a reference direction, a data link for transmitting data representing said angle to said angle measuring apparatus, and means for driving said alignment device in accordance with the transmitted data to locate said target.

Said reference direction may be the (vertical) direction of the earth's gravitational field or the (horizontal) direction of the earth's magnetic field. These reference directions can be regarded as uniform in the great majority of cases but in, for example, large-scale or geodetic surveys, appropriate compensations may be introduced through one or more of the computerised central functions of the system.

In use of the gravity-based system, the operator aims the sight on the target device at the angle measuring apparatus and then, by means ωhich may include one or more angle sensors, tilt sensors and electronic circuitry, the angle of elevation between the target device and the angle measuring apparatus ^"is estimated and data transmitted by means of a data link to the angle measuring apparatus. By means of this estimated vertical elevation angle, the alignment device of the angle measuring apparatus is auto¬ matically rotated to a suitable angular position on its horizontal axis of rotation. By then making an angular sweep about the vertical axis, the target is brought within the field of view of the alignment device, and accurate measurement of the target position can commence. Then, the results of the accurate measurement may be transmitted back to the target device by means of a data link and displayed to the operator.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:-

Figure 1 shows an angle measuring apparatus and a target device, forming a gravity-based position finding system for use in land surveying;

Figure 2 is a perspective view of the angle measuring apparatus forming part of the position finding system;

Figure 3 is a view of the target device and its support; and

Figure 4 shows another position-finding system according to the invention, using the earth's magnetic field as the reference base.

According to the present invention, and as shown in Figure 1, only one operator R is required, at the target 10, ' since the angle measuring apparatus 11 functions auto¬ matically once set in position.

The angle measuring apparatus is shown in Figure 2 and includes an alignment device 12 which is rotatable about horizontal and vertical axes 13 and 14 by motorised drives 15 and 16, respectively. The angular positions of the alignment device are set and measured, ,using angular scales 17 and 1B. The angular scales would typically have fiπely- spaced radial divisions, movements of which are sensed using well known opto-electronic techniques.

The alignment device 12 is adapted to sense the angular position of the target 10 within a limited field of view, and one suitable device is shown in the Specification of French Patent No. 2,164,180. The alignment device may receive electro-magnetic radiation transmitted from the target device 10 and detected by a directionally- ^'sensitive detector built into the alignment device. The detector suitably comprises an objective lens focussing onto one or more photocells which gives an output indica- tive of the focussed image; a CCD may be employed for this purpose. Alternatively, the direction of radiation may be reversed, so that a narrow beam of radiation is transmitted, from the alignment device and received by a simple detector or an array of simple detectors built into the target, and appropriate signals are fed back to the angle measuring apparatus over the data link. Alternatively again, radiation may be reflected back to the alignment device by one or more reflectors built into the target. It will be appreciated that the signals generated by the target detectors or the reflections from the target reflectors are definitive of the position of the target. When such detectors are employed, it is advan¬ tageous to use a beam of coherent radiation and there are known techniques, used in aircraft or missile guidance

systems, by which such a beam or beams, which may have circular or elongated cross sections, may be scanned over a small range of angle to improve the effective field of view of the alignment device. Any of these detecting systems may be used not only for locating the target but also for precisely locating the centre of the target, as hereinafter described.

Referring to Figure 3, the target device 10 has a target 19 and a sight 20, which are included in a unit which can be manually rotated about axis 21, the angle of rotation b'eing sensed electronically by, for example, a rotary potentiometer. The target and sight unit are mounted on a support 22 which may be hand-held and of a length suitable to bring the sight to eye level. In many cases, the point the position of which is to be determined, will be at ground level and the lower end of support 22 would then be rested on this point.

At least one tilt sensor 23 is built into the target device or its support. The position for the tilt sensor shown in Figure 3 is close to ground level; this allows the tilt sensor(s) to be relatively unaffected by angular accelarations of the hand-held support about the point on which it is resting. Alternatively, tilt sensors may be provided at two different heights above ground level so that both the tilt and the angular acceleration of the support may be instantaneously determined, and correction applied to allow for movement of the support.

In use of the position measuring system according to the invention, the operator R aim^'s the sight 20 at the angle measuring device 11; if practical, the operator aims at the centre of the alignment device 12. The angle A between the axis of the sight which is sight line S, and the earth's gravitational field shown as vertical line G is determined approximately, using the tilt sensor 23 together with the (potentiometer) angle sensor on the axis 21.

Data representing angle A is then transmitted by means of a data link to the angle measuring apparatus, and this data is used to operate motor 15 to drive the alignment device to the appropriate angle of elevation as shown in Figure 1. The motor 16 is then energised to drive the alignment device through a single angular sweep about its vertical axis 14.

When the target 19 enters the field of view of the alignment device, a further signal is generated to stop the motor 16 which might briefly reverse on a feedback, signal if appropriate. Accurate measurement then commences by the alignment device being driven to locate and lock on to the centre of the target. The results ,of the measurements of the vertical angle (A) and the horizontal angle can be assessed by the operator over the data link, and possibly processed in a known manner to give positional coordinates of the target.

If the operator were to make an eroneous or highly inaccurate aim through the sight 20, the scanning alignment device would fail to locate the target and an appropriate warning signal would be transmitted on completion of a 360 sweep.

A radio system could provide a suitable data link between the target device and the angle measuring apparatus. At the target, transmitting and receiving aerial(s) could be built into the support 22 and, for example, a small telescopic aerial may be mounted on the angle measuring apparatus. The data could be, and in a prototype has successfully been, transmitted over an electrical or fibre-optic cable; however, this albeit simple arrangement would not be very practical in many situations where, for e-xample, th-e two ends of the system are very distant or separated by awkward terrain. The angle sensor could possibly be eliminated if the sight is rigidly mounted to the support and angular measurements are made from the tilt sensor alone, or if rotation about axis 21 can be locked in one or more known positions; however, this makes the equipment less convenient to use, particularly when working on a steep slope, and also requires the tilt sensor 23 to be accurate over a wider range of inclination. Another alternative is to eliminate the tilt sensor(s) and to rely on the operator's judgement to position the support 22 at a constant angle to the vertical, possibly with the aid of a levelling device such as a spirit level. However, this arrangement is more demanding on the operator and is likely to be less accurate than a system which includes the tilt sensor 23.

The angle measuring apparatus 11 is either set and left with its vertical axis of rotation 14 as close as possible to the true vertical, i.e line G, or at a known inclination to true vertical, or alternatively it is provided .with a tilt sensor 24 which is used to monitor the inclination of the axis 14 to true vertical and to correct the results to allow for this inclination. By mounting the tilt sensor 24, as shown, on a part of the apparatus which rotates about a vertical axis, a single tilt sensor can be used to monitor tilt in any direction and also it is possible to eliminate some errors by making pairs of tilt measurements with the sensor rotated end for end between measurements. The tilt sensor 24 can also serve to sense any disturbance of the unattended instrument and initiate a warning signal to the operator.

Modern surveying theodolites are often provided with integral means for distance measuring so that both the angle and distance coordinates of a remote target may be determined by a single instrument. This type of combined instrument is known as a total station unit. The angle measuring apparatus shown in Figure 2 may measure only the angular coordinates of a target, or it may include an integral distance measuring function so that it can perform the function of a total station unit, or it may be electronically interfaced to a separate distance-measuring device. .If the angle measuring apparatus is to perform the function of a total station unit, then the target device could include a retroreflector to return an electromagnetic beam to the angle measuring apparatus for the purpose of electronic distance measurement, using techniques which are well-known. This retroreflector could be mounted within, or in close proximity to, the target 19.

The system as so far described, determines the position of the target 19. If it i-s required to determine the position of the point on which the support 22 rests, this is possible by combining the measured position of the target with the known length of the support and the inclination of the support to ^"the vertical. The inclination of the support to the-'vertical may be determined by use of tilt sensor 23 or by relying on the operator to hold the support.22 vertical or at a known inclination, possibly with the aid of a device such as a spirit level.

It may be required to measure the position of a point which is not at ground level, for example a point on a wall. For this purpose the target device or its support may be fitted with an extension in a position such as shown in broken lines at 25 or 26. This extension would be placed in contact with the point the position of which is to be measured and the measurement of the position of the target 19 would then be extrapolated to this point, possibly making use of the readings from the tilt sensor 23 and/or the (potentiometer) angle sensor on axis 21, together with the known length and position of the extension. If the extension at position 26 is maintained accurately in line with the line of sight from the target to the angle measuring apparatus, readings from the tilt sensor and angle sensor are not required in carrying out this extrapolation. The extension at position 26 could be formed by swinging the support 22 about the axis 21 and locking the target device to the support.

The embodiment of the invention described above uses the earth's gravitational field to provide a reference direction which is, for practical purposes,the same at the position of the target device as at the 'position of the angle measuring apparatus. However, other reference directions could be used in place of the direction of the earth's gravitational field. For example, use could be made of the earth's magnetic field and the directions relative to this field could be determined by means of magnetic compasses which could be of an electronic type. In this case, and as shown in Figure 4, a signal indicative of the angle B between the line of sight S from the alignment device 12 to the target 19 and the earth's magnetic field shown as (horizontal) line N, or a component of the earth's magnetic field, would be transmitted by a data link from the target device 10 to the angle .measuring apparatus 11. The alignment device 12 of the angle measuring device would then be rotated about its vertical axis 14 to a suitable angular position, and the target is brought within the field of view of the alignment device by making a vertical sweep about its horizontal axis 13. Accurate measurements of the target position would then be made by the apparatus 11.

Other reference directions which could, in certain circumstances, be employed, include light from the sun, wind and the rotation of the earth.

The system could also make use of two reference directions which are not parallel; for example, the direction of the earth's magnetic field or a component thereof, and the direction of the earth's gravitational field. In this case, data signals indicative of two angles, such as A and B, would be transmitted from the target device 10 to the angle measuring apparatus 11, and this signal would be used to rotate the alignment device of the angle measuring apparatus about both axes of rotation 13 and 14 so as to bring the target within the field of view of the align¬ ment device, either directly or with an angular sweep through a small angle about one or both axes of rotation.

Another means to avoid the need for an angular sweep of the alignment device 12 or to reduce the angle swept, is to include a second means of direction-finding within the system, for example, by including radio direction finding apparatus within a system based on the use of the earth's gravitational field. In this case, a signal indicative of the vertical elevation angle between the target device 10 and the angle measuring apparatus 11 would be used to set the angular position of the alignment device about a horizontal axis of rotation, and a radio direction finding apparatus would be used to set the angular position of the alignment device about a vertical axis of rotation. The radio direction finding apparatus could comprise a transmitter positioned at or near the target device and a directionally-sensitive receiver positioned at or near the angle measuring apparatus. The principles of such radio direction finding apparatus are well known.

Claims

CLAIMS .

1. A position measurement system, comprising an angle measuring apparatus capable of unattended operation and having an alignment device, a target device for use attended by an operator at a position remote from the angle measuring apparatus and having sight means for establishing the line of direction of said angle measuring apparatus, means for determining the angle between said. line of direction and a leference direction, a data link for transmitting data representing said angle to said angle measuring apparatus, and means for driving said alignment device in accordance with the transmitted data to locate, said target.

2. A position measurement system as claimed in Claim 1, wherein said angle measuring apparatus comprises an alignment device which is rotatable about horizontal and vertical axes by respective motor .drives.

3c A position measurement system as claimed in Claim 1 or Claim 2, wherein said target device comprises a target and a sight for locating the angle measuring apparatus.

4. A position measurement system as claimed in any preceding Claim, wherein position-sensitive sensor means are provided at said alignment device and/or at said target, for use in precisely measuring the angular positions of the target.

5. A position measurement system as claimed in any preceding Claim, wherein tilt sensor means are provided at said target device and/or at said angle measurement apparatus.

6. A position measurement system as claimed in any preceding

Claim, wherein said reference direction is the earth's gravitational field, and said alignment device makes a sweep about a vertical axis to locate said target.

7. A position measurement system as claimed in any of Claims 1 to 5, wherein said reference direction is the earth's magnetic field or a component thereof, and said alignment device makes a sweep about a horizontal axis to locate said target.

8. A position measurement system according to any preceding Claim, in which two reference directions are employed.

9. A position measurement system according to Claim 8, wherein radio-direction-finding means are provided at said direction finding apparatus to -function as a second reference direction.

10. A position measurement system according to any preceding Claim, wherein said target device is mounted on a support of known dimensions, whereby the position of a point adjacent the target can be determined.