GB2217454A

GB2217454A - Position measurement system

Info

Publication number: GB2217454A
Application number: GB8800988A
Authority: GB
Inventors: John Perry
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-01-18
Filing date: 1988-01-18
Publication date: 1989-10-25
Also published as: WO1989006783A1; GB8800988D0

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 KEASUREI{2NT SYST This i invention relates to a e position measurement system. An application to land surveying is described but other applications are possible, for example in setting up large machinery.

Position measurement systems often include an apparatus for measuring the angle between points by use of a gimbal led alignment system, angular scales and a remote target. The surveyors theodolite is such an apparatus, the alignment system being a sighting telescope which is manually directed to bring a target into the center' of a field of view. To achieve good accuracy the alignment system requires good resolution and this is often associated with a small field of view which can cause difficulty in initialy 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 arrangement is not applicable to a position finding system for which the angle measuring apparatus is unattended and required to operate automatically, the human operator being at the target end of the system.

For such a system an automated method may be required for initially bringing a target within the field of view of an alignment system.

The invention provides a position measurement system as claimed in each of claims 1 to A prefered embodiment of the invention includes a sighting device, which may be of comparatively low angular resolution, which is attached to a target device. There is also provided a means of determining the inclination to vertical of both a mounting for this sighting device and of the angle measuring device. When the system is in use the operator aims the sighting device at the angle measuring device and then, by means of suitable electronic circuitry and angle sensors, the vertical elevation angle between the target device and the angle measuring device is estimated and transmitted by a data link to the angle measuring device.Using this estimated vertical elevation angle the alignment system of the angle measuring device can now be automatically rotated to a suitable angular position about its horrezontal axis of rotation such that by then making a single anglular sweep about the vertical axis the target is brought within the field of view of the alignment system and accurate angular measurement of the target position can commence.

A specific embodiment ol the invention will now be described by way of example with reference to the accompanying drawing in which l Figure 1 shows, diagramatically, an angle measuring device forming part of a land surveying system; Figure 2 shows, diagramatically, a target and target support for this angle measuring device.

Modern surveying theodolites are often provided with integral apparatus for electronic distance measuring so that both the angular 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. The angle measuring device shown in figure 1 may be a theodolite or may be part of a total station.

Surveying theodolites and total stations generaly require two operators, one to operate the instrument and the other to position a remote target. The angle measuring device shown in figure 1 avoids the need for one operator since it operates automatically and unattented once set up. The angle measuring device includes vertical and horrizontal axis of rotation 1 and 2 respectively, vertical and horizontal electronically sensed angular scales 3 and 4 respectively and an alignment system 5. It differs from conventional instruments in that manual controls for making rotations about the vertical and horizontal axis are replaced by servo motors 6 and 7 respectively, and the alignment system is c a type suitable for electronic sensing of the target position within the field of view.Such an alignment system may use electromagnetic radiation transmitted from the target position and recieved by a detector built into the angle measuring device.

Alternatively the direction of propagation of radiation may be reversed, for example by projecting a laser beam from the angle measuring device and recieving it at a detector built into the target. There are known techniques, used in guidance systems, by which a laser beam or beams which may be fan shaped are scanned over a range of angle to improve the effective field of view of such a system.

The angle measuring device is either set and left with its vertical axis of rotation as close as possible to true vertical, or alternatively it is provided with a tilt sensor 8 which is used to monitor the inclination of the vertical axis of rotation to true vertical and to correct the results for this inclination. By mounting the tilt sensor 8 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 o tilt measurements with the sensor rotated end for end between measurments. The tilt sensor 8 can also serve to sense any disturbance of the unattended instrument and initiate a warning signal to the operator.

The target 9 for the alignment system is mounted in proximity to the sight 10 forming a unit which can be manually rotated about a horizontal axis 11, the angle of rotation being sensed electronically. The target and sight unit are mounted on a support 12 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 12 would then be rested on this point. A tilt sensor(s) 13 are mounted on, or built into the support 12. The position of this tilt sensor(s) may be close to ground level as shown, this allows the tilt sensors to be relatively unaffected by angular accelerations 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.

Electronic circuitry is provided to determine the vertical angle of the sight using the tilt sensor together with the angle sensor on the axis 11. This angle sensor could be eliminated if the sight is rigidly mounted to the support, or if the rotation about axis 11 can be locked in one or more known positions.

However this makes the equipment less convienient to use, particularly when working on a steep slope, and also requires the tilt sensor(s) 13 to be accurate over a wider range of inclination. Another alternative is to eliminate the tilt sensor(s) 13 and to rely on the operators judgement to position the support at a constant angle to 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(s) 13.

A radio system could provide the necessary data link between the target end of the system and the angle measuring device. At the target end of the system the antenna for the data link could be built into the support 12.

If the angle measuring device is of the total station type, that is if it provides for electronic distance measuring, the target would normally include a retro reflector or there would be a retro-reflector mounted in proximity to it. This retro-reflector would return an electromagnetic beam to the angle measuring device allowing distance measurement by known techniques.

The system as so far described determines the position of the target. If it is required to determine the position of the point on which the support 12 is rested 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. For this purpose the tilt sensor(s) 13 must be able to measure inclination about two horrezontal axis, whereas for measurement of the target position only, measurement of tilt about only one axis is sufficient.

It may be required to measure the position of a point above ground level on a vertical surface, for example a point on a wall. For this purpose the support 12 may be extended as shown dotted 14, or alternatively an extension may be attached to the target and sight unit, extending as shown dotted 15. The end of either of these extensions would be rested on the point the position of which is to be determined and the measured position of the target would then be corrected to allow for the known length and angular position of the extension.

When measurements are made using extensions 14 or 15 the accuracy of the results is partially dependent on the accuracy with which the sight 10 is aimed. When taking measurements of the position of only the target the accuracy with which the sight is aimed does not affect the accuracy of the result, but may prevent a result being obtained if it is grossly in error. This is also true when the measurement is extended, using tilt sensors, to a point on the target support which is vertically in line with the target;

Claims

CLAIIC

1 A position measurement system comprising an automatic angle measuring device capable of operation unattended, and a target device, for use attended by an operator, in association with the automatic angle measuring device at a position which is remote from the automatic angle measuring device, characterised by the target device being adapted to be aimed by the operator at the angle measuring device and to transmit a signal thereto indicative of the vertical elevation angle of the direction of aim, the angle measuring device being responsive to the said signal to determine the angular coordinates of the target device by sweeping horizontally at the given vertical elevation angle.

2 A position measurement system as claimed in claim 1 wherein tilt sensor means are attached to a support of the target device for use to obtain an estimate of the vertical elevation angle between the angle measuring device and the target device.

3 A position measurement system as claimed in claim 2 wherein the tilt sensor means attached to the target support have a secondary function in allowing the measured position of the target to be used to determine the position of another point on the target support.