GB2173311A

GB2173311A - Digitizing equipment

Info

Publication number: GB2173311A
Application number: GB08606796A
Authority: GB
Inventors: Ward William Carson
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-03-20
Filing date: 1986-03-19
Publication date: 1986-10-08
Also published as: NO851107L; GB8606796D0

Abstract

Two-dimensional coordinates (X,Y) of a movable point (C) in a fixed plane are computed by measuring the lengths of two sides of a triangle formed by the movable point (C) and two fixed points (A,B). Two arms (8,9) linked at a point (10) overlying the movable point have straight tracks and reflective scales (4,5), and slide along steering devices (6,7) which pivot on a common base. Transducers (2,3) respond to the reflective scales (4,5) to produce signals representing the lengths of the arms lying between the steering device pivots and the pivot (10) linking the arms. <IMAGE>

Description

SPECIFICATION Digitizing equipment The invention at hand concerns a procedure and an instrument for measurement of the twodimensional coordinates of a movable point in a coordinate system where registration of this information is transferred to coded information that can be stored or processed by a computer.

Such an apparatus is utilized, for example, for coordinate measurements and further digitising of movable points and is especially applicable for maps, drawings, and photographs. Results of measurements from such maps or drawings often exist in the form of point-data which describes a line or curve and it is common to transfer such data to a computer for further processing, such as plotting in the form of a drawing from a digital plotter. Results of measurements from aerial photographs can, if they are very precise, be used in a computer to determine object coordinates, which is the main application for the invention at hand.

There are different known apparatuses for this purpose, but, they are based on principles that make them expensive to produce compared with this apparatus when the same precision shall be attained. Such apparatuses are usually called mechanically constrained XY-digitising equipment, in comparison to digitiser tables with cursor. The applicant knows of 3 main types of mechanically constrained digitising equipment that have come into practical utilization and are of commercial interest. The most common design utilizes one encoder to measure a linear displacement along X-axis, and another encoder to measure a linear displacement along the Y-axis, in a rectangular coordinate system. A second design makes use of one encoder to measure the angle displacement about a fixed point and another encoder to measure the linear displacement along the radius in a polar coordinate system.A third design, more explicitly described in Norwegian patent 126,596, utilises two encoders to measure angle displacement of two different angles. In this design two measured angle displacements are combined together with two fixed lengths in a measuring apparatus providing for the determination of the two-dimensional coordinates of a moveable point.

Certainly a digitising table with a cursor is reasonable digitising equipment, but, it has limited accuracy which at best is +0,050 mm and can not be utilized for exact resolution of pointcoordinates in photograph measuring.

The invention provides a method of measuring the position of a moveable point in a fixed plane, comprising measuring the distances in the plane between two fixed points and the moveable point, and calculating the said position from the said measured distances and the predetermined distance between the two fixed points.

According to a further aspect, the invention provides apparatus for measuring the position of a moveable point in a fixed plane, comprising means defining a triangle in the plane having a base of fixed length and two sides of variable length, such that the apex of the triangle may be made to follow the moveable point, and transducer means for measuring the said two variable lengths, whereby the position of the moveable point may be determined from the said variable lengths and the fixed length.

The invention is capable of providing an instrument which is of a simpler construction and is essentially less expensive to produce than known mechanically constrained digitising equipment, while at the same time it exhibits extremely good accuracy-0,010 mm or better. A prototype has been found to give measurement results with 0,006 mm accuracy.

This increased accuracy derives from the arrangement whereby the lengths of two sides of a triangle are measured in relation to two fixed vertices and one movable vertex in the triangle.

Computing means, preferably digitising equipment, compute the two-dimensional coordinates of the movable point, based on the determination of the variable lengths of the two sides of the triangle, and the predetermined distance, i.e. the length of the third side. In a given triangle ABC two sides AC and BC are movable and one side AB is constant, where consequently corner C constitutes a movable point.In this manner point C's coordinates in relation to an XY-coordinate system with the origin in point A and the- Y-axis an AB are given by the equations: Y=((AC+BC) * (AC -BC)+(AB AB))/(2 * AB) x=V(AC * AC-Y * Y) An embodiment of the invention, will be more closely described in the following with reference to the enclosed drawings, where: Figure 1 shows schematically the instrument seen from above and where a coordinate system XY is superposed, Figure 2 shows schematically a vertical cross-section of the transducers seen from ll-ll in Fig.

1, Figure 3 shows schematically a vertical longitudinal section of the transducers seen from Ill-Ill in Fig. 1, Figure 4 shows schematically a vertical section through the corner C, seen from IV-IV in Fig.

1, and, Figure 5 shows a perspective sketch of one configuration of the apparatus in accordance with the invention.

With reference to Fig. 1, this shows an embodiment of the invention where two encoders are arranged to register the two-dimensional coordinates of a movable point. The apparatus is arranged in the form of a triangle ABC, where one side AB is always a constant length, while the other two sides AC, BC are displaceable such that the corners A, B in the triangle are always fixed, while point C is movable.

In the description of the operation of the apparatus it is found appropriate to refer to three vectors stated as Na, Nb and Nc. These vectors are defined as those normal to the plane, formed by the points A, B and C, which pass through these points such that Na goes through A, Nb goes through B, and Nc goes through C.

The triangle contains a coordinate system X,Y, where the Y-axis passes through corners A and B in the triangle and the X-axis is normal to the Y-axis with the origin in corner A. A fixed reference bracket 1 defines side AB in the triangle. Two arms, 8 and 9, constitute the movable sides AC, BC in the triangle. The arms are linked together and rotate in relation to each other about axis Nc in corner C and also rotate about axes Na, Nb, respectively, in corners A and B.

Scanners, 2 and 3, are arranged at each of the corners A and B. Graded scales, 4 and 5, for example linear incremental scales, are arranged on each of the arms, 8 and 9. Together, the scanners and the scales constitute the encoders. The scanners, 2 and 3, send signals to the graded scales, 4 and 5, on the arms, 8 and 9, which reflect back to the scanners and produce signals in proportion to changes of the scales position in a manner which is in and of itself known.

In Fig. 2 and 3 a restraining and guiding device 6 (or 7) is shown: device which restrains a scanner 2 at the same time as it guides arm 8 with the help of the guide rollers 15. Device 6 rotates about shaft 13, connected to a base 14, which in turn is secured to reference bracket 1.

Axle 13 is fastened to base 14 for rotation of device 6 about vertical axis Na through a bearing assembly 11. The steering rollers 15 go in tracks in arms 8 and 9 such that these can not move vertically or horizontally to the arms, but only in the direction of the length of the arms.

As shown in Fig. 4, arms 8 and 9 are coupled together in a linked manner about bearing assembly 10 and a vertically arranged axle. Axis Nc in bearing asseembly 10 defines a movable point C in the triangle. Movable point C is guided manually or by other means to the point which is to be measured.

Reference bracket 1 serves as a fixed base in the system, and, it is built as a firm base for mounting of the two restrainers 6 and 7 for scanners 2 and 3. When the instrument is to be fastened to any kind of base such as a drawing table, a desk, or a table with back lighting, the attachment is made to this fixed reference bracket 1.

There is a large selection of encoder systems which can be used with this measuring apparatus. One choice is an optical, linear encoder system, with an optical/electronic scanner and reflecting scale. Another choice is a scanner/scale system used in a linear, magnetic system. A third choice is a rotary encoder, geared to a scale such that it will give out electronic signals in relation to the scale's linear displacement. All of these, or other encoder systems, can be used in this measuring apparatus. In principle the scanner in the encoder system must sense the linear displacement of a scale and provide signals which can be processed and digitised. Beyond this requirement the choice of encoder scanners is open.

An essential feature of the restraining and guiding devices, 6 and 7, is that these restrain arms, 8 and 9, in a fixed relationship to themselves such that any angular movement of the arms causes rotation of devices 6 and 7 about their respective axes Na and Nb. The arms 8 and 9 must also be free for movement through devices 6 and 7 such that they pass back and forth in a plane which is parallel to the main plane formed by points A, B and C.

In the example shown, arms 8 and 9 are of equal length, and are substantially equal in length to the predetermined distance AB. However, while this gives optimum accuracy, it is not an essential feature.

Claims

1. A method of measuring the position of a moveable point in a fixed plane, comprising measuring the distances in the plane between two fixed points and the moveable point, and calculating the said position from the said measured distances and the predetermined distance between the two fixed points.

2. A method according to Claim 1, wherein the distances between the two fixed points and the moveable point are determined by measuring the linear displacement over each respective fixed point of a straight arm, the two straight arms being linked together at a pivot joint of the moveable point.

3. A method according to Claim 1, using apparatus comprising two straight arms linked by a pivot joint; two steering devices, each steering device constraining a respective arm to lengthwise sliding movement through a different fixed point and to pivotal movement about the fixed point in a plane parallel to the said fixed plane, the fixed points of the steering devices being spaced apart by the predetermined distance; and transducer means for measuring the sliding movement of each arm through the fixed point of the associated steering device to produce signals representative of the distances parallel to the fixed plane between each of the fixed points and the said pivot joint, comprising positioning the said pivot joint over the movable point, and determining from the said signals and from the predetermined distance the position of the movable point.

4. Apparatus for measuring the position of a moveable point in a fixed plane, comprising means defining a triangle in the plane having a base of fixed length and two sides of variable length, such that apex of the triangle may be made to follow the moveable point, and transducer means for measuring the said two variable lengths, whereby the position of the moveable point may be determined from the said variable lengths and the fixed length.

5. Apparatus according to Claim 4, wherein the triangle-defining means comprises two straight arms linked by a pivot joint defining the apex, and two steering devices each steering device constraining a respective arm to lengthwise sliding movement through a fixed point and to pivotal movement about the fixed point in a plane parallel to the said fixed plane, the fixed points of the steering devices being the vertices at the base of the triangle; and wherein the transducer means measure the sliding movement of each arm through the fixed point of the associated steering device to produce signals representative of the distances parallel to the fixed plane between each of the fixed points and the said pivot joint, whereby the pivot joint may be made to follow the movable point whose position may then be determined from the said signals and from the fixed length.

6. Apparatus according to Claim 5 further comprising means responsive to the fixed length and to the said signals to compute the position of the movable point in terms of twodimensional coordinates.

7. Apparatus according to Claim 5 or 6, wherein each arm has a straight steering track for sliding or rolling engagement with the associated steering device.

8. Apparatus according to any of Claims 5 to 7, wherein the arms are linked at their ends, and their lengths are substantially equal.

9. Apparatus according to Claim 8, wherein the lengths of the arms are substantially equal to the said fixed length between the fixed points.

10. Apparatus according to any of Claims 5 to 9, wherein each arm comprises a straight reflective scale along its length for measurement of its sliding movement by the transducer means.

11. Apparatus according to Claim 10 as appendant to Claim 7, wherein the reflective scale of each arm is parallel and closely adjacent the said steering track.

12. Apparatus according to Claim 10 or Claim 11, wherein the transducer means comprises a scanner associated with each steering device to scan the reflective scale and to produce a signal representative of the point on the scale lying adjacent the steering device.

13. Apparatus according to any of Claims 5 to 12, wherein each steering device is mounted on a common platform through a bearing assembly at the fixed point which allows the steering device to rotate in a plane parallel to the fixed plane.

14. Apparatus according to any preceding claim, wherein the pivot joint linking the two arms comprises a bearing assembly.

15. Procedure for measurement of the two-dimensional coordinates of a movable point in a coordinate system where registration of this information is transferred to coded information which is stored or processed in a computer, characterized by registration taking place by measuring the linear displacement of two sides of a triangle in relation to two fixed corner points and a movable corner point in the triangle.

16. Apparatus for measurement of the two-dimensional coordinates of a movable point in a coordinate system where registration of this information is transferred to coded information which is stored or processed in a computer, characterized by transducers, for example linear incremental encoders, being arranged to measure, respectively, the linear displacement of two sides in a triangle in relation to two fixed corner points and a movable corner point in the triangle.

17. Apparatus for measuring the position of a movable point in a fixed plane, substantially as described herein with reference to the accompanying drawings.

18. A method of measuring the position of a movable point in a fixed plane, substantially as described herein with reference to the accompanying drawings.