GB2186371A - Measurement apparatus and methods - Google Patents

Abstract

A dimension of an object is measured by using a video camera 18, 25 viewing the object via a mirror 20 on an arm 33. The camera and arm can be independently rotated about the camera axis 23 by motors 26, 35 so as to view each end of the object. Potentiometers 30, 38 provide an electric signal related to the angular movement and from which a measure of the dimension is devised. The mirror station also pivots the mirror about pivot 32 through cam follower 40 cooperable with fixed cam 41 so as to maintain an image of an elongate object horizontal. The object may be a nuclear fuel rod and the optical system may be mounted in a cell wall with an external visual monitor. <IMAGE>

Description

SPECIFICATION Measurement apparatus and methods This invention relatesto methods and apparatusfor measurement, particularly but not exclusively for measuring the lengths of elongate articles.

According to one aspect ofthe invention a method of measuring a dimension of an object comprises viewing one end of the object th rough an optical system, moving the optical system so as to viewthe other end of the object, and deriving from the movement a measure of the distance between said ends.

The measure may be derived by obtaining an electrical signal in response to the movement, and deriving from the signal a measure of the distance.

According to another aspect of the invention apparatusfor measuring a dimension of an object comprises an optical system, the system comprising a video camera adapted to view the object through a mirror, means for moving the system between positions in which opposite ends of the object are viewed, and means for deriving from the movement a measure of the distance between said ends. Means may be provided for rotating the camera about a central axis, and means may be provided for rotating the mirror about said axis and for pivoting the mirror about a pivot axis at right angles to the said central axis.

The rotation of the mirror may be arranged to effectthe pivoting of the mirrorthrough cam means.

The cam means may comprise a fixed cam co-operable with a cam follower moveable with the mirror.

Potentiometers may be responsive to rotation of the camera and of the mirror and arranged to obtain electric signals for deriving said measure.

The invention maybe performed in various ways and one specific embodiment with possible modifications will now be described by way of example with reference to the accompanying schematic drawings, in which: Figure I is a perspective view of a measuring apparatus; Figure2shows an optical diagram; and Figure 3 is a side view of a camera and mirror arrangement.

The apparatus is primarily intended for measuring the length of a nuclearfuel rod but can be used to measure the lengths of other elongate objects.

Afuel rod 10 whose length is to be measured is contained in a tray 11 on a support 12. The rod 10 is in a containment cell 13 having a shield wall part of which is shown at 14, with windows 15. In the present case a measuring apparatus is mounted in an aperture in thethewall 14. The apparatus comprises a tube 16, a close fit in the aperture,the tube having a lining 17.

Mounted in the tube is a closed circuit video camera 18 having control apparatus 19, and a mirror 20 having control apparatus 21.

In the tube 1 6there is a main frame 22 which supports for rotation about axis 23 in bearing 24 the video camera 18 with iens 25. The frame supports a DCmotor26which rotatesthecamerathrough a gearbox 27 and gearing 28, the angular position of the camera being indicated by a position servo potentiometer30. The camera is connected to a control unit49 outside the wall 14 having a monitor screen 47 and the potentiometer 30 is connected to a suitable indicator48 outside the wall.

The mirror 20 is silvered on the face 31 facing the camera and can pivot at 32 on arm 33 rotatable in bearing 34 in frame 22 on axis 23.

The frame 22 supports a DC motor 35 arranged to rotatethe mirror20 through gearbox 36 and gearing 37. The angular position ofthe mirror in relationto the axis 23 is obtained using a precision servo potentiometer 38 mounted on frame 22. One edge of the rectangular mirror 20 is provided with a cam follower40 co-operable with a cam 41 on the frame 22. Thus as the mirror is rotated about the axis 23 the shape of the cam 41 is such as to change the angle of the plane ofthe mirror in relation to axis 23 by pivoting about pivot 32.

Figure 3 shows a schematic arrangement of the camera and mirror drive assemblies. Both drives are achieved using a DC motor and positional information is obtained from a precision servo potentiometer. Backlash due to gears is eliminated by using toothed drive belts 50 and toothed gears on the motors, potentiometers, camera drive and mirror drive.

Fine control of mirror position is achieved by using a DC motor 35 with tachometer feedback. This provides a feedback signal proportional to velocity and ensures accurate adjustment at very slow speeds while allowing high speeds to be used to travel from one end of the rod to the other, as mentioned below.

The mirror is supported by means of U bracket33 enabling the mirrorto both tilt and rotate. Mirror rotation due to action of the motor 35 causes the cam follower40 attached to the mirror to move overthe profile of pre-determined cam 41. Thus mirror tilt is a function of mirror rotation.

Operator control ofthe system is limited to a lateral movementjoy-stick52 in unit 49 with centre biasing. Movement ofthe joy-stick to left or right causes the mirrorto rotate clockwise or anti-clockwise respectively and velocity of rotation is proportional to the amount of movement.

The cam follower and cam arrangementensures that the rod 10 appears central on the monitor 47 at all times. Feedback potential from the servo potentiometer 38 at the mirror is used to determine the correct position ofthe camera 18 and via suitable amplifiers and offsets it controls any rotation which may be necessary. In practice the camera must rotate through approximately 1.5 times the Mirror rotation in orderthatthe rod image remains horizontal on the monitor screen.

Thewindowtube l7isasealing deviceallowing viewing into the cell via a suitable window material but preventing contamination of the camera mirror assembly or air movement through the aperture.

The instrument frame 22 is supported at an outer flange 60 Figure 1 using a system of locking screws which allow a lateral and vertical tilt adjustment of 1 .5' from the horizontal axis of the aperture. This givesi 1.25 inches movement at the mirror assembly for example.

The frame 22 is adjustable radially to +5 ofthe normal. This normal is taken as perpendiculartotray 11 which is 10'47' anticlockwise of true vertical (viewed from flange 60 into the cell 13).

The tilt and rotation settings are made relative to a second locating flange thus ensuring easy removal and replacement ofthe instrument frame 22 at the second flange fixing points without having to readjust the location settings.

Design calculations are based on the mirrortilt pivot 32 being positioned atthe inner surface 14a of the cell wall. Frame adjustments are provided to achieve this mirror position as well as a longitudinal adjustment of camera position to within n 10 cm at the calculated mean position.

Figure 2 showsthe importantdimensionsto be used in calculating the required rotation of mirror and camera and alsotheangleofmirrortilt.

The centre line ofthe rod 10 is at 70; the rod (or tray) is shown atAB; K is the distance of line 70 from the vertical wall surface 14a. Line 71 is horizontal.

A' B' is the projection of AB on surface 14a; line 72 is the projection of line 71 on surface 14a and is at 10 47',forexample,to lineA'B'.

Line MH is at rightanglesto line 70, M being point 32. Line MH' is at rightanglesto line A' B'.

+ indicates the angle of mirror rotation from line MH'. 2a, is the angle of light path deflection required when viewing a rod 10witha mirrorrotation of+.

Projection ofthetriangles MHAand MHB onto the inner surface 14a ofthe cell wall gives triangles MH1A1 and MH1B1.Angles H1MA1 and H' MB1 are the required mirror rotation angles to successfully scan the camera from one end of tray 1 to the other. Line MH1 formsthe perpendicular intersection with A1 B1 and is a "normal" reference line aboutwhich rotation angles are calculated.

Angle of mirror rotation + is given bytan - (A1 H1 -:H1M) andforthe dimensions given in Figure 2, 4 24.70 Anticlockwise when viewing pointA1 and + = 9.30 Clockwise when viewing point B1.

All rotation directions are taken looking into the cell from the instrument frame flange.

The mirrortilt angle must be half of the angle through which the line of camera view is deflected to view the fuel rod, shown in Figure 2 as 2 a.

2Y = tan (H1M-:H1H) However this angle varies with rotation of mirror +, since the distance from M to A1 B1 follows the relationship MH7 -. Cos+.

Thus deflection angle follows the relationship tan-1(MH' . (HH' x Cos+)).

Mirrortiltangle is thus 0.5 x tan-1 (MH1 -: (HH1 x Cos +)).

Design of a suitable cam to achieve this variation in tiltangle is straightforward but relies on knowledge ofthe cam follower position in relation to the axis of mirror rotation and the axis of mirrortilt.

Assuming a distance from the front surface ofthe vidicontube 18to the mirror tilt axis of 442 mum and a diagonal picture angle ofthe 135 mm lens to be 4.40 then the minimum sized mirror required would be an ellipse with major and minor axes 48.6 mm and 23 mm respectively.

In practice the minimum recommended size of mirror is 100 mm by 50 mm which gives some latitude in lens choice and camera position. The tilting pivot point 32 should ideally be 45 mm from the top ofthe mirror in this case.

In view of the possible difference in the dimensions between theoretical and actual and the effect that these will have on mirror rotation and tilt angle there are a number of interchangeable cams with slightly differing profiles to cover most eventualities. It will assistthe initial setting up ofthe instrument if these cams have some method of indexing the 0" position for mirror rotation.

The arrangement involves the scanning of a video camera such that both ends ofthefuel rod 10 may be viewed in turn and the amount of scan from one end to the other is translated electronically into a distance corresponding to the length of the rod in question.

Practical restrictions dictate that the camera be outside the confines of the cell and out of direct line of sight of the fuel rod. Therefore viewing of the rod is via an angled mirror and the scanning is accomplished by suitable rotation ofthat mirror.

With reference to Figure 2,the mirror is rotated about an axis 23 which passes through the optical axis ofthe camera to bring the image of any point on the rod to the vertical cursor on the monitor47. The mirror must also be suitably titled in orderthatthe camera always views the centre line of the rod.

Figure 2 shows the deflection angle produced by the mirrorto be 2a and this angle varies from a minimum to 2010, when viewing perpendiculartothe rod, to a maximum of 2(xl at the end of the rod furthestfrom the perpendicular. To provide such deflectionsthe mirror mustbe atan angle of aoand a1 respectively to the optical axis 23 and this angle is a function of mirror rotation angle P- In addition, when viewing a plane object via a mirrorthe image as seen in the mirror can have an apparent tilt caused by viewing the object at an acute angle. To prevent ambiguity it is advisable to correct this tilt which then results in the rod appearing horizontal on the monitor at all times. This correction is achieved by rotating the camera by a proportional amount in excess ofthe mirror rotation.

The angle of mirror rotation is converted to a change in voltage by a precision servo potentiometer and this, in conjunction with a microprocessor, provides an accurate measurement of rod length.

Claims (11)

1. A method of measuring a dimension of an object comprising viewing one end of the object through an optical system, moving the optical system so as to view the other end ofthe object, and deriving from the movement a measure of the distance between the ends.

2. A method as claimed in claim 1, in which the measure is derived by obtaining an electrical signal in response to the movement, and deriving from the signal a measure of the distance.

3. Apparatus for measuring a dimension of an object comprising an optical system, the system comprising a video camera adapted to viewthe objectthrough a mirror, means for moving the system between positions in which opposite ends of the object are viewed, and means for deriving from the movement a measure of the distance between said ends.

4. Apparatus as claimed in claim 3, comprising means for rotating the camera about a central axis, and means for rotating the mirror about said axis and for pivoting the mirror about a pivot axis at right angles to the central axis.

5. Apparatus as claimed in claim 4, in which the rotation of the mirror is arranged to effect the pivoting ofthe mirror th rough cam means.

6. Apparatus as claimed in claim 5, in which the cam means comprises a fixed cam coopeerable with a cam follower moveablewith the mirror.

7. Apparatus as claimed in any of claims 4 to 6, including potentiometers responsive to rotation of the camera and ofthe mirror and arranged to obtain electric signals for deriving said measure.

8. Apparatusasclaimed in anyofclaims3to7, including avisual display deviceforviewing an image of the object.

9. Apparatus as claimed in claim 8, in which the camera and mirror are in an aperature in a cell wall and the display device is outside the wall.

10. Amethod of measuring adimensionofan object substantially as hereinbefore described.

11. Apparatusformeasuring a dimension ofan object substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.