GB2096312A

GB2096312A - Detecting or measuring movement

Info

Publication number: GB2096312A
Application number: GB8110954A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1981-04-08
Filing date: 1981-04-08
Publication date: 1982-10-13
Also published as: GB2096312B

Abstract

In order to detect when a moving member 1 reaches a pre-determined velocity or has stopped, it has attached to it a strip 2 bearing a series of retro-reflective triangular shapes 2A. Three optical detectors 7A, 7B and 7C view the strip 2 through an optical system 5 and, because of the tapered shape of each triangle, the output from each optical detector, after differentiation, indicates the velocity of movement. A logic circuit 11 provides a "majority decision" indicating whether the outputs from a majority of the detectors 7A, 7B and 7C indicates that the velocity is below a pre-set level. This means that false indications provided when one of the detectors (detector 7C as illustrated) is viewing an area between two of the triangles are ignored. The detection apparatus may be mounted on a vehicle and the strip 2 on a rail. The detectors 7A, 7B and 7C may alternatively be tapered and the shapes 2A in the form of straight lines. <IMAGE>

Description

SPECIFICATION Apparatus for detecting or measuring movement of an object This invention relates to apparatus for detecting or measuring movement of an object.

The invention provides apparatus for detecting or measuring movement of an object relative to the apparatus or vice versa, which object carries a series of patterns or shapes, the apparatus comprising: at least three detectors each constructed and arranged to receive at any one time radiation from a particular region of said series, the amount of such received radiation varying as a result of said movement; recognition means associated with an output of each detector to recognise a characteristic thereof associated with a particular type of movement: and logic means for indicating when more than a given proportion of the recognition means recognise said characteristic.

The aforementioned "type of movement" can be, for example: a particular velocity or range of velocities (e.g. a velocity close to or equal to zero); a direction of movement; or a particular acceleration or range of accelerations.

The invention arose in the design of an optical system for detecting the instant when a moving machine part either comes to a halt or reaches a pre-set velocity. In this sytem the series of patterns or shapes is marked with retro-reflective material on a sheet which is secured permanently or temporarily on the moving machine part. The patterns of shapes can be a series of triangles, diamond shapes, or other tapered configurations spaced in the direction of movement.

Aiternatively, the patterns could be areas of varying reflective properties. In another possibility the patterns or shapes could be formed by a series of lines, each extending laterally with respect to the direction of movement. In this latter case it would be necessary for each of the detectors to be specially designed, so as to have, for example, a triangular, rhomboid, or other tapered aperture or shape so that the amount of light received by it varies as an image of the patterns or shapes moves relative to it.

The aforementioned recognition means is preferably designed to recognise a condition when the rate of change of the output of the associated detector is above or below a predetermined value, this condition occurring either when the velocity of the machine part is above or below a particular value or when the relevant detector is being illuminated by a region between two adjacent patterns or shapes in the aforementioned series.

One way in which the invention may be performed will now be disclosed by way of example with reference to the accompanying drawing of an apparatus constructed in accordance with the invention.

Referring to the drawing, which is very schematic, the purpose of the illustrated apparatus is to measure the time taken for a moving part 1 of a machine to reach a safe, very slow, velocity after actuation of a braking mechanism (not shown). For this purpose a strip of paper 2 is fixed, e.g. by contact adhesive, to the member 1. The strip 2 has printed on it a series of triangular shapes in retro-reflective material.

These are arranged in the direction of movement (shown by the arrow) of the member 1 as illustrated. Spaced from the member 1 and mounted on a tripod, not shown, is the measuring instrument which is mounted in a casing 3. This casing has an aperture 4 associated with an optical system represented by the convex lens 5.

The lens 5 projects light from a light source L1, via a half-silvered mirror M onto the strip 2 and projects an image of part of the strip 2 onto a member 6 carrying three longitudinal optical detectors 7A, 7B and 7C.

Considering firstly the detector 7B it will be seen that this is illuminated by a central part of the image of one of the triangles 2A. As the member 1 moves in the downward direction illustrated, the amount of the detector 7B which is illuminated will increase because of the tapered shape of the triangle 2A. Thus, the output from 7B will increase. This increasing output is amplified at 8B and the rate of increase is measured by a differentiator 9B. This rate of increase is a measure of the speed of movement of the member 1 and is compared in a comparator 1 OB with a voltage level V ref. The comparator 1 OB produces an output of logic level 1 when the speed is below a very low value considered to be safe for the apparatus concerned and this logic signal is passed to a logic circuit 11 which will be described later.

The outputs from detectors 7A and 7C are similar to that from 7B and are treated in exactly the same way by circuit components 8A, 9A and 1 OA and, 8C, 9C and 1 OC.

At the time when the apparatus is in the position illustrated it will be seen that the detector 7C is being illuminated by an intermediate portion of the pattern between two of the triangles. The output from detector 7C may therefore be increasing or decreasing and will not give a useful indication about the velocity of movement of the member 1. Let us suppose, however, that this output is decreasing. The output from differentiator 9C will therefore be negative and as a consequence of this the output of the comparator 10 will be at logic level zero.

The logic circuit 11 is designed so that it produces an output at level 1 when and only when a majority of its inputs are at level 1. Thus, in the situation discussed, where both comparators 1 OA and 1 OB produce an output at level 1, the output of the logic circuit is 1, this causing the timer 12 to stop. The timer 1 2 has previously been started by a control signal applied at terminal 1 3 at an instant when a braking operation acting on the member 1 is commenced. The timer 12 thus measures the time taken for the member 1 to come to an approximate halt.

Whilst, for simplicity of description, a system using only three detectors 7A, 7B and 7C has been described, alternative systems might have four or more detectors. Also, whilst in the illustrated system the logic circuit 11 is designed to indicate when a majority (i.e. more than 50%) of the detectors recognise the said given characteristic; alternative logic circuits could be designed to indicate when more than say 75% or some other given proportion of the detectors recognise said characteristic.

In an alternative form of the invention an apparatus like that illustrated could be mounted on a vehicle or other moving body. A pattern like that shown on strip 2 could be fixed to a stationary rail or other member close to which the moving body passes. In anotherform of the invention it would be possible to replace the triangular shapes 2A by a series of straight lines. In such a modified version of the invention the detectors 7A, 7B and 7C would have to be tapered, or set behind a tapered aperture or otherwise be arranged to receive radiation incident on a tapered area.

In yet another application of the invention a signal like that which appears at the output of the illustrated logic circuit 11 could be used as a control signal to control the said movement. The invention is thus applicable to the field of control systems and can be used for example to regulate the velocity of or direction of movement of motors, generators, conveyors and other machine parts.

Claims

1. Apparatus for detecting or measuring movement of an object relative to the apparatus or vice versa, which object carries a series of patterns or shapes, the apparatus comprising at least three detectors, each constructed and arranged to receive at any one time radiation from a particular region of said series, the amount of such received radiation varying as a result of said movement: recognition means associated with an output of each detector to recognise a characteristic thereof associated with a particular type of movement; and logic means for indicating when more than a given proportion of the recognition means recognises said characteristic.

2. Apparatus according to claim 1 in which the patterns or shapes are tapered.

3. Apparatus according to any preceding claim in which each detector is constructed or arranged to receive radiation incident on a tapered area.

4. Apparatus according to any preceding claim in which each recognition means is constructed to recognise when the rate of change of the output of its associated detector is above or below a predetermined value.

5. Apparatus according to any preceding claim in which the said proportion is 50%.

6. Apparatus according to claim 1 and substantially as described with reference to the accompanying drawings.