GB2062222A

GB2062222A - Speed measuring arrangement

Info

Publication number: GB2062222A
Application number: GB8034823A
Authority: GB
Original assignee: GEC Elliott Automation Ltd
Current assignee: Roadside Technology Services Ltd
Priority date: 1979-10-29
Filing date: 1980-10-29
Publication date: 1981-05-20
Also published as: GB2062222B

Abstract

A speed measuring arrangement for measuring the rotational speed of a rotating toothed wheel 2 comprises a light source 3 on one side of the wheel and a linear array of light sensitive elements 4 on the other side. The teeth move along the array thereby progressively obscuring successive elements of the array from the light source. The position of a tooth edge is determined from the transition in output between adjacent obscured and unobscured light sensitive elements. Storage means 8 connected to the array stores the positions of transitions on the array sampled at predetermined time intervals. The difference between successive positions (represented as counts) of a transition is evaluated by circuit 11 and the speed, calculated in circuit 12, is displayed at 13. <IMAGE>

Description

SPECIFICATION Speed measuring arrangement This invention relates to a speed measuring arrangement particularly, but not exclusively, for measuring the angular velocity of a rotating shaft.

According to the present invention, a speed measuring arrangement comprises means for projecting a beam of light on to a linear array of light-sensitive elements, a member movable along the linear array so as to obscure the light-sensitive elements progressively, storage means connected to the elements of said array for storing an indication of the energisation of the respective elements, timing means arranged to control the operation of said storage means so as to provide at predetermined intervals a stored indication of the position on the array of a transition between obscured and unobscured light-sensitive elements, and differencing means providing a representation of the difference between successive positions of said transition, said representation indicating the speed of said member along said array.

Preferably the length of the array is greater than twice the distance moved by a transition during one of the predetermined intervals to ensure at least two successive stored indications of each transition.

The member may be a tooth of a wheel, the representation then providing an indication of the wheel rotational velocity.

The tooth is preferably one of a number fixed relative to one another and of such size and spacing that before one said transition has left the array another has arrived on it, the distance between any two such successive transitions, plus the distance moved by a transition during one of said predetermined intervals, being together less than the length of the array so that a speed indication can be obtained from the passage of every transition along the array.

In the latter case, and where two or more transitions repeatedly occur simultaneously on the array, the arrangement may be such that said differencing means is responsive to the difference between successive positions of the most recently arrived transition.

The storage and differencing means preferably both comprise digital circuitry.

The timing means may be controllable to provide a selected value of said intervals, said differencing means then being responsive to the value of the interval so as to provide a direct speed indication.

One embodiment of a speed measuring arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of a light-source/sensor arrangement with intervening toothed wheel and associated circuitry, and Figure 2 is an illustration of the patterns of light on the array at successive sampling intervals together with the corresponding tooth position.

Referring to Figure 1, a linear array 4 composed of a large number of light sensitive elements is positioned relative to a light source 3 so that the emitted light falls upon the array 4.

A toothed wheel 2 mounted on a rotating shaft 1 is positioned in the light path so that light from the source 3 to the array 4 is interrupted by the teeth 5 of the toothed wheel 2 so that the shadow of the edge of a tooth moves across the array and its position can be detected by the array. The emitted light from the source 3 is parallel to the axis of rotation of the shaft 1.

The dimensions of the array are determined in relation to the probable range of velocity to be measured and to the spacing of successive tooth edges in a manner to be explained.

The elements of the array 4 are connected to a store 8, the store 8 being operative only when triggered by a clock pulse from timing control means 9. Thus the store 8 and the timing control means 9 act as a staticiser storing the outputs of the array 4 at regular sampling intervals.

Referring to Figures 2(a)-(d), the diagrams represent the successive instantaneous positions of the teeth 5 in relation to the linear array 4 when the outputs of the light sensitive elements of the array are stored. The broken lines in each diagram denote the shadows on the array caused by the teeth 5 obscuring the array from the incident light.

The unbroken lines denote the illuminated sections of the linear array 4 corresponding to gaps between the teeth 5. For clarity, the position and direction of motion of the teeth 5 are shown in each diagram corresponding to the illuminated and shadowed parts of the array 4 at that instant.

It will be noted that Xa, X2 and X3 represent successive positions of the leading edge of one tooth 5, while Y, and Y2 represent successive positions of the trailing edge of the same tooth.

Clearly, a tooth edge is detectable from the transition between a succession of energised elements of the array 4 and an adjacent succession of un-energised elements. After staticising the shadow pattern in the store 8, a transition detector 10 scans the store 8 for any pair of adjacent store elements containing opposite binary bits and produces an output pulse for each such pair and consequently for each shadow transition. Each of these transition outputs is recorded as a count corresponding to the position of the transition in further digital circuitry 11 which simultaneously extracts a difference count from the difference between the current and preceding transition counts.Thus, at the instant illustrated by Figure 2(a), a number X is stored by the circuitry 11. At the instant illustrated in Figure 2(b), the tooth edge has moved across the array in the intervening time interval to a position which gives a transition count X2 which is similarly stored in the circuitry 11 and the difference count X1 - X2 is extracted.

The circuitry 11 is, in turn, connected to speed calculation circuitry 12, which divides the difference count by a factor proportional to the sampling interval, to give: angular speed of shaft = k(X1 - X2) where k is a constant factor incorporating the spacing of the elements the radial distance of the array from the shaft and the sampling interval. The speed is displayed by display circuitry 13 connected to the speed calculating circuitry 12.

The sampling interval must be small enough to ensure that a particular tooth edge transition is recorded at least twice as the tooth edge traverses the array. With a given sampling interval and maximum tooth velocity this requirement can be considered as specifying a minimum array length of at least twice the distance moved (the sampling distance) in a sampling interval.

Apart from the necessity of obtaining at least two samplings of each tooth edge, it is also desirable, if rapid changes of velocity are to be detected and advantage is to be taken of a high sampling rate, that a velocity calculation can be obtained from every sampling. Thus, on the last appearance of a tooth edge transition on the array (giving a velocity calculation from the stored count of its previous appearance) a shadow transition must also be present from the following toothedge (whether this is the trailing edge of the same tooth or the leading edge of the following tooth).

To ensure this condition the length of the array must be at least as great as the sum of the distance between successive tooth edges and the sampling distance. If the tooth length (circumferential) is equal to the gap length the distance between successive edges is the 'edgepitch', and if one is greater than the other then the greater of the two must be taken in the above calculation.

When the transition detector 10 detects more than one transition in a particular sampling it passes both of these on to the circuitry 11. The circuitry 11 selects the leading one of these transitions for deriving the difference count but stores the position of the most recently arrived transition for the purpose of the next difference count. Thus, in Figure 2(c) the number X3 would be used in the differencing circuitry 11, and the number1 would be stored.

Referring to Figure 2(d), the transition detector 10 would detect the pass on the transition count Y2 which would then be used with the previously stored Y1 in the differencing circuitry 11.

If the array 4 is of such extent that more than two transitions are detected by the detector 10 in any one sampling, means must be provided for selecting the two most recently arrived transitions (or other identifiable pair) to process for the speed calculation.

Clearly, the sampling rate must be sufficiently high that there is no ambiguity in the recognition of a particular tooth edge.

The arrangement so far described relates to the rotary speed measurement of shafts and other rotary machinery. Cleary however, the invention is applicable to the calculation and display of the speed of linear members where such members have a shape which can cast a progressive shadow on a linear optical array.

Claims

1. A speed measuring arrangement comprising means for projecting a beam of light on to a linear array of light-sensitive elements, a member movable along the linear array so as to obscure the light-sensitive elements progressively, storage means connected to the elements of said array for storing an indication of the energisation of the respective elements, timing means arranged to control the operation of said storage means so as to provide at predetermined intervals a stored indication of the position on the array of a transition between obscured and unobscured light-sensitive elements, and differencing means providing a representation of the difference between successive positions of said transition, said representation indicating the speed of said member along said array.

2. A speed measuring arrangement comprising means for projecting a beam of light on to a linear array of light-sensitive elements, a member movable along the linear array so as to obscure the light sensitive elements progressively, storage means connected to the elements of said array for storing an indication of the energisation of the respective elements, timing means arranged to control the operation of said storage means so as to provide at predetermined intervals a stored indication of the position on the array of a transition between obscured and unobscured light-sensitive elements, the length of the array being at least twice the distance moved by a transition during one of said predetermined intervals to ensure at least two successive stored indications of each transition, and differencing means providing a representation of the difference between the successive positions of said transition, said representation indicating the speed of said member along said array.

3. A speed measuring arrangement according to Claim 1 or Claim 2 wherein said member is a tooth of a wheel, and said representation provides an indication of the wheel rotational velocity.

4. A speed measuring arrangement according to any preceding claim wherein said member is one of a number fixed relative to one another and of such size and spacing that before one said transition has left the array another has arrived on it, the distance between any two successive transitions, plus the distance moved by a transition during one of said predetermined intervals, being together less than the length of the array.

5. A speed measuring arrangement according to Claim 4 wherein for two or more transitions occurring on the array simultaneously, the arrangement is such that said differencing means is responsive to the difference between successive positions of the most recently arrived transition.

6. A speed measuring arrangement according to any preceding claim wherein said storage means and said differencing means both comprise digital circuitry.

7. A speed measuring arrangement according to any preceding claim wherein said timing means is controllable to provide a selected value of said intervals and said differencing means is responsive to the value of the interval to provide a direct speed indication.

8. A speed measuring arrangement according to any preceding claim substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.