GB1568421A - Arrangement for use in measuring the number of rotations of a rotating device - Google Patents

Abstract

Mounted on the axis (AC) of a flow meter which rotates at a speed proportional to the rate of flow is an interrupter (FB) with segments (FL) whose angular width and the angular width of their spacings are equal, and which are scanned by detectors (PW1, PW2, PW3). The latter are arranged with respect to the interrupter segments (FL) in such a way that upon rotation of the interrupter by, in each case, a specific angle, only a single detector signal occurs or vanishes. The detector signals are fed to a discriminator (ZSP1, ZSP2, VGL1, VGL2, VGL3, SP), in which the permissible changes to the combinations of the detector signals are stored. By comparing the detector signal combinations with the stored signal combinations, it establishes whether the detector signal combinations and their changes are permissible. Given a permissible change, it transmits a quantity pulse to a volumetric meter, while in the case of an impermissible change it transmits an error signal. Due to the fact that the combination of the detector signals and their change is monitored, a high security is achieved with respect to mismeasurements. <IMAGE>

Description

(54) ARRANGEMENT FOR USE IN MEASURING THE NUMBER OF ROTATIONS OF A ROTATING DEVICE (71) We, SIEMENS AKTIENGESELLSCHAFT, a German company of Berlin & Munich, Germany, Fed. Rep. do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an arrangement for use in measuring the number of rotations of a rotating device.

According to the invention there is provided an arrangement for use in measuring the number of rotations of a rotating device, the arrangement comprising: at least two detectors, for sensing angular displacement relative to the detectors of interrupter segments coupled with such a rotating device; bidirectional counting means; means for supplying pulses to the counting means in response to predetermined combinations of signals supplied by the detectors, said means being connected between the detectors and the counting means; storage means, for storing such predetermined combinations of signals supplied by the detectors, and the sequence thereof, during normal rotation of such interrupter segments, and for temporarily storing each combination of signals supplied by the detectors in use; and comparison means, for comparing such a temporarilystored combination of signals with a successively-occurring combination of signals and for producing an error signal if such a successively-occurring combination of signals is equal neither to its preceding, temporarily-stored, combination of signals, nor to the predetermined combination of signals stored by the storage means and occurring, in said sequence, either before or after said preceding, temporarily-stored, combination of signals; the arrangement being such that, in use, if such a successively-occurring combination of signals is equal to the predetermined combination of signals stored by the storage means and occurring, in said sequence, after said preceding, temporarily-stored, combination of signals, then the number of pulses counted by the counting means is increased by one, and, if such a successively-occurring combination of signals is equal to the predetermined combination of signals stored by the storage means and occurring, in said sequence, before said preceding, temporarilystored, combination of signals, then the number of pulses counted by the counting means is decreased by one.

Each temporarily stored combination or the newly acquired signal combination can be conveyed to an adder or a subtracter which produce the corresponding adjacent combinations. Alternatively, the storage means could include a store comprising cells, the addresses of these cells being respective combinations of such predetermined combinations of signals supplied by the detectors, and the contents of each cell being those predetermined combinations of signals which occur, in said sequence, before and after the respective predetermined combination of signals forming the address of that cell, the arrangement being such that, in use, each occurrring combination of signals is compared with the predetermined combinations of signals contained in the cell addressed by the preceding combination of signals.

Alternatively, the arrangement could be such that, in use, the predetermined combinations of signals contained in the cell addressed by each occurring combination of signals are compared with the preceding combination of signals.

The bidirectional counting means could comprise a specific cell of a store, which cell is addressed by said means for supplying pulses to the counter when the number of pulses counted by the counting means is to be increased or decreased by one in use.

The arrangement could be used in combination with a rotating device and interrupter segments coupled with the rotating device, preferably in the form of a flow meter, for measuring fluid or gas quantities.

The arrangement could further comprise a plurality of additional detectors, for use in measuring the number of rotations of a plurality of associated rotating devices, the arrangement being such that, in use, the first-mentioned detectors and said additional detectors are connected cyclically with said storage means and said comparison means.

Alternatively, the arrangement could be such that, in use, the first-mentioned detectors and said additional detectors are connected with said storage means and said comparison means in dependence on the number of pulses supplied to said counting means. Such arrangements could be used in combination with a plurality of rotating devices and a plurality of interrupter segments coupled with the rotating devices, preferably in the form of a plurality of flow meters, for measuring fluid or gas quantities.

At least parts of said counting means, storage means, and comparison means could be parts of a microprocessor.

Advantageously, the arrangement could be such that, in use, the number of pulses counted by the counting means is multiplied by a calibration factor.

The detectors and interruptors may be different types. For example, the segments of the interrupter and the dectectors can be the sliding contacts and brushes of mechanical contacts. Expediently, however, the segments are scanned contactlessly, e.g. by the fact that the interrupter is a diaphragm and the detectors are light-sensitive elements such as photo-elements or photo-resistances, which may be illuminated through openings in the diaphragm by one or several light sources. Advantageously, the diaphragm is a vane wheel diaphragm. The diaphragm may also take the form of a hollow cylinder, slotted in axial direction on its periphery. The light source is expediently disposed in the centre of the hollow cylinder and the lightsensitive elements outside. The number of segments may be different from that of the detectors.

The detectors supply a specific signal combination in every position of the interrupter.

The signal combinations and their sequence while the interrupter is continuously rotating depends on the number of segments, their angles, their mutual position and the number and distribution of the detectors. Where there is a given number of detectors, that of the interrupters is selected such that while the interrupter makes one revolution, the signal combinations form a continuous prog- ressive code, i.e. occur in a sequence such that only one detector signal changes each time. An error caused by manufacturing tolerances cannot then occur on the transfer of one signal combination to another. Scanning errors can be determined by checking whether several signals change at the same time.

Arrangements which fulfil this requirement are advantageously constructed such that with an identical angular width of the segments and their mutual distances, the angular distance of the detectors is the same as the angular width of the segments divided by the number of detectors. Alternatively, the angular distance between neighbouring detectors may be determined by the angle formed by dividing the circumference of the interrupter segments by the product of the number of interrupter segments and the number of detectors, except that the angular distance between at least one pair of neighbouring detectors is larger by an angle formed by dividing the circumference of the interrupter segments by the number of interrupter segments.

When the interrupter makes a revolution, a larger number of signal combinations appears than there are interrupter segments present. In contrast to known arrangements, having only one detector, a lower rotational speed of the interrupter may therefore be selected when a volume pulse has to correspond to a specific fluid quantity, and less wear can thereby be obtained, or, if the rotational speed is to be retained, the interrupter segments can be made wider and therefore greater manufacturing tolerances can be permitted.

If, for example, there are four evenly distributed interrupter segments of 45" width respectively and also three detectors whose angular distances are 30 + n x 90" (n = 0, 1, 2, 3), then when the interrupter rotates by the width of one segment and a segment distance, six different signal combinations are obtained one after the other, of which each successive one differs by one signal. With every signal modification, a pulse is supplied to the volume counter whose reading therefore corresponds to the quantity which has flowed through the flow meter. Instead of this, a precedure may be followed where only every sixth change in the signal combination trips a counting pulse for the volume counter.

This counting pulse can be produced when a specific signal combination changes to another specific signal combination. The three lowest value points are taken up by the detectors for representation.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic representation of an embodiment, Figure 2 shows a preferred form of the interrupter and the detectors, and Figures 3a and 3b show pulse diagrams and signal combinations produced with the arrangement according to Figure 2.

Referring to Figure 1, R denotes a pipeline through which a fluid or gas flows. The through-flow quantity of fluid or gas is to be determined. To do this, a flow meter DM, for example a rotary piston meter, is connected into the pipeline R and from this meter is run an axle AC on which a vane diaphragm FB with vanes FL is mounted. On one side of the diaphragm, above the area of the vanes FL, are luminescent diodes LD1, LD2 and LD3 which illuminate photo-resistances PW1, PW2 and PW3 through the interspaces between the vanes FL.

Figure 2 shows a diagrammatic view of the vane diaphragm FB. The diaphragm FB has four vanes FL1, FL2, FL3 and FL4 which all have the same angular width of 45 , and between which there are interspaces similarly with angular widths of 45". Generally, a diaphragm with a much greater number of vanes is used. Here, the photoresistances PW1, PW2 and PW3, which can each be illuminated through the interspaces by one light source, for example by a luminescent diode or by a common light source, have angular separations of 1200 in each case. In the position of the vane diaphragm FB shown in Figure 2, the photo-resistances PW1 and PW3 are illuminated and the photoresistance PW2 is masked by the vane FL2.If the output signal of an illuminated photoresistance is logic "1" and that of a masked photo-resistance is logic "0", then the output signals of the photo-resistances PW1 and PW3 are "1" and that of the photo-resistance PW2 is "0". When the vane diaphragm FB rotates in a clockwise direction from its position shown in Figure 2, the photo-resistance PW3 is masked first, then the photoresistance PW2 appears in the interspace between the vanes FL1 and FL2, and then the photo-resistance PW1 is masked by the vane FL4, etc. The signals thus being produced are represented in Figure 3a as waveforms, in which the vertical broken line corresponds to the position of the diaphragm FB shown in Figure 2.The top waveform pw 1 represents the outout signal of the photo-resistance PW1, waveformpw2 shows that of the photo-resistance PW2, and waveform pw3 that of the photo-resistance PW3. The pulse edges are sloped because of the finite dimensions of the photoresistances PW1, PW2 and PW3. It is clear from the waveforms pwl to pw3 that the output signals of the photo-resistances always change one after the other but that two output signals never change at the same time. If the vane diaphragm FB remains stationary in a position in which a photoresistance is half illuminated, a pulse shaper connected after this photo-resistance may determine either one or the other signal reading and pass this reading on. However, this results in a digitalization error which is no greater than that usual in the digitalization of analog quantities.If two signals were changed simultaneously, however, a considerably greater error could arise.

Since the vanes FL and the interspaces between the vanes have the same angular width in each case, the signal combinations produced are repeated after each rotation of the vane diaphragm FB by 90". In the course of this rotation through 90" six different signal combinations are produced which are repeated four times in the course of a full rotation of the vane diaphragm FB. To make this clear, signal readings pw' 1, pw '2, pw '3 of the photo-resistances PW1, PW2 and PW3 are represented in Figure 3b in the form of a table. In this table, the signal readings given in the columns of the table correspond with the signals represented graphically above in the waveformspwl,pw2 andpw3.

The signal combinations shown in Figure 3 may be obtained not only with the arrangement of photo-resistances shown in Figure 2, but they can also be obtained with any other arrangement in which one or several photoresistances are staggered by 90 , 1800 or 270 relative to their respective positions shown in Figure 2. As an example, the photo-resistance PW2 may be disposed at the position indicated by an arrow PW'2, or at positions PW"2 or PW"'2.

Referring to Figure 1, the output signals of the photo-resistances PW1, PW2 and PW3 are fed to a preamplifier and pulse shaper VV to the outputs of which the inputs of a first intermediate store ZSP1 are connected.

When a pulse occurs on a clock pulse line T1, the output signals of the preamplifier and pulse shaper VV are transferred to the intermediate store ZSP1 and then passed to the inputs of a second intermediate store ZSP2, into which they are entered when a clock pulse occurs on a clock pulse line T2.

The frequency of the clock pulses must be at least equal to the highest possible modification frequency of the signal combinations produced by the photo-resistances PW, PW2 and PW3. The clock pulses occurring on lines T1 and T2 have the same frequency but are relatively phase-shifted. After the occurrence of a clock pulse on line T2, and until the occurrence of the next clock pulse on line T1, the contents of the two intermediate stores ZSP1 and ZSP2 are the same.

Thereafter the contents may be different.

The output signals of the first intermediate store ZSP1 are also fed to first inputs E1V1 E1V2, E1V3 of comparators VGL, VGL2 and VGL3 respectively. To second inputs E2V1 of the comparator VGL1 are connected the output signals of the second intermediate store ZSP2. As long as the signal combination stored in the latter is the same as the signal combination newly acquired by the intermediate store ZSP1, the comparator VGL1 will produce a "1" output signal.

If the signal combination held in the intermediate store ZSP2 is different from that newly acquired by the intermediate store ZSP1, the output signal of comparator VGL1 changes from "1" to "0" as pulses occur on the clock pulse line T1. This output signal modification is conveyed to a volume counter VZ as a counting pulse. Providing that its counting input is cleared, this counter VZ adds the output signal pulses, so that its reading is 6 after a rotation of the vane diaphragm FB through 90 , and is 24 after a full rotation. These digital readings provide a measure of the quantity of fluid or gas which has flowed through the pipeline R.In order to represent this quantity in one of the standard dimensions, such as cubic centimetres or litres, the reading of the counter VZ is controlled with the counting pulses and is multiplied in a first multiplication device MPZ1 by a calibration factor supplied via a line EF. This calibration factor is adapted to the flow meter DM, the number and arrangement of the vanes FL of the vane diaphragm FB and the photo-resistances PW1, PW2 and PW3. The multiplication result, which represents the respective quantity of fluid or gas which has flowed through the pipeline R in a standard dimension, is transferred to a volume register VR with which a volume indicator unit VAZ is connected. This indicator VAZ presents the measurement result in decimal figures together with dimensional information.

Connected with the output of the volume register VR, furthermore, is a second multiplication device MPZ2 which is similarly controlled by the counting pulses and which multiplies the measured volume by a basic price, supplied via a line GP, for each unit of volume, so that its output signals, conveyed to a price register PR, represent the price of the quantity of fluid or gas indicated by the volume indicator unit VAZ. This price is presented digitally by a price indicator unit PAZ.

Apart from generating the counting pulses for the volume counter VZ, the second intermediate store ZSP2 and the comparator VGL1 in conjunction with the two further comparators VGL2 and VGL3 and a store SP, provide the function of error checking.

The store SP comprises six cells each with a storage capacity of 6 bits. The addresses of these cells are the signal combinations produced by the photo-resistances PW1, PW2 and PW3 and given in Figure 3b. With the occurrence of each valid signal combination, therefore, a store cell is addressed. The first three bits of each store cell specify the signal combination which, in the normal sequence of the signal combinations, occurs before the combination forming the address of that store cell. In the second half of the store cell is stored the signal combination which should appear after the signal combination which forms the address of that store cell. The first three bits are supplied to the second inputs E2V2 of the comparator VGL2 and the last three bits, to the second inputs E2V3 of the comparator VGL3.The contents of the store cells are compared in these comparators VGL2 and VGL3 with the newly acquired signal combinations which are fed to their first inputs, E1V2 and E1V3 respectively. In the case of error freedom, each signal combination newly acquired by the first intermediate store ZSP1 must either be the same as the signal combination acquired previously (and held in the. second intermediate store ZSP2) or be the same as the signal combination which normally follows, or in the case of opposite flow direction, the one which normally precedes.In these three cases, one of the comparators VGL1, VGL2 or VGL3 establishes consistency of the signal combinations supplied to it and provides a "1" signal to a NOR device, NOR, whose output signal is therefore zero.Howver, if a signal combination occurs which does not belong to the six possible combinations, or which ought not yet to appear in the light of the signal combination held in the second intermediate store ZSP2, all the comparators produce a "0" signal and the NOR device NOR produces an error signal on a line F2. The comparators VGL2 and VGL3, moreover, control the counting direction of the volume counter VZ.

The checking of the signal combinations and the counting direction control of the volume counter VZ will now be explained in more detail for a particular example. Suppose the signal combination "100" is held in the second intermediate store ZSP2. This is applied to the second inputs E2V1 of the comparator VGL1 and address inputs ADE of the store SP. The second inputs E2V2 of the comparator VGL2 therefore receive the combination "101" and the second inputs E2V3 of the comparator VGL3, the combination "110". Suppose the through-flow of fluid or gas through the pipeline R is so small that, at the next clock pulse supplied to the first intermediate store ZSP1, the combination produced by the photo-resistances PW1, PW2 and PW3 is still the same as that held in the second intermediate store ZSP2. In this case, the comparator VGL1 responds and produces a "1" signal which is inverted in the NOR element NOR, meaning that no error signal appears on the line F2. Also, no counting pulse is fed to the input of the volume counter VZ. As the vane diaphragm FB rotates, if no error occurs then after the combination "100", the combination "110" appears; that is, the combination which is applied to the second inputs E2V3 of the comparator VGL3 by the store SP. This combination is transferred with the next clock pulse on line T1 into the first intermediate store ZSP1. The comparator VGL3 thereupon responds by producing a "1" signal which is fed to the NOR element NOR, and an input V to the volume counter VZ, whereby the latter is controlled to count forwards.With the next clock pulse on line T2, the combination "110" is transferred into the second intermediate store ZSP2 and the cell of the store SP with the address "110" is addressed. This causes the signals "100" to be applied to the second inputs E2C2 of comparator VGL2 and the signals "010" to the second inputs E2V3 of comparator VGL3. The comparator VGL3 thereupon switches its output signal from "1" to "0" with the result that the counter VZ is blocked. The "0" signal on line F2 is maintained by the comparator VGL1 establishing consistancy of the signal combinations supplied to it. This state is maintained until the next signal combination "010" is acquired by the intermediate store ZSP2.

The comparator VGL3 clears the input V again for forward counting of the volume counter VZ, and the comparator VGL1 produces a counting pulse. In this way, with each modification of the signal combinations produced by the photo-resistances PW1, PW2 and PW3, the reading of the counter VZ is raised by 1.

When the fluid or gas flows backwards through the pipeline R, i.e. opposite the direction indicated by the arrows in Figure 1, the vane diaphragm FB rotates in the opposite direction. In this case, signal combinations specified in Figure 3b are run through from right to left. Thus, for example, the combination "011" is succeeded by the combination "010", i.e. the combination which is applied to the second inputs E2V2 of the comparator VGL2. The latter therefore activates an input R of the volume counter VZ which is thereby cleared for backwards counting. The counting pulse arriving simultaneously from the comparator VGL1 consequently produces a lowering of the reading of the volume counter VZ by 1.Supposing the combination "110" is held in the second intermediate store ZSP2, the combinations "100" and "010" are therefore supplied to comparators VGL2 and VGL3. If, as the result of an error, the combination "001" now appears at the output of the first intermediate store ZSP1, none of the comparators VGL1, VGL2 and VGL3 responds.

Their three output signals are zero, the counter VZ is blocked and the NOR ELE MENT NOR switches a "1" signal on to line F2 which produces an error reading in a unit FAZ.

The arrangement shown in Figure 1 can be modified in various ways. For example, the counter VZ may be designed such that it has a counting pulse input for forwards counting and a counting pulse input for backwards counting. In this case the comparator VGL1 does not need to be connected to the counter VZ. The pulse input for forwards counting is connected to comparator VGL3 and the input for backwards counting to comparator VGL2. The address inputs ADE of the store SP may also be actuated by the first intermediate store ZSP1 and the first inputs E1V2 and E1V3 of comparators VGL2 and VGL3 respectively may be connected to the second intermediate store ZSP2.

Instead of comparators VGL1, VGL2 and VGL3, a single comparator may be used to which the three signal combinations are applied one after another. In a preferred form of construction, the intermediate stores ZSP1, ZSP2, the comparator or comparators VGL1, VGL2, VGL3, the store SP, the volume counter VZ, the multiplication devices MPZ1 and MPZ2, and also the volume register VR and price register PR are replaced, at least partially, by a microprocessor, it being possible to realise the functions of these individual devices with one programme.

The arrangement shown in Figure 1 may be extended in order to measure the quantitues of fluids or gases flowing through several pipelines by assigning to each pipeline a flow meter with a vane diaphragm, an illuminating device and photo-resistances, and also an intermediate store, volume counter and the indication devices V Z, PAZ, FAZ with which the remaining devices of comparators, stores and multipliers are connected according to a time division multiplex method. Such an arrangement could be realised particularly advantageously with a microprocessor.

It can happen, primarily when the multipliers have to multiply the contents of several volume counters and volume registers, that with a high flow-through of the fluids or gases to be measured, the calculating speed is no longer adequate to carry out a multiplication for each volume pulse. In this case it is sufficient to multiply the reading of the volume counter and the content of the volume register only after a specific number of pulses in each case. For this purpose, in the supply line to the control inputs of the multipliers MPZ1 and MPZ2, a controllable frequency divider FT is provided which actuates itself when a certain input frequency is exceeded.

WHAT WE CLAIM IS: 1. An arrangement for use in measuring the number of rotations of a rotating device, the arrangement comprising: at least two detectors, for sensing angular displacement relative to the detectors of interrupter segments coupled with such a rotating device; bidirectional counting means; means for

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. applied to the second inputs E2V3 of the comparator VGL3 by the store SP. This combination is transferred with the next clock pulse on line T1 into the first intermediate store ZSP1. The comparator VGL3 thereupon responds by producing a "1" signal which is fed to the NOR element NOR, and an input V to the volume counter VZ, whereby the latter is controlled to count forwards. With the next clock pulse on line T2, the combination "110" is transferred into the second intermediate store ZSP2 and the cell of the store SP with the address "110" is addressed. This causes the signals "100" to be applied to the second inputs E2C2 of comparator VGL2 and the signals "010" to the second inputs E2V3 of comparator VGL3.The comparator VGL3 thereupon switches its output signal from "1" to "0" with the result that the counter VZ is blocked. The "0" signal on line F2 is maintained by the comparator VGL1 establishing consistancy of the signal combinations supplied to it. This state is maintained until the next signal combination "010" is acquired by the intermediate store ZSP2. The comparator VGL3 clears the input V again for forward counting of the volume counter VZ, and the comparator VGL1 produces a counting pulse. In this way, with each modification of the signal combinations produced by the photo-resistances PW1, PW2 and PW3, the reading of the counter VZ is raised by 1. When the fluid or gas flows backwards through the pipeline R, i.e. opposite the direction indicated by the arrows in Figure 1, the vane diaphragm FB rotates in the opposite direction. In this case, signal combinations specified in Figure 3b are run through from right to left. Thus, for example, the combination "011" is succeeded by the combination "010", i.e. the combination which is applied to the second inputs E2V2 of the comparator VGL2. The latter therefore activates an input R of the volume counter VZ which is thereby cleared for backwards counting. The counting pulse arriving simultaneously from the comparator VGL1 consequently produces a lowering of the reading of the volume counter VZ by 1.Supposing the combination "110" is held in the second intermediate store ZSP2, the combinations "100" and "010" are therefore supplied to comparators VGL2 and VGL3. If, as the result of an error, the combination "001" now appears at the output of the first intermediate store ZSP1, none of the comparators VGL1, VGL2 and VGL3 responds. Their three output signals are zero, the counter VZ is blocked and the NOR ELE MENT NOR switches a "1" signal on to line F2 which produces an error reading in a unit FAZ. The arrangement shown in Figure 1 can be modified in various ways. For example, the counter VZ may be designed such that it has a counting pulse input for forwards counting and a counting pulse input for backwards counting. In this case the comparator VGL1 does not need to be connected to the counter VZ. The pulse input for forwards counting is connected to comparator VGL3 and the input for backwards counting to comparator VGL2. The address inputs ADE of the store SP may also be actuated by the first intermediate store ZSP1 and the first inputs E1V2 and E1V3 of comparators VGL2 and VGL3 respectively may be connected to the second intermediate store ZSP2. Instead of comparators VGL1, VGL2 and VGL3, a single comparator may be used to which the three signal combinations are applied one after another. In a preferred form of construction, the intermediate stores ZSP1, ZSP2, the comparator or comparators VGL1, VGL2, VGL3, the store SP, the volume counter VZ, the multiplication devices MPZ1 and MPZ2, and also the volume register VR and price register PR are replaced, at least partially, by a microprocessor, it being possible to realise the functions of these individual devices with one programme. The arrangement shown in Figure 1 may be extended in order to measure the quantitues of fluids or gases flowing through several pipelines by assigning to each pipeline a flow meter with a vane diaphragm, an illuminating device and photo-resistances, and also an intermediate store, volume counter and the indication devices V Z, PAZ, FAZ with which the remaining devices of comparators, stores and multipliers are connected according to a time division multiplex method. Such an arrangement could be realised particularly advantageously with a microprocessor. It can happen, primarily when the multipliers have to multiply the contents of several volume counters and volume registers, that with a high flow-through of the fluids or gases to be measured, the calculating speed is no longer adequate to carry out a multiplication for each volume pulse. In this case it is sufficient to multiply the reading of the volume counter and the content of the volume register only after a specific number of pulses in each case. For this purpose, in the supply line to the control inputs of the multipliers MPZ1 and MPZ2, a controllable frequency divider FT is provided which actuates itself when a certain input frequency is exceeded. WHAT WE CLAIM IS:

1. An arrangement for use in measuring the number of rotations of a rotating device, the arrangement comprising: at least two detectors, for sensing angular displacement relative to the detectors of interrupter segments coupled with such a rotating device; bidirectional counting means; means for

supplying pulses to the counting means in response to predetermined combinations of signals supplied by the detectors, said means being connected between the detectors and the counting means; storage means, for storing such predetermined combinations of signals supplied by the detectors, and the sequence thereof, during normal rotation of such interrupter segments, and for temporarily storing each combination of signals supplied by the detectors in use; and comparison means, for comparing such a temporarilystored combination of signals with a successively-occurring combination of signals and for producing an error signal if such a successively-occurring combination of signals is equal neither to its preceding, temporarily-stored, combination of signals, nor to the predetermined combination of signals stored by the storage means and occurring, in said sequence, either before or after said preceding, temporarily-stored, combination of signals; the arrangement being such that, in use, if such a successively-occurring combination of signals is equal to the predetermined combination of signals stored by the storage means and occurring, in said sequence, after said preceding, temporarily-stored, combination of signals, then the number of pulses counted by the counting means is increased by one, and if such a successively-occurring combination of signals is equal to the predetermined combination of signals stored by the storage means and occurring, in said sequence, before said preceding, temporarily-stored, combination of signals, then the number of pulses counted by the counting means is decreased by one.

2. An arrangement according to claim 1, wherein the storage means includes a store comprising cells, the addresses of these cells being respective combinations of such predetermined combinations of signals supplied by the detectors, and the contents of each cell being those predetermined combinations of signals which occur, in said sequence, before and after the respective predetermined combination of signals forming the address of that cell. the arrangement being such that, in use, each occurring combination of signals is compared with the predetermined combinations of signals contained in the cell addressed by the preceding combination of signals.

3. An arrangement according to claim 1, wherein the storage means includes a store comprising cells, the addresses of these cells being respective combinations of such predetermined combinations of signals supplied by the detectors, and the contents of each cell being those predetermined combinations of signals which occur, in said sequence, before and after the respective predetermined combination of signals forming the address of that cell, the arrangement being such that, in use, the predetermined combinations of signals contained in the cell addressed by each occurring combination of signals are compared with the preceding combination of signals.

4. An arrangement according to any preceding claim, wherein the bidirectional counting means comprises a specific cell of a store, which cell is addressed by said means for supplying pulses to the counter when the number of pulses counted by the counting means is to be increased or decreased by one in use.

5. An arrangement according to any preceding claim, in combination with a rotating device and interrupter segments coupled with the rotating device.

6. An arrangement according to claim 5, in the form of a flow meter, for measuring fluid or gas quantities.

7. An arrangement according to any of claims 1 to 4, further comprising a plurality of additional detectors, for use in measuring the number of rotations of a plurality of associated rotating devices, the arrangement being such that, in use, the first-mentioned detectors and said additional detectors are connected cyclically with said storage means and said comparison means.

8. An arrangement according to any of claims 1 to 4, further comprising a plurality of additional detectors, for use in measuring the number of rotations of a plurality of associated rotating devices, the arrangement being such that, in use, the first-mentioned detectors and said additional detectors are connected with said storage means and said comparison means in dependence on the number of pulses supplied to said counting means.

9. An arrangement according to claim 7 or 8, in combination with a plurality of rotating devices and a plurality of interrupter segments coupled with the rotating devices.

10. An arrangement according to claim 9, in the form of a plurality of flow meters, for measuring fluid or gas quantities.

11. An arrangement according to any preceding claim, wherein at least parts of said counting means, storage means, and comparison means are parts of a microprocessor.

12. An arrangement according to any preceding claim, wherein, in use, the number of pulses counted by the counting means is multiplied by a calibration factor.

13. An arrangement according to claim 5, 6, 9 or 10, or claim 11 or 12 as dependent on claim 5, 6, 9 or 10, wherein the angular distance between neighbouring detectors is determined by the angle formed by dividing the circumference of the interrupter segments by the product of the number of interrupter segments and the number of detectors, except that the angular distance between at least one pair of neighbouring detec tors is larger by an angle formed by dividing the circumference of the interrupter segments by the number of interrupter segments.

14. An arrangement for use in measuring the number of rotations of a rotating device substantially as herein described with reference to the accompanying drawings.