APPARATUS FOR MEASURING PREFERABLY THE ROTATIONAL SPEED OF ROTATING BODIES

Abstract: Apparatus for measuring preferably the rotational speed of rotating bodies and comprising a sectored disc (3) with transparent and non-transparent sectors placed on the apparatus, a light source (4) placed on one side of the sectored disc and a sensor (5) placed in the light beam transmitted through the disc and means for transforming output signals from the sensor into information signals relating to the speed and/or acceleration of the rotating body. In order to enable the apparatus to measure speed in an interval around zero using only one stationary light source (4) and a corresponding stationary sensor (5), the light source comprises a number of separate light emitters (6) adapted to emit light pulses of displaced phase, and the sensor comprises a field of sensitivity corresponding to the width of one transparent and one non-transparent sector.

Description: Apparatus for measuring preferably the rotational speed of rotating bodies. Technical field of the invention The present invention relates to an apparatus for measuring preferably the rotational speed of rotating bodies and comprising a sectored disc with transparent and non-transparent sectors placed on the apparatus, a light source placed on one side of the sectored disc and a sensor placed in the light beam transmitted through the disc, and means for transforming output signals from the sensor into information signals relating to the speed and/or acceleration of the rotating body. The ap¬ paratus is preferably for use as a tachometer, but may be used also in connection with linear movements if a suit- able and corresponding adaptation of the sectored disc- is made. The expression 'light source' relates in the present context to any emitter of visible or invisible light or other electromagnetic waves that radiate and may be cut off or reflected in the same way as visible light. Technical background of the invention In tachometers of the above type the attainment of a high resolution at low speed is connected with problems. At such speed the number of pulses received by the sensor is low, and the sampling frequency and the pulse fre¬ quency will have the same order of magnitude. Correspond¬ ingly, a tachometer,which in order to obtain a reasonable accuracy at low speed has a fine sectoring of the sectored disc, will give rise to problems at high speed owing to the maximum frequency of. the sensor. In order to solve these problems it has been suggested, cf. IEEE - Trans¬ actions on Instrumentation and Measurement, Vol. 114-2 No. 2, May 1972, to place the light source and the sensor on a shaft in extension of the axis of the rotating body and to drive the light source at a known speed in the same or the opposite direction direction as the rotating body, depending on the nature of the problem, if it is a lack of resolution at slow speed or if the maximum fre¬ quency of the sensor at high speed is exceeded or if a minimum frequency is to be kept at slow speed. The out¬ put signals from the sensor are processed digitally, and depending on the specific logic in the connected circuits, the speed and/or the acceleration of the body is monitor¬ ed. A drawback of the known system is that both the energy .supply and the output signal from the sensor must be trans¬ ferred from the rotating shaft which will entail induction of noise in the transmitted signals and not least give problems owing to wear. Summary of the invention The object of the invention is to avoid these draw- backs. In accordance with the invention this is obtained if the light source comprises a number of separate light emitters adapted to emit light pulses of displaced phase and if the sensor comprises a field of sensitivity cor¬ responding to the width of one transparent and one non- transparent sector. The phase-displaced light pulses form on the light emitters a kind of "electric newspaper", on which the moving pulse trains will be perceived as a moving light source. The stationary light source divided into separate light emitting elements thereby obtains the same proper¬ ties as a moving light source, but without the drawbacks related to its rotating movement. In order to obtain an output signal which is analogous with a signal received from a rotating light source on a correspondingly rotating sensor, the extension of the sensor must corre¬ spond to one transparent plus one non-transparent sector. The sensor may be divided into separate elements if only the total extension of its sensitive area corresponds to a transparent and a non-transparent sector on the sector¬ ed disc. If the sensor is divided into separate elements, the output signals should be .added by connecting the elements to a common wire. Light emitting elements are cheap, and the use of a number of such elements does not give economic problems, and correspondingly, a pulse ge¬ nerator for supplying the necessary pulses for energizing the light emitters is a conventional and comparatively cheap component. Preferably, according to the invention four light emitters are used. By using just four elements the pulses are in pairs displaced 180 degrees in phase and may con¬ sequently be produced by means of a single generator, which simplifies the electronic part of the apparatus. Using a larger number provides a more smooth signal from the sensor, especially at normal trigging level. The light emitters may according to the invention be arranged in a spaced relationship along a circle around the axis of the rotating body, the sensor being split into a corresponding number of elements. In this arrangement, the practical design of the light emitters may be simpli¬ fied, as more space is available around each element. Certainly, it is not the same portion of the sectored disc that successively shadows the four emitters, but the light received on the sensor which is cut up correspondingly will vary in the same way as if the light emitters were placed side by side in close relationship and matching a one-piece sensor having the required field of sensitivity. According to the invention the light pulses have such a duration and shape that the light intensity of the superposed pulses are substantially constant. This en¬ sures a well-defined shape of the pulses received by the sensor and ensures a stable registration in the logical circuit, in which the output from the sensor is processed. In a further development according to the invention the light emitters are- arranged in two series, each of whom is connected to a pulse generator in such a way that in the two series the pulse trains have a different, pos¬ sibly angular, velocity. In accordance with this arrange¬ ment the range of measurement may be widened, as the range of measurement may incorporate the field, in which one or the other series of light emitters emit the light at the same angular velocity as the rotating body. In this range useful signals may be obtained from the other series of light emitters and corresponding sensor elements. Alternatively, the light emitters may be arranged in two series and connected to a common pulse generator, but with a different angular spacing of the two series. This arrangement provides a widening of the measurement range without increasing the costs of the pulse generator. According to the invention at least one of the light emitters in the two series may be common, especially if the light emitters are connected with a common pulse ge¬ nerator and two different angular velocities are required of the pulse trains. This may simplify the practical ar- rangement without detracting from the function. In most cases even two light emitters may be common in the two series. Short description of the drawings In the following specification the invention will be described in detail with reference to the accompanying drawingsshowing an embodiment of the apparatus according to the invention. In the drawings fig. 1 schematically illustrates an embodiment of the invention, fig. 2 is a section through a sectored disc, the sensor and the light diodes, fig. 3 is a diagram of the light emittance from the light diodes over a period of time, and ______fig_. ^ 4 a diagram of a signal received by the sen¬ sor. Preferred embodiments In fig. 1 an apparatus for measuring the speed of a rotating body is schematically shown. The rotating body in the figure is represented by means of a shaft 1. The body rotates with a velocity V, represented by the arrow 2. On the shaft a sectored disc 3 is mounted, said disc having four wings separated by cut-out or trans¬ parent sectors having the same width as the wings. Under the sectored disc 3 a light emitter 4 is arranged, and above the disc and opposite the light emitter a sensor for receiving light from the emitter 4 through the trans¬ parent sectors of the disc 4 is placed. The light emitter 4 is connected to a (not shown) source of energy and the sensor is connected with a (not shown) device which is able to convert the output signals from the sensor to information signals relating to the rotational speed of the body and/or its acceleration. Such a device is de¬ scribed in IEEE Transactions on Instrumentation and Measurement, Vol. 114-2 No. 2, May 1972. In the known ap- paratus the light emitter 4 and the sensor 5 are arranged to rotate around the extension of the rotational axis of the body at a predetermined, constant speed for i.a. to allow measurements of the rotational speed also when it is zero. As the mounting of a light source and a sensor on a rotating device entails a number of drawbacks, it is an object of the invention to provide a design for a stationary light emitter and sensor. In the apparatus according to the invention the light emitter comprises a number of, preferably at least four, light diodes 6 connected to a source of energy consisting of a pulse generator, which provides each of the light diodes with phase-displaced pulses making the light diodes emit light alternately in such a way that so to say a train of light emissions passes along the four diodes. As shown in fig. 2 the light emitter 4' and the sensor 5* are arranged oppositely with respect to the sectored disc 3', and the sensor must have a range of sensivity at least corresponding to one transparent and one non-transparent sector on the disc 3* . In the present context it is not necessary that the light emitter emits visible light, as any type of electromagnetic waves that may be cut off like visible light may be used in the apparatus according to the invention. The pattern of the pulses from the four diodes a,b,c and d is shown in relation to time. If fig. 2 and 3 are studied together the resulting signal received on sensor 5' may be obtained. This signal is shown in fig. 4. It will be seen that the shape is dependent on the stationary position of the sectored disc, a change of position providing a displacement in phase, but no change in frequency. A minor change of the shape will occur, but at normal trigger level this change will have minor in- fluence. If, however, the sectored disc is moved, a change in frequency will occur, but the shape will remain the same. This means that the output signal may be processed to provide information about the speed and/or the acceleration of the body in a way analogous to the processing in the known apparatus, in which the light emitter and the sensor are arranged on rotating means on the extension of the ro¬ tational axis of the body. By means of a change in the frequency of the phase-displaced pulses from the pulse ge¬ nerator, the range of measurement of the apparatus may be widened and particularly also be comprising the range around zero. The number of light diodes is as mentioned preferably four, but may be increased or possibly reduced at the cost of measuring accuracy. The light pulses should be of such a shape that if they are superposed the resulting light emittance will be constant. This is obtained by light pulses which as shown in fig. 3 is of a trapezoidal shape, which pulses through the sectored disc result in an impulse of substantially saw-tooth shape on the sensor. A perfect saw-tooth shape is favourable as the straight flanks provide a precise monitoring of the light signal in the processing means for the output signal from the sensor also in case the trigging level is not optimized. In fig. 1 the four light diodes 6 are shown in a close relation as a single light source 4. This arrange¬ ment is in practice less appropriate i.a. because some precautions have to be taken for screening the light emit¬ ters in such a way that only a portion of the disc is ex¬ posed corresponding to one transparent and one non-trans¬ parent sector. The four light diodes may just as well be distributed along a circle around the shaft 1. The sensor must then be cut up into a corresponding number of separate elements, the total sensitive field of which still having to correspond to a transparent plus a non-transparent sector on the disc. The possibility of distributing the sensor and the light source may also be utilized for ar- ranging two series of light emitters and sensors in the apparatus, the angular velocity being different for the two series or even opposite. If the angular velocities of the two series are identical, but opposite, the two series may be supplied from the same pulse generator or one or more of the light diodes may be common in the two series. Apart from the mechanical simplification of the apparatus it is easier to adjust. Furthermore, a suitable - possibly automatic - change-over may be incorporated in the pro¬ cessing means for the output from the sensor, whereby an extremely wide range of measurement of the apparatus is obtained, which - because the light source and the sensor is stationary - is insensitive for noise phenomina in the transmission of the signals. The width of the sectors on the sectored disc may be adapted to the range of measure- ment in which the apparatus is used, and the frequency of the pulse trains may also be optimized for obtaining a high accuracy of the measurements in the normal range of operation. Industrial utilization of the invention The apparatus has been described above in connection with the measuring of the speed and/or the acceleration 'gUREA OMPI '^7ά of a rotating body, but the apparatus may also analogously be adapted to the measuring of speed and/or acceleration _,____o_f_a_ _ine__ry movable body. The sectored disc must be re¬ placed by a linear ruler placed on the body or along its 5 path of movement. The diodes in the light source are placed in line along the ruler and are connected in a row to the pulse generator. The wide range of measurement and the simple con¬ struction makes the apparatus suitable both where high 10 accuracy is required, such as in case of machine tools, robots, antennas and aircrafts, or where requirements are less demanding with respect to general speed indication, speed control, for example in transport equipment or plants, process equipment, etc. 15 20 25 30 35

Claims: WHAT IS CLAIMED IS: 1. Apparatus for measuring preferably the rotational speed of rotating bodies and comprising a sectored disc (3) with transparent and non-transparent sectors placed on the apparatus, a light source (4) placed on one side of the sectored disc and a sensor (5) placed in the light beam transmitted through the disc and means for transform¬ ing output signals from the sensor into information sig- nals relating to the speed and/or acceleration of the ro¬ tating body, c h a r a c t e r i z e d in that the light source comprises a number of separate light emitters (6) adapted to emit light pulses of displaced phase and the sensor comprises a field of sensitivity corresponding to the width of one transparent and one non-transparent sector. 2. Apparatus according to claim 1, c h a r a c t e r¬ i z e d in that the number of light emitters is four. 3. Apparatus according to claim l or 2, c h a r a c - e r i z e d in that the light emitters are arranged in spaced relationship along a circle around the rotating body, the sensor being cut up into corresponding elements. 4. Apparatus according to claim 1, c h a r a c t e r¬ i z e d in that the light pulses have such a duration and shape that the light intensity of the superposed light pulses is substantially constant. 5. Apparatus according to claim 3, c h a r a c t e r¬ i z e d in that the light emitters are arranged in two se¬ ries which are mutually connected with a pulse generator in such a way that in the two series pulse trains are ge¬ nerated, the phase-displacement of which is different, possibly opposite. 6. Apparatus according to claim 3, c h a r a c t e r¬ i z e d in that the light emitters are placed in two se- ries mutually connected with a pulse generator, the ar¬ rangement being such that the light emitters of the two series are placed with different angular spacing. -fϋRE-C OMPI ^ W1P0 7. Apparatus according to claim 6, c h a r a c t¬ e r i z e d in that at least one of the light emit¬ ters is common for the two series.

Inventors: PLOUG OLE, PLOUG, OLE
Applicants: PLOUG OLE, PLOUG, OLE
Application year: 1984
Main classification: G01P3
IPC codes: G01P3