A METHOD AND A DEVICE FOR DETERMINING THE ANGULAR VELOCITY OF A ROTATING CYLINDRICAL SHAFT OF ELECTRICALLY CONDUCTIVE MATERIAL

Abstract: A method and a device for determining the angular velocity of a rotating, electrically conductive cylindrical shaft (1). A homogeneous magnetic field (3) is directed perpendicularly to the shaft in such a way that the magnetic field lines extend in the cross section plane of the shaft, perpendicular to the symmetry line (8) of the shaft. When the shaft is rotating, axial eddy currents are induced in the shaft which outside the shaft also generate a magnetic field of dipole type with a component (6) in the cross section plane of the shaft and perpendicular to the originally applied magnetic field. The magnitude of this component is determined by the angular velocity of the rotating shaft, and a measure of the angular velocity of the shaft is obtained by detecting this component with the aid of a measuring winding (7) in a plane parallel to the homogeneous magnetic field and which passes through the symmetry line of the shaft.

Description: A method and a device for determining the angular velocity of a rotating cylindrical shaft nf electrically conductive material TECHNICAL FIELD The present invention relates to a method and a device in the form of a sensor for contactless determination of the angular velocity of a rotating cylindrical shaft. One con- dition for the method to function is that the shaft is of electrically conductive material. Determining or measuring the angular velocity of a rotating shaft is a basic requirement in the majority of industrial applications. There may thus be a need to determine the angular velocity for setting the desired rotational speed of the shaft and the associated working machine. Another need may be connected with supervision of the maximally allowed angular velocity. One condition for the control of paper machines, rolling mills, etc., is access to a measure or a value in the form of a signal corresponding to the rota¬ tional speed of the machines included, that is, of the angular velocity of the respective shafts. BACKGROUND ART One of the most commonly occurring angular velocity sensors is a so-called tachometer, which is an electric generator which is connected in some way to the shaft of the machine in question. In its simplest form, the tachometer is a dc generator which delivers a signal proportional to the angular velocity of the shaft. Since many industrial processes are nowadays controlled by digital means, also angular velocity sensors based on pulse technique have become increasingly more common . In this type of sensor, a number of pulses per revolution of the rotating shaft are obtained. By counting the pulses, the angular velocity can thus be determined. These pulse sensors may be based on magnetic, optical, or another type of pulses. Angular velocity sensors based on the above principles have been part of the state of the art for a long time and are of a fundamentally very simple nature. No reference to this state of the art is therefore considered necessary. SUMMARY OF THE INVENTION One principle, which belongs to classical electrotechnics and as such has been known for a very long time, is the fact that, if an electric conductor moves in a magnetic field, currents are induced in the conductor. These currents themselves give rise to a magnetic field. A method and a device in the form of an angular velocity sensor according to the invention utilize this effect for achieving a possibility of obtaining a measure of the rotational speed of the conductor and, in this particular case, of a shaft. As stated above, the starting-point is a cylindrical shaft of electrically conductive material. The shaft is to rotate in a homogeneous magnetic field directed perpendicularly to the shaft in such a way that the magnetic field lines run in the cross-section plane of the shaft, perpendicular to the symmetry line of the shaft. When the shaft rotates, axial eddy currents are induced in the shaft. These currents, in turn, result in a magnetic field of dipole type being indu¬ ced outside the shaft, with a component perpendicular to the shaft, that is, in the cross-section plane of the shaft, and perpendicular to the originally applied magnetic field. The magnitude of this component depends on the angular velocity of the rotating shaft and may thus be used for determining this velocity. This is true independently of whether the applied homogeneous magnetizing field is a static field or a time-varying field. The magnetizing field may be achieved with the aid of win¬ dings which are located on a bobbin concentric with the shaft and which are supplied with a constant or time-varying current depending on the embodiment used. A static magnetic field may also be achieved with the aid of permanent magnets fixed to a yoke arranged outside the shaft. For sensing the magnitude of the above-mentioned component, which is perpen¬ dicular to the shaft, corresponding to the angular velocity of the shaft, in case of a time-varying magnetizing field, a sensing winding arranged around the shaft in a yoke may be used. If the device is based on a static magnetic field, the angular velocity component may be sensed in a different way, for example with a Hall sensor. With a sensing winding in the yoke, speed variations, that is, acceleration/- deceleration, may be determined. The conducting cylindrical shaft need not necessarily be solid but may be tubular or consist of a plurality of con¬ centric layers . BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows a preferred embodiment of a sensor according to the invention. Figure 2 shows the embodiment of the winding coils included in the sensor. DESCRIPTION OF THE PREFERRED EMBODIMENTS The fundamental design of a preferred embodiment of a sensor according to the invention is clear from Figure 1. A cylin¬ drical electrically conductive shaft 1 is concentrically surrounded by a bobbin 2, on which the necessary windings for magnetization and sensing are wound. The homogeneous magnetic field 3, which is to be directed perpendicularly to the shaft, is achieved with the aid of two excitation windings 4 and 5. The induced magnetic field may 6 with a component perpendicular to both the shaft and the homogeneous field may be detected as a voltage induced in a measuring winding 7 in a plane parallel to the homogeneous magnetic field and passing through the symmetry line 8 of the cylindrical shaft. By connecting an instrument 9 to this winding, the angular velocity of the shaft can thus be read, and possibly the induced voltage may be used for purposes as those stated in the foregoing. The sensor is preferably surrounded by a shielding shell 10. In a preferred embodiment, the windings are designed as practically identical winding coils, which is clear from Figure 2. As is clear, that part of the excitation winding 4, which contributes to the generation of the homogeneous magnetic field, comprises parts 4a and 4b disposed on a generatrix to the bobbin, as well as parts 4c and 4d peri- perally disposed with respect to the bobbin. The correspon¬ ding parts for the excitation winding 5 comprises the parts 5a, 5b, 5c and 5d. To achieve the homogeneous magnetic field, the two excitation windings are to be placed symme- trically on respective sides of an axial plane through the symmetry line 9 of the shaft . In a corresponding manner, the measuring winding 7 comprises parts 7a and 7b disposed on the generatrix to the bobbin, as well as parts 7c and 7d peripherally disposed with respect to the bobbin. Since the winding is intended to measure the induced field component, directed perpendicularly to the homogeneous magnetic field, the parts 7a and 7b of the measuring which are located on a generatrix to the bobbin are to lie in a plane parallel to the homogeneous magnetic field and passing through the symmetry line 9 of the shaft. In a preferred embodiment, in which a shell is used which is made of a material with a relatively high permeability and a high resistivity, the parts of the excitation windings and the measuring winding which are located on a generatrix to the bobbin are to be placed as is clear from Figure 1, that is, with a circular division of 60 degrees. This means, thus, that the angle between the winding parts 4a and 4b as well as 5a and 5b is 120 degrees. The angle between the winding parts 7a and 7b is 180 degrees. With such a location of the windings, the best magnetic homogeneity is obtained inside the bobbin and the shell and the greatest possible output signal is obtained from the measuring winding. To facilitate the winding work and to ensure a fixed loca¬ tion of the windings on the bobbin, it may be provided with circular flanges with slots 11 according to Figure 1, or with longitudinal flanges disposed as generatrices. The connection of magnetization current to the two excita¬ tion windings is, of course, a trivial measure and is there¬ fore omitted so as not to burden Figure 2. The same is true of terminals to be able to measure the voltage induced in the measuring winding. Within the scope of the invention, there are alternative similar methods for application of the windings. The win- dings may, for example, be formed in a fixture and then be glued or otherwise attached to the bobbin. With this method, thus, no flanges are necessary, which simplifies the manu¬ facture of the bobbin. As mentioned above, the best homogeneity in the magnetic field is obtained when the windings are disposed with a circular division of 60 degrees. If desirable for practical or other reasons, the scope of the invention allows devia¬ ting from the optimal location with a largely maintained field configuration.

Claims: CLAIMS 1. A method for determining the angular velocity of a rota¬ ting, electrically conductive cylindrical shaft (1), characterized in that a homogeneous magnetic field (3) is directed perpendicularly to the shaft in such a way that the magnetic field lines extend in the cross-section plane of the shaft, perpendicular to the symmetry line (8) of the shaft, whereby, when the shaft is rotating, axial eddy currents are induced in the shaft which outside the shaft generate a magnetic field of dipole type with a component (6) in the cross-section plane of the shaft and perpendi¬ cular to the originally applied magnetic field, and that the magnitude of this component is determined by the angular velocity of the rotating shaft, and that a measure of the angular velocity of the shaft is obtained by detecting this component with the aid of a measuring winding (7) in a plane parallel to the homogeneous magnetic field and which passes through the symmetry line of the shaft. 2. A method according to claim 1, characterized in that the homogeneous magnetic field (3) is generated with the aid of two or more excitation windings (4, 5) and that the windings are fixed to a bobbin (2) which is concentric with the shaft. 3. A method according to claim 1, characterized in that the homogeneous magnetic field is arranged as a time-varying magnetic field. 4. A method according to claim 1, characterized in that the homogeneous magnetic field is arranged as a static magnetic field. 5. A method according to claim 1, characterized in that the electrically conductive cylindrical shaft (1) may be designed as a solid or tubular shaft or may comprise a plurality of concentric layers. 6. A device in the form of a sensor for carrying out the method according to claim 1 for determining the angular velocity of a rotating, electrically conductive cylindrical shaft (1), characterized in that on a bobbin (2), concen- trically placed around the shaft, there are arranged two or more excitation windings (4, 5) for generating a homogeneous magnetic field (3) directed perpendicularly to the shaft, and a measuring winding (6) arranged in a plane parallel to the homogeneous magnetic field and extending through the symmetry line (8) of the shaft for detecting that component of the magnetic field, induced outside the shaft because of eddy currents in the shaft, which is perpendicular to both the shaft and the homogeneous magnetic field. 7. A device in the form of a sensor according to claim 6, characterized in that the shaft (1) is designed as a solid or tubular shaft or it may comprise a plurality of concen¬ tric layers. 8. A device in the form of a sensor according to claim 6, characterized in that the bobbin (2) is concentrically surrounded by a shielding shell (10) . 9. A device in the form of a sensor according to claim 6, characterized in that the two excitation windings (4, 5) and the measuring winding (7) are designed as winding coils comprising parts (4a, 4b, 5a, 5b, 7a and 7b) disposed on a generatrix to the bobbin as well as parts (4c, 4d, 5c, 5d, 7c and 7d) peripherally disposed with respect to the bobbin. 10. A device in the form of a sensor according to claim 6, characterized in that the angle between the parts (4a and 4b, 5a and 5b) associated with the excitation windings and disposed on a generatrix is 120 degrees, and that the angle between those parts (7a and 7b) of the measuring winding which are disposed on the generatrix is 180 degrees. 11. A device in the form of a sensor according to claim 6, characterized in that the angle between each part (4a and 5a, 5a and 7b, 7b and 5b, 5b and 4b, 4b and 7a) of the windings disposed on a generatrix is 60 degrees. 12. A device in the form of a sensor according to claim 6, characterised in that those parts (7a, 7b) of the measu¬ ring winding (7) which are disposed on a generatrix are arranged midway between those parts (4a and 4b, 5a and 5b) of excitations windings (4, 5) which are disposed on a generatrix.

Inventors: HAMBERG JOHAN, HAMBERG, JOHAN
Applicants: ASEA BROWN BOVERI, HAMBERG JOHAN, ASEA BROWN BOVERI AB, HAMBERG, JOHAN
Application year: 1996
Main classification: G01P3
IPC codes: G01P3, G01P15