Abstract:
A device for contactless measurement of a rotor temperature includes a magnet supported on the rotor, electric coils which are arranged at the stator with which the rotor cooperates, and an electronic unit for measuring a voltage induced in the coil and including a component that converts the measured voltage into a temperature-proportional signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a device for contactless measurement of rotor temperatures and including a magnet provided on the rotor and an electrical coil arranged on a stator with which the rotor cooperates, and electronic means for measuring a voltage signal applied to the coil. 
   2. Description of the Prior Art 
   The measurement of temperatures of moving objects is a very common problem encountered in technology. When the objects are rotors rotating at very high speeds, only contactless measurement methods are considered. Additional boundary conditions further limit the applicability of known measuring methods. One such boundary condition can be operation under vacuum, for example. High-speed rotating mechanical vacuum pumps (e.g., turbomolecular pumps) and centrifuges can be taken as representative for many applications. The importance of temperature measurement and monitoring becomes particularly clear when considering the example of turbomolecular pumps in which specific rotor temperatures may not be exceeded for safe operation of the rotor. An exact and reproducible temperature monitoring is indispensable for this purpose. 
   In many areas of technology, pyrometers are used for contactless temperature measurement. These pyrometers measure the radiation coming from the rotor as a measure of its temperature. An example for an arrangement of this kind is shown in EP 1 348 940 A2. The radiation sensed by sensors depends not only on the temperature but also to a great extent on the surface condition of the rotor. Temperature measurements may be corrupted when pumping vaporous media particularly in processing technology, in that the surface of a pump rotor is subject to changes caused by condensation, deposit of solid particles, or chemical reactions. Further, expensive compensating electronics are required because of the dependency of the readings on the ambient temperature and housing temperature. The sometimes high sensitivity to high-energy ionized radiation limits the range of uses. 
   DE-OS 43 09 018 describes an arrangement for contactless measurement of rotor temperatures which uses ferromagnetic materials. These ferromagnetic materials are attached to the rotor of the arrangement and, along with additional components at the stator, form a magnetic circuit. Its characteristics change suddenly when the Curie temperature of the ferromagnet is exceeded. With this arrangement, it is only possible to determine that the Curie temperature has been exceeded. Several temperatures can be determined only by using several ferromagnets with different Curie temperatures. The difficulty arising in the use of an arrangement of this kind on a larger scale consists in procuring ferromagnetic materials with a reproducible Curie temperature. Further, it is possible to take measurements over necessary temperature ranges only by devoting considerable resources with the resulting high costs. 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention to provide a device for measuring rotor temperatures in a noncontacting manner which delivers accurate, and reproducible results by simple means and which avoids the drawbacks discussed above. 
   This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a temperature measuring device in which electronic measuring means measures the voltage that is induced in the electrical coil provided on the stator, and converts the measured voltage in a temperature proportional signal. 
   In contrast to the prior art, the device according to the invention makes it possible to measure the temperature of the rotor over a broad range, and only a single device is needed for this purpose. The temperature measurement is not dependent on the Curie temperature of the ferromagnet, which facilitates production of the device because the requirement for a constant and exact Curie temperature for a large number of ferromagnets is eliminated. Further, the temperature measurement is not dependent upon the surface quality of the rotor and stator so that deposits are of practically no significance. 
   In a further development of the invention, the temperature-proportional signal is a voltage, which simplifies the further processing of the signal in an electronic unit. 
   According to another embodiment of the invention, the operating temperature of the magnet lies within the temperature range in which the dependency of the magnetization on temperature is linear and reversible. Therefore, the measurement can be repeated as often as desired in practice. It is possible to continue the determination of temperature even after a temperature threshold has been reached. Accuracy is increased and conversion of the signal voltage to a temperature-proportional signal is facilitated. 
   According to a further development of the invention, a fixed gap is provided between two coils between which the ferromagnet moves on its circular path in rotating around the rotor axis. This measure increases the accuracy of the measurement because it prevents a change in the gap between the magnet and the measurement coil. This change is generally caused by the expansion of the rotor material as the temperature increases. 
   According to yet another embodiment of the invention, a plurality of pairs of coils are arranged over the circular path so that the measuring accuracy can be further increased. These pairs of coils are coupled by a series connection or parallel connection. 
   According to a still further embodiment of the present invention, the device is based, in an advantageous manner, on already existing components for carrying out temperature measurements, i.e., as a magnet for temperature measurement, a drive magnet is used. The expenditure on components is considerably reduced in an advantageous manner by using the motor magnet and motor coils. 
   The advantages of the device according to the invention are especially pronounced when it is used in a turbomolecular pump with a high-speed rotor because of the arrangement of few components on the rotor. This makes it possible to place them at different locations on the rotor and, therefore, allows greater freedom in the choice of measurement location. The embodiment with a constant gap is likewise advantageous because it eliminates the influence of heat-induced length changes. The latter occur precisely in turbomolecular pumps because the rotor works mostly in atmosphere-free space and, therefore, has only poor heat dissipation. Further, the lack of sensitivity to deposits of particles on the surface of the device in a turbomolecular pump is particularly advantageous because these pumps are often used in processing technology. 
   The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show: 
       FIG. 1  a diagram showing the temperature dependency of the magnetization of a magnet; 
       FIG. 2  a cross-sectional axial view of a first embodiment of a measuring device according to the present invention; 
       FIG. 3  a cross-sectional view illustrating an advantageous embodiment of the invention which makes use of the drive arrangement for a shaft, for example, in a turbomolecular pump; and 
       FIG. 4  a schematic diaphragm of the electric circuit for the arrangement according to  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a diagram illustrating the dependency of the magnetic polarization M (magnetization) of a permanent magnet on temperature T, in the present case, the rotor temperature. This temperature dependency follows curve  1 . As the temperature increases, the magnetic polarization decreases. In area I, this decrease is linear within a very close approximation and, further, is reversible. In area II, it drops off increasingly sharply until, finally, there is no further polarization. This non-polarization point  2  occurs at the material-specific Curie temperature TC. The magnets of the device according to the present invention are advantageously used in area I. 
   A first suitable device according to the present invention for measuring temperature is shown in  FIG. 2 . The rotor is mounted on a shaft  201  and has a disk-shaped part  202 . A ring  203 , in which a ferromagnet  204  is arranged at a point on the circumference, is fitted to this disk-shaped part  202 . Alternatively, a protuberance at which the ferromagnet is arranged can also be provided instead of a ring. When the rotor rotates, it travels on a path around the rotor axis. Along this rotational path, it passes the coil  205  in which it induces a voltage at the moment it passes the coil  205 . This voltage is measured by the voltage measuring electronics  207  and is converted into a temperature-proportional signal by a part  208  of the electronics. This signal is then communicated, e.g., to a microcontroller  209 , for further processing. 
   This first measuring device can be improved by arranging a second coil  206  at a fixed distance from the first coil  205  in such a way that the magnet  204  on its circular path passes the intermediate space between the coils  205 ,  206 . This noticeably improves the accuracy: The rotor, particularly the disk  202 , expands when heated. This increases the distance of the ferromagnet  204  from the axis of rotation and it accordingly moves past the coil  205  at a closer distance, and the air gap decreases. Now, if a second coil  206  is arranged as shown, the distance from the coil  205  decreases, but the distance from the coil  206  increases so that the air gap remains constant on the whole. Therefore, the induced signal does not depend on the rotor displacement disturbance variable. 
   A second suitable measuring device is shown in  FIGS. 3 and 4 .  FIG. 3  shows the shaft  301  of a rotor. Permanent magnets  302  are fastened to it. Coils  303  generate a magnetic field which sets the shaft in rotation when suitable current is applied to the coils.  FIG. 4  shows an arrangement that operates in this way. Three coils of the drive are designated by  403 . Current is applied to these coils in a manner known per se by drive electronics  402 , where, at any one time, one of the coils  403  is without current. The currentless coil can, therefore, serve as a detector coil for the voltage measuring device  405 . This voltage is subsequently rectified by a rectifier circuit  406  so that a DC voltage is communicated to a microcontroller  409  as a temperature signal for further processing. Power supply electronics  401  are used to supply electrical energy to the drive electronics  402 . 
   Though the present invention was shown and described with references to the preferred embodiments, such are merely illustrative of the present invention and are not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiments or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.