Patent Publication Number: US-2005127905-A1

Title: Eddy current sensors

Description:
BACKGROUND OF THE INVENTION  
      This invention relates to eddy current sensors for sensing the movement of an electrically conductive member along a path. Such sensors are used as speed or torque sensing probes for example for sensing the speed of the blades of the compressor or turbine of a gas turbine. Torque in a rotating member may be assessed by using two sensing probes for sensing the rotation of the member at two axially-spaced positions and determining the phase difference between the outputs of the probes.  
     PRIOR ART  
      An example of such a probe is disclosed in GB 2 265 221. The probe uses a strong and large magnet to form a strong magnetic field. When a metal object such as a turbine blade passes through the field, eddy currents are generated in the blade. The probe includes a pick-up coil for detecting the very small magnetic fields generated by the eddy currents induced in the tip of a moving blade. To do this task, the coil needs to have a large number of turns, typically 5,000, wound on a core or pole piece made of a soft magnetic material having a high permeability. As a result of this coil construction, its inductance is very large, typically 1 or 2 henries.  
     SUMMARY OF THE INVENTION  
      For gas turbine applications, the probe is mounted outside on the turbine casing, resulting in a large air gap between the sensor and the tips of the blades. To increase the signal picked up in the coil, the magnetic field should be as strong as possible at the operating distance. This is achieved by selecting a very long wide magnet.  
      According to the present invention, there is provided an inductive sensor for sensing the movement of an electrically conductive member as the member moves along a path, comprising a magnet having a pole adjacent the path of the member for generating a magnetic flux pattern in the path of the member, and an eddy current detector element positioned adjacent the said path to receive magnetic pulses caused by eddy currents generated in the said member as it moves through the flux pattern generated by the magnet pole wherein the detector element has minimal magnetic susceptibility so as to leave substantially undistorted the flux pattern generated by the magnetic pole in the path of the member. Conveniently, the detector element is in the form of a pick-up coil devoid of a core of any soft magnetic material and thus effectively an air-cored coil.  
      With the known arrangement, the pick-up coil has to be as close to the magnet as possible to minimise space and to maximums the signal picked up. As a result the magnetic field is attracted to the core of the pick-up coil so the strength of the field through which the blade tips pass is reduced. Thus, the portions of the blade passing at a distance of typically 10 to 12 mm from the magnet are not exposed to a strong magnetic field and the resulting eddy currents are weakened.  
      The magnetic core of the coil increases the coil inductance resulting in a low resonance frequency and high impedance of the coil. The presence of resonance in the coil distorts the output signal amplitude of the probe. The signal may well have superimposed on it an oscillating signal resulting in several zero crossings giving a false speed reading. Moreover, the current in a coil having a large inductance decays very slowly affecting the amplitude of the subsequent signals and changing the position of the zero crossings.  
      Further sensors with coils having a large impedance are much more susceptible to electromagnetic interference. A sensor having a large inductance cannot be used in areas which must be intrinsically safe because the inductance stores a lot of energy and could generate a spark igniting an explosive gas mixture.  
      To solve the above problems the invention removes the conventional soft magnetic core from the probe coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be further described by way of example with reference to the accompanying diagrammatic drawings in which:  
       FIG. 1  shows a blade passing through the magnetic flux of a permanent magnet in the absence of a pick-up coil;  
       FIG. 2  shows a blade passing a known type of probe;  
       FIGS. 3, 4  and  5  are views of three alternative arrangements corresponding to that shown in  FIG. 2 ;  
       FIGS. 6 and 7  are perspective views of arrangements of the kinds shown in  FIGS. 3 and 5  respectively for use with compressor blades of a gas turbine;  
       FIG. 8  is a diagrammatic tangential view of an arrangement similar to that of  FIG. 6  but in which the blades are angled relative to their direction of motion;  
       FIG. 9  shows a modification of  FIG. 8 ; and  
       FIG. 10  is a simplified exploded perspective view of a probe embodying the invention. 
    
    
     BACKGROUND DESCRIPTION  
       FIG. 1  shows a turbine blade  1 , moving past a pole  2  of a magnet  3  (typically a permanent magnet) which generates a flux pattern indicated at  4 . In  FIG. 1 , there is no pick-up coil so the flux pattern  4  extends well into the path of the turbine blade  1  with the result that eddy currents are generated in the turbine blade  1 .  
       FIG. 2  shows the known type of probe. The second pole  15  of the magnet  13  is fixed to a yoke  16  which in turn is fixed to the high permeability core  17  of a pick-up coil  18 . As a result of the magnetic circuit formed by the magnet  13 , yoke  16  and core  17 , the flux pattern  14  is concentrated close to the pole  12  and the end  19  of the core  17  is largely missed by the turbine blade  11 , bearing in mind the inevitable large spacing between the probe and the turbine blades  11 . When the core  17  is placed close to the magnet  13  it will “short circuit” the magnetic flux. This means that the strength of the magnetic flux  14  through which the blade passes will be reduced. As a result, the eddy currents generated in the turbine blades  11  are considerably smaller than those generated in the system shown in  FIG. 1 .  
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIGS. 3 and 6  show one arrangement in accordance with the invention. Here the magnet  23  has a pick-up coil  28   a  adjacent it on one side and a further pick-up coil  28   b  on the opposite side. Both the coils  28   a  and  28   b  are formed without any core of magnetic material and have their axis parallel to the magnetic axis of the magnet.  FIG. 4  shows a modification of the arrangement shown in  FIG. 3 , having only a single coil  28 . This configuration allows space for a larger magnet, whereas the arrangement of  FIG. 3  is suitable for use where the probe is to fit into a circular envelope.  
      As shown in  FIG. 5  for some applications the coil  38  can be wound around the permanent magnet  33 , thus providing a very compact construction but with a different signal output to that of  FIGS. 3 and 4 . Although made of magnetic material, the permanent magnet  33  cannot act as a core for the coil in the manner of a soft-iron core and so the coil acts as if it had no magnetic core.  
      This arrangement is also shown in  FIG. 7 . In  FIGS. 6 and 7 , the blades  21 ,  31  are compressor blades thus having thin and long cross-sections. Accordingly, the magnets  23 ,  33  and coils  28 ,  38  are elongated to conform to Is the general shape of the compressor blades  21 ,  31 .  
      In the arrangement shown in  FIG. 8 , the blades  41  are angled relative to their direction of travel  40  and the assembly of magnet  43  and pick-up coils  48   a, b , is correspondingly angled.  
      In the modified arrangement shown in  FIG. 9 , however, the longer sides of the magnet  53  are parallel to the direction of travel  40 , with the coils  58   a, b , positioned on each side. This arrangement is found to produce a stronger signal than the arrangement shown in  FIG. 8 .  
       FIG. 10  shows a mounting arrangement for the sensor to form a probe which thus comprises a housing  60  of non-magnetic material having a generally cylindrical body  61 , which in use projects through the casing of an engine, and a mounting flange  62  by means of which the probe is secured to the engine casing. The magnet is here formed by a pair of cylindrical permanent magnets  63  of the same polarity received in boxes  64  in the body  61 .  
      The body  61  has two further boxes  67   a ,  67   b , one on each side of the pair of magnets  63 , to receive the pick-up coils  68   a ,  68   b . The cylindrical surface of the body  61  is formed with channels  69  leading to apertures  70  in the mounting flange  62  to receive the connecting wired for the coils.  
      By providing the or each coil with a further coil without a core in a configuration as shown in GB 2 265 221 it is possible to make the probe less susceptible to the interference of external magnetic fields.  
      Thus, a further coil (not shown) again without a core of soft magnetic material, maybe positioned coaxially with the or each pick up coil  28 ,  38 ,  48 ,  58  or  68  but further from the path of the blades. By connecting the pick up and further coils in opposition, the effects of external electromagnetic field may be largely cancelled out with only minor reduction in the pick up signal since the further coil will only pick up a much reduced signal as the result of its greater distance from the path of the blades.  
      The probes shown in FIGS.  3  to  10  may be used to measure the speed of any metallic non-ferromagnetic object with at least one discrete member such as the tooth of a gearwheel, a phonic wheel, a shaft with a slot or protrusion or the blades of a fan in which eddy currents are generated in the member as it rotates through the flux pattern.  
      While the magnets will normally be permanent magnets for convenience, it would be possible to use electromagnets provided that they are fully saturated in use.