Magnetic non-destructive method and apparatus for measurement of cross sectional area and detection of local flaws in elongated ferrous objects in response to longitudinally spaced sensors in an inter-pole area

Abstract

A magnetic non-destructive method and an apparatus for measurement of cross sectional area of elongated ferrous objects such as steel wire ropes and for detecting local flaws is disclosed. A section of a wire rope is magnetized by longitudinally spaced magnetic poles. A magnetic field parameter, e.g. magnetic flux density, is measured in, by at least, one pair of points between the poles of magnetizing device (in an inter-pole area) at the object under test surface. The pair of points is formed by two sensors placed in the inter-pole area along a direct line parallel to the rope axis. The rope cross sectional area corresponds to a sum of the sensor pair signals. Local flaws, such as broken wires and pitting corrosion in the rope, is detected by a first differences of signals of the sensor pair. At least one additional magneto-sensitive sensor is located radially inward of the poles and weight coefficient A depending on a nominal value of the rope cross sectional area is subtracted from the sum of signals of the sensor pair thereby providing a second difference of the signals corresponding to the rope cross sectional area. The coefficient A value is chosen to get the minimum value of the second signal difference while the magnetizing device and all the sensors are placed onto the rope having a nominal value of a cross sectional area. A sensor unit in the inter-pole area includes a magnetic core in form of three longitudinally spaced ferrous elements. Pairs of the sensors are located in the gaps along a direct line parallel to the rope. Two embodiments of the magnetic heads are disclosed: the hollow cylinder-shaped one and the U-shaped one.

1. A method of monitoring a cross sectional area of an elongated ferrous object having a longitudinal axis, the method comprising:
 (a) magnetizing longitudinal section of the object between two magnetic poles spaced apart along the longitudinal axis to magnetically saturate a section of the object;  
 (b) measuring a magnetic field parameter with first and second sensors located at a first and a second  longitudinally spaced apart location  locations intermediate the two magnetic poles; and  
 (c) determining one of a change in a cross sectional area of the object and presence of a local flaw in the object corresponding to the measured parameter.  
 
     
     
       2. The method of  claim 1 , wherein determining the presence of a local flaw includes subtracting a first sensor signal from a second sensor signal. 
     
     
       3. The method of  claim 1 , wherein determining the change in cross sectional area includes adding a first sensor signal to a second sensor signal. 
     
     
       4. The method of  claim 1 , further comprising measuring the magnetic field parameter at symmetric locations relative to the spaced apart poles along the longitudinal axis. 
     
     
       5. The method of  claim 1 , further comprising measuring a magnetic field parameter at an additional sensor located intermediate one pole and an adjacent section of the object, adjusting a signal from the additional sensor by a known coefficient to provide a resultant and subtracting the resultant from a summation of a first sensor and a second sensor signal. 
     
     
       6. An apparatus for monitoring a cross sectional area of an elongated ferrous object having a longitudinal axis, comprising:
 (a) a pair of magnetic poles spaced along the longitudinal axis to define an inter-pole area;  
 (b) a first pair of longitudinally spaced apart magnetic sensors in the inter-pole area; and  
 (c) a processor connected to the magnetic sensors to provide a signal corresponding to one of a MA or LF in the elongated ferrous object.  
 
     
     
       7. The apparatus of  claim 6 , further comprising an additional magnetic sensor radial inward of a magnetic pole at the same longitudinal location as the magnetic pole. 
     
     
       8. The apparatus of  claim 6 , further comprising a second pair of longitudinally spaced magnetic sensors in the inter-pole area. 
     
     
       9. The apparatus of  claim 8 , wherein the first pair of sensors are diametrically opposed to the second pair of magnetic sensors. 
     
     
       10. The apparatus of  claim 6 , wherein the processor is selected to add the signals from the magnetic sensors to determine the MA. 
     
     
       11. The apparatus of  claim 6 , wherein the processor is selected to subtract the signals from the magnetic sensors to determine the LF. 
     
     
       12. The apparatus of  claim 6 , wherein the magnetic sensors are Hall effect sensors. 
     
     
       13. A method for the magnetic non-destructive detection of a local flaw an elongated ferrous object having a longitudinal axis, the method comprising:
 (a) longitudinally magnetizing a section of the object section by a magnetizing device having longitudinally spaced apart poles directed toward the object to substantially magnetically saturate the object section;  
 (b) measuring a magnetic field parameter within an inter-pole area longitudinally intermediate the spaced apart poles, in at least one pair of spaced points within the inter-pole area by a pair of magneto-sensitive sensors disposed along a line parallel to the longitudinal axis; and  
 (c) deducting signals of the sensors forming the pair from other to provide a first signal difference corresponding to the presence of a local flaw in the object.  
 
     
     
       14. A method for the magnetic non-destructive measurement of a cross sectional area of an elongated ferrous object having a longitudinal axis, the method comprising:
 (a) longitudinally magnetizing a section of the object section by a magnetizing device having poles longitudinally spaced apart poles directed toward the object to substantially magnetically saturate the object section;  
 (b) measuring a magnetic field parameter within an inter-pole area at a surface of the object, in at least one pair of longitudinally spaced points within the inter-pole area by a pair of magneto-sensitive sensors disposed along a line parallel to the longitudinal axis; and  
 (c) determining a cross sectional area dependent value corresponding to a sum of signals of the pair of magneto-sensitive sensors.  
 
     
     
       15. The method of  claim 14 , wherein the magnetic field parameter is measured at points symmetrical relative to a longitudinal canter of the spaced poles. 
     
     
       16. The method of  claim 14 , further comprising modifying the sum by subtracting an additional signal from a sensor radially intermediate one of the poles and object. 
     
     
       17. The method of  claim 16 , further comprising multiplying the additional signal by a weight coefficient. 
     
     
       18. The method of  claim 16 , further comprising corresponding the weight coefficient to a nominal value of the object. 
     
     
       19. The method of  claim 17 , further comprising selecting a value of the weight coefficient to provide a minimum value of a second difference of the sensor signals while the magnetizing device and the first sensor pair are disposed to test an object having a nominal value of cross sectional area. 
     
     
       20. Apparatus for a magnetic non-detective measurement of cross sectional area of elongated ferrous objects and for a detection of a local flaw, comprising:
 (a) a pair of longitudinally spaced magnetic poles directed toward a channel through which the object passes, the poles sufficient to substantially magnetically saturate the object, the poles defining an inter-pole area; and  
 (b) a sensor unit located in the inter-pole area, the sensor unit including a magnetic core and a pair of magneto-sensitive sensors, the magnetic core having three longitudinally spaced ferrous elements, wherein two of the spaced ferrous elements are symmetrically spaced about a remaining one of the spaced ferrous elements to form a pair of gaps, the pair of sensors longitudinally spaced and located in the pair of gaps, respectively along a line parallel to the longitudinal axis.  
 
     
     
       21. The apparatus of  claim 20 , further comprising a signal processor connected to the sensor unit. 
     
     
       22. The apparatus of  claim 20 , further comprising a non-ferrous shell located radially inside the magnetic poles and the sensor unit, the shell sized to form a channel through which the object passes. 
     
     
       23. The apparatus of  claim 20 , wherein the spaced ferrous elements include ferrous inserts located inside the ferrous elements. 
     
     
       24. The apparatus of  claim 20 , further comprising inserts disposed radially inward of the poles. 
     
     
       25. The apparatus of  claim 20 , further comprising at least one magneto-sensitive sensor disposed radially inward of one of the magnetic poles. 
     
     
       26. The apparatus of  claim 20 , wherein the magneto-sensitive sensors are Hall effect sensors. 
     
     
       27. The apparatus of  claim 20 , wherein the spaced ferrous elements and the pair of sensors are symmetrically located about a longitudinal center point of the inter-pole area.