Eddy current inspection of industrial machining processes has evolved over the past several decades to the point where it is now a well understood and effectively used technology capable of making a positive and consistent impact on quality assurance-but it can do more.
While eddy current is used to inspect for cracks in machined surfaces, voids in castings, thread presence and quality, hardness differences, chemistry changes, surface roughness, thickness variations and part runout, there is a little used aspect of its capability-part profiling. Part profiling is the review of the eddy current characteristics of a part with respect to a position variable.
In a profiling application, either a dual- or single-element eddy current sensor is passed over the surface of a part and its position, with respect to that part is measured with a linear displacement device such as a linear potentiometer or an linear variable differential transformer (LVDT). As the sensor is moved, the eddy current profile of the material adjacent to it is recorded with respect to its position. The resulting output of both sensors is then plotted on a Cartesian coordinate system with position on the X axis and the associated materials' eddy current profile on the Y axis. Because eddy current inspection is a comparison means of testing, the eddy current profile vs. position of a known good part is compared to that of the part under test. If the profile of these two parts match within user set limits, the inspected part is assessed to be good and accepted; if not, the part is rejected.
To understand the myriad of additional applications that can be obtained with an eddy current system, it may help to examine a few potential inspection scenarios.
Crankshaft oil holes: During machining operations, a gun drilling sequence is used to drill holes for oil flow in a crankshaft. Oil flow holes are drilled through each main journal, and then intersection oil flow holes are drilled from each of the rod journals into the main journal "through hole." Because of the relative position of the rod journals with respect to the main journals, the rod journal intersecting holes are up to 6 inches long, and drilled at compound angles through the crankshaft counterweight structure. This complex gun drilling procedure occasionally resulted in incomplete drilling operations because of crankshaft misalignment or broken drill bits. An inspection technique was needed that could inspect each main journal oil hole and all associated rod journal oil flow holes to determine if they intersect.
Solution: Using an eddy current profiling technique to profile the inside of each main journal through hole, it could be determined that the rod journal holes intersected with it and that the intersecting holes broke through correctly. By plotting the eddy current profile of the inside of the main journal holes against the eddy current sensor position within the hole relative to one surface of the main journal, the breakthrough of each intersecting rod journal oil hole was clearly discerned. (See Fig. 1a and 1b.)
Aluminum casting voids: After a boring operation into an aluminum casting, it was necessary to determine if any voids existed on the inner surface of the bored hole that could eventually result in a leak path after a preformed nozzle was pressed into it.
Solution: Using a dual-element eddy current sensor to profile the inside of the bored hole, it was possible to discern voids that were less then 1 millimeter in diameter and 0.25 millimeter deep. (See Fig. 2a and 2b.)
Machined grooves: After a machining operation on a centerless ground shaft, it was necessary to discern the location and relative size of two 10-millimeter grooves.
Solution: By using a profiling technique that extracted the eddy current profile of the shaft with respect to its end, the position and relative size of both grooves could be determined. (See Fig. 3a and 3b.)
Camshaft hardness: A cast camshaft was flame hardened and ground. After the grinding operation it was necessary to verify that both the camshaft main journals and each cam journal had, in fact, been hardened.
Solution: An eddy current profiling operation using a single element coil was effectively used to verify both the hardness of each area of interest as well as the length of the flame hardening operation along the camshaft length.
Eddy current profiling requires a number of components, including an eddy current sensor, drive electronics and signal diagnostic software.
Any profiling operation begins with the eddy current sensor. These sensors come in two basic types, single element and dual element. The decision as to which type to use requires a review of both the material and machining process that is used to create the part. To explore this further, an understanding of fundamental principles is necessary.
Eddy current inspection simultaneously monitors four material parameters, all of which relate to base materials conductivity. These are chemistry, hardness, geometry and temperature. In most cases temperature can be negated from the test results, as all parts inspected will be at, or near, the same temperature. The other three parameters, however, must be considered during sensor type selection.
In most cases part geometry is the parameter that is to be reviewed. If a single element eddy current sensor is to be used, however, changes in base material chemistry and hardness can appear to be changes in geometry in the resulting eddy current vs. position graph. Cast iron and nodular iron exhibit substantial problems in this area and profiling these materials is often difficult unless part chemistry and hardness changes can be negated from the part signature. In contrast, most steel and aluminum materials can be effectively profiled with single element sensors because there are only small variations in base materials characteristics with which the eddy current system must deal.
If small defects are to be discerned, however, any base material variation will appear as "noise" that can detract from the capability of the inspection system to detect small part details or defects. A dual-element eddy current sensor provides the capability to subtract chemistry and hardness variations from the extracted signature. This can be accomplished by using the difference between the signatures of each of the elements. This difference will be void of all but geometry information as the sensor is moved over the surface of the part. For small defects, especially in materials that are nonhomogeneous in base material hardness and chemistry, a dual-element sensor is an excellent choice. In designing an eddy current sensor several things must be considered:
Fill ratio: The fill ratio is defined as the ratio between the outer diameter (OD) of the sensor and the inner diameter (ID) of the part for a hole, or conversely the OD of the part to the ID of the sensor if the sensor is a coil.
Coil shielding: Because eddy current sensing is based on the propagation of an electromagnetic field, depending on the type and size of defect to be detected, some sensor shielding may be required. Also, when a dual-element sensor is selected, shielding is often required between the individual elements to eliminate cross talk between them.
Coil spacing: The size of the feature or defect to be detected will determine the spacing in a dual-element sensor. Also, in materials that have a large variation in the base materials' chemistry and hardness, sensor elements should usually be spaced as close as possible to reduce the possibility of detecting base materials changes as geometry differences.
Air core vs. ferrite: Several approaches to the design of the eddy current detection coil are available. One is to use a coil of wire wound on a non-magnetically conductive bobbin while the second involves winding a coil on a magnetically conductive ferrite core. The choice of an air core or ferrite core, and the selection of the specific type of ferrite depend on the type of defect to be detected, the base materials characteristics and the size of the defect or feature to be examined. Often the specifics of the sensor design will be best determined empirically.
Eddy current sensor drives electronics
The second major hardware requirement is the eddy current sensor drive electronics, an essential component of the profiling system. During the evaluation of the materials characteristics, two signal parameters that can be extracted from the sensor need to be reviewed. These parameters are the amplitude change of the output signal from the probe and phase shift of the output signal when compared to the reference oscillator.
In any well-designed profiling system, both of these signals must be evaluated as each is capable of detecting different features or defects. If both of these parameters are not available from the electronics, essential information can be missed. The frequency of the drive signal is also important. The penetration of the eddy currents into the material relates directly to the frequency of the drive signal. Low frequencies penetrate farther into the material than higher frequencies and are therefore more likely to detect subsurface defects or base materials changes that appear as "noise" that must be dealt with in signature evaluation.
Because eddy current profiling requires a dynamic review of the part's surface, it is important to verify that the signals produced by the sensor as it moves across the parts surface stay within their linear ranges and do not saturate the front-end amplifiers. This is typically accomplished with some sort of health and status monitor that provides an error signal if an "out of range" condition in any portion of the electronics occurs. All quality profiling systems will include this feature and that health and status signal should be recorded along with both the position signal and the eddy current signature. If a health and status error occurs, the associated eddy current signal is suspect and the part should either be reprofiled or rejected.
Signal diagnostics software
By far the most important element in any profiling system is the data acquisition and analysis software used to evaluate the extracted signals and create the position vs. eddy current signature graph. This software is as varied as the applications and can make the difference between a successful and unsuccessful inspection. Common essential elements of this software include:
Data resolution, which is the number of data points taken for each profile. This must be high enough to accurately represent the analog signal extracted from the probe, but not so high that the time necessary to analyze the signal is extensive.
Data smoothing, which is a means to smooth or average adjacent data points. This should be employed to eliminate the effects of ground noise or electromagnetic interference (EMI) originating within the test environment.
Data reduction and analysis techniques. When the data is to be analyzed to locate part defects or features, a number of detection approaches can be employed. Using threshold detection, defects or features can be detected if the signal exceeds a preset threshold. With limit detection, defects of features can be detected if the signal transitions outside of preset or "learned" high and low limits.
A third option is signal derivative evaluation. With this technique, the derivative of the signal is reviewed with respect to its shape or zero cross and features or defects can often be discerned. In addition, signal integral evaluation can be used. This is the integral of the extracted eddy current signal that can also be employed to discern defects or features. Additional approaches such as zero cross, slope, absolute value, maxima or minima and average can also be used.
As quality inspection of machining operations increases in importance and zero defects mechanical fabrication techniques become required, a new weapon in the quality assurance arsenal is eddy current profiling. This technique has just recently come into its own and now represents a means by which mechanical inspections can be conducted and the results stored in computer memory for future statistical process control analysis. Eddy current profiling is fast, accurate and has the potential to produce results not available from any other inspection method. NDT