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Development of Nonlinear Harmonic Sensors for Detection of Mechanical Damage
Project Number

The goal of this work is to design, fabricate, and test the feasibility of a nonlinear harmonic (NLH) sensor system, deployed on a pipeline inspection pig, for use in detecting third party mechanical damage to transmission pipelines.


Southwest Research Institute (SwRI) – NLH sensor design, prototype construction and system integration, testing

San Antonio, Texas 78238


Third party damage is the leading cause of reported incidents on transmission pipelines. Currently available inspection technologies are not able to reliably detect or characterize defects resulting from such incidents. New technology from the Southwest Research Institute (SwRI) has shown to be effective in detecting areas of plastically strained steel (such as dents and gouges) in pipelines. SwRI's nonlinear harmonics technology will provide pipeline operators with more specific information about mechanical damage, thus improving their ability to predict the likelihood of dangerous pipeline failures.

The nonlinear harmonics method is an approach that uses a time-varying magnetic field to sense the magnetic properties (e.g., permeability) of a component. This method is based on applying an alternating sinusoidal magnetic field at a given frequency. Because of magnetic hysteresis and the nonlinear permeability of ferromagnetic material, the magnetic induction on the material becomes distorted. The distorted magnetic induction waveform contains odd harmonic frequencies of the applied magnetic field. With the nonlinear harmonics method, these harmonic frequencies are detected and their amplitudes are related to the magnetic condition of the material being tested. 

Image of actual nonlinear harmonic sensors
Image of actual nonlinear harmonic sensors



Nonlinear harmonics, an AC magnetic method for detecting local anomalies of stress and plastic deformation, shows promise of improved characterization of mechanical damage defects such as gouged dents, even though the dents may have re-rounded. The work in this project has produced a sensor design, electronic design, and sensor suspension design that are directly adaptable to a multitechnology in-line inspection system.

Accomplishments (most recent listed first)
  • Designed and optimized sensor array and evaluated it using a set of known pipeline defects,
  • Designed and fabricated prototype sensors (elements include: sensor structure, electrical connection, mechanical suspension linkage, and environmental protection),
  • Designed and evaluated prototype electronics,
  • Modified standard data display system to show NLH signals in correct format and tested system,
  • Combined all components of the NLH in-line inspection tool into a functioning prototype unit.

A plate with a stress-rising defect was scanned using a relatively standard, robust sensor mounting assembly with circuits connected to a high performance commercial recorder and with electronics for detection that could easily be integrated with it. This data was processed and displayed with standard commercial software. SwRI had previously scanned the same plate with a 2-axis gimbaled probe and laboratory equipment, and the sensor orientation was the same as that for the prototype scans: aligned with the long axis of the plate. Comparison of the 30 kHz magnitude data showed the same major features, however some of the minor features were missing in the prototype data. Differences in the 30 kHz data could be caused by:

  • differences in the10 kHz drive level,
  • lift-off that affected both 10 kHz and 30 kHz,
  • a need for different calibration,
  • a difference between the prototype sensor,
  • or some unseen difference between the lock-in amplifier and prototype detection electronics.

It should be noted that this project characterized only one example of mechanical damage on a single sample of pipe. Commercializing this methodology will require performing similar experiments with a large number of known defects of varying severity, measuring the tool response, and developing grading algorithms to characterize the severity of the mechanical damage. 

Nonlinear harmonic sensor and associated circuitry
Nonlinear harmonic sensor and associated circuitry


Current Status

This project is completed and all deliverables have been received.

Project Start
Project End
DOE Contribution


Performer Contribution


Contact Information

NETL – Daniel Driscoll ( or 304-285-4717)
SwRI – Alfred Crouch ( or 210-522-3157)

Additional Information

Final Report [PDF-2564KB] - March 2004

Pertinent Publications:
“Use of Nonlinear Harmonic Sensors for Detection and Characterization of Pipeline Mechanical Damage”, Alfred E. Crouch, Proceedings of the Natural Gas Technologies II Conference (Phoenix, AZ), February 2004.