Eddy current instrumentation for the next millennium. Are a dozen frequencies enough?
Introducing frequency transforms, new for the new millennium. When you have dozens of frequencies to inspect a heat exchanger tube and apply direct color to frequency conversion, you add the new dimension of frequency to the defect signal. The defect signal is then viewed in the frequency domain as below.
Low to High High to Low 3D XYF
Red is used to indicate the lowest frequency because red is the lowest frequency in the color spectrum. The colors yellow, green, blue, and violet are used to indicate increasing frequency, with violet being the highest frequency, because it is the highest frequency in the color spectrum.
Currently, there are three ways of viewing frequency transforms, shown above. In the left and centered figures above, full lissajous signal has been used. The left transform, the low frequency, has been drawn first, and the high frequency last, hence the label low to high. In the transform in the center, frequencies have been drawn in the opposite order from high to low, the high to low transform. In the transform to the right, lissajous figures have been eliminated and the signal end points have been connected, giving a color signature for the signal. Each signal type has a unique signature. This is truly a three dimensional image, as we are viewing the impedance plane with color representing frequency to add a third dimension, the 3D XYF transform.
Several examples of frequency transforms are given below:
The transform of the dent signal is shown above. This is a very distinctive pattern as all the lissajous figures are horizontal. In the 3D XYF transform it is seen that as the frequency increases, the amplitude of the signal increases. This is obvious in the high to low transform, but not in the low to high transform due to overwrite. The 3D XYF transform is easier to read because the signal does not get overwritten.
In the transform for the hole shown above, the amplitude of the signal remains constant over the frequency range. The angle rotates clockwise from the lowest frequencies, reaching a maximum value in the mid-frequencies, and then rotates counterclockwise to the highest frequency.
The O.D. Pit transform is shown above. In the 3D XYF transform it is seen that as frequency increases, the signal rotates clockwise and the amplitude decreases.
The transform shown above is for an I.D. Pit. In the 3D XYF transform the I.D. Pit signal rotates clockwise initially as frequency increases, and like the hole, moves counterclockwise at the highest frequencies. The difference between the transform for the hole and the I. D. Pit is that the I. D. Pit signal increases in amplitude with increasing frequency.
If you analyze eddy current signals looking at just the lissajous figure, it's very easy to confuse the signal from a through wall hole (or a very deep pit) with the signal from a magnetic inclusion or a roll stop. From the three lissajous figures shown above, can you pick out which signal is the hole with a high degree of confidence?
The frequency transform shown above is from a magnetic inclusion. As the frequency increases, there is a slight clockwise rotation. It is this clockwise rotation, specifically at the highest frequency, differentiates this transform from the transform for an I. D. Pit.
The transform above is from the through wall hole. The nearly constant amplitude and change in angle rotation from clockwise to counterclockwise are the signature of a hole signal.
The transform above is from a roll stop. As the frequencies increase, there is an increase in amplitude, and at the highest frequencies, a very notable counterclockwise rotation. This is the signature of a roll stop.
Using frequency transforms, the difference between these three similar signals, is as clear as black and white.
Frequency transforms that have been shown in this brochure use 25 frequencies. Both differential and absolute are available. Therefore, you might be asking yourself How do I adjust 50 channels? That's 25 frequency controls, 50 phase controls, 50 gain controls. This must be an overwhelmingly difficult task.
Setting up the ect 48 LT for an inspection is as easy as 1, 2, 3.
| The ect 48 LT Preliminary Specifications | |
| Main Features | Performance |
| Virtually unlimited frequencies, 1 kHz to 5 MHz | Sampling rate adjustable up to 8 kHz |
| Adjustable in 1 Hz Increments | 500 Hz Band Width |
| Frequency transforms | Up to 2 meters per second probe speed |
| Automatic calibration | Data Storage |
| Automatic calibration table generation | Built-in 4.2 GByte optical disk drive |
| Automatic defect detection and analysis | 16-bit Data Conversion |
| Differential and absolute simultaneously | 16-bit True Electronic Balance |
| Simultaneous injection | Probe Types |
| Physical Characteristics | Differential, Absolute, and Send/Receive with |
| Size:12.0 H x 15.5 W x 8.5 D Inches | Industry Standard 4-Pin MS Connector. |
| Weight:22 pounds | Computer Specifications |
| Power Requirements | Screen Size:15.1 inches Viewable |
| Voltage: 90/130 and 185/270 Vac | Screen Resolution: 1024 x 768 |
| Auto Selected | Processor: Pentium 586/200 MHz |
| Frequency:50 to 60 Hz | RAM: 16 MBytes |
| Power:200 Watts Maximum | Hard Drive: 2 GByte |
| Code Compliance | Floppy Drive: 3-1/2 Inch, 1.44 MByte |
| Meets or exceeds all ASME Section V and XI | Keyboard: 102 Keys |
| requirements. | Screen Type: Active TFT Color |
