Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-07-20
2001-05-22
Chapman, John E. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06234026
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates in general to the use of electromagnetic acoustic transducers (EMATs) for testing materials and, in particular, to a new and useful method and apparatus for performing non-destructive testing using EMATs which avoids the problem of “main bang,” that is, the overloading of the EMAT receiver by the electrical interference created by the high energy transmitter pulse.
In most cases, prior art EMAT testing systems use the same EMAT coil to transmit waves and to receive reflected waves. Resistance was added to the EMAT coil, in some cases, in order to reduce the Q of the tuned circuit, reducing the duration of the main bang or overload signal, but at the same time reducing signal amplitude and signal-to-noise ratio. In other cases separate non-overlapping coils were used to transmit and receive. This can cause problems because the coils are not at the same location for transmitting and receiving.
When performing nondestructive testing using EMATs, there is typically a region adjacent the EMAT from which no reflections can be received due to main bang. Main bang is a large signal in the receiver output created by the transmitter pulse. This is such a large signal that it completely overloads the receiver, preventing it from detecting any reflections during this time. The duration of the main bang includes the length of the transmit pulse, and a period of time immediately after transmitting, during which the transmit pulse decays to a level small enough so that it no longer interferes with the detection of reflections. The duration of the main bang is a significant impediment for many potential applications of EMATs at the present time.
SUMMARY OF THE INVENTION
An object of the present invention is to reduce the duration of the main bang in an EMAT based system. The method and apparatus according to the invention has been tested and reduces main bang duration by a factor of two.
Another object of the present invention is to achieve reduction of main bang by using separate coils for transmitting and receiving EMAT waves through the work piece to be tested, by minimizing the coupling between these two coils, and by separating and isolating the transmitter and receiver electronics. This reduces Q of the tuned circuits used to match the EMAT coils to the EMAT electronics.
By constructing an EMAT with separate coils for transmitting and receiving, and by adding an electrostatic shield between the transmitter coil and the receiver coil, capacitive coupling of the large transmit pulse to the receiver coil is greatly reduced. The electrostatic shield can be formed by using a very thin layer of high resistivity nonmagnetic metal such as titanium, or a highly conductive nonmagnetic metal such as copper with gaps etched in the layer to prevent eddy currents from being produced in the shield layer which would attenuate the actual EMAT signal. For meander coil EMATs, offsetting the transmitter EMAT coil and receiver EMAT coil by half the wire-to-wire spacing or by a quarter wavelength of the wave, minimizes the magnetic coupling between these two coils, again reducing the duration of the main bang.
In order to optimize power transfer to the EMAT coil, typically, the inductance of the EMAT coil is resonated by placing a capacitor in parallel with it. By selecting the capacitor so that it resonates with the coil inductance at the desired frequency of operation, the impedance becomes real, allowing the maximum transfer of power to the resistance of the EMAT coil. Often a considerable length of cable is connected between the EMAT coil and the matching network. Typically these cables add considerable inductance and little additional resistance to the EMAT circuit. This tends to raise the Q, which is defined as the inductive impedance (Z
L
=2 &pgr;fL) divided by the resistance of the circuit (Q=Z
L
/R). Higher Q results in the tuned circuit “ringing” longer after the excitation drive has been removed, causing increased mainbang duration.
By placing the tuning capacitor as close as possible to the EMAT coil, and using a printed flex circuit stripline to connect the EMAT coil to the capacitor, the additional inductance is minimized. A flexible stripline is constructed by placing two narrow strips of thin copper on either side of a thin flexible plastic such as Kapton® (a trademark for a polyamide plastic). For example the copper may be 0.25″ wide by 0.001″ thick and the Kapton® plastic 0.001″ thick. This provides much lower Q resonant circuits, which results in reduced main bang duration while providing the same or somewhat improved signal-to-noise (by reducing the resistance of the connecting cable). By using a combination of these techniques the main bang duration can be substantially reduced.
Testing indicates that the main bang duration can be reduced by as much as a factor of two using this technique as opposed to using the same coil for both transmitting and receiving. In addition, this technique has been found to be very effective at reducing sensitivity to electrical noise. The electrostatic shield serves to prevent electrical noise on the part from being capacitively coupled to the receiver EMAT coil.
As alternatives, the coils and shield could be constructed using printed circuit techniques or manually constructed using hand wound coils. The electrostatic shield could be a copper layer with narrow gaps in the copper to prevent the generation of eddy currents in the copper layer so as to prevent attenuating the magnetically coupled EMAT signals.
Accordingly, one aspect of the present invention is to provide an EMAT arrangement which comprises at least one EMAT coil having opposite coil ends, a tuning capacitor and a stripline cable connected between the coil ends and the capacitor for minimizing losses between the coil and the capacitor.
Another aspect of the present invention is to provide separate transmitter and receiver EMATs which are separated from each other by an electrostatic shield.
Yet another aspect of the present invention is to provide such an arrangement where the separate coils each have multiple sections which are separated by a pitch distance, with the coils being offset from each other by one-half the pitch distance or by one-quarter of the wavelength of the wave meant to be transmitted from the transmitter coil.
Yet still another aspect of the present invention is to provide an EMAT arrangement which includes a preamp connected to the receiver EMAT coil, and a matching network between the EMAT electronics and the transmitter EMAT coil for producing the acoustic signals.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.
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Hancock Jimmy W.
MacLauchlan Daniel T.
Baraona R. C.
Chapman John E.
Marich Eric
McDermott Technology Inc.
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