The Focal Law Wizard

OmniScan MX2 Training Program

2OmniScan MX2 Training Focal Law Wizard Overview The focal law wizard is the third wizard in our process after all

parameters of the material, probe, and wedge have been entered in the part and set up wizards.

The focal law wizard is designed for a step by step method of populating the beam parameters. These parameters are also available in the focal law sub menu.

The focal law wizard is where beam formation, steering, and focusing are programmed for the current group.

The wizard will walk the user through each step for group creation to include:

Selection of group type (Sector, linear, linear at 0 degrees). Selection of longitudinal or shear wave velocity. Selection of element quantity and position (Aperture). Selection of angular coverage and resolution. (45-70 degrees, .5 degree) Selection of focal depth. Selection of gate position.

3OmniScan MX2 Training Focal Law Wizard Overview Each step in the wizard process is completed prior to selecting next. Each task is

explained in the next series of slides. Below is the focal law wizard progression.

4Focal Law Wizard Law Configuration The three options for law configuration are:

1. Sectorial.2. Linear.3. Linear at 0 degrees.

The linear at 0 degrees option is used for corrosion and composite C-scan inspections and it is limited to one group only in the .Ops file.

Other option such as linear spread can be imported from an external focal law calculator.

The legs option is relative to the ray tracing feature of the report and is addressed in a later section. This is normally set to 1 or 2 legs for weld inspection and will limit ray tracing in gate A to the number of legs selected.

5Focal Law Wizard Wave Type The wave type is a function of velocity for the material type selected in the part and

weld wizard. The wave type is also relative to the wedge. Wedges are normally designed with an

incident angle for either shear wave or longitudinal inspection. Shear wave wedges are designed with an approximate incident angle of 36 degrees

for a natural 55 degree refracted angle in carbon steel. This allows for good beam steering from approximately 35-70 degrees.

Longitudinal angle beam wedges are designed with an approximate incident angle of 20 degrees for a natural 60 degree refracted angle in carbon steel. This allows for good beam steering from approximately 30-80 degrees.

Longitudinal straight beam wedges are designed with a 0 degree incident angle for a +\- approximately 30 degree beam steering or linear configuration.

6Focal Law Wizard Probe Element Quantity The probe element selection step in the wizard

will determine both the size of the beams (Aperture) and the position of the beams on the probe (1st element).

Element quantity selection cannot exceed the total elements available in the probe or the maximum pulsers available in the MX2 acquisition module (16:XXX or 32:XXX).

The aperture is equivalent to the diameter or size of a conventional probe. Element quantity X element pitch = aperture.

The element quantity must be sufficient for beam steering and energy which is usually a minimum of 4 elements for straight beam and 8 elements for angle beam.

Increasing the element quantity increases the aperture which in turn extends the near field and ability to focus farther.

7Focal Law Wizard Probe Element Quantity cont.Question: How can I determine if the element quantity is sufficient for beam formation?Answer: The exact same way we verify beam parameters in conventional UT. The

quality of the A-scan (S/N ratio) and the verification of the angle and exit point using a standard reference block. The limits of beam steering and aperture for any probe\wedge combination are dependent on many parameters and cannot be predicted by the software.

8Focal Law Wizard Probe Element Quantity cont.Question: How can I reproduce the beam size of a 10mm conventional probe using

phased array?Answer: Aperture (Size) = element pitch X element quantity. For a .6mm pitch probe, 9.6mm = .6mm pitch X 16 elements. For a 1mm pitch probe, 10mm = 1mm pitch X 10 elements.

9Focal Law Wizard Probe Element Quantity cont.Question: In the previous example, which probe can be calibrated to a wider sector scan?

(45-65, 40-70, etc)Answer: The .6mm pitch probe would have a wider range of beam steering due to the

smaller element pitch. Both apertures would have the same near field length (Max focal depth) but the .6mm X 16 aperture set up would have improved focusing within the same range because it is using more elements.

9.6mm = .6mm pitch X 16 elements.

10mm = 1mm pitch X 10 elements.

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Focal Law Wizard Probe Element Quantity cont.Question: The application requires reproducing the same 12mm probe size that was

used with a conventional UT probe. What is the advantage of a 16:XXX instrument over a 32:XXX instrument given the optimum probe?

Answer: The 32:XXX using more elements of a smaller pitch probe will have

significantly improved beam steering range, energy, and focusing.

32 elements X .6mm = 19.2mm aperture (32:XXX acquisition module)16 elements X 1.2mm pitch = 19.2mm aperture (16:128 acquisition module)

Because the near field calculation is the same for both examples, the maximum focal length would be the same. But more elements of a smaller pitch within the same aperture improves focusing significantly.

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Focal Law Wizard Probe Element Quantity cont.

27 elements

.6 mm pitch probe = 16 mm aperture (32:XXX) 16 elements

1 mm pitch probe = 16 mm aperture (16:XXX)

Below we see two examples that demonstrate the benefit of more elements of a smaller pitch with the same size aperture. (16:XXX vs. 32:XXX)

The signal in the red gate A is a crack tip of a 20% through wall ID connected crack in a 25mm thick carbon steel weld. The gain is increased so that the crack tip signal is at 80% amplitude.

The 32:XXX acquisition module example shows a clear improvement in sizing accuracy by producing a clearer image of the crack tip and improved signal to noise ratio than that of a 16:XXX acquisition module.

Using more elements of a smaller pitch required a probe with more elements and a compatible acquisition module for the MX2 with more pulsers.

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Focal Law Wizard Probe 1st Element Position The position of the beam set within the probe is defined by the first

element position. For probes with a large element quantity (64, 128) the aperture can be

programmed at any position. It is a common inspection strategy to use two sector scan groups from the

same probe for coverage. One from the front of the probe for 1st leg coverage of the ID (First element position 49), and one from the back of the probe for 2nd leg coverage of the OD (First element position 1).

Element 1

Element 49

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Focal Law Wizard Probe 1st Element Position cont.Question:

For manual inspection is it better to program the sector scan at the front or back of the probe\wedge?

Answer: The front of the wedge. In the example below using a start element position of 49 allows the 16 element aperture beam to exit the wedge as close to the weld as possible.

Element 49 is the last element on the probe that will still allow a 16 element aperture. Total probe elements (64) aperture (16) + 1 element = maximum first element position (49).

First element 49

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Focal Law Wizard Beam Min and Max Angle Depending if sector or linear was selected, the beam angle form will change

for the required parameters. For a linear scan only one angle is available in the focal law wizard. The minimum and maximum steering angles are defined by the ability to

maintain angle and velocity, and ability to calibrate it for sensitivity and TOF. This is a function of physics, not software. For a given

probe\wedge\aperture, programming an angle beyond the steering range or first\second critical angle (Snells law) will result in a poor quality A-scan, mode conversion due to velocity change, angle error, and inability to calibrate.

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Focal Law Wizard Beam Min and Max Angle cont. Beam steering is limited by the probe element size and aperture, the

wavelength, physics of UT (Snells law) and most importantly, the ability to calibrate each A-scan in the group to the satisfaction of the application or procedure.

The phased array calibration process, like conventional UT, includes maintaining the velocity at fixed angles (Does not mode convert) and ability to correct the wedge delay, sensitivity, and build a TCG (If required) for every A-scan in the group.

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Focal Law Wizard Beam Min and Max Angle cont.Question: Why cant the software predict what is an unacceptable steering range for a

given probe\wedge\aperture combination? 20-85 degrees?Answer: Because the beam does not terminate when it reaches a maximum steering

angle. It slowly deteriorates as it approaches or exceeds the steering range. Depending on the sound path required, the size of the calibration reflector, the required A-scan quality for the application, etc., what is acceptable for one customer may not be acceptable for another and the software does not attempt to limit it.

Also, for some applications there is still value in detection in a beam or group of beams even though they have mode converted, cannot be calibrated, and the depth and surface readings are not valid. (Which thing does not belong here inspection).

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Focal Law Wizard Beam Min and Max Angle cont.Question: What factors most affect the ability of a probe\wedge\aperture combination to

improve beam steering?Answer:Probe element pitch and beam aperture. The smaller the element pitch and

aperture, the better the beam steering up to the limits of physics (Snells law) and the critical angles.

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Focal Law Wizard Beam Angle Step Resolution The beam angle step resolution is directly relevant to the number of A-

scans generated within the first and last angle of a sector scan. More focal laws or higher A-scan density within the sector scan improves

sizing and flaw characterization. The step resolution is normally .5, 1, or 2 degrees but is also possible at a

smaller resolution such as .1, .2, or .25 degrees. When using a small step resolution such as .1, .2 or .25, the 256 focal law

limit of the MX2 is quickly reached for multi probe or multi group inspections.

More A-scans in the Ops file as a result of wide angle coverage or small step resolution will also increase the data file size, reduce PRF, and reduce acquisition speed. It is important to program the group for the needs of the application. More is not always better.

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Focal Law Wizard Beam Angle Step Resolution cont. Below we can see the affect that beam angle step resolution has on the

resulting number of focal laws or A-scans generated for a similar 45-70 degree sector scan.

14 total A-scans (45, 47, 49.....)

26 total A-scans (45, 46, 47.....)

51 total A-scans (45, 45.5, 46, 46.5.....)

101 total A-scans (45, 45.25, 45.5..)

251 total A-scans (45, 45.1, 45.2, 45.3..)

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Focal Law Wizard Beam Angle Step Resolution cont. In this example there are 4 groups of phased array data and 4 groups of TOFD data

to provide 1mm detection and sizing for a 100mm thick ASME vessel inspection. The two phased array groups covering the ID are at a 1 degree resolution. The two phased array groups skipping off the ID for OD coverage are at a degree

resolution. Because the OD coverage groups have a longer sound path, the groups are

programmed at degree resolution (2X as many A-scans) to maintain the same sizing ability as the ID groups.

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Focal Law Wizard Beam Angle Step Resolution cont. Below the affect of beam angle resolution for a typical 45-70 degree S-scan on a

30mm V weld is displayed. Beam angle resolution is directly related to sizing accuracy and ability to characterize

flaws. The ability to distinguish between a crack and root non-fusion, or porosity and lack of

fusion is greatly enhanced by a higher A-scan density within the S-scan. For inspections where flaw characterization and sizing is not a consideration the

scan speed and file size can be optimized by reducing the angle resolution to 1 or 2 degrees creating fewer A-scans within the S-scan.

45-70 degree S-scan at 1 degree resolution 45-70 degree S-scan at 1/2 degree resolution

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Focal Law Wizard Beam Angle Step Resolution cont. The image to the right is a

crack tip detected at the same scan sensitivity for a 1 degree resolution and a .25 degree resolution sector scan group.

The ability to size and characterize flaws is improved with the increased A-scan density within the sector scan.

The .25 degree resolution group will more clearly locate and size the crack tip data due to more A-scans within the same angle range (45-65 degrees).

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Focal Law Wizard Beam Angle Focus Depth In phased array inspection, the beam size, angle, and focal plane are capable of

being manipulated within the limits of physics, the software, and the hardware. The OmniScan MX2 only supports depth focusing. Focusing of any other type can be achieved by importing the focal laws from an

external calculator. The maximum distance that the beam can be focused is defined by the near field

calculation below. Any number entered into the focus depth field that is greater than the near field will

result in an unfocused beam.

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Focal Law Wizard Beam Angle Focus Depth cont. The benefits of a 32:XXX acquisition module over a 16:XXX are enormous

when long focal lengths are required for an application. Maximum estimated near field calculations in sound path for common

probes in carbon steel without consideration of the wedge:

5Mhz .6mm probes (A10, A2, A12) 10 elements = 44mm 12 elements = 46mm 16 elements = 54mm 20 elements = 69mm 24 elements = 90mm 28 elements = 117mm 32 elements = 148mm

5Mhz 1mm pitch probes (A14, PWZ3) 10 elements = 54mm 12 elements = 66mm 16 elements = 102mm 20 elements = 153mm 24 elements = 217mm 28 elements = 294mm 32 elements = 384mm

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Focal Law Wizard Beam Angle Focus Depth cont.Question:

What is the difference between a 16:XXX and a 32:XXX MX2 acquisition module with regard to beam formation?

Answer:Ability to create larger apertures, focus farther, and improved flaw characterization and imaging. Maximum focal depth (Near field) is listed for each aperture below without considering the wedge.

5Mhz .6mm probes (A10, A2, A12 probes) 10 elements = 44mm 12 elements = 46mm 16 elements = 54mm 20 elements = 69mm 24 elements = 90mm 28 elements = 117mm 32 elements = 148mm

5Mhz 1mm pitch probes (A14, PWZ3 probes) 10 elements = 54mm 12 elements = 66mm 16 elements = 102mm 20 elements = 153mm 24 elements = 217mm 28 elements = 294mm 32 elements = 384mm

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Focal Law Wizard Set Gate A Position The gate position is not related to the focal law calculator. It is included in

this step of the wizard as a shortcut for the normal setup process. The gate position is critical because it will generate the C-scan and

determine the signal to calculate readings such as DA, PA, SA. In the example below on a 25mm thick weld inspection, the gate starts at

the area of interest and has a width that would terminate at the end of the second skip.

The red box around the gate parameter indicated it is relative to gate A. Gate B is green and gate I is yellow.

Gate functions are covered in detail in a later section.

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OmniScan MX2 Training Use of Focal Law Wizard1. Select Menu>Wizard>Focal Law>Start.2. Law Configuration = Sectorial.3. Legs = 2.4. Select Next.5. Wave type = Shear wave.6. Select Next.7. Element Quantity = 168. First Element = 1 9. Select Next.10. Min Angle = 45, Max Angle = 70, Angle Step = 1, Focus Depth = 50mm.11. Select Next.12. Gate Start = 5mm, Gate Width = 20mm13. Select Next.14. Select Generate.

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OmniScan MX2 Training Use of Focal Law Wizard cont. Upon successful completion of the part and weld, group set up, and

focal law wizards the default display of A-S-C is displayed with default UT settings.

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OmniScan MX2 Training Focal Law Wizard Movie

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