Gates
See Also: Gate Features
Gates and Regions are spans of time on a digitized waveform from which data is extracted. At minimum, all gates have a Start Time and a Length. Most gates also have a Threshold to determine a level at which to keep data. Gates that do not have thresholds are called Regions (with one exception).
A collection of gates is called a Gate Tree. This is due to the parent-child relationships among gates, giving rise to a "tree" type structure.
Gate positions and configuration can be done through the Setup form (in Basic Mode), or on the Gate Settings tab of the Digital Oscilloscope. The Setup form should suffice for most configurations. When using the Gate Settings tab on the Scope, more complex options and features are available.
Important differences in the input fields between the Setup form and the Gate Settings Tab:
- The Start (us) is relative to the parent gate: thus, if a data gate is tracking a Follower, its start time of 10 us means it will be drawn 10 us from the crossing point of the Follower. This is true in both methods; however, on the Setup tab, gates that have no parent have an absolute start time (e.g. from Time 0), whereas on the Gate Settings tab, this time is relative to the Absolute Gate position.
- On the Setup tab, the Thresholds are specified in %FSH; on the Gate Settings Tab they are specified as percent values from 0 to 1.
ODIS defines several gate types with specific functions.
Absolute Gate
The Absolute Gate is the root of the Gate Tree. it is represented by a red region at the bottom of the Digital Oscilloscope.
By moving the absolute gate, the entire gate tree can be shifted without moving gates individually.
The Absolute Gate is not often used for collection: instead, it is the reference to which all other gates are positioned. However, if desired, the Absolute Gate region can be used to define a fixed Waveform Collection Region.
Note: If not in use, keep the length of the Absolute gate small.
Follower Gates
|
Follower gates track a signal that crosses the follower threshold. A gate that tracks a follower gate will maintain a constant distance from the crossing point to the start of the specified gate. Follower gates track either the first Rising Edge or the first Falling Edge, depending on the setup specified by the user. A Follower gate is displayed as a single line on the Digital Oscilloscope. The gate color can be specified by the user. A Data gate that tracks a follower will have a small dot of color that identifies its parent, and hash marks that indicate the direction of the edge to track (unless gate annotations are turned off in Configuration). In the example below, the data gate (represented by the red thresholds) is shifted to the right as the crossing point on the follower (represented by the yellow threshold) shifts right. A second follower gate can be set to track the first follower gate, in order to track an additional signal. This second follower is called a Sub-Surface Follower. Follower gates are used to indicate material transitions: on the Scope, a dark vertical line will be drawn showing the location of the transition. Also, if the Time axis of the Scope is changed to Distance, the material velocity transitions will occur at the followers. To configure a Follower gate, a Start, Length, and a Threshold are specified. The user can also specify whether the gate looks for a Rising Edge or a Falling Edge crossing. |
||
Data Gates
Data gates are used to collect Peak and Time of Flight (TOF) information. ODIS allows the user to configure multiple data gates.
Data gates are often set to track a follower gate, in order to keep the signal of interest within the gate.
A Data gate is represented on the Digital Oscilloscope as a positive and an identical negative threshold. Only signals at or above the threshold are recorded. If the gate is tracking a Follower, the left tip of the thresholds will have a dot that is the same color as the Follower.
To create a Data Gate, turn on "Data1" or "Data2" on the Setup form (Basic Mode), or create a Data gate using the Scope tools (Advanced mode)
Data Gates also contain features for generating Void images or PCM Phase Inversion Detection.
SALI Gates
Scanning Acoustic Layered Image Gates (SALI) are used to rapidly configure a group of Data gates over a region of time. This allows the user to do multiple tests simultaneously, in order to find the region in time that generates the best data. Functionally, a single SALI Gate is identical to a Data Gate.
A SALI gate region is represented on the Digital Oscilloscope as a highlighted section of the waveform, with the selected gate within the region highlighted. As with Data Gates, if the SALI gate is tracking a Follower, the tip of the SALI gate thresholds will have the Follower's color.
Along with the normal Data Gate settings, the user can control the number of SALI gates and whether they overlap. There is a limit to the number of SALI gates that can overlap the same time region: if too many SALI gates overlap, the scope will display an "Invalid Gate Setup" message.
Groups of SALI gates can be set independently: up to 26 groupings are allowed (in Basic Mode, only four (A-D) are shown on the Setup form).
If the user decides that a particular SALI gate is the only on needed, it can be converted to a Data Gate by pressing the "Set" button (located at the bottom right of the Setup window when SALI gate is selected). This will convert the selected SALI gate to the Data1 or Data2 gate (depending on the user selection). This will also remove any remaining SALI gates. Note: this feature is only available in Basic Mode.
Relative Threshold Gates
Relative Threshold Gates (RTG) allow a user to set thresholds based on a relative amplitude, rather than a fixed threshold. By setting a Relative threshold gate appropriately, phase inversion can be detected.
The RTG gate is represented on the Scope by a box with three extra thresholds above the box and two below.
The Relative Threshold gate contains the following settings:
- Data Detect Threshold: this is the threshold at which Absolute Peak Amplitude data is collected. On the Scope, it is represented by a box. Below this threshold, data is ignored and no test for Phase Inversion will be performed.
- Consider Threshold: a second threshold at which the algorithm will check for Phase Inversion. This should be set at or above the Data Detect threshold. On the Scope, Consider Threshold is represented by two dark dashed lines.
- Positive Relative Thresholds 1 and 2: these are the thresholds that determine the intensity of the Phase Inversion. If threshold 2 is crossed, the resulting RTG map pixel will be red, and if threshold 1 is crossed the pixel will be yellow. On the Scope, these are represented by two positive-going solid lines.
- Negative Relative Threshold: this gate checks for a negative crossing. The existence and/or position of the negative threshold crossing is critical for the determination of Phase Inversion. On the Scope, this threshold is represented by a single negative-going solid line.
Note that the inputs for the Data Detect and Consider thresholds are in Volts or %FSH, whereas the Positive and Negative Relative thresholds are in percent (%). This is because the voltage value of the relative thresholds will vary based on the Amplitude of the signal. For example, if you have a absolute peak signal of 0.5V and a Relative Threshold of 50%, the threshold will be set at 0.25V. If the absolute peak signal rises to 0.6V, the Relative Threshold moves to 0.3V. Thus, relative threshold position is a percentage of a peak, and not a fixed voltage.
Relative Threshold Gates generate two sets of data: an Absolute Peak Amplitude map, and a Threshold Map, showing places where higher or lower intensity phase inversion is detected. High phase inversion is shown in red, and low phase inversion is shown in yellow.
on the Digital Oscilloscope, if a phase inversion is found, the depth indicator above the signal will show as red or yellow, depending on the intensity of the signal.
See Also: Phase Inversion Detection
Dynamic Gating
A Dynamic Gate is a data gate that changes size based on the position of two Follower gates. As the difference between the crossing points in the followers changes, the Dynamic gate changes to maintain a constant distance between them. The second Follower gate must track the first follower gate for this feature to work.
FFT Region
The FFT region is used to generate an FFT of the waveform. The FFT is used to generate frequency features using the Magnitude Spectrum.
FFT options can be configured under the "FFT Settings" tab of the Digital Oscilloscope
FFT Gates
When displaying the Magnitude Spectrum of the FFT, the user can create Frequency Gates that cover certain frequency ranges.
Waveform Collection Region (B-Scans)
The Waveform Collection region is used to collect raw waveform data. The waveform data can be displayed as a B-Scan or used for Post Processing.