Simcenter Testlab Signature (formerly called LMS Test.Lab) has “triggered start/stop” feature for time data throughput acquisition. This article describes how these features work, especially to facilitate the consistent capturing of transient events.
A transient event is a short duration, high amplitude change in a measurement signal. Examples include vibration caused by an explosion, sound created by clapping, or input loads produced by hitting a bump while driving a car.
With a user initiated measurement, it can be difficult to capture a transient event. For example, if driving along and listening for a transient event, pressing the start measurement button only when it is heard will actually miss the event in the data recording.
A better approach would be to have a defined trigger level with a throughput “prestart time”. The acquisition would start immediately when the desired level is reached, and the “prestart time” buffer would contain data leading up to the transient event as shown in Figure 1.
Figure 1: Transient time signal with trigger level creating throughput prestart and measurement duration
This prestart buffer is appended to the beginning of the measurement. The final recording will have the original measurement duration and the prestart data combined together. For example, if the measurement duration was set to 3 seconds, and the prestart was 3 seconds, the total recording time in the throughput file would be 6 seconds.
2. Simcenter Testlab Settings
To utilize triggers in Simcenter Testlab Signature, there are several available settings in the Tracking Setup and Measure worksheets.
2.1 Tracking Setup Worksheet
In the Tracking Setup worksheet, turn on “Use triggered start” and “Edit throughput prestart”, and set the duration, as shown in Figure 2.
Figure 2: Throughput prestart and measurement duration definition in Tracking Setup worksheet
The duration is the amount of time that is recorded once the trigger level is achieved.
To set the trigger level, press the “More…” button next to “Use triggered start”. The level and channel for the trigger can be set as shown in Figure 3.
Figure 3: Trigger level settings under “More…” button of triggered start
When the ‘Level’ setting is exceeded on the indicated channel, the recording will start. If ‘Slope’ is set to ‘Up’, then the signal must start below and then rise above the trigger level for the measurement to start.
2.2 Measure Worksheet
In the ‘Measure’ worksheet, a few more options can be helpful. Click on the “More…” button (located on the middle right of screen) and change the following options as needed (Figure 4):
Processing Mode: Change this option from “Inline Processing” to “Delayed Processing’ to make the turnaround time between re-arming and acquiring new runs as short as possible. With “Delayed Processing”, no FFT data will be calculated from the throughput file after the acquisition is finished. Instead, the data can be processed later.
Monitoring: This sets the amount of time signal displayed on the screen when viewing signals from the ‘Online Data -> Fixed sampling -> Monitor -> Time History’ folder.
Automatically Accept measurement: Turn this option on to avoid being prompted with the question “Would you like to keep this run?” at the end of each acquisition.
Normal measure sequence: If set higher than the default of 1, the software will automatically re-arm and reacquire after each measurement acquisition. For example, if this was set to 4, then the system would take four separate acquisitions automatically (assuming step 3 is peformed) after the user presses the start button once.
Figure 4: Options from “More…” button in Measure worksheet
Now a triggered measurement can be started. After pressing the Start button, the system will behave differently because triggers have been defined. Instead of measuring right away, a message “Ready for Time” will appear, indicating the systems is waiting to acquire based on a trigger (Figure 5).
Figure 5: When a trigger is defined, the message “Ready for Time” indicates that it is waiting for a trigger before acquiring
During arming the prestart buffer is filled. So if the prestart was 20 seconds, it would take an extra 20 seconds than normal to fill the buffer before the measurement could proceed.
3. Results and Conclusion
By using the trigger, the transient events in different acquisitions will be aligned in time as shown in Figure 6.
Figure 6: Transient events from different measurements are aligned in time by use of triggered measurements
By using triggers, the transient events contained in different acquisitions are captured. This enables the events to be easily overlaid and consistent analysis to be performed. For example, a Shock Response Spectrum (SRS) analysis or Wavelet Analysis might be performed on the transient data.