Time Waveform Replication

A Time Waveform Replication (TWR) Test is often conducted with data from a moving vehicle

A Time Waveform Replication (TWR) Test is often conducted with data from a moving vehicle

In Time Waveform Replication tests, a pre-defined time domain waveform is replicated and the structure under test is excited using this waveform. The response of the unit under test is measured and the output signal is varied in closed loop to maintain the input control signal same as that of the pre-defined waveform. The algorithm used for the replication of a waveform is similar to the algorithm used in the Shock tests.

Multiple Time domain waveform can be stored and replicated in a TWR test. Initially, the test calculates the impulse response of the system in a similar way as performed in the Shock tests. It is assumed that the test system is linear, which means that its response to any input can be predicted from its Frequency Response function. In the control process, this FRF is continually estimated and updated, and used to calculate the output drive signal. The output causes the vibration test system to respond by matching the control signal to the pre-defined field data time waveform.

Not all waveforms collected in the field however are easily reproduced.  Shaker restrictions, including displacement and velocity limitations may prevent sections of field data from exact shaker replication.  To address this issue, CI has developed EDM Waveform Editor.  Waveform Editor is a powerful tool offered with CI's TWR VCS software package that allows operators to edit or modify all or part of the waveform so that it will fall within the shakers capabilities while still preserving the overall shape, length, and transients within the data.

TWR Features
The following are the important test features of the TWR tests:

  • Number of Channels: Up to 8 Channels can be enabled for monitoring and data recording.

  • Multiple Profiles: Large number of Waveform recordings (subject to the available Flash memory) can be supplied simultaneously to automatically run one after the other on the test specimen.

  • Maximum number of points: A complete flash memory space can be used for storing the profile (currently 3.7 GB), which could store about 1 billion points of data at a sampling rate of 200 samples/sec. It can replicate a waveform of about 50 days.

  • Maximum replicable frequency: Waveforms of up to 30 kHz can be replicated.

  • Maximum Sampling Rates: Waveforms of any sampling rate could be imported into the Waveform Editor tool and could be converted to the waveforms with desired sampling rates that can be accepted by the TWR.

Safety Features
In TWR tests, there are a number of safety features that help prevent damage to the shaker and related equipment. During a shake test, 5 different types of checks are performed and an event is triggered if any of these checks fail. The response actions to these events can be customized under Event-Action Rules. The 3 checks are 1) Maximum shaker drive voltage limiting, 2) Channel overload or loss detection, and 3) TWR abort limits.

The system also detects when input channels are overloaded or lost which can indicate a sensor fault or an accidental disconnect. The test will be aborted if this occurs.

In the event of an accidental network disconnection or power loss, the hardware is able to save test data and state information to non-volatile memory to protect against loss. For a network disconnection, the test can continue to run in Black Box mode or save all data and execute an orderly shutdown.

TWR Abort Limit
TWR abort limits are similar to the abort lines in Shock and are defined in the time domain. If the level of the control signal, within the vicinity of the waveform output, falls outside these limits then an abort event is triggered. 



Figure 1. TWR Control Profile

The top of the window shows an acceleration waveform in green. The abort limits are red lines.

The bottom pane has the abort high and low limits which can be set by the user. The user also has options to define different abort limits at different points of time of the time domain waveform. Further details about the different settings available for abort settings are listed in the “Editing Profile and Abort Settings” section below.