Transient Time History Control
Transient Time History outputs a pre-defined, short duration waveform. The closed-loop control algorithm ensures that the control signal input matches the specified waveform shape. The output is repeated in a set interval.
The shape can be any of the usual waveform shapes, such as sinusoidal, triangular, and trapezoidal, or can be white noise. The shape can also be imported from a file. Compensation adjustments can be applied to make the net acceleration or velocity zero at the end of the period.
The waveform is defined under the TTH Test Profile section of the Test configuration window. It can be imported from a file or created from a standard signal type (sine, triangle, and so on). Then it can be modified by multiplication with a data window or re-scaled.
To create a new waveform, click in Import from File to locate a file to import the data points from. Or, click Generate Function to define it from a standard signal function, which brings up the Waveform Model window.
Waveform Type supported are: Sine, triangle, chirp, white noise, triangle burst, bellcore, sine beat, and door slam.
Chirp is a sine wave that sweeps continuously from the start frequency to the end frequency within one block.
The Triangle Bursts conform to commonly used standards for CD player testing in the American and European automotive markets.
Bellcore 1&2, 3, and 4 are the seismic time histories required by the Bellcore test specification. When using the Bellcore signals, the RRS (Required Response Spectrum) of the specification is available as a display to allow direct comparison with the control SRS.
Sine Beat is interference between two sine wave of slightly different frequencies, perceived as periodic variations whose rate is the difference between the two frequencies.
Door Slam includes two parameters, acceleration amplitude and decay rate. When the retractor is mounted on a test rig to simulate the vibration effect of vehicle door slamming, and impacted by a force sufficient to activate the vehicle sensor and cause lock up of the retractor, the webbing shall be extractable after one second.
Figure 1. Transient Waveform shown in Acceleration/Velocity/Displacement
Compensation can be applied to the waveform to change its velocity and displacement characteristics. It is important that the entire waveform has zero average displacement and velocity, otherwise the shaker limits would be exceeded after multiple waveform periods. Compensation can also be used to apply a high-pass or low-pass filter to the acceleration waveform.
There are 6 waveform compensation methods that can be used: Remove Acc. DC, Remove Vel. DC, High-Pass filter, Brick Wall High-Pass filter, Low-Pass filter, and rescale.
Figure 2. Compensation Methods
To use one of the methods, select it and press Insert. Any number of these items can be inserted. Change the item order by clicking and dragging in the items within the list.
Remove Acc. DC applies a DC offset to the acceleration waveform to make the average value zero.
Remove Vel. DC applies a DC offset to the Velocity to make the average velocity zero.
High Pass filter inserts a high-pass filter with a user-definable cutoff.
Brick Wall High Pass inserts a brick wall high-pass filter, which has a much sharper cutoff slope.
Low Pass filter inserts a low-pass filter with a user-definable cutoff
Re-Scale rescales the amplitude values of the waveform by a user specified factor.