Generating reference signals¶

A test is planned before it is run, and a response is predicted before there is a record to read it from. Both start from the signals the references will carry: random shaped to a specification, bursts sized to a capture, an impulse or a shock pulse, a sine sweep. Generate Signal makes them as time histories at the reference DOFs, in the quantity they are (a force, a voltage, an acceleration), beside the object they were made from.

What a signal is made from¶

Select one object and press Generate Signal on the bar. The pane beside the plot offers only the kinds that object makes. Rows picked in a channel table's grid also offer it, and the signal is made at the picked rows that are reference channels; a picked response row is left out.

Selected	Kinds	At
A specification or a PSD	random, burst random, periodic random	its DOFs, drawn to its levels and cross terms
A sine specification	sine	its control DOFs, following its tone
A shock specification	SRS synthesis	its DOFs, a sum of wavelets meeting its spectrum, checked against the shaker's shock ratings (Synthesizing a shock to its spectrum)
A channel table	random, burst random, periodic random, impulse, classical pulse	its reference channels, the random kinds to a flat band and level typed in the pane

A channel table's reference channels are the rows whose role is reference, and they have to say what they measure (the channel type), all the same quantity: a signal is one quantity. The level and the peak typed for a channel table are in each reference channel's own unit, as the table writes it: a level of 1.0 is 1 lbf²/Hz on a channel in lbf and 1 N²/Hz on a channel in N. A channel with no unit is read in SI. The pane shows the unit beside the box when the channels share one.

The kinds¶

Random is stationary and Gaussian, with the target's PSD at every DOF and, with Cross terms ticked, the coherence and phase its cross terms state. It is built a frame at a time in the frequency domain, each line at the target's level with a random phase, and the frames overlap by half under a half-cycle sine window, so the level is the same from the first sample to the last, there is no seam where frames meet, and the record never repeats. The frame sets the line spacing the target is drawn at; Auto picks lines no coarser than 1 Hz or a quarter of the lowest frequency written.

Burst random is the same signal gated on for a share of each frame (On) and quiet for the rest, with a cosine ramp at each end of the burst (Taper). Each frame holds one burst from its start, so a capture of the frame's length holds the burst and the response dying away. The level while a burst is on is the target's; averaged over the whole frame, the PSD is the target times the share on.

Periodic random repeats one random frame. Read with a rectangular window at the frame's length, every line is exactly the target's and nothing leaks between them. Realization draws a new frame every so many periods; All keeps one frame throughout (pseudo-random).

An impulse is a half-sine of a given Peak and Width, starting at Starts, the shape of the force a hammer tip makes. Its width sets the band it excites. The spectrum of a half-sine of width T is flat at low frequency, falls 3 dB by 0.59/T and 10 dB by 1.02/T, and reaches its first zero at 1.5/T, so a 1 ms impulse excites up to about 1 kHz and a 0.25 ms impulse up to about 4 kHz. The pane gives the three frequencies for the width typed, and the plot draws the impulse's spectrum under its trace, as the record holds it: the magnitude of its Fourier transform, in the quantity times seconds (N·s for a force), whose level at low frequency is the impulse's area, 2·peak·width/π. The width has to span at least two samples. A classical pulse is a half-sine, a terminal-peak sawtooth or a trapezoid, at its Peak and Duration, starting at Starts.

Sine follows a sine specification's tone: its frequency at the tone's rates, dwells included, and its amplitude and phase between breakpoints as the controller follows them.

An SRS synthesis is iterated whole, since a shorter one would meet a different spectrum, and off the window's thread once the settings rest; Synthesizing a shock to its spectrum says how it is made and read.

Read the way it was made¶

A generated signal carries the analysis settings it was made for, and its averaging opens on them. Random is read in frames of the length it was drawn at, under a Hann window, overlapped by half. Burst random is read in one rectangular frame per burst, and periodic random in one per period, with no overlap: 50 bursts are 50 averages, each leakage-free. An impulse or a pulse is the whole record in one rectangular frame. A sine has no frames of its own and opens with the usual defaults.

Preview, then Generate¶

The plot previews the signal as its settings change: for the random kinds, the preview's PSD over the target; for an impulse, its trace and its spectrum; for the others, its trace. The preview is a short stretch of the signal, up to 131,072 samples per DOF, so that it answers while a number is being typed. The rows under the settings give the samples the full signal holds, its line spacing and the preview's rms.

A setting the signal cannot have is said in the pane, not raised: a band at or above the Nyquist frequency names the rate it needs, and a target no set of signals can have (coherences that cannot all hold together) names the first line where it fails.

Generate makes the full signal as a time history in the source's group. Its settings are kept with it, and the random seed with them: Seed set to Drawn picks one on Generate and keeps it, so the same record can be made again. When the source changes (a level, a tone, a reference channel), the signal is marked as computed from something that has since changed, and Recompute makes it again from the same settings and seed.

From a script¶

import visualdynamics

project = visualdynamics.Project.open('test.vdyn')
drives = project.generate_signal('Random Specification', 'random',
                                 duration=60.0, sample_rate=5120.0,
                                 seed=1)
tap = project.generate_signal('Channel Table', 'impulse',
                              duration=2.0, sample_rate=10240.0,
                              at=0.1, peak=500.0, width=0.0005)

The settings are the fields of core.signals.SignalSettings, and the generators behind each kind (random_history, burst_random_history, periodic_random_history, impulse_history, sine_history) can be called on their own, with flat_spectrum for a target typed in. impulse_rolloff(width) gives an impulse's −3 dB, −10 dB and first-zero frequencies, and impulse_spectrum(history) its spectrum.