Technical Note
Control System Dynamic Range
The Advantages Offered by the High Performance of SD2550 Shaker Control Systems
Introduction
Choosing a digital Shaker Control
system to operate with either an electrodynamic or hydraulic shaker,
requires careful consideration of a number of issues. In addition
to cost, the two most important factors for a customer to evaluate
are safety and performance. Obviously, a system
that does not have adequate built-in safety characteristics should
not be considered, due to the expense of test equipment, product
being tested, and hazards to personnel.
Key factors from a performance standpoint
include:
- Dynamic range
- Equalization speed
- Control accuracy
- Testing/control features
The first, dynamic range, may prove
to be the most important. The structural dynamics of a shaker
fixture and test article often present a challenging control problem.
Due to the wide dynamic range, that is, the amplitude ratio between
the amplitude of the highest resonance and the lowest anti-resonance,
accurate control requires a Shaker Control system with superior
performance. Failure to control the test structure's full dynamic
range not only results in out-of-compliance tests but it also
causes overtest conditions or even test article damage.
Figure 1 illustrates the severity of
the problem. The dynamic range for this shaker fixture exceeds
88 dB. A simple box design, this fixture holds printed circuit
boards for vibration stress testing.
Figure 1: Spectrum for shaker box-fixture
to hold circuit cards
Designed for Superior Performance
Spectral Dynamics has applied its many
years of vibration testing experience to vigorously address the
requirements for usable dynamic range. As a result, SD2550 Vibration
Control Systems make clever use of dedicated hardware at key points
in the control loop, including a fully programmable output subsystem.
The output subsystem design utilizes a 16-bit Digital-to-Analog
Converter (DAC) coupled with digital smoothing filters and a 24-bit
amplitude attenuator. Smoothing filters insure signal purity
and minimize harmonic distortion. The 24-bit attenuator allows
the SD2550 to adjust the full-scale output voltage range in steps
as fine as 0.1 dB. Fine output adjustment capability provides
accurate control at both full and low test excitations levels.
Also, autoranging of both input and
output attenuators maximizes the usefulness of available signal
strength and optimizes processing of these signals in the control
loop. Automatic input ranging protects against overloading the
input subsystem and consequential clipping and distortion of the
control accelerometer signals. Autoranging also sets the full-scale
voltage range to maximize the accuracy of the 16-bit ADCs (Analog
to Digital Converters) and retain the full input dynamic range.
Simultaneously, autoranging and scaling of the output DAC provides
maximum resolution of the drive signal and assures that no unwanted
output signal clipping will take place.
The result is an incredible control
dynamic range, up to 90 dB, provided as a standard feature. Naturally,
shaker system noise, charge amplifier settings, and several other
factors may work against this. However, the more dynamic range
the system can generate, the more chance there is of successfully
completing a critical vibration testing program.
A Performance Proof Test
Testing with a stable, active dual peak-notch
filter, placed as a test article in the feedback loop of the control
system, demonstrates the tremendous range of Spectral Dynamic's
control systems. The filter design models a mechanical system
with two resonance-anti-resonance pairs. Figure 2 shows the filter's
frequency response. This filter exhibits a maximum positive gain
of over 9 dB, maximum attenuation of almost 82 dB, and a total
dynamic range over 90 dB. The highest filter peak occurs at a
frequency of 177 Hz and the lowest notch occurs at a frequency
of 998 Hz.
Figure 2: Frequency response function
of active peak-notch filter used in feedback loop
Closed-loop random tests with the peak-notch
filter used a flat reference from 20-2,000 Hz,
with 800 lines of control resolution
and 160 degrees of freedom. Total level for the reference is 2
grms.
Figure 3 shows the connection of the
peak-notch filter in the feed-back loop. The control system drive
output connects to the filter input and to input channel 2 for
measurement as an auxiliary input. Response from the filter feeds
back to control system channel 1 as the accelerometer control
signal. Conducted tests used an input channel sensitivity of
10 mV/g.
Figure 3: Closed-loop random control
test setup
Figure 4 shows a sample of the actual
control spectrum achieved with this test setup. The tolerance
lines superimposed on the display represent ±2 dB, about
the standard spectrum shape. Note that the control achieved was
typically within ±1.5 dB. This is further emphasized by
looking at the error spectrum shown in Figure 5. Once again,
±2 dB power tolerances are superimposed on the error spectrum
indicating excellent control even in the area of the most severe
dynamic response.
Figure 4: Actual control spectrum
with 90 dB of dynamics in the control loop
Figure 5: Error spectrum of control
with + 2 dB tolerances
Perhaps the most telling "proof
of performance" in this type of test is to observe the control
system's drive spectrum. This tells how hard the control system
is working in order to achieve the desired feedback control.
It also reveals if the drive clips or has, in some other way,
run out of dynamic range. Figure 6 superimposes the drive spectrum
as reported by the control system and measured directly as an
auxiliary input channel. Note the good agreement between the
drive spectrum plots. As Figure 6 indicates, the control system
produces an output voltage swing of 90 dB in order to compensate
for the peak-notch filter dynamics.
Figure 6: Drive spectrum as reported
by control system and measured as an auxiliary input channel
Conclusion
Your digital shaker control system can
never have too much dynamic range. Even though not every test
article demands 90 dB of dynamic range, a reserve of dynamic range
assures that you can run those difficult tests every day - not
just on good days. Note also that the test case shown here is
difficult or impossible to control with fewer than 800 lines of
control resolution. Spectral Dynamics' state-of-the-art vibration
control systems, offer up to 3200 lines and 90 dB dynamic range
for controlling even the most difficult shaker fixtures and test
articles.
|
