- What are Random's definition and testing Overall Grms Limits?
- How do I set the proper voltage range in Random?
- What is a monitor (or auxiliary or control) channel?
- Why can't the drive (or monitor or control) channel be used in the transfer function H(f) table?
- In the random vibration control application, what is the "Prestored Drive" option and how is it used?
- What do the different status panel colors indicate in Random?
- What does the limit parameter 'Sigma Clipping' refer to?
How do I set the proper voltage range in Random?
The Random application is always in Auto Range Mode meaning the voltage is continuously checking the current frame and monitoring the gain range for the subsequent frame. The software makes sure that the system is not continuously changing gain ranges, so the change in signal level has to be significant enough to warrant a change. The sequence starts off at full level (10V) and then analyzes each frame until the desired level is met. If the signal changes during the test (ramp up) then it automatically changes to the appropriate gain range.
An auxiliary channel is an measured (single) data channel which can produce averaged results in either a linear or exponential averaged format. The averaging parameters are set via the user interface. All "active" data channels can be considered as auxiliary channels. The control pseudo channel source is 1 or more auxiliary channels which are "combined" together (using spectral averaging and the control combination type selection). The results are continuously exponentially averaged to produce an estimate for the "control" channel. The averaging type is fixed as exponential and the properties are dynamically tailored to the control bandwidth and other parameters. Since the control channel may represent the (combined) results of several auxiliary channels, no phase information (relative to auxiliary or drive channel data) is available from the control channel. The monitor pseudo channel source uses the same auxiliary channels as the control channel (the combination is done the same as with the control channel). The averaging method of the monitor channel , however, matches the same averaging parameters as used with auxiliary channels. The monitor channel is provided to allow comparisons based on the same auxiliary averaging parameters, between the auxiliary channels used in control and other auxiliary channels. As with the control channel, no phase information (relative to auxiliary or drive channel data) is available from the monitor channel.
At present, the pseudo drive channel contains no true phase information because this is a synthesized data source. The actual output is performed by a randomized output sequence that continuously updates. Since this data source is not really "measured", then H(f) (or frequency response function) estimates (such as between the auxiliary and drive channels) would not reflect the true dynamic phase information. The user should "T" the drive signal into an auxiliary channel and then setup the H(f) table as appropriate should transfer functions with respect to the drive channel be required. Since the control and monitor channels are pseudo channels, no phase information is available for these channels since these channels are not "physically" measured, as mentioned in "What is a Monitor Channel" FAQ. Therefore, no H(f) definitions are meaningful using these pseudo channels. If the control (and hence the monitor channel) is based on a single channel, then the H(f) table can be setup to calculate the necessary transfer function using the appropriate channel selections.
In the random vibration control application, what is the "Prestored Drive" option and how is it used?
The random vibration control application utilizes dynamic measurements to constantly modify the spectral content of the shaker "drive" function. The time domain drive function is a constantly randomized time history which has the required spectral attributes to insure that the measured control points remain in spectral compliance. The generation of the spectral drive function begins with the low level equalization phase of the test and then continues until the end of the test. Due to the nature of the data acquisition process, the drive generation/modification process can take many (usually exponentially averaged) measurements to converge to a stable drive spectrum. The drive spectrum is constantly monitored and modified to ensure control compliance during the test. The intent of the "Prestored Drive" option is to start the test application with an already converged drive spectrum rather than forcing the control system to regenerate the spectrum. By starting with a prestored drive spectrum, the test can be allowed to start essentially at the transition to full level. This can significantly decrease the test startup times, especially for high resolution or low bandwidth test cases. Successful use of this feature generally is a function of the linearity of the shaker/amplifier system. The user has 2 options in determining and loading the source of the prestored drive spectrum. One simple method is to use the previous test's drive spectrum. This (first) option (PREV) assumes that the prior test was successful and that a drive spectrum is available. This spectrum option would not be available for the first test run. The drive spectrum is automatically stored/updated (locally) at the conclusion of each successfully completed test run. Should this option be selected, the first test run will execute without using the prestored drive feature. All SUBSEQUENT tests will invoke the prestored drive feature by utilizing the previous test's drive spectrum. The second option (YES/LOAD) is to select a drive spectrum from a previously stored test run's data file. This option allows an opportunity to have a drive spectrum loaded from a prior test run even if the current test is the first run. In either option case, the test specifications must be the same as the currently loaded run where the prestored drive is to be utilized. In other words, the number of lines, bandwidth and reference profile must be the same as that employed in the run from which the prestored drive spectrum was extracted. If the reference profile is modified, the system will reset any active prestored drive request to the PREV option. Finally, as a safety precaution, the user may elect to have the application request a verification at the start of any prestored drive enabled test run. This ensures that this option is not inadvertently left enabled when the user may have wanted to begin the test using the normal convergence process for drive spectrum generation.
The color of the status panels indicate if a channel is on/off in the random, sine on random, and random on random applications.
|Grey||Channel is inactive or not available.|
|Green||Channel is active.|
What does the limit parameter 'Sigma Clipping' refer to?
A random based time domain signal's magnitudes are generally characterized by probability distributions. These distributions typically define the probability fraction (i.e. amount of "time" or likelihood) that the signal amplitude occurs within a given amplitude range. In particular, random vibration contain peak magnitudes that occur according to a Gaussian distribution. This means that the signal amplitudes spend most of time near the low amplitude limits and, very infrequently, the time history may have much larger peaks. The frequency content of the random signal can be shaped to match the control system's needs, but the time history basically follows a Gaussian definitions. The generated time domain output can have a peak to RMS ratio of up to 6.5:1 . Since most output devices, as well as shaker amplifiers, are sensitive to peak values, the user may establish the maximum allowable amplitude excursions as a multiple of the signal's RMS level (sigma). This allows the user to tailor the peak to RMS ratio of the drive time history waveform.
Sigma clipping is used to prevent high magnitude voltage signals from being sent to the shaker system. It is used to protect either the shaker amplifier from an over voltage condition or possibly to limit the instantaneous acceleration seen on the test article. Sigma clipping limits the peak values of the drive signal by establishing a maximum level that is set by the user relative to the 1 sigma, or RMS level of the drive waveform.
Sigma clipping should be utilized with care since the effects of severe signal truncation may produce unintended consequences. Various internal digital processing windows and output smoothing filters generally help smooth the effects of sigma clipping, but extreme clipping limits can cause control problems in certain situations. Sigma clipping can produce non-linear effects when extreme limits are utilized (e.g. less than 3.0 or so). Extreme clipping effects are similar to increased noise or broadband non-correlated excitations (i.e. structure rattling). One will find that a sigma clip level in the 3 to 4 range usually produces good control and good peak to RMS ratios.
What are Random's definition and testing Overall Grms Limits?
Puma(X) has the capability to set Alarm and Abort levels on the overall Grms of the control spectrum. The appropriate program dialog is located under Setup/Limits/Safety Limits, under the Grms group of entries.
The levels may be defined as an absolute level or as a relative dB level with respect to the reference profile Grms. In general, either option would specify the maximum levels to be monitored as an Alarm and an Abort. The level specified is a floating point number greater than 0. However, one should note that there are basic limitations regarding Grms level accuracy that depend on control DOF, basic DUT and shaker stability as well as system noise levels and system gain stability. For example, a typical 120 DOF test on a wire, the control Grms value can be expected to be maintained within about a 0.25 dB variance (about 3%) of the reference profile Grms. Note also that this Grms calculation is bandlimited to the profile’s specified frequency range which typically is a subset of the acquisition range.
The actual variance can also be affected by reference profile definitions when large differences in spectral content are employed.
The dB level option specification can also be used to define a bipolar range that would establish both maximum as well as minimum level boundaries for the Alarm and Abort level checks. This Puma(X) feature is not available when using the absolute Grms level option. The bipolar option also needs to be activated via Safety tuning parameter not normally enabled for standard program operations. The standard default dB definitions are for maximum levels for alarm and abort.
You can verify the dB option using about a 0.25 dB alarm level to watch the variability of the Grms level during control (on a wire). Be aware that changes in test level can cause some damped oscillations of the Grms level immediately following the level change, but this is expected due to the averaging required during test operations.