• What are the explanations for the various sine resonance dwell types?

Answer: The resonance dwell feature in the sine application allows the user to specify (via a user entered table) a set of frequencies and levels that the output will follow. The output drive excitation is monitored via closed loop control to ensure that the level is constantly within the user specified parameters. The level may be specified in terms of acceleration, velocity or displacement. As an additional feature, the drive output can be made to either dwell at a fixed frequency (FIXED option) or allow the frequency to vary in order to maintain a constant drive phase angle relative to a selected reference signal (frequency tracking options). There are four options available when using the frequency tracking based dwell. The PHASE option allows the user to manually specify the starting phase angle (as usually extracted from a previous test run). The AUTO option forces the initial startup of the sine dwell to read the current phase angle and then track the drive frequency based on this initial phase angle measurement. Since test to test variations can slightly modify the initial phase angle, this setup option is most often preferred over the manual specification. The CONTINUE option allows the user to effectively utilize multiple dwell levels while performing a frequency tracked test. The CONTINUE option transitions to each new level without shutting down the drive excitation. An additional option "CONTINUE W/RESET" forces the drive output to shutdown between level changes. Note that both of these "continue" options utilize tracked frequency w/AUTO phase detection only. Since these tests continue at the previous test line's ending frequency, no specification of a starting frequency is allowed. All minimum and maximum dwell limit parameters are also carried over from the prior test setup line.

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• What are the digitizing options (i.e. sampling types) available in RMA?

Answer: There are 2 types of digitizing methods utilized by the RMA package: fixed and tracking.  The choice of which method to utilize depends on the input characteristics and available hardware options.

FIXED SAMPLING Fixed sampling refers to the classical method of digitizing analog data streams by using a low pass filter at a specific bandwidth followed by a fixed sample rate converter. The user typically specifies the desired bandwidth and the software selects the proper low pass filter and sample rate to match. This method forces all of the data to be sampled at the basic digitizing rate irrespective of the operating conditions (e.g. machine rpm). The results of this processing is a digitized time history using a constant delta T (time) increment. This procedure is most often utilized for cases where the test object's primary motion/operation is stationary or slowly varying (relative to the amount of time per data frame). When the rpm change (rpm slew) becomes more significant during the acquired data frame, then the spectral results (including order tracks, etc.) become more "smeared". Increasing the number of spectral lines, to decrease the spectral resolution (i.e. a "finer" or smaller delta frequency) often only increases the problem as the increased resolution also increases the data frame time. The data frame time can be calculated via several methods:

Data_Frame_Time => 1.0 / Delta_Freq => Num_Lines / Bandwidth => Frame_size / Sample_Rate

TRACKING (SYNCHRONOUS) SAMPLING Synchronous or tracking sampling requires that the sample rate (and hence the low pass filter cutoffs) be continuously adjusted with respect to input rpm changes. As the input rpm changes, so does the sample rate and low pass filter cutoff change to maintain a synchronous relationship between the data samples and fundamental rpm values. In this case, the sampling time increment is continuously modified from data point to data point. Instead of using a delta time increment, the data domain is often referred to the angle or order domain. In this domain, true order related events are completely synchronous to the reference rpm and do not display the typical smearing observed by fixed sample rate processing. The ability to accurately observe order related events often requires that the sample rate be constantly adjustable to maintain the proper digitizing perspective. For tracking sampling, since the delta time increment is constantly changing, the data frame time is also constantly changing. The user typically specifies the desired maximum order and the software selects the proper tracking ratios to be utilized during the sample rate/filter tracking process. Tracking sampling is also useful for fixed speed cases where accuracy of order extraction is important since there is no "smearing" of the order data due to slight fundamental speed variations or drift. Of particular importance is the fact that both the sample rate and associated low pass filter cutoffs are continuously modified thus maintaining alias-free data at all times. Tracking sampling can only be utilized when the SD tacho hardware is available. The quality of the tracking sampling is tied to the quality of the input tacho signal and the maximum order required.

(See additional FAQ about tracking sampling performance)

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