Rotational Analysis with no Tachometer Data
The identification of cyclic events in rotational analysis is often key to identifying problems. It is good practice to use a suitable sensor for capturing rotational speed. However, this is…
titanium and steel gears and cogwheels
Analysis of Dynamic Engine Vibration – Angular Vibration in High Performance Engines
The analysis of dynamic engine vibration and the accurate measurement of angular vibration is a non-trivial task, as a more in-depth analysis of boundary conditions reveals. Tools for engine vibration…
Tire background texture
Using Source Contribution Analysis to analyze complex structural dynamics issues
Using Source Contribution Analysis (SCA) and Structural Animation (STA) in the DATS software for the analysis of complex structural dynamics This post uses Source Contribution Analysis (SCA) techniques and a…
Vehicle AC
Analysis of Airflow Instabilities in HVAC units
HVAC (Heat Ventilation Air Conditioning) units are devices used in various applications, such as cars, buildings, aircraft etc. They facilitate the transport and conditioning (heating, cooling, changing of humidity) of…
How Do I Upsample and Downsample My Data?
This post covers how to upsample and downsample data and the possible pitfalls of this process. Before we cover the technical details let us first explain what we mean by upsample…
The Effect of the Nyquist Theory on Rotational Order Analysis
Nyquist theory is generally understood, but this understanding usually relates to time sampling and the conversion to the frequency domain. Rotational order analysis and the effect of the Nyquist frequency…
What is Operating Deflection Shape (ODS) Analysis?
Operating Deflection Shape (ODS) analysis is a method used for visualisation of the vibration patterns of a machine or structure caused by unknown operating forces. This is different from the study of the…
A Different Way Of Averaging – Time Synchronous Averaging
Assuming one wants a frequency spectrum from an acquired time measurement, it is generally accepted that averaging of a signal in the time domain is not very useful due to…
Quantifying Signals – Peak, Peak-to-Peak, & RMS metrics
Anytime you measure something which is changing with time, there are multiple ways to quantify the signal. For the purpose of this discussion, we will be talking about how to describe the…
Where Does The Noise In A Signal Come From?
When one thinks of noise in a signal, it is generally associated with having been added in some way to the amplitude of a signal. This is not always the…
What is Synchronous (Angular) Sampling?
The term synchronous data is usually applied to vibration or acoustic data that is captured from an item of rotating equipment at regularly spaced angle intervals as distinct from regularly spaced time intervals. The rotating part could be an engine, a gear wheel, a drive shaft, a turbine rotor, a propeller, a turbocharger or any other type of rotary mechanical device. Typically these items are subjected to out-of-balance forces that cause them to vibrate at frequencies that are multiples of the fundamental (once per revolution) rotation speed frequency. (more…)
Understanding Filter Characteristics
Recently when discussing with an engineering student the characteristics of filters, it became clear that some confusion exists around this subject area. This note attempts to explain the differences between types of filter and the effects of the parameters of those filters. (more…)
Aliasing, Orders and Wagon Wheels
These days most people collecting engineering and scientific data digitally have heard of and know of the implications of the sample rate and the highest observable frequency in order to avoid aliasing. For those people who are perhaps unfamiliar with the phenomenon of aliasing then an Appendix is included below which illustrates the phenomenon.
In saying that most people are aware of the relationship concerning sample rate and aliasing this generally means they are aware of it when dealing with constant time step sampling where digital values are measured at equal increments of time. There is far less familiarity with the relevant relationship when dealing with orders, where an order is a multiple of the rotational rate of the shaft. For example second order is a rate that is exactly twice the current rotational speed of the shaft. What we are considering here then is the relationship between the rate at which we collect data from a rotating shaft and the highest order to avoid aliasing.
The relationship depends on how we do our sampling as we could sample at constant time steps (equi-time step sampling), or at equal angles spaced around the shaft (equi-angular or synchronous sampling). We will consider both of these but first let us recall the relationship for regular equi-time step sampling and the highest frequency permissible to avoid aliasing. This is often known as Shannons Theorem [Learn more about Claude E Shannon].
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Figure 4: Energy corrected spectrum
Amplitude And Energy Correction – A Brief Summary
Amplitude and energy correction has been and is a continuing point of confusion for many people calculating spectra from time domain signals using Fourier transform methods. The first thing to say, the information contained in data presented as amplitude and energy corrected spectra is equivalent. The only difference is the scaling of the numbers calculated.
Data Windows : What, why and when?
Before we discuss the use of data windows, we should first remind ourselves of three basic properties of the FFT (Fast Fourier Transform) process.
- First, energy information in signal must be preserved during transformation. That is, the energy measured on time signal must equal the energy measured on the frequency representation of that signal.
- Second, an FFT converts the signal representation between time and frequency domains. The time domain representation shows when something happens and the frequency domain representation shows how often something happens.
- And finally, an FFT assumes that the signal is repetitive and continuous.
High Pass Filtering And Tacho Signals
It is sometimes necessary to perform high pass filtering to eliminate low frequency signals. These may arise for instance from whole body vibrations when perhaps our interest is in higher frequency components from a substructure such as an engine or gearbox mounting. The vibration levels are speed sensitive and the usual scheme is to record a once per revolution ‘tacho’ signal with the vibration data. The tacho signal, which ideally is a nice regular pulse train, is processed to find rotational speed and hence to select which part of the vibration signal is to be frequency analyzed. The most common form of analysis is a waterfall type such as shown below.
