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# Vibration Glossary

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Technical information for Reliability tests

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### Vibration Glossary

1. 1. Vibration Glossary Acceleration A vector quantity that specifies the rate of change of velocity. It is expressed in m/s² or gravitational units (gn). 1gn = 9.80665 m/s². Note: m/s² should be written as m s-² Accelerometer A transducer (sensor) whose output is proportional to acceleration. The output is commonly measured in pC/g (pico Coulombs per gravitational unit). A/D Converter. A device that changes an analogue voltage or current into a digital signal Amplitude Defines the magnitude of a changing level. Its value can be a peak, rms or average value. When referring to displacement it can be given as a peak or peak to peak value, which is referenced to a mean or datum point. Angular Frequency Also known as circular frequency to describe the torsional vibration frequency in radians per second. Anti-resonance Anti-resonance exists in a system when, with a constant excitation, any change in the frequency causes a decrease in the response of the system. Armature structural resonance (electrodynamic vibrators): Armature structural resonance is the resonant frequency at which the acceleration measured at the centre of the table becomes 90° out of phase with the exciting force. The armature current is used as a reference for the exciting force, as the exciting force is always proportional to the armature current. ASD. Acceleration spectral density describes the average power seen in a random profile. Also see PSD. Bandwidth. For a vibration test profile the bandwidth is a fixed range between the lowest and highest required frequency in Hz. Broad band In vibration terms would be considered to be a random noise whose bandwidth is greater than 500Hz. 2010 Page 1 of 7 LDS Test and Measurement Ltd
2. 2. Vibration Glossary Calibration As applied to accelerometers is a method of determining its charge sensitivity, normally by comparing it to a standard accelerometer whose calibrated value is traceable back to an international standard. The use of a fopal wedge is only a method of verification as it relies on the accuracy of the user and the test equipment used. Closed Loop Control. A way of controlling a non linear system by using constant feedback to the controller from a measurement point. Coherence. A measure of the similarity of vibration at two locations. Compliance The reciprocal of stiffness, i.e. displacement divided by force. Compression In Sine, the maximum rate of the drive correction is called the compression rate (or compression speed) and is usually expressed as decibels per second (dB/sec). Cross axial motion A motion which is not in the direction of the exciting motion. This is usually caused by test items and fixtures which are offset. Crest factor The ratio of the peak to rms value of a waveform. In the case of a sine wave this ratio is √ 2, thus to change from an rms value to a peak value multiply by √ 2 (1.414213562). However in a true random waveform the crest factor is infinity as theoretically its level can, at some time, be infinity. For the purpose of vibration testing a value of 3 is generally used since this will cover 99.73% of the conditions using Gaussian distribution. Critica Damping The minimum viscous damping that will allow a system to return to its original position without oscillation Damping (material) Is a substance or material used to dissipate vibration energy. Damping (factor) Is a means of expressing the effectiveness of a damping method. Damping - Under damped Is where little damping is applied (less then 0.1) and the system continues to ring for a prolonged time period. 2010 Page 2 of 7 LDS Test and Measurement Ltd
3. 3. Vibration Glossary Damping – Critically damped Is where a system will return to the equilibrium position in the shortest possible time and has a damping ratio of 1. Damping – Over damped Is where a system has a large damping ratio preventing it from returning to an equilibrium position in a reasonable time. Decade A decade is the interval between two frequencies having a ratio of 10:1. i.e. 10 100 1000 10000 are decades, but also are 25 250 2500. Number of decades = log frequency (upper ) frequency ( lower ) decibel (dB) Is a logarithmic relationship of two values. Although originally intended for units of power it is commonly used for linear units such as voltage and gn etc. N ( dB) power = 10 log 10 P2 P1 N ( dB)linear = 20 log 10 V2 V1 Degrees of freedom In mechanical engineering defines the number of directions that an object is free to move. For example if an object is only able to move up and down it would be described as having a single degree of freedom. Displacement Displacement is a vector quantity that specifies the change of position of a body or point. It can be described as a peak or peak to peak value. Distortion Distortion is an undesired change in waveform produced by non-linearity in a system. DSA Dynamic signal analyzer used for real time analysis of noise and vibration signals. Typically displaying frequency, phase and time domain dimensions. Duration of a shock / bump / transient pulse is the period of time to go from zero to maximum to zero acceleration, usually defined in milliseconds (ms). Dynamic Range. Dynamic Range is the ratio of the maximum level of a parameter to the minimum detectable value – normally measured in dB 2010 Page 3 of 7 LDS Test and Measurement Ltd
4. 4. Vibration Glossary EU Engineering units define any measurement with a recognisable calibrated unit. For example mm or inches for Displacement, m/s or inches/s for Velocity and gn or m/s² for Acceleration. Excitation An external force applied to a system that causes a response from the system FEA Finite element analysis is a computer-aided design technique for predicting dynamic behaviour. FFT The Fast Fourier Transform is a method for calculating the Discrete FT, extremely efficiently, to analyze a time domain waveform in the frequency domain. Filter A filter generally refers to a means of allowing a frequency range to pass, by attenuating the unwanted parts of a signal. A filter has specific characteristics which should be referred to when used for different vibration types. Filter Types Low-Pass, attenuates high frequencies High-Pass, attenuates low frequencies Band-Pass, attenuates both high and low frequencies FMEA Failure mode and effects analysis procedure to analyze and classify the severity of a potential system failure. Force A force of one Newton is the force required to accelerate one kilogram at one meter per second per second. Force can also be expressed in pound force (lbf) or kilogram force (kgf) and can be a peak or rms (root mean square) value. i.e. lbf rms. Force (system) Defines the maximum force a vibration system can provide. It is normally derived from the product of the maximum permitted acceleration (rms or peak) and the armature mass. Force (required) Is determined by multiplying the total moving mass (test item, armature, fixture, etc.) by the acceleration required. Frequency response. The usable frequency range of a device can be determined by measuring the manner in which the device responds to an applied frequency spectrum. 2010 Page 4 of 7 LDS Test and Measurement Ltd
5. 5. Vibration Glossary G gn A term used to describe the level of acceleration. The expression "vibrate at a level of 5g" should be expressed as "vibrate at a level of 5gn peak", meaning the maximum value will be 5 gravitational units or in other words 49.0325 m s-2 peak. Gravitational unit. The internationally agreed constant 1gn = 9.80665 m/s-² .or in Imperial terms 386.088527 in/sec² = 32.174 ft/s². HALT Highly accelerated life test. A way of stressing a new product, during development, to vibration levels that are much higher than real field measurements. Used to identify weaknesses and potentially improve the reliability of a product. Harmonic Component frequencies of a fundamental signal that are equally spaced and periodic. A harmonic is a component whose frequency is an integral multiple of the fundamental frequency. For example, the second harmonic of 100Hz is 200Hz. Impulse The integral of force over a time interval. Jerk Is a differential of acceleration. i.e. the rate of change of acceleration with time. Mass Mass is a property of an object which is related to gravity. For example if an object has a mass of 1kg, its weight on earth would be 1kg, but on the moon its weight would be 0.165kg but its mass is still 1kg. Modal Analysis A way of analyzing a system to find the natural frequencies and damping parameters. A model of the system will provide the systems modal shapes over the frequency analysis range. Octave An octave is the interval between any two frequencies differing in magnitude by a factor of two (20 to 40 Hz, 500 to 1000Hz, etc.). fu log fl Number of octaves = log 2 Oscillation Oscillation is the variation, usually with time of the magnitude of a quantity with respect to a specified reference, when the magnitude is alternatively greater and smaller than the reference. 2010 Page 5 of 7 LDS Test and Measurement Ltd
6. 6. Vibration Glossary Periodic (e.g. sinusoidal motion): Periodic implies that it is repetitive and therefore its state can be predicted. Piezoelectric (PE) Accelerometer A transducer that generates a linear output charge, from its crystal element, during vibration. Piezoresistive (PR) Accelerometer A transducer whose strain gage elements electrical resistance changes in proportion to applied mechanical strain. All PR transducers are passive and require an external power supply or signal conditioner. Power spectral density PSD describes the power of a random vibration intensity. It is expressed as a root mean square value of an acceleration with a given band width. The terms commonly used are g²/Hz (m/s²)²/Hz g/√Hz. These terms can also be written as grms²/Hz m²/s³ or m² s-3 . Random A varying level, whose level is unpredictable. Resonance Resonance exists in a system when, with a constant excitation, any change in the frequency of excitation causes an increase in the response of the system. Shock A rapid change of state from a known condition. Simple harmonic motion The simplest form of vibratory motion, where the amplitude of motion varies sinusiodally with time. Stiffness Stiffness is the ratio of change of force (or torque) to the corresponding change in translational (or rotational) deflection of an elastic element. That is the measure of resistance to an applied force such as bending, compressing or stretching. Sub-harmonic A sub-harmonic is a component whose frequency is an integral sub-multiple of the fundamental frequency. For example, the second sub-harmonic of 100Hz is 50Hz. Transmissibility A ratio of a systems amplitude response to an amplitude input excitation 2010 Page 6 of 7 LDS Test and Measurement Ltd
7. 7. Vibration Glossary Tracking filter In Sine, a narrow band-pass filter that is tuned to the centre frequency of the operating test frequency to track the applied signal. Uncertainty of measurement This is a doubt that exists about the result of any measurement. It can be calculated from a chain of system errors such as amplitude, frequency and linearity. Velocity Velocity is the rate of change of displacement, in other words speed. It is expressed in meters per second or millimeters per second (m/s or mm/s), but should be written as m s-1. Vibration A body, or point, is said to vibrate when it executes an oscillatory motion about a position of equilibrium. 2010 Page 7 of 7 LDS Test and Measurement Ltd