Condition Monitoring Systems

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LastH

In Dynamic Signal Analyzers, the LastH file contains the last system transfer function used by the test for the equalization of the drive signal.

Latent defect

A flaw (in a part or assembly) and/or workmanship that is dormant, not immediately apparent visually or by electrical test, yet can result in failure. See Patent Defect.

Lateral Location

The definition of various points along the shaft axis of rotation.

Lateral sensitivity

Sensitivity of a transducer in a direction perpendicular to the normal measurement axis. See also transverse sensitivity.

Lateral Vibration

See Radial Vibration.

Layout File

In SignalCalc Dynamic Signal Analyzers: A binary file within a Test or Run folder that defines all graphical parameters for a specific (user named) arrangement of display windows.

Layout Manager

In SignalCalc Dynamic Signal Analyzers: A control dialog affecting all (named) Layouts used within a Test or Run; facilitates creation, deletion, renaming and automatic usage.

Leakage

Smearing of frequency components caused during the process of computing the FFT from a digitized block of time data. Weighting or Windowing functions such as Hanning reduce or remove leakage).

LCD

Liquid Crystal Display

Leakage

An unfortunate result of sampling with finite intervals. Results in smearing of frequency components. Improved by windowing (e.g. Hanning). / In DSAs, a result of finite time record length that results in smearing of frequency components. Its effects are greatly reduced by the use of weighted window functions such as flat top and Hanning.

LED

Light emitting Diode.

Level

The (usually base 10) logarithm of the ratio between a quantity and a reference quantity. For acoustic measurements the reference quantity is 20 micro-pascals. For acceleration measurements, one reference is 1 micro-g.

The level of a signal is its amplitude, alternatively the amplitude expressed on a decibel scale relative to a reference value.

Life Cycle Testing

Subjecting products to stresses similar to those anticipated in actual service while collecting engineering data related to life expectancy, reliability, specification compliance, or product improvements. Usually aimed at determining the products’ mean time between failures or MTBF.

Life cycle history

A time history of events and conditions associated with an item of equipment from its release from manufacturing to its removal from service. The life cycle should include the various phases that an item will encounter it its life, such as: handling, shipping and storage prior to use; mission profiles while in use; phases between missions, such as standby time or storage, transfer to and from repair sites and alternate locations; and geographical locations of expected deployment.

Life units

A measure of use duration applicable to the item. Measures include time, cycles, distance, rounds fired, attempts to operate, etc.

Limiting

In Vibration Control: The ability to limit the Control spectrum based on a measurement signal approaching a Limit spectrum. Limiting is done on a spectral line basis.

Line Spacing

In an FFT spectrum, the frequency difference between two adjacent bin centers or lines.

Linear

Having or being a response or output that is directly proportional to the input.

Linear Averaging

See Time Averaging.

Linear system

A system is linear if its magnitude of response is directly proportional to its magnitude of excitation, for every part of the system.

Linearity

The closeness of a calibration curve to a specified straight line, preferably passing through zero. Commonly specified as a % of full scale. / The response characteristics of a linear system remain constant with input level. That is, if the response to input a is A, and the response to input b is B, then the response of a linear system to input (a + b) will be (A + B). An example of a non-linear system is one whose response is limited by mechanical stop, such as occurs when a bearing mount is loose. / The closeness to which a curve approximates a straight line or the deviation of an instrument’s response from a straight line.

Lines

Common term used to describe the filters of a DSA (e.g., 400 line analyzer).

Limit Control Spectrum

In Vibration Control, the ability to limit the Control spectrum based on a measurement signal approaching a Limit spectrum. Limiting is done on a spectral line basis.

LN2

Liquid nitrogen, often used for rapid cooling of environmental test chambers.

Logarithmic

A function the raises a number by an exponent to produce a given number. For example, the logarithm of 100 to the base 10 is 2.

Longitudinal Vibration

Longitudinal or Compressional or Axial Vibration is in the same direction as the transfer of energy. Transverse Vibration is perpendicular to that direction.

Loudness

The human ranking of an auditory sensation, usually in terms ranging from soft to loud, expressed in sones (not in decibels).

Low-Pass Filter

A filter whose transmission band extends from dc to an upper cutoff frequency.

A filter that passes signals with less than 3 dB attenuation up to its cut-off frequency, and attenuates the signal above that frequency. The attenuation slope is called the roll off, q.v. An anti-aliasing filter is an example of a low pass filter.