RDP Home Page Banner

Glossary of technical terms


This Glossary of Terms provides details of most of the technical terms used in this website. The definitions are in the context of the LVDTs and other transducers in our range of products and may not relate to other different types of sensors or usage of these terms. If you discover a term that is not covered here and you would like an explanation then please contact us and we would be happy to help.
Bandwidth

Bandwidth in the context of an amplifier is the band of frequency over which the amplifier can operate. A rapidly varying signal when applied to an amplifier needs to be amplified by the same amount irrespective of the frequency of that signal. In all RDP amplifiers or amplified transducers the lower frequency is 0Hz, the specification therefore details the maximum mechanical signal frequency that the amplifier can faithfully reproduce. Please note that the sensor itself may have a lower maximum frequency than the amplifier.

Calibration

Calibration is the process of verifying and/or setting the output of a transducer against known values. There are many different types of calibration, some use information from new transducer calibration certificates to configure amplifiers others require that a transducer is directly compared with a standard in order that the transducer can be accurately loaded, pressurised or moved. RDP are able to calibrate the majority of the LVDT, displacement, load and pressure transducers that we sell.
On an LVDT transducer the process involves moving the sensor by known amounts and configuring the amplifier or digital display to read the value that is being applied. The first and most important step is to find the centre of the LVDTs range. The output of an LVDT rises (either becoming more positive or more negative) from a central zero or null point. All RDP LVDTs have a mechanical travel that is bigger than their calibrated range. It is therefore important that the position of this null point is found and used as the datum point for the calibration otherwise there is a risk that the transducer will be used in a non-linear region beyond the calibrated range of the transducer and into the 'over travel' zone. To find this point the amplifier must be zeroed, that is without any input the amplifier must be adjusted to give zero output. When the transducer is connected the transducer can then be moved until the output of the amplifier is again zero thus finding the electrical zero point of the transducer. From this point the transducer should be accurately moved to inward and outward full scale and the amplifier adjusted to give the required output at these positions.

Centre of range

The centre of range is the datum centre of the calibrated range of an LVDT or DC LVDT and is a nominal value. LVDT transducers are inherently centre zero but with built-in electronics this can be changed using the amplifier to give for example 4-20mA or 0-10V output. With amplified transducers RDP have taken care of the correct calibration so this value is only a guide for installation but if you are calibrating an unamplified LVDT with an amplifier it is essential that the centre of the measurement range is found and used as a datum for calibration. For more information see the 'calibration' entry in this glossary.

Compensated Temperature Range

The Compensated Temperature Range of a sensor is the range of temperature over which the transducer meets its specification. Outside this range (but within the Operating Temperature Range) the sensor can be used but may not meet its published specification.

Fatigue Life

Fatigue life is primarily a factor in some load cell applications. At their heart our load cells are basically a mechanical structure. The deflection of this structure is measured on RDP products using strain gauges. The fatigue life is a measure of the load cell´s ability to withstand this repeated bending and assumes that the frequency of the load cycles is low enough that there is no excessive heat build-up in the load cell.

Hysteresis

Hysteresis is the measure of the residual signal that is left when a sensor, initially at zero, has full scale applied and removed. There is no repeatability error on an LVDT.

Linearity Error

The linearity of a transducer is the measure of the actual output of the transducer compared with a straight line. For RDP LVDT transducers we use the Best Fit Straight Line (or least squares) method to express linearity as this produces the best real world measure of the performance of the LVDT sensor. Linearity error is expressed as % FS (percentage of full scale). For example a ±5mm LVDT with a measured linearity of ±0.1% FS would have a maximum deviation of 10mm (the full scale) multiplied by 0.1% which gives ±0.01mm from the best fit straight line.
For LVDT transducers the standard linearity error is the worst linearity error on any of the transducers in the range (this is usually but not always ±0.5% FS). On some models and ranges there may be improved linearity grades available as an option. For example "±0.5/±0.25/±0.1" in the "Linearity error" column indicates that for that range ±0.25% FS and ±0.1% FS are available as options in addition to the standard ±0.5% FS.
For an amplifier or digital display the linearity error is the error introduced by the amplifier to a perfectly linear input.

Load Cell

A Load Cell is a sensor that measure load or force and converts it into an electrical signal. There are typically 3 types; Compression, tension/compression and tension load cells.
A compression load cell is typically a cylindrical device with a raised load button on top. The load cell is placed onto a flat, hard surface and then gives on output proportional to the load applied to the load button.
A tension/compression (or universal) load cell usually has the load applied to threads. This means that the force can be applies towards the cell (so compression) or away from the cell (so tension). The output of a universal load cell usually goes positive or negative to indicate whether the applied force is tension or compression
On a tension load cell the load is usually applied in tension to threads on the top and bottom of the load cell.

LVDT Transducer

LVDT stands for Linear Variable Differential Transformer. An LVDT is a device that is used to accurately measure position or extension and convert this into an electrical signal that can be read by other devices. Because an LVDT works by a magnetic transfer they are very reliable, and robust and can be designed to work in wet or even submersible environments and to work at high temperature and radiation. They are typically used in a factory to measure the size of manufactured components.

Maximum Frequency

The Maximum frequency of a sensor is the maximum mechanical sinusoidal input that the sensor can withstand. Above this the sensor may incorrectly record the values or be damaged or both.
For an unguided or captive guided LVDT transducer this is usually limed to 20% of the excitation frequency or the bandwidth of the LVDT amplifier (whichever is lower). On longer range transducers the limit may be the strength of the mechanical joints in the armature components.
For spring return transducers the limit is the strength of the spring with respect to the mass of the moving armature assembly and typically for a ±5mm LVDT the armature will begin to lose contact on the transition between inward and outward movement (where the spring is decelerating the moving mass) at around 10Hz. For longer range transducers this will be lower.

Operating Temperature Range

The Operating Temperature Range of a sensor is the band of temperatures to which the sensor can be exposed (either in use or in storage) without damaging the sensor. See also Compensated Temperature range in this glossary.

Over-range Capacity

The Over-range Capacity is primarily a specification for load cells and pressure transducers. The specified value is the amount, as a percentage of full-scale that the sensor will tolerate above its full scale value. For example a 100N load cell with a 50% Over-Range Capacity can withstand a static force of 150N. Above full-scale load changes must be slow and definitely must not have any transient (high speed or shock) components. Many of our load cells have a very fast response so even very short transient loads above full-scale have the potential to cause damage.

Over-Travel

All RDP LVDTs have a mechanical dead stop at at least one end of their mechanical range. Therefor it is necessary to have an over-travel region to allow the full calibrated range of the transducer to be used and allow some contingency for manufacturing tolerances or tolerances in the application.
The inward-over-travel is the nominal distance the armature moves beyond inward full-scale and the mechanical end-stop.
The outward-over-travel is the nominal distance the armature moves beyond outward full-scale and the mechanical end-stop although with unguided types the outward over travel is infinite.
Force applied to the armature at either the inward or outward end-stop positions may cause damage to the sensor.
The output of the transducer will continue to change in the over-travel region but it will not be within the calibrated range of the sensor and so the linearity error of the measurements will be increased.

Pressure Transducer

A pressure transducer measures the pressure of a gas or liquid, usually this is applied to the threaded pressure port on the transducer (so the transducer is screwed into something and measures the pressure inside). There are three main types; gauge, absolute and differential.
A gauge pressure transducer measures the pressure in the pressure port relative to the ambient pressure.
An absolute pressure transducer measures the pressure in the pressure port relative to a vacuum, so if the pressure port is vented to atmosphere it will measure the ambient pressure. Barometric pressure transducers are absolute pressure transducers that give an output over the normal range of atmospheric pressure change.
A differential pressure transducer gives an output proportional to the difference between the pressures applied to its two pressure ports.

Range

The range of a transducer it the difference between the highest and lowest value that it can measure. For a unidirectional transducer (such as a gauge pressure transducer) the range is the maximum allowable pressure (for example 500psi).
For a bidirectional transducer (such as a tension/compression load cell or an LVDT) the range is the difference between the negative full scale and the positive full scale. So for a ±10mm LVDT, the range Is 20mm (-10 to +10).

Why are some LVDT ranges +/-


LVDTs are inherently centre zero devices; the output is zero at the centre of the stroke and rises in one direction and falls in the other. For this reason historically they have their range expressed as ±.

Repeatability

The repeatability of a sensor is the ability of the sensor to give the same output each time the sensor is presented with the same physical condition. The repeatability of an unguided LVDT is absolute; there is no error due to repeatability. The addition of bearings to an LVDT creates a repeatability error of about ±0.003mm.

Resolution

The resolution of a transducer is a measure of the smallest physical change that the transducer capable of detecting. The resolution of our LVDT Displacement Transducers, Load Cells and Pressure Transducers is infinite, so they will detect even the smallest changes.

Ripple

The output ripple is the small oscillation on the output of an amplifier or LVDT with a built-in amplifier. On both strain gauge and LVDT amplifiers there is an element of power supply noise in the output of the amplifier.
On an internal or external LVDT amplifier there is also breakthrough of the oscillator which is used to make the LVDT work. The frequency of the oscillator ripple is usually 10 kHz or more and so is invisible to the majority monitoring units and usually well above the maximum frequency at which the LVDT sensor can operate.

Sample Frequency

Sample Frequency and Sample Rate are in this context related to the frequency or rate at which a signal is sampled by an Analogue to Digital Converter or Data Logger.
All RDP LVDT transducers, Load Cells and Pressure transducers are analogue; that is to say that there is no digitisation process in the signal conversion so the bandwidth of the output signal is limited by either the mechanical frequency response of the sensor or the bandwidth of the (internal) amplifier, whichever is lower.
Consider as an example a spring in series with a load cell and exercised by a hydraulic cylinder such that the load cell measures the force applied to compress the spring.
If the cylinder is driven with, for example a 10Hz sinusoidal oscillation then the output of the Load Cell would also be a 10Hz sinusoidal signal proportional to load. If you wanted to sample this signal with a logger then you would need to determine the sample rate required to accurately sample the analogue signals. If you sampled at 10Hz then you would not capture anything useful as each sample could always coincide with the point at which the load was zero so your data would read zero. If you were to sample at 1MHz then you would capture every point accurately but you would have a very large amount of data. If you were to sample at 1kHz then you would be sampling every cycle of the load 100 times. Depending on your accuracy requirement you may feel that this would give a reasonable compromise between accuracy (making sure that you capture even the rapidly changing components of the sine wave) and quantity of data.
Continuing this logic therefore, you may decide that the digital sample frequency needs to be 100 times greater than the maximum analogue frequency of the signal. We´ve only considered sinusoidal movement here, for square or triangular movement this may need to be higher but if you don´t need high accuracy then it could be lower.
The important point here is that sample frequency and analogue bandwidth are not the same thing; you can sample an analogue signal as often as you like but you need to weigh up the merits of a higher sample rate (so a lot of similar data) versus accurately capturing the important points of the analogue signal.

Sensitivity

The sensitivity of a transducer is a measure of the output change from the sensor for a given physical change.
In the case of an LVDT the LVDT sensor is moved using precision calibration equipment over its full range and the output of the LVDT is measured. The result is that the sensitivity (expressed in output change per mm or similar) is measured and defined on the calibration certificate of the transducer. On an unamplified LVDT it is possible that the sensitivity may be slightly different if used with a different amplifier so the LVDT should be re-calibrated with the amplifier it will be used with. On a DC LVDT the output is less affected by the amplifier it is connected to. All RDP LVDTs are supplied with a calibration slip showing the sensitivity and linearity of the LVDT transducer.

Temperature Coefficient

The temperature coefficient of a transducer is a measure of the change in the output of the sensor which is caused by a change in temperature and is express in percentage change per unit change in temperature. Temperature coefficient can be broadly split into two components; gain (span) drift and zero drift.
Gain, span or sensitivity drift is the change in the sensitivity of a transducer which is caused by temperature change and is largely caused by changes in the resistance of the electrical conductors that are used in the sensor as well as thermal expansion of the components.
Zero drift is caused almost exclusively by the expansion or contraction of the (mainly metal) components that make up the sensor. For most of our LVDTs this is not specified simply because it varies so much depending on sensor design, mounting and time but mainly because it is normally significantly swamped by the expansion and contraction of the metal structure of the user´s installation.