Data Acquisition Glossary

September 30th, 2005

 

This glossary courtesy of Windmill Software.

A

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Alternating Current (ac)

Electric current whose flow alternates in direction. The number of times the current changes direction in one second is called the frequency. The usual waveform of ac is sinusoidal.

Alumel

Trade name for an alloy of nickel with up to 5% aluminium, manganese and silicon, used with chromel in K-type thermocouples.

Ampere (A)

SI unit of electric current.

Amplifier

A circuit that produces a larger output power, voltage or current than was applied at its input.

Amplitude

The size or magnitude of a signal.

Analogue-to-Digital (A-D) Converter

Converts an analogue signal (such as a voltage signal from a temperature sensor) into a digital signal suitable for input to a computer.

Anti-Alias Filter

An anti-alias (or anti-aliasing) filter allows through the lower frequency components of a signal but stops higher frequencies, in either the signal or noise, from introducing distortion. Anti-alias filters are specified according to the sampling rate of the system and there must be one filter per input signal.

Argument

Input parameter to a program.

ASCII

American Standard Code for Information Interchange. Coding for text files.

B

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Backbone

The major multi-channel link in a network, from which smaller links branch.

Background noise

Extraneous signals that might be confused with the required measurement.

Batch process

Any process on which operations are carried out on a limited number of articles, as opposed to continuous process.

Bathymetry

Measurement of the depths of features at the bottom of the sea, especially by echo-sounding.

C

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Capacitance

The ability to store an electrical charge, or, more precisely, the ratio of the total charge on a capacitor to its potential. The unit is the Farad.

CE

Conformite Europeene. A mark that is affixed to a product to designate that it is in full compliance with all applicable European Union legal requirements.

Chromel

An alloy of nickel with about 10% chromium, used with Alumel in K-type thermocouples.

Cold Junction

The reference junction of a thermocouple which is kept at a constant temperature.

COM port

A connection on a computer into which a serial device may be plugged.

Common-Mode Signal

A signal applied simultaneously to both inputs of a differential amplifier.

Common-Mode Rejection Ratio (cmrr)

The ability of the differential amplifier to obtain the difference between the + and - inputs whilst rejecting the signal common to both.

Contact emf

Electromotive force which arises at the contact of dissimilar metals at the same temperature, or the same metal at different temperatures.

Contact Rating

Refers to the power that can be safely switched with a relay. Quoted for non-reactive load, that is without capacitance or inductance.

Constantan

An alloy of 40% nickel and 60% copper, with a high volume resistivity and almost negligible temperature coefficient. Used with copper in T-type thermocouples.

Continuous Process

Method of producing an article continuously.

CMOS

Complimentary metal-oxide semiconductor.

Current

Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise pick-up. Before A-D conversion the current signals are usually turned into voltage signals by a current-sensing resistor.

D

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Data Acquisition

The automatic collection of data from sensors, instruments and devices: in a factory, laboratory or in the field.

Data Logging

Making measurements and recording readings against time.

DCE

DCE stands for Data Circuit-Terminating Equipment or Data Communications Equipment. It is part of the RS232 standard and represents, for example, an instrument or modem attached to your PC.

Device

A peripheral which connects to the computer. Mice, keyboards, printers, data acquisition instruments, modules and cards are all devices.

Differential Amplifier

One whose output is proportional to the difference between two inputs.

Differential Inputs

Using differential inputs can reduce noise picked up by the signal leads. For each input signal there are two signal wires. A third connector allows the signals to be referenced to ground. The measurement is the difference in voltage between the two wires: any voltage common to both wires is removed.

Digital-to-Analogue (D-A) Converter

Used to produce analogue output signals. These may be control signals or synthesised waveforms.

Direct Current (dc)

Current which flows in one direction.

Drift

Slow variation of a performance characteristic such as gain, frequency or power output. May be due to, for instance, temperature or ageing. Usually only significant when measuring low-level signals (a few millivolts) over long periods of time, or in difficult environmental conditions.

Driver

A program that controls a device. Each device has its own set of commands that its driver understands, and can translate for other software like Windmill’s Logger and Chart.

Dynamic Data Exchange (DDE)

A standard Microsoft Windows protocol that defines a way for Windows applications to share information with one another.

DTE

DTE stands for Data Terminal Equipment. It is part of the RS232 standard and represents, for example, the PC.

Dynamic Data Exchange (DDE)

A standard Microsoft Windows protocol that defines a way for Windows applications to share information with one another.

E

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Electromotive Force (emf)

Difference of potential produced by sources of electrical energy which can be used to drive currents through external circuits. Unit is the volt.

Endurance limit

In fatigue testing, the number of cycles which may be withstood
without failure at a particular level of stress.

EIA

Electronic Industries Association.

Ethernet

A local area network to which you can connect data acquisition
devices.

E-Type Thermocouple

Chromel-constantan thermocouple with a temperature range of 0 to 800 ^oC.

Excitation

The voltage or current applied to a transducer.

F

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Fast Fourier Transfer (FFT)

An analysis algorithm - given a finite set of data points, the FFT expresses the data in terms of its component frequencies.

FIFO buffer

A first in, first out, store. The first value placed in the buffer
(queue) is the first value subsequently read.

Filtering

Attenuates components of a signal that are undesired: reduces
noise errors in a signal.

Frequency

Measured in hertz (cycles per second), rate of repetition of changes.

Frequency Counter

Counts digital pulses over a defined gate time. A typical gate
time is between 0.1 and 10 seconds.

Front panel

The front surface of a unit, generally containing switches and
indicator lights.

Full Scale Output

The difference between the minimum output (normally zero) of a data acquisition device and the rated capacity.

G

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Gain

Amplification of a circuit.

GIS

Geographic information system. Where data is assembled, stored, displayed and identified according to its location.

GPIB

General Purpose Interface Bus. Also known as IEEE-488 bus. The GPIB standard was designed to connect several instruments to computers for data acquisition and control. Data can be transferred over GPIB at 200 000 bytes per second, over distances of 2 metres.

Ground

See earth.

Ground-Truthed

When data is collected by remote sensing techniques, ground-truthing confirms that the information is accurate. Ground-truthing is collecting data by non-remote sensing means.

H

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HART

Highway Addressable Remote Terminal. Provides digital communication to microprocessor-based (smart) analogue process control instruments.

Hertz (Hz)

Cycles per second unit of frequency.

Hexadecimal

Counting system based on 16.

Human machine interface (HMI)

Also known as man machine interface. The communication between the computer system and the people who use it.

I

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I

Symbol for electric current.

I/O

Input/Output. A data acquisition system monitors signals through its inputs, and sends control signals through its outputs.

IC

Integrated Circuit (electronic components fabricated on a semiconductor substrate which cannot be divided without losing its function).

IEEE

Institute of Electrical and Electronics Engineers. An American Society that, amongst other things, establishes international standards in the computing, electronic and telecommunications
fields.

IEEE-488 Bus

See GPIB.

IML

Interface Management Language. A programming language used to communicate with measurement instruments.

Inductance

The magnitude of a magnetic field created by a circuit carrying a current. This can cause higher voltages in the circuit.

Input

Data entering a device from the environment. A signal being monitored by a data acquisition system.

Instrument

Any item of electrical or electronic equipment which is designed to carry out a specific function or set of functions. For example an electronic balance, a gas analyser or a chromatograph.

Integrated Circuit

Electronic components fabricated on a semiconductor substrate. An integrated circuit cannot be divided without losing its function.

Integer

A positive or negative whole number, or 0.

Interface

A shared boundary. It might be a piece of hardware used between two pieces of equipment, or a software display communicating between the computer system and the people who use it.

Interpreter

Software enabling a computer to run programs statement by statement.

Interrupt

An external signal causing the execution of a program to be suspended.

Integration Time

The time over which an integrating A-D converter averages the input signal. If chosen appropriately will average over a complete mains cycle thereby helping to reduce mains frequency interference.

Inverter

A dc-ac converter.

ISA

Industry Standard Architecture. An ISA expansion slot lets you plug data acquisition boards into PCs.

ISO

International Organization for Standardization, which is made up of national members. A member is the "most representative of standardisation in its country". For example BSI (British
Standards Institute), DIN (Deutsches Institut für Normung) and ANSI (American National Standards Institute).

Isolation

Two circuits are isolated when there is no direct electrical connection between them.

Isolation to Earth or System

A high transient voltage at one input may damage not only the input circuit, but the rest of the data acquisition hardware, and, by propagating through the signal conditioning and A-D circuits,
eventually damage the computer system as well. You can prevent this type of damage by isolating the input from the earth of the data acquisition and computer hardware.

Isolation Between Inputs

A transient at an input can also propagate to other equipment connected to that input. This is prevented by providing isolation between inputs.

J

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J-Type Thermocouple

Iron-constantan thermocouple with a temperature range of 0 to 750 ^oC.

K

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k

A symbol for a thousand, from kilo.

K

A unit of stored data. 1K = 2¹⁰ = 1024. Also stands for a degree on the Kelvin temperature scale.

K-Type Thermocouple

Chromel-Alumel thermocouple with a temperature range of -200 to 1200 ^oC.

L

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LAN

Local area network. A data communication system connecting devices in the same vicinity. Data is transferred without the use of public communications. Examples of LANs are Ethernet,
token ring and Modbus.

LIFO

Last in first out. Describes a stack method of data storage.

LIMS

Laboratory information management system.

Linearity

Ideally an A-D or D-A converter converts the input or output range into equal steps. In practice the steps are not exactly equal. Linearity, or non-linearity, is a measure of how close the steps
approach equality.

M

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m

Symbol for metre and milli- (thousandth).

M

Symbol for mega (million).

Mains

Source of electrical power, normally the electricity supply system.

Mains Frequency

Electricity ac supply frequency; 50 Hz in UK, 60 Hz in US.

MES

Manufacturing Execution System.

Metre

The SI fundamental unit of length, equal to 1.093 yards. (Meter in US.)

MIS

Manufacturing Information System.

MMI

Man Machine Interface. Also known as human machine interface. The communication between the computer system and the people who use it.

Modular

Form of construction in which hardware or software units, often with differing functions, are quickly interchangeable.

Multiplexing

Where each signal is switched in turn to a single analogue-to-digital converter. As opposed to where one A-D converter is used for each signal in simultaneous sampling.

Modbus

An industrial networking system that uses peer-to-peer communications.

N

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N-Type Thermocouple

Nicrosil-Nisil thermocouple with a temperature range of -200 to 1200 ^oC.

Network

The physical interconnection of devices sharing a communications protocol.

Node

A device with a direct point of access to a network.

Noise

Any unwanted electrical signals contaminating the signal to be measured. This noise may be electronic noise which is an artefact of semiconductor construction techniques and is not reducible.
Alternatively the noise may be caused by environmental factors. This type of noise can be the result of poor positioning or screening of signal wiring. This may result in mains frequency or RF pickup contaminating the required signal.

Non-Conforming

A product outside manufacturing limits but not necessarily defective.

Non-Destructive Testing

Method of inspecting materials and products without affecting their subsequent properties and performance. Abbreviation NDT.

NTSC

The US National Television System Committee and their standard colour television system. Modified or 4.43 NTSC has the colour subscarrier at the European frequency of 4.43 MHz instead of 3.58 MHz.

Nyquist Theorem

The Nyquist theorem demands that a signal be sampled at at least twice its maximum frequency. To get an accurate picture of a waveform however, a sampling rate of 10 - 20 times the highest frequency is better.

O

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OEM

Original Equipment Manufacturer. A company which makes basic computer hardware for other manufacturers to build into their products.

Offset Error

If you get a reading other than zero for a zero condition then you have an offset error: every reading will be inaccurate by this amount.

Ohm

SI unit of resistance, such that one ampere through it produces a potential difference of one voltage.

On-Off Control

A simple control system which is either on or off.

Output

Information leaving a device.

P

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PAL

Phase Alternation Line
The colour television coding system generally used for European broadcasting.

PC

Personal computer. Generally applied to computers conforming to the IBM designed architecture.

PCI

Peripheral Component Interconnect
A local bus standard developed by in 1992. PCI cards plug into your computer and are configured through software. They do not have jumpers or switches.

PCMCIA

Personal Computer Memory Card International Association
Industry group that developed the specification for credit card-sized plug-in cards for laptop computers.

Peer-to-Peer Communication

A communication between networked devices in which any device
can initiate data transfer.

PID

Proportional gain, integral action time and derivative action
time. PID software, for example, compares an analogue input
value with a set point and if there’s a discrepancy outputs an
appropriate analogue or digital control value, according the PID
calculations.

PI&D

Piping and instrumentation diagram.

PLC

Programmable Logic Controller.

Pole

A relay contact.

Port

The external connector on a device.

Positive Temperature Coefficient

An increase in resistance due to an increase in temperature.

Protocol

A set of rules used in data communications.

Pulse

A temporary change in voltage of any length.

Q

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QA

Quality assurance.

R

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Ramp Voltage

A steadily rising voltage.

Range

The maximum and minimum allowable full-scale signal (input or
output).

Relay

Electromechanical device that opens or closes contacts when a
current is passed through a coil.

Reed Relay

Consists of two thin magnetic strips (reeds). When a coil close
to the reeds is energised, they are magnetised and drawn
together making a connection between leads attached to the reeds.

Resistance Temperature Device (RTD)

Resistance temperature devices (or detectors) rely on the principle that the resistance of a metal increases with temperature. When made of platinum, they may be known as platinum resistance thermometers (PRTs).

Resolution

A measure of the smallest change that can be detected.

Repeatability

The ability of an instrument to give the same reading under repeated identical conditions.

rms

Root mean square. The square root of the sum of the squares of a
set of quantities divided by the total number of quantities.
Used when monitoring ac (alternating current) signals. Many
power supplies, for example, issue an ac signal. This needs to
be converted to a dc (direct current) signal for the PC
interface. The solution is a signal conditioning input that
produces a dc signal proportional to the rms of the amplitude of
the input signal. The rms operation means the reading will
always be positive.

RS232

An EIA (Electronic Industries Association) standard that defines
a protocol for serial data communications. An RS232 link will
run at up 38400 baud (bits per second) over short distances, and
at lower speeds as the distance increases. You can plug the
RS232 lead directly into the computer’s serial (COM) port.

RS485

Another EIA protocol for serial communications. Allows several
devices to be connected to a single cable, distributed over a
wide area.

S

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Sample and Hold

A component of a type of analogue-to-digital converter. The analogue signal is frozen in a sample and hold circuit to prevent it changing during digitisation.

Scan

Normal channel scanning in a data acquisition system involves stepping round and reading each input channel in turn. The scan will return to the first channel once all the channels have been sampled.

SCADA

Supervisory control and data acquisition - a large scale software package usually used to monitor and control a manufacturing process.

Seebeck Effect

The principle that describes how a thermocouple works. In a circuit in which there are junctions between dissimilar metals, an electromotive force (voltage) is set up when the junctions are at different temperatures.

Self-Calibrating

A data acquisition card or module with a stable on-board reference voltage that software can use for automatic recalibration.

Sensitivity

A measure of the minimum change in an input signal that an instrument can detect.

Sensor

A device that can detect a change in a physical quantity (light or pressure for example) and produce a corresponding electrical signal.

Serial Communication

Where data is transferred one bit at a time.

Settling Time

When a change in signal occurs, the time taken for the input or output channel to settle to its new value.

Set Point

Value of a controlled variable, departure from which causes a controller to operate to reduce the error and restore the intended steady state.

SI

International system of units. Abbreviation for Systeme International (d’Unites).

Signal Conditioning

Makes a signal suitable for input to an analogue-to-digital converter. For example, a signal may be filtered to remove noise, or amplified to meet the range of the A-D converter.

Signal

General term referring to a conveyor of information.

Single-Ended Input

An analogue input that is measured with respect to a common earth. Single ended inputs are only suitable for signals that are of good size - 100 mV full scale or above.

Simultaneous Sampling

When all analogue signals are read simultaneously. This is achieved by providing each input with its own A-D converter, and initiating sampling from a single clock. It ensures that there is no reduction in sampling rate when more signals are connected.

Sine Wave

Waveform of a single frequency, indefinitely repeated in time. In practice there must be a transient at the start and finish of such a wave.

Slew Rate

The maximum rate of change of an output signal.

Slave

A networked device that is controlled by another, master, device.

Software Trigger

A programmed event, such as a specific key press or mouse click, that triggers data capture.

Spike

Short pulse of voltage or current - usually undesirable.

Square Wave

Wave that alternates between two fixed values. Has very rapid (theoretically zero) rise and fall times.

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Stability

The ability of an instrument or sensor to maintain a constant output when a constant input is applied.

Strain

When a material is distorted by forces acting on it, it is said to be strained. Strain is the ratio of change in dimension to original dimension.

Strain Gauge

A sensor that experiences a change in resistance when it is stretched or strained. It is attached to the body subjected to the strain.

Successive Approximation

A technique used in A-D converters to measure an analogue signal. It compares the signal with progressively smaller values, each step getting nearer the actual voltage.

Surge

A large, momentary, increase in the voltage on a power line.

System

Combination of several pieces of equipment to perform in a particular manner.

T

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Talker

A device on the GPIB (general purpose interface bus) that sends information to a Listener on the bus.

Thermal

Coefficient of Resistance The change in resistance of a semiconductor per unit change in temperature, over a specific range of temperature.

Thermal Conductivity

A measure of the rate of flow of thermal energy through a material in the presence of a temperature gradient. Materials with high electrical conductivities tend to have high thermal conductivities.

Thermocouple

Popular temperature sensor because of its low cost, wide operating range and ruggedness. Consists of two dissimilar metals joined together, making a continuous loop. When one junction has a different temperature from the other an electromotive force (voltage) occurs. There are several types of thermocouples, constructed from different metals and with differing temperature ranges and accuracies.

Thermistor

A temperature sensor. The name comes from thermal resistor. It is a semiconductor that exhibits a large change in resistance as a function of temperature. Most thermistors exhibit a negative temperature coefficient, where resistance decreases as temperature rises. These are referred to as NTC thermistors.

Throughput

Number of results produced per unit time.

Time stamp

Information added to data to indicate the time at which it was collected.

Transducer

A device which converts a physical quantity into an electrical signal. Examples include thermocouples and photocells. Most sensors are also transducers.

Transient

A short surge of current or voltage, often occurring before steady-state conditions have become established.

Transient

A short surge of voltage or current, often occurring before steady-state conditions have become established.

Truncation

Rejection of the final digits in a number, thus lessening the precision but not necessarily the accuracy.

TTL

Abbreviation for transistor-transistor logic. Referring to logic circuits consisting of two or more directly interconnected transistors, to provide conditional switching capability.

TTL-Compatible

For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 40 microA, and a logic 0 for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 microA; and a logic 0 by 0 to 0.6 V with a current sink capability of at least 16 mA.

T-Type Thermocouple

Copper-constantan thermocouple with a temperature range of -200 to 400 ^oC.

Twisted Pair

Cable that consists of individual wires wrapped around each other for carrying telephone or computer data. Reduces pickup noise levels in signals.

U

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Unipolar

A signal range that is always positive or always negative, for example 0 to +10 V.

UPS

Uninterruptible Power Supply. Used to keep critical equipment, including computers, running in the event of a mains power failure.

USB

Universal Serial Bus A serial bus gradually replacing RS232 on PCs because of its higher speed. Generally fitted as standard in new PCs.

V

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VAC

AC Voltage

Velocity

The rate of change of displacement; dx/dt.

Volt

SI unit of potential difference such that the potential difference across a conductor is 1 volt when 1 ampere of current in it dissipates 1 watt of power. Named after Count Alessandro Volta (1745-1827). Symbol V.

Voltage

The value of an electromotive force or potential difference expressed in volts.

Voltage-to-Frequency Converter

A device that converts an analogue input voltage into a sequence of digital pulses with a frequency that is proportional to the input voltage.

W

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WAN

Wide area network. A network of circuits spanning a large region which is used to transmit data.

Wheatstone Bridge

A network of four resistances, an emf (voltage) source, and an indicator connected such that when the four resistances are matched, the indicator will show a zero deflection or "null" reading. Prototype of most other bridge circuits.

Wi-Fi

Wireless Fidelity. A wireless communications network conforming to IEEE 802.11 specifications.

X

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x-axis

Conventionally, the horizontal axis of any type of graph.

Y

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y-axis

Conventionally, the axis perpendicular to and in the horizontal plane through the x-axis of any type of graph.

Z

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z-axis

Conventionally, the vertical axis in any three-dimensional co-ordinate system.

Zero Balance

With transducers like strain gauges, the output is large compared to the changes caused by the strain. Setting a zero balance subtracts an offset (actually a fraction of the bridge excitation voltage), so the changes caused by the strain can be accurately measured.

Entry Filed under: Data Acquisition