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Windmill - Data Acquisition and Control Glossary
Category: Technical and IT > Data Acquisition
Date & country: 18/09/2007,
Words: 222

Absolute Accuracy
How closely the measured value approaches the real value. For more on accuracy in temperature measurements, see Issue 50 of our Monitor Newsletter.

ADC - Analogue-to-Digital Converter
Converts an analogue signal (such as a voltage signal from a temperature sensor) into a digital signal suitable for input to a computer. See Issues 3 and 4 of our newsletter for more information on A-D converters.

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.

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.

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

The size or magnitude of a signal.

Analogue Input
An infinitely variable signal going into a data acquisition device. This is generally a voltage signal. Thermocouples, strain gauge bridge circuits and gas concentration probes, for example, all produce an analogue voltage. Alternatively the signal may be a milliamp current. In this case the data acquisition hardware will convert the current to a voltage before accepting it.

Analogue Output
A waveform or control signal generated by the data acquisition and control equipment. See Issue 36 of our newsletter for more information.

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. See Issues 3 and 4 of our newsletter for more information on A-D converters.

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. See Issue 8 of our newsletter for more information.

Input parameter to a program.

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

B-Type Thermocouple
Platinum-rhodium thermocouple with a temperature range of 600 to >1700 oC.

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.

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

Calibration compares a data acquisition device's performance to an accuracy standard, and adjusts the performance as necessary.

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.

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.

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

Complimentary metal-oxide semiconductor.

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 Rejection Ratio (cmrr)
The ability of the differential amplifier to obtain the difference between the + and - inputs whilst rejecting the signal common to both. For more information see Issue 11 of Monitor.

Common-Mode Signal
A signal applied simultaneously to both inputs of a differential amplifier.

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.

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.

Continuous Process
Method of producing an article continuously.

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. The unit is the Ampere (A).

Current Sink
The maximum current that data acquisition output channels can dissipate.

Current Source
The maximum current that can be supplied by the data acquisition device for output signals.

DAC - Digital-to-Analogue Converter
Used to produce analogue output signals. These may be control signals or synthesised waveforms.

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 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.

A logrithmic measure of the ratio between two quantities. Symbol dB.

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. For more information see Issue 11 of our Monitor Newsletter.

Digital Input
A digital signal going into a data acquisition device.

Digital Output
A digital signal generated by the data acquisition and control equipment. A digital signal has only 2 states. Software controls each digital output by just one bit - setting the digital line high or low. For more information see Issue 71 of our Monitor Newsletter.

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.

Diesel particulate matter, or defects per million.

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.

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.

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.

Dynamic Range
The ratio of the largest to the smallest signal that can be measured at one time. Normally expressed in Decibels (dB). The maximum signal is generally the analogue-to-digital converter's full scale signal. The dynamic range of a DAQ device is important when both large and small signals are to be measured. Dynamic Range (dB) = 20 x log (Max Voltage / Min Voltage).

E-Type Thermocouple
Chromel-constantan thermocouple with a temperature range of 0 to 800 oC.

Electronic Industries Association.

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.

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

The voltage or current applied to a transducer.

Unit of capacitance.

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.

Attenuates components of a signal that are undesired: reduces noise errors in a signal. More details on filters are in Issue 8 of Monitor.

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.

Amplification of a circuit.

Gain Range
The maximum and minimum voltage that will be digitised by the A-D converter is sometimes called the gain range.

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

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.

See earth.

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.

Hardware Trigger
A trigger is something that causes a data acquisition system to start collecting data. When not generated by software it is known as a hardware or external trigger. For more information see Issue 45 of the Monitor newsletter.

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

Hertz (Hz)
Cycles per second unit of frequency.

Counting system based on 16.

High Pass Filter
When a low-level transducer signal is superimposed on a large dc output voltage, a high-pass filter might be useful. This attenuates (removes) low frequencies. Using a cut-off frequency of, say, 4 Hz, will eliminate the dc voltage which has a frequency of zero. A high-pass filter will remove 'drift'. This can be a particular problem with biological and chemical signals, but not usually with modern electronic signals. See Issue 8 of our newsletter for more information.

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

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

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

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

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

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

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

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.

A positive or negative whole number, or 0.

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

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.

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.

Software enabling a computer to run programs statement by statement.

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

A dc-ac converter.

Infrared Data Association. A Nonprofit organisation whose goal is to develop globally adopted specifications for infrared wireless communication.

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

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).

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

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.

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.

J-Type Thermocouple
Iron-constantan thermocouple with a temperature range of 0 to 750 oC.