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


K-Type Thermocouple
Chromel-Alumel thermocouple with a temperature range of -200 to 1200 oC.

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.

Load Cell
A transducer which converts a force into an electrical signal. It normally comprises four strain gauges in a wheatstone bridge arrangement.

Low Pass Filter
This lets through the lower frequencies and attenuates the higher frequencies. Choose the cut-off frequency to be compatible with the unwanted frequencies, the frequencies present in the signal you are measuring, and the sampling rate of the analogue-to-digital converter. See Issue 8 of our newsletter for more information.

LVDT
Linear Variable Differential Transformer. Used in measuring devices that convert changes in physical position to an electrical output.

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.

Modbus
An industrial networking system that uses peer-to-peer communications. More details on Modbus are in Issue 32 of Monitor.

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.

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.

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.

Output Sink Current
See Current Sink.

Output Source Current
See Current Source.

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.

PI&D
Piping and instrumentation diagram.

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.

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.

QA
Quality assurance.

Ramp Voltage
A steadily rising voltage.

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

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.

Relative Accuracy
How accurately a change in signal is measured. See also Absolute Accuracy.

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

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

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). See Issue 5 of our newsletter for more information on RTDs.

Resolution
A measure of the smallest change that can be detected.

Response Time
The time a system takes to respond to a given input. For example: the time between software sending a message to an instrument and the instrument sending a reply, or the time a sensor takes to indicate a change in 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. More details are in Issue 32 of Monitor.

RTU
Remote Terminal Unit. A data acquisition device at a remote location which transmits data back to, and accepts commands from, a central PC (or other controller).

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. For more information on A-D converters see Issue 3 of Monitor.

Sampling Rate
The number of samples, or readings, per second of an analogue signal.

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

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.

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 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.

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.

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

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

Signal
General term referring to a conveyor of information.

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.

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.

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.

Sink Current
See Current Sink.

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

Slew Rate
The maximum rate of change of an output signal.

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

Source Current
See Current Source.

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.
___ ___


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. More details are in Monitor Issue 4.

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

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

TCP-IP
Transmission Control Protocol/Internet Protocol. Communications protocols used to connect hosts on the internet.

Text Format
Text format refers to information in the ASCII character set. It is unformatted. Each byte in the file contains one character that conforms to the standard ASCII code.

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.

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.

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. More details of thermocouples are in Issue 5 of our newsletter, Monitor.

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.