lectute instrumentation and process control data acquisition

Dr. ABDULLAH KHAN DURRANI

DATA ACQUISITION for Instrumentation and control

Introduction


Definition

Data acquisition is the process by which

physical phenomena from the real world

are transformed into electrical signals that

are measured and converted into a digital

format for processing, analysis, and

storage by a computer.

data acquisition (DAQ) system is designed not

only to acquire data, but to act on it as well.


DAQ and Control

Controlis the process by which digital control signals from the system hardware are convened to a signal format for use by control devices such as actuators and relays. These devices then control a system or process. Where a system is referred to as a data acquisition system or DAQ system, it is possible that it includes control functions as well.


Elements of a data acquisition system

Sensors and transducers Field wiring Signal conditioning Data acquisition hardware PC (operating system) Data acquisition software


Basic elements

Sensors and transducers


Data Acquisition and Processing


Sensors and Transducers

• Transducers and sensors provide the actual interface between the real world and the data acquisition system by converting physical phenomena into electrical signals that the

• signal conditioning and/or data acquisition hardware can accept.


Give the names of Transducers

• ?


wiring and communications cabling

Field wiring represents the physical connectionfrom the transducers and sensors to thesignal conditioning hardware and/or data acquisition hardware. When the signal conditioning and/or data acquisition hardware is remotely located from the PC, then the field wiring provides the physical link between these hardware elements and the host computer.


Signal conditioning

• Filtering• Amplification• Linearization• Isolation• Excitation


• Filtering• In noisy environments, it is very difficult

for very small signals received from sensors

• such as thermocouples and strain gauges (in the order of mV), to survive without the

• sensor data being compromised.


• Amplification• Having filtered the required input signal, it must

be amplified to increase the resolution.• The maximum resolution is obtained by

amplifying the input signal so that the maximum• voltage swing of the input signal equals the input

range of the analog-to-digital converter• (ADC), contained within the data acquisition

hardware.


• Linearization• Many transducers, such as thermocouples,

display a non-linear relationship to the

• physical quantity they are required to measure. The method of linearizing these input

• signals varies between signal conditioning products.


• Isolation• Signal conditioning equipment can also be used to

provide isolation of transducer signals• from the computer where there is a possibility that

high voltage transients may occur• within the system being monitored, either due to

electrostatic discharge or electrical• failure. Isolation protects expensive computer

equipment


• Excitation• Signal conditioning products also provide

excitation for some transducers. For example:

• strain gauges, thermistors and RTDs, require external voltage or current excitation signals.


Functions of Acquisition hardware

1- The input, processing and conversion to digital format, using ADCs, of analogsignal data measured from a system or process – the data is then transferred toa computer for display, storage and analysis2- The input of digital signals,3- The processing, conversion to analog format,

using DACs,4- output of digital control signals


Hardware /Links with Computer• Ports for data acquisitionRS232IEEE-488 (GPIB (General Purpose

Interface Bus)Printer portSound Card portsSpecially designed BUS CardsDAQ cards


Software

application software can be a full screen

interactive panel, a dedicated input/output

control program, a data logger, a

communications handler, or a combination of

all of these.


Options for software

• Program the registers of the data acquisition hardware directly

• Utilize low-level driver software, usually provided with the hardware, to develop a software application for the specific tasks required

• Utilize off-the-shelf application software(third party packages such as LabVIEW and Labtech

Notebook provide a graphical interface for programming)


PC

• Depending on the particular application, the microprocessor speed, hard disk access

• time, disk capacity and the types of data transfer available, can all have an impact on the

• speed at which the computer is able to continuously acquire data.


Classification of Signals

The Output signal has a relationship with the physical phenomenon.For Example, value of e.m.f obtained from a thermocouple, has relationship with the temperature

Voltage or current output signal from transducers has some direct relationship with the physical phenomena they are designed to measure.


Digital signals/ binary signals

A digital, or binary, signal can have only two possible specified levels or states; an ‘on’ state, in which the signal is at its highest level, and an ‘off’ state, in which the signal is at its lowest level.

Exaples:- the output voltage signal of a transistor-to-transistor logic (TTL), Control devices, such as relays, and indicators such as LEDs,


Digital pulse trains

• a sequence of digital pulses• a digital pulse can have only two defined

levels or states.• For Example:- Output of level indicator, Control of speed and position of a stepper

motor


Analog signals

Analog signals contain information within the variation in the magnitude of the signal with respect to time.

information contained in the signal is dependent on whether the magnitude of the analog signal is varying slowly or quickly with respect to time.

For Example:-Temperature and Pressure measurement, control hardware like a valve actuator,


Analog DC signals


Analog Signals Conversion

• DAQ hardware would only be required to convert the signal level to a digital form for processing by the computer using an analog-to-digital converter (ADC). Low speed A/D boards would be capable of measuring this class of signal.


Analog Signal


Sensors and transducers• A transducer is a device that converts one form

of energy or physical quantity into another, in accordance with some defined relationship.

• In data acquisition systems, transducers sense physical phenomena and provide electrical signals that the system can accept. For example, thermocouples, resistive temperature detectors (RTDs), thermistors, and IC sensors convert temperature into an analog voltage signal, while flow transducers produce digital pulse trains whose frequency depends on the speed of flow.


Categories of Transducers

• Active transducers convert non-electrical energy into an electrical output signal. They do not require external excitation to operate. Thermocouples are an example of an active transducer.

• Passive transducers change an electrical network value, such as resistance, inductance or capacitance, according to changes in the physical quantity being measured. Strain gauges (resistive change to stress) and LVDTs (inductance change to displacement) are two examples of this.


Transducer characteristics

• Accuracy (how close a measurement is to the actual value)

• Sensitivity (change in the output signal from a transducer to a specified change in the input variable)

• Repeatability (close the repeated measurements)

• Range (and maximum measurable values of a process variable)


Thermocouples


Signal Conditioning• Filtering of signals• Cut-off frequency >This is the transition

frequency at which the filter takes effect. It may be the high-pass cut-off or the low-pass cut-off frequency and is usually defined as the frequency at which the normalized gain drops 3 dB below unity.

• Roll-off >This is the slope of the amplitude versus the frequency graph at the region of the cut-off frequency. This characteristic distinguishes an ideal filter from a practical (non-ideal) filter. The roll-off is usually measured on a logarithmic scale in units of decibels (dB).


Low pass filters

• Low pass filters pass low frequency components of the signal and filter out high frequency components above a specific high frequency.


Signals Data after Filtering


Signal circuit isolation


How Computer Takes INPUT signals

• Interrupts are the mechanism by which the CPU of a computer can attend to important events such as keystrokes or characters arriving at the COM port only when they occur. This allows the CPU to execute a program and only service such I/O devices as needed


Interrupts

• Hardware interruptsThese are generated electrically by I/O devices

that require attention from the CPU.• Software interruptsThere are 256 possible interrupt types that can

be generated by software.• Processor exceptionsExceptions are generated when an illegal

operation is performed in software (for example divide by zero).


Programmable interrupt controller(s)


Direct Memory Access (DMA)

• Microprocessor controls data transfers within the PC (using the IN(port) and OUT(port) instructions.

• In many I/O interfacing applications and certainly in data acquisition systems, it is often necessary to transfer data to or from an interface at data rates higher than those possible using simple programmed I/O loops.


DMA contd.

• Transferring screen information to the ‘video card adapter’ on board memory

• Transferring data from a remote I/O device (data acquisition board) to the PC’s memory

• Direct memory access (DMA) facilitates the maximum data transfer rate and microprocessor concurrence.


Computer Operations• Memory-read: data transfer from a memory

device to the CPU• Memory-write: data transfer from the CPU to

a memory device• I/O-read: data transfer from an I/O device to

the CPU• I/O-write: data transfer from the CPU to an

I/O device• DMA Write I/O: data transfer from a memory

device to an I/O device• DMA Read I/O: data transfer from an I/O

device to a memory device


Communication I/O devices

• Serial Port

• Parallel Port

• PCI Bus

• EISA Bus


Computer Interfacing


Plug in Data Acquisition board

lectute instrumentation and process control data acquisition

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