Dr. ABDULLAH KHAN DURRANI
DATA ACQUISITION for Instrumentation and control
Introduction
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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.
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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.
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Elements of a data acquisition system
Sensors and transducers Field wiring Signal conditioning Data acquisition hardware PC (operating system) Data acquisition software
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Basic elements
Sensors and transducers
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Data Acquisition and Processing
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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.
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Give the names of Transducers
• ?
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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.
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Signal conditioning
• Filtering• Amplification• Linearization• Isolation• Excitation
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• 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.
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• 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.
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• 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.
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• 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
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• Excitation• Signal conditioning products also provide
excitation for some transducers. For example:
• strain gauges, thermistors and RTDs, require external voltage or current excitation signals.
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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
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Hardware /Links with Computer• Ports for data acquisitionRS232IEEE-488 (GPIB (General Purpose
Interface Bus)Printer portSound Card portsSpecially designed BUS CardsDAQ cards
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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.
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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)
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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.
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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.
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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,
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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
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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,
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Analog DC signals
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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.
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Analog Signal
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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.
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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.
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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)
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Thermocouples
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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).
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Low pass filters
• Low pass filters pass low frequency components of the signal and filter out high frequency components above a specific high frequency.
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Signals Data after Filtering
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Signal circuit isolation
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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
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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).
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Programmable interrupt controller(s)
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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.
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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.
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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
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Communication I/O devices
• Serial Port
• Parallel Port
• PCI Bus
• EISA Bus
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Computer Interfacing
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Plug in Data Acquisition board