DATA ACQUISITION for Instrumentation and control

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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

• Control
• is 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 connection
• from the transducers and sensors to the
• signal 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 analog
• signal data measured from a system or
• process the data is then transferred to
• a computer for display, storage and
• analysis
• 2- 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 acquisition
• RS232
• IEEE-488 (GPIB (General Purpose Interface Bus)
• Printer port
• Sound Card ports
• Specially designed BUS Cards
• DAQ 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 gtThis 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 gtThis 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 interrupts
• These are generated electrically by I/O devices
  that require attention from the CPU.
• Software interrupts
• There are 256 possible interrupt types that can
  be generated by software.
• Processor exceptions
• Exceptions 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 PCs 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