Advanced Programming for 3D Applications CE00383-3

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Advanced Programming for 3DApplicationsCE00383-3
Data Acquisition Lecture 10
Bob Hobbs Staffordshire university
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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
• convert physical phenomena into electrical
  signals that the signal conditioning and/or data
  acquisition hardware can accept.

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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 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
• 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.

Examples- 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 convert temperature into an analog
  voltage signal
• 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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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.

Signal after filters
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How Computer Takes INPUT signals

• Interrupts
• 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
• 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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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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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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Serial Communication

• RS-232
• USB
• Synchronous and Asynchronous

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Serial Communication Hardware

• UART (Universal Asynchronous Receiver
  Transmitter) -- Translates data between parallel
  and serial forms
• Included in Tmote microcontroller
• RX, TX, and FIFO buffers
• Line driver -- Converts circuit level voltages
  to line voltages and vice versa
• USB controller

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Serial Communication Parameters

• Baud Rate
• Start Bit
• Data Bits 5 to 8
• Parity Error check (Even, Odd, none)
• Stop Bit(s)
• Flow Control (DTR/DSR, RTS/CTS, Xon/Xoff, none)
• Start and Stop bits not necessary for synchronous
  communication

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Serial Communication Example Parameters

• Baud rate 115200 bps
• 8 Data Bits
• No parity
• 1 Stop Bit
• No Flow Control

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Serial Communication Signals

• Transmitted Data (TxD) 
• Received Data (RxD)
• Ground  (GND)

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Serial CommunicationSignals

• Request To Send (RTS) 
• Asserted (set to 0) by sender to
• prepare receiver to receive data.
• Clear To Send (CTS) 
• Asserted by receiver to acknowledge RTS and allow
  transmission.
• Data Terminal Ready (DTR) 
• Asserted by device to indicate that it is ready
  to be connected. If the device is a modem, this
  may "wake up" the modem, bringing it out of a
  power saving mode.
• Data Set Ready (DSR) 
• Asserted by host to indicate an active
  connection.
• Data Carrier Detect (DCD) 
• Asserted by host when a connection has been
  established with remote equipment.

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Flow Control

• Flow control avoids overflow
• Can Eliminate the need for flow control by
• Regulating speeds
• Packet size smaller than buffers

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Signal Processing

• Data acquired can be in a variety of forms
• DA hardware has to process the signal
• Device driver manages conformed signal

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Analog Digital Signal

• Analog signal must be converted into Digital form
  (Discrete) before DSP techniques can be applied.
  The analog signal is basically denoted as xt or
  xat because it varied by time. The analog
  signal comes in form of sinusoid (sine or cosine
  wave).
• The Analog signal is digitized by using
  Integrated Electronic Circuit device called an
  Analog-to-Digital Converter (ADC). The output of
  ADC will be in the form of binary number that
  represents the analog signal such as electrical
  voltage.

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Analog Digital Signal

• The analog signal are always come with noise.
  Thus the noise filtering is needed before the
  signal goes to ADC. The filtering can be done by
  using DSP techniques.
• The special purpose microprocessors are designed
  to carry out application of DSP. It is named as
  Digital Signal Processors (DSPs) and used in real
  time application.

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Digital Signal Processing

• DSPs are programmable devices and capable of
  carrying out millions of instruction per second.
• It is vital to know how Digital Signal Processing
  work before we go to DSPs (The diagram of the
  process is shown in Figure 1 and 2).
• The signals and systems must come together. The
  systems are needed to operate the signals. For
  example, we need to use Thermometer to measure
  Temperature, Microphone to carry out analog
  signal (human voice) and convert it to electrical
  signal, Charge-Couple Device (CCD) used in in
  Camera or Digital Camera to convert image to
  picture and so on. In general, the system is
  characterized by the type of operation that it
  performs on the signal.

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Discrete Signals

• Figure 1 Digitized process of signal

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Figure 2 Complete Process of Digital Signal
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• From the diagram, it can be seen that ADC and DAC
  are 2 vital devices used in signal processing to
  convert the signal from analog to discrete
  (digital) and vice versa.
• ADC is basically consists of Sampler, Quantizer
  and Coder. All this elements are built up by CMOS
  Switched-Capacitor (for Sampling), Op-Amp (Signal
  Amplification) Comparator (Quantizer).

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• Quantization is the conversion of discrete-time
  continuous-valued to discrete-time discrete-
  valued (digital) signal. The difference of this
  is called Quantization Error.
• The coder in ADC will convert the output of the
  Quantizer to b-bit binary sequence that can be
  read by DSPs (Digital Signal Processors).
• The DAC, will perform a reverse operation of ADC
  in order to generate back analog signal.

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• DSP CAN BE APPLIED IN THE FOLLOWING FIELDS
• gt COMMUNICATION SYSTEMS
• (MOBILE PHONE, SATELLITE, RADAR, SONAR,
• INTERNET)
• gt ENTERTAINMENT ELECTRONICS
• (RADIO, TV, Hi-Fi, CD/VCD/DVD
  PLAYER,MP3),
• gt MULTIMEDIA
• (ACOUSTICS, IMAGE, SPEECH RECOGNIZATION
• VIDEO CODING, DIGITAL CAMERA),

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• gt MEDICAL INSTRUMENT
• (ECG Electrocardiogram provides
• information about the condition of
  patients heart),
• gt GEOPHYSICS (Seismology)
• (Apparatus used to measure earth
  movement)
• gt INSTRUMENTATION
• (ELECTRONIC TESTER Such as Digital
  Multimeter,
• Oscilloscope)
• gt IC TECHNOLOGY
• gt DATA COMPRESSION

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Signal and noise
Signals convey relevant information but what is
relevant can be very relative!
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Continuous and discrete signals
Continous signal xa(t)
Discrete signal (sequence) xn
T sampling period fs 1/T
sampling rate
xn xa(nT)
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Basic sequences
Unit step
Unit impulse
Sinusoidal
Exponential
Periodic
Random
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Examples of digital signals
Periodic . . . . Random
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Speech signals
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one more
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more examples
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more examples
Earthquake prediction???