Data Acquisition and Data Logging

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Data Acquisition and Data Logging

• Some basics

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What is data acquisition and data logging?

• Data acquisition is simply the gathering of
  information about a system or process
• Information may be gathered for various reasons
• knowledge
• research and development
• systems or process monitoring
• systems or process control
• Data acquisition requires one or more transducers
  (sensors) to sense the system or process, and a
  signal measuring instrument

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What is data acquisition and data logging?

• Real Time Data Acquisition relates to data being
  acquired and used in the same time frame, such as
  when monitoring or controlling a system or
  process
• Data Logging is when acquired data is stored or
  logged for later use or analysis. Involves some
  form of memory.

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What is data acquisition and data logging?

• Data acquisition and data logging is not a
  complex or mysterious science as viewed by some
• Data acquisition in its simplest form could be to
  measure the length of a piece of string with a
  ruler
• Data logging in its simplest form could be to
  write that measured length down on a sheet of
  paper

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A data acquisition or data logging system

• A data acquisition or data logging system
  generally includes the following components
• sensors or transducers which provide the
  fundamental information of the parameters to be
  measured
• a device to convert the primary signal from the
  sensors into a form compatible with information
  processing systems
• a computer or other controller which supervises
  the overall system, and manages the generated
  data

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Plan your task

• Before you start
• What am I trying to measure?
• How am I going to measure?
• How accurately do I want measure?
• How often do I need to measure?
• During
• Calculations, Data reduction, Alarms
• After
• How am I going to recover my data?
• What am I going to do with my data?
• Post processing

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Parameters to measure

• There are many different parameters for which
  sensors or transducers are commonly available
• temperature
• pressure, force, mass, weight
• velocity, acceleration, vibration
• strain, stress, distortion, fatigue
• flow, volume, level
• length, width, depth, thickness, displacement
• state, pulse, counter
• composition, concentration

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Parameters to measure

• Selecting suitable sensors is often the most
  difficult task

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Types of sensors

• Sensors produce an electrical output that is
  proportional to the quantity of the parameter
  being sensed
• Electrical output from sensors can be
• Voltage, direct or alternating
• Current, current loop
• Resistance, conductance
• Frequency
• Binary states or pulses (counts)
• Serial data - RS232, RS422, RS485, SDI-12
• Parallel data - BCD, Gray Code, quadrature
  encoded

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Characteristics of sensors

• General characteristics to consider
• Accuracy
• Degree of agreement between the measured value
  and the true value
• Linearity
• How well the measured data points fit a straight
  line
• Repeatability
• The degree of variance of successive readings of
  a measured value.
• Match the sensor characteristics to the parameter
  being measured

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Support for sensors

• Sensors often require support that must be
  provided by the data acquisition system or data
  logger
• sensor excitation
• signal conditioning
• compensation for cable effects
• management of references, zeros, etc
• scaling of the electrical output to parameter
  units
• Ability of data acquisition system or data logger
  to support sensors can determine the
  effectiveness of a project

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Reading information from sensors

• Many sensors produce an analog signal, such as
  voltage, current, resistance, frequency, etc.
• Analog signals are measured by a process of
  Analog to Digital Conversion or ADC
• Important characteristics of ADC are
• accuracy
• resolution
• linearity
• repeatability
• speed
• common mode range
• electrical noise rejection

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Resolution, accuracy, linearity, repeatability

• These terms are often confused, however they have
  very different and important meaning
• Apply to both the ADC and to sensors
• Important to consider when matching sensors and
  data acquisition systems to requirements

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Resolution, accuracy, linearity, repeatability

• Resolution is the degree to which the ADC can
  determine that two readings are different.
• usually expressed as a binary proportion of the
  measuring range
• for example a 16 bit ADC resolves to 165535 of
  range, for a 1 Volt range this is 15.26 microvolt
• small data loggers commonly have 12 bit
  resolution, larger systems have 16 bit resolution
  or greater
• resolution is usually fixed, but may vary with
  sampling speed

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Resolution, accuracy, linearity, repeatability

• Accuracy is the degree to which the ADC can
  measure the true value of a signal.
• usually expressed as full scale of measuring
  range
• for example an accuracy of 0.1 for a 1 Volt
  measuring range is 1 millivolt
• accuracy will often vary with operating
  conditions such as ambient temperature, signal
  type, environmental noise, etc.

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Resolution, accuracy, linearity, repeatability

• Linearity is the degree to which the relationship
  between the true value of the signal and the
  measured value of the signal remain constant over
  the measuring range.
• usually expressed as full scale of measuring
  range
• for example a linearity of 0.01 for a 1 Volt
  measuring range is 0.1 millivolt
• linearity will often vary with operating
  conditions such as ambient temperature, signal
  type, environmental noise, etc.

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Resolution, accuracy, linearity, repeatability

• Repeatability is the degree to which the ADC will
  measure the same value for the same signal level
  on successive occasions.
• usually expressed as full scale of measuring
  range
• for example a repeatability of 0.01 for a 1 Volt
  measuring range is 0.1 millivolt
• repeatability will often vary with operating
  conditions such as ambient temperature, signal
  type, environmental noise, etc.

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Common mode range

• Common mode range is a measure of the ability of
  the ADC to measure a signal separate from local
  ground

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Electrical noise rejection

• Noise rejection is the ability of the ADC to
  eliminate local environmental electrical noise
  from the measurements

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Sampling Speed

• Sampling speed is the number of readings per unit
  time of a single channel, or of a group of
  channels
• Expressed as samples/second (Hz, KHz)
• Varies from less than 1 Hz to 100s KHz for
  different data acquisition systems
• May be variable within the same system

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Sampling Speed

• Appropriate sampling speed is dependent on many
  factors
• what type of information is to be captured -
  short term events, long term trends, both?
• when is information to be captured?
• what is the level of importance of various
  information?
• Aim to set a sampling speed to
• maximize sensitivity of data.
• minimize redundancy of data.
• optimize generation of reports.
• simplify archiving of data.

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Number of channels

• Most data acquisition systems have a single
  analog to digital converter, and many input
  channels
• Input channels are selected for measurement by a
  switch or multiplexer (mux), which may be
  sequential or random. Switching may be by relay
  or solid state.
• Only one channel can be measured at a time
• Consideration must be given to sensor bounce
  when a sensor is selected for measurement

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Data recovery and transfer

• Data can be recovered from data acquisition
  systems and data loggers in various ways.
• Serial comms interface via direct connect,
  modem, cell phone, radio, satellite
• PCMCIA - modem, cell phone, LAN, memory card,
  USB memory device
• Network port - Ethernet, field bus, proprietary,
  etc
• Universal Serial Bus (USB)
• Data can be transferred and published using
• Local Ethernet network
• Intranet - email, web pages
• Internet - email, world wide web pages

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Data processing and reporting

• Collected data is analyzed and reported using any
  of
• generic packages such as
• ASCII text editors
• Spreadsheets
• Databases
• general purpose data processing packages
• general purpose SCADA packages
• proprietary host software packages

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Other functionality

• In addition to the core functionality of data
  acquisition, modern data acquisition and data
  logging systems have other functionality
  including
• real-time calculations, statistics, FFT, etc.
• alarms testing for out of range conditions.
• control feedback to the measured system or
  process.
• a range of communications and memory options etc.

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Form factor for dataacquisition systems

• Data acquisition systems can be of a number of
  forms
• Pocket loggers - small, battery powered, stand
  alone devices with simple functionality
• Intelligent data loggers - stand alone devices
  with own intelligence, sophisticated data
  manipulation, alarms, backed up power supply,
  displays, etc. Accessed continuously or
  periodically by a PC.
• Plug in cards - plugged into the ISA bus or USB
  port of a PC to provide basic data acquisition
  functionality. The PC provides power, control,
  data storage, etc.

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Exercise

• Write down the following
• Types of sensors you will use.
• The accuracy / repeatability required.
• Sensor power requirements.
• Sensor scaling.
• Rate of measurements that need to be made.
• The method of powering the logger, sensors and
  communications systems.
• The memory requirements of the application.
• The method of communicating with the logger.
• The method of displaying, distributing and
  analysing the data collected.