ELECTROTECHNOLOGY III

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ELECTROTECHNOLOGY III

• Farah Khalid
• REAL TIME SYSTEMS

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A REAL TIME SYSTEM

• A RTS consists of four parts
• Physical process An environment which is
  optimised by certain controlled parameters .
• Sensors That which observe these parameters.
• Computer It interprets from the information
  provided by the sensor and plans what is done
  next.
• Actuators Acts on the decision of the computer.

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Eg. A Manufaturing plant that needs to control
temperature

• Physical Process Temperature
• Sensor Temperature transducer ( A device or
  material that changes linearly with temperature)
• Computer An embedded microprocessor or PC that
  records the changes of the sensor, checks if the
  temp. is in the correct range, if not plans what
  to do next ?
• Actuators Corrective mechanism for less or more
  temperature.
• Heating Element Increases the temperature
• Coolant Decreases the temperature

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Eg. Aircraft Autopilotfor maintaining level
flight.

• Physical process Airplane, surrounding air,
  navigation radio sources.
• Sensor Airspeed, attitude, control-surface
  positions, control panel, etc.
• Computer Embedded microprocessor or
  general-purpose computer.
• Great care needs to be taken when writing the
  software (robust and reliable)
• Actuator Hydraulics and servos for positioning
  control surfaces (ruder, flaps, etc.).

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COMPLEXITY and RELIABILITY

• There can be a wide range of complexity involved
  
• Simple Washing machines, burglar alarms
• Moderate Automobile engine systems
• Complex Aircraft autopilot systems,
  pharmaceutical control systems
• HENCE
• Use proven design methodologies.
• Introduce new techniques slowly.
• Design systems to be fault tolerant.
• A fault-tolerant system can continue to operate
  properly despite faults.
• Design systems to fail safe.
• Failure will result in minimal damage.

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Reliability-Assurance Problem

• Acceptable error rate must be very low.
• Example if an avionics system causes a plane to
  crash one out of a million landings then how many
  would die per year?
• Testing cannot assure a sufficiently reliable
  system.
• Example How much would it cost to land an
  airliner one million times
• (to test a device)?

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REAL TIME SYSTEMS

• The following steps are typical
• 1 A transducer converts process state to a raw
  electrical quantity.
• 2 A conditioning circuit converts the raw
  electrical quantity into a useful electrical
  quantity.
• 3 An analog-to-digital converter (ADC) converts
  the useful electrical quantity to information.
• 4 A buffer and interface store, format, and
  present the information to a computer which then
  interprets and selects the next course of action.
• 5 A digital-to-analog converter (DAC) converts
  binary information into useful electrical
  quantity
• 6 An Actuator coverts the electrical quantity
  into a desired form to control the process
  variable.

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PROCESS VARIABLE

• The process state is the current condition of the
  process, down to in-finitesimal detail.
• The process variable is a part or
  characterization of a process state, usually in
  terms of a common measure.
• For example, consider a coffee maker.
• Process state amount of water in carafe, water
  temperature, chemical description of water in
  carafe, type of coffee beans, etc.
• Process variable temperature of water.
• Process variable value 70 C. (do not know the
  exact value can be determined to a high degree of
  precision.)

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TRANSDUCER

• Device which converts a physical quantity from
  one form to another.Usually from a physical
  quantity which is a process variable to some
  useful electrical quantity.
• For example, a transducer might convert
  temperature to resistance.
• Transducer Modelling
• Mapping (function) from process variable to
  electrical quantity.
• Symbol Ht denotes the function.
• Let x be a process variable.
• Then Ht(x) is the output of the transducer with
  function Ht.

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EG. TRANSDUCER

• A variable resistor can be used as a transducer.
• Consider a variable resistor which consists of a
  slider that move 5mm while resistance varies
  linearly from 0 to 10kO.
• Process variable position of slider, x.
• Mapping Ht(x) x 10 kO/5mm
• Process variable value determined from transducer
  output.
• Let y Ht(x) where Ht and x are as above.
• Quantity y is a resistance.
• The position x is found by Ht-1 (y) y 5mm/10kO
• The process of finding the inverse is equivalent
  to solving for x in the equation y x 10 kO/5mm

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CONDITIONING CIRCUIT

• The output of a transducer is a raw electrical
  quantity.
• It might have to be amplified or otherwise
  processed.
• This is done by conditioning circuits.
• Conditioning circuits might have to do one or
  more of the following
• Amplify a tiny voltage.
• Convert resistance to voltage.
• Detect tiny changes in resistance (e.g., 1001
  to 1002 ).
• Add an offset to the transducer output.
• Correct for nonlinearities in the transducer
  function.
• Other functions.

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• Notation
• The symbol Hc will be used for the conditioning
  circuit's function.
• An amplifier is a simple conditioning circuit
• Hc(v) Av
• where A, the gain, is a dimensionless number.
• x is a process variable
• Ht(x) is the transducer output
• Hc(Ht(x)) is the conditioning circuit output.
• Sensors
• The combination of transducer and conditioning
  circuit is referred to as a sensor.

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ADCs

• Conditioning-circuit output is usually fed to an
  ADC.
• An analog-to-digital converter converts
  electrical quantities to information.
• Input is usually a voltage, output is usually an
  integer.
• Symbol HADC(v) will be used for an ADC function.
• Standard ADC Function
• HADC(hb)(v) v (2b 1)/h)
• where h is a voltage and b is an integer.
• This ADC would convert voltages in the range 0 to
  h (inclusive) to a binary number from 0 to 2b-1
• For example, HADC(10V8)(5 V) 127
• and HADC(17V16)(1.3V) 5011 .