PERFORMANCE SPECIFICATION AND COMPONENT MATCHING

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PERFORMANCE SPECIFICATION AND COMPONENT MATCHING

• BY
• RAJESH KOMMISETTY
• SRUJAN KUMAR .D
• YADA SAIRAJ

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PROLOGUE

• When two or more components are
  interconnected ,the behavior of individual
  components in the overall system can deviate
  significantly from their behavior when each
  component operates independently.
• Therefore matching of components
  in a multi-component system, particularly with
  respect to their impedance and component
  matching, should be done carefully in order to
  improve system performance and accuracy.
• Hence selection of available
  components for a particular application or a new
  design should rely heavily on performance
  specifications for these components
• although the discussion is
  primarily limited to components in a measurement
  system, the ideas are applicable to many other
  types of components in a control system

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A CASE IN POINT

• Sensor Transducer
• Used in feedback control systems

Transmittable variable
Transducer
Signal sensor
Measurand
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• The sensor transducer stages of a
  typical measuring device are represented
  schematically in the above figure as an example
  consider the operation of a piezo-electric
  accelerometer. In this case acceleration is the
  measurand. It is first converted into inertia
  force through mass element and is exerted on a
  piezo-electric crystal within which a
  strain(stress) is generated. This is considered
  the sensing stage at the output of the
  accelerometer. This stress-to-charge conversion
  or stress-to-voltage conversion can be
  interpreted as the transducer stage.
• a complex measuring device can have
  more than one sensing stage. More often, the
  measurand goes through several transducer stages
  before it is available for control and actuating
  purposes.

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• TWO GENRES IN TRANSDUCERS
• Passive Transducers
• passive transducers derive its power from
  a measured signal (measurand)
• Ex-piezo-electric, photo-voltaic
  transducers ,.etc
• Active transducers
• active transducers requires a separate
  power source (power supply)
• Ex-potentiometer,resistive transducer
  ,.etc

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PARAMETERS FOR PERFORMANCE SPECIFICATION

• Fast response
• High gain or low output impedance
• Stability
• Static linearity
• Absence of loading effects and matching of
  impedances
• High input impedances

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• All of these properties are based on
  dynamic behavior of the measuring device. In
  particular, items 1 through 4 can be specified in
  terms of the device (response), either in the
  time domain or in the frequency domain. Items 2,
  5 and 6 can be specified using the impedance
  characteristics of device.

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SPECIFICATIONS IN TIME DOMAIN

• Response parameters for the time domain
  specification of performance

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• The above figure shows a typical
  response in the dominant mode of a device. Note
  that the curve is normalized with respect to the
  steady state value.

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• Rise time
• This is the time taken to pass the
  steady-state value or often defined as the time
  taken to pass 90 percent of the steadystate
  value.
• Delay time
• This is usually defined as the time
  taken to reach 50 percent of the steady state
  value for the first time
• Peak time
• This is time at the first peak.
• Settling time
• This is the time taken for the device
  response to settle down with in a certain
  percentage ( ex- ()or(-)2 ) of the steady
  state value.
• Percentage overshoot (P.O)
• This is defined as
• P.O 100( Mp -1 ) using normalized
  unit step response curve
• Mp is the peak value
• Steady-state error
• This is the deviation of the actual
  steady state value from the desired value. Steady
  state error can be completely eliminated using
  integral control

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SPECIFICATIONS IN FREQUENCY DOMAIN

• Response parameters for the frequency domain
  specification of the performance.

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• The above figure shows a representative
  frequency transfer function of a device. This
  constitutes gain and phase angle plots using
  frequency as the independent variable. This pair
  of plots is commonly known as the Bode diagram

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• Useful frequency range
• This corresponds to the flat region
  (static region) in the gain curve and the zero
  phase lead region in the phase curve. operation
  in this range of a measuring device implies
  measurement of a signal whose significant
  frequency content is limited to this band. In
  that case, faithful measurement and fast response
  are guaranteed, because measuring device dynamics
  do not corrupt the measurement.
• Instrument bandwidth
• This is the measure of the useful
  frequency range of an instrument.
• Control bandwidth
• This is used to specify speed of
  control. It is an important specification in both
  analog control and digital control.
• Static gain (DC gain)
• This is the gain (transfer function
  magnitude) of a measuring instrument within the
  useful range ( or at low frequencies ) of the
  instrument. It is also termed as DC gain.

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Hysteresis

• Non linear devices may produce hysteresis. In
  this case the input or output changes, depending
  on the direction of the motion, resulting in a
  hysteresis loop.
• If a dc current passes through the coil, a
  magnetic field is generated. As the current is
  increased from zero, the field strength will also
  increase. Now, if the current is decreased back
  to zero, the field strength will not return to
  zero because of residual magnetism in Ferro
  magnetic core.
• So, to demagnetize, we have to apply negative
  current. Then the field strength will be back to
  zero.

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• Jump phenomenon
• Some non linear devices exhibit an instability
  know as the jump phenomenon (or fold catastrophe)
  in the frequency response function curve
• Limit cycles
• Non linear devices may produce limit cycles. A
  limit cycle is a closed trajectory in the state
  space that corresponds to sustained oscillations
  with out decay or growth.
• Frequency creation
• At steady state, non linear devices can create
  frequencies that are not present in the
  excitation signals. These frequencies might be
  harmonics, sub harmonics, or non harmonics.

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Impedance characteristics

• When components such as measuring instruments,
  control boards, process hardware, and signal
  conditioning equipment are interconnected, it is
  necessary to match impedances properly at each
  interface in order to realize their rated
  performance level.
• In mechanical and electrical systems, loading
  errors can appear as phase distortions as well.
• Another adverse effect of improper impedance
  consideration is inadequate output signal levels,
  which make signal processing and transmission
  very difficult.

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Impedance matching amplifiers

• Impedance matching amplifier has very high input
  impedance, very low output impedance, and almost
  unity gain.
• Impedance matching amplifiers are, in fact,
  operational amplifiers with feed back.
• Operational amplifiers are voltage amplifiers
  with very high gain K, high input impedance Z-I,
  low output impedance Z-0.
• Op-amps originally made with conventional
  transistors, diodes and resistors are now
  available as miniature units with integrated
  circuit elements.

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Voltage followers

• Voltage followers are impedance matching
  amplifiers with very high input impedance, very
  low output impedance and almost with unity gain.
• For these reasons they are suitable for use with
  high output impedance sensors such as piezo
  electric devices.
• The principle of Capacitance feedback is utilized
  in Charge amplifiers.
• Charge amplifiers are commonly used for
  conditioning output signals from piezo electric
  transducers.

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Ground loop noise

• In devices that handle low level signals,
  electrical noise can create excessive error.
  Here, the noise is caused by fluctuating magnetic
  fields due to near by ac lines.
• This can be avoided either by taking precautions
  not to have strong magnetic fields and
  fluctuating currents near delicate instruments or
  by using fiber optic signal transmission.

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INSTRUMENT RATING PARAMETERS

• 1.Sensitivity
• 2.Dynamic range
• 3.Resolution
• 4.Linearity
• 5.Zero drift and Full scale drift
• 6.Useful frequency range
• 7.Bandwidth
• 8.Input and Output impedance

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• Sensitivity incremental output
• incremental input
• Dynamic range ratio range of operation
• resolution
• Resolution is the smallest change in
• signal that can be detected and
• accurately indicated by a transducer.

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• Linearity is determined by the calibration curve
  of an instrument.
• Zero drift is the drift from null reading of the
  instrument when the measurand is maintained study
  for a long period.
• CAUSES instrument instability, ambient
  changes, changes in power
  supply, parameter changes in
  instrument.

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• Useful Frequency Range is the flat gain curve and
  a zero phase curve in the frequency response
  characteristics of an instrument. It is the
  measure of instrument bandwidth
• Bandwidth of an instrument determines the maximum
  speed or frequency at which the instrument is
  capable of operating.
• high bandwidth implies faster speed or
  response. It is determined by dominant natural
  frequency or resonant frequency.

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ACCURACY

• Measurement accuracy determines the closeness of
  measured value to the true value.
• Error (measured value) - (true value).
• Correction (true value) - (measured
  value).
• Two types of errors are deterministic error and
  random error.

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PRECISION

• Reproducibility of an instrument reading
  determines the precision of the instrument.
• Precision measurement range
• standard deviation of error (se)
• Readings of an instrument may have a large mean
  value or error, but if the standard deviation is
  small, it has high precision.

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ERROR ANALYSIS

• Some of the difficulties in error analysis are
• True value is usually unknown.
• Instrument reading may contain random error which
  cannot be determined exactly.
• The error may be a complex function of many
  variables.
• The instrument may be made up of many components
  that have complex inter-relations and each
  component may contribute to overall error.

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• If true value is known, there is no need to
  measure it and if it is unknown , it is
  impossible to determine exactly how inaccurate a
  particular reading is.
• This situation can be addressed to some extent by
  statistical representation of errors, which takes
  us to second item listed.
• Third and fourth items can be addressed by error
  combination in multi-variable system and by error
  propagation in complex multi-component systems.

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STATISTICAL PROCESS CONTROL (SPC)

• The use of statistical signals to improve
  performance of a process is called SPC.
• According to the normal distribution, the
  boundries(-3s and 3s)drawn about the mean value
  may be considered as control limits or action
  lines in SPC.
• STEPS
• 1.Collect measurements of appropriate response
  variables of the process.

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• 2.Compute the mean value of the data, the upper
  and lower control limits.
• 3.Plot the measured data and the two control
  limits on a control chart.
• 4.If measurements fall outside control limits,
  take corrective actions and repeat the control
  cycle.
• If the measurements fall within the control
  limits, the process is said to be in statistical
  control.

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DISCUSSIONS

• 1. Explain briefly about specifications in time
  domain and frequency domain.
• 2. What is magnetic hysteresis?
• 3. Write a short notes on rating parameters.

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References

• Control sensors and actuators by C.W Desilva
• www.wikipedia.org

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