Control%20System%20Instrumentation

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Control System Instrumentation
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Standard Instrument Signals

• Pneumatic (air pressure) 3 15 psig
• Electrical 4 20 mA
• I/P or E/P transducer

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Transducers and Transmitters
Chapter 9

• Figure 9.3 illustrates the general configuration
  of a measurement transducer it typically
  consists of a sensing element combined with a
  driving element (transmitter).
• Since about 1960, electronic instrumentation has
  come into widespread use.

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Sensors The book briefly discusses commonly used
sensors for the most important process variables.
(See text.)
Transmitters

• A transmitter usually converts the sensor output
  to a signal level appropriate for input to a
  controller, such as 4 to 20 mA.
• Transmitters are generally designed to be direct
  acting.
• In addition, most commercial transmitters have an
  adjustable input range (or span).
• For example, a temperature transmitter might be
  adjusted so that the input range of a platinum
  resistance element (the sensor) is 50 to 150 C.

Chapter 9
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Chapter 9
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Chapter 9
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Range and Scale Factor
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Transfer Function Nonlinear Case
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Chapter 9
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• Measurement / Transmission Lags
• Temperature sensor
• make as small as possible (location, materials
  for thermowell)
• Pneumatic transmission lines
• usually pure time delay, measure experimentally
  (no time delays for electronic lines) less
  common today compared to electronic transmissions.

Chapter 9
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Transmitter/Controller
Chapter 9
May need additional transducers for Gm if its
output is in mA or psi. In the above case, Gc
is dimensionless (volts/volts).
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Measurement Errors

• Systematic errors
• Drift slowly changing instrument output when
  input is constant.
• Nonlinearity
• Hysteresis or backlash
• Dead band
• Dynamic error
• Random errors

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Chapter 9
Figure 9.15 Nonideal instrument behavior (a)
hysteresis, (b) dead band.
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Chapter 9
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Chapter 9
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Precision, Resolution, Accuracy and Repeatability

• Precision can be interpreted as the number of
  significant digits in measurement, but more
  accurately it refers to the least significant
  digit which contains valid information, e.g.,
  0.01 in the present case. Therefore, 0.33 is more
  precise than 0.3.
• Resolution is defined as the smallest change in
  the input that will result in a significant
  change in the transducer output.
• Repeatability is /- 0.02 in the present case.
• Accuracy is 0.39-0.250.14, i.e., maximum error.

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Final Control Elements

• The most-common manipulated variables to be
  adjusted are (1) energy flow rates, and (2)
  material flow rates.
• Type (1) transducer heating element
• Type (2) transducer control valve (pump drive,
  screw conveyer, blower, etc.)

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Chapter 9
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Control Valve Characteristics (Inherent)
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Chapter 9
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Chapter 9
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Pressure Drop Across Control Valve Installed
On-Line

• In practical applications, one must take other
  flow obstructions into account for actual valve
  performance.

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Design Guideline
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Chapter 9
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Design Calculation for a Linear Valve
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Rangeability (Turn-Down Ratio)
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Example
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Installed Valve Characteristics

• Desired behavior the flow rate is a linear
  function of valve lift.
• Let us assume that the control valve has linear
  trim and it is necessary to increase the flow
  rate. If ?p through exchanger did not change,
  then valve would behave linearly (true for low
  flow rates), since it takes most of ?p . For
  higher flow rates, ?p through exchanger will be
  important, changing effective valve
  characteristics (valve must open more than
  expected ? nonlinear behavior).

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Linear Valve Behavior
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Equal-Percentage Valve Characteristics
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Chapter 9
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Chapter 9
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Control Valve Transfer Function