Control System Instrumentation - Process Control - Lecture Slides, Slides of Process Control

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Control System Instrumentation

Standard Instrument Signals

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

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

Range and Scale Factor

range 50 to 150

scale factor 0.16 mA /

150 50

m 0.

C

C

G s





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.

( ) 1

( ) 1

m s s m m s s

T s m C

T s s U A

τ τ

= =

Chapter 9

τ m

Measurement Errors

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

Figure 9.15 Nonideal instrument behavior: (a) hysteresis, (b) dead band.

Chapter 9

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 0.39-0.25=0.14, i.e., maximum error.

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