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Control System Instrumentation

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Presentation on theme: "Control System Instrumentation"— Presentation transcript:

1 Control System Instrumentation

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

3 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.

4 Chapter 9 Sensors Transmitters
The book briefly discusses commonly used sensors for the most important process variables. (See text.) Transmitters Chapter 9 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.

5 Chapter 9

6 Chapter 9

7 Range and Scale Factor


9 Transfer Function – Nonlinear Case

10 Chapter 9

11 Chapter 9 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

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

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

14 Chapter 9 Figure 9.15 Nonideal instrument behavior: (a) hysteresis, (b) dead band.

15 Chapter 9

16 Chapter 9

17 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 +/ in the present case. Accuracy is =0.14, i.e., maximum error.

18 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.)

19 Chapter 9


21 Control Valve Characteristics (Inherent)

22 Chapter 9

23 Chapter 9

24 Pressure Drop Across Control Valve Installed On-Line
In practical applications, one must take other flow obstructions into account for actual valve performance.


26 Design Guideline

27 Chapter 9


29 Design Calculation for a Linear Valve

30 Rangeability (Turn-Down Ratio)

31 Example

32 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).

33 Linear Valve Behavior

34 Equal-Percentage Valve Characteristics

35 Chapter 9

36 Chapter 9

37 Control Valve Transfer Function

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