Control System Instrumentation

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

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.

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.

Chapter 9

Chapter 9

Range and Scale Factor

Transfer Function – Nonlinear Case

Chapter 9

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

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).

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

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

Chapter 9

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.

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.)

Chapter 9

Control Valve Characteristics (Inherent)

Chapter 9

Chapter 9

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

Design Guideline

Chapter 9

Design Calculation for a Linear Valve

Rangeability (Turn-Down Ratio)

Example

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).

Linear Valve Behavior

Equal-Percentage Valve Characteristics

Chapter 9

Chapter 9

Control Valve Transfer Function