Current-to-pressure transducer

Fig. 6. Impact of field bus on wiring costs, where A/D and D/A correspond to analogue-to-digital and digital-to-analogue converters, respectively LCU = local control unit and I/P = current-to-pressure transducer, (a) DCS without remote terminal rooms (b) DCS with remote terminal rooms and...

G = current-to-pressure transducer(I/P) H = actuator and I = final control element. Some devices combine A, B, and C, and (--) represents the line... 

Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve control devices. In some applications, several auxiliary devices are used together on the same control valve. For example, mounted on the control valve, one may find a current-to-pressure transducer, a valve positioner, a volume booster relay, a solenoid valve, a trip valve, a limit switch, a process controller, and/or a stem position transmitter. Figure 8-80 shows a valve positioner mounted on tne yoke leg of a spring and diaphragm actuator. 

Transducers The current-to-pressure transducer (I/P transducer) is a conversion interface that accepts a standard 4- to 20-mA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range [normally 0.2 to 1.0 bar (3 to 15 psig) or, less frequently, 0.4 to 2.0 bar (6 to 30 psig)]. The output pressure generated by the transducer is connected directly to the pressure connection on a spring-opposed diaphragm actuator or to the input of a pneumatic valve positioner. 

With the exception of the dead-band booster, the application of booster relays has diminished somewhat by the increased use of current-to-pressure transducers, electropneumatic positioners, and electronic control systems. Transducers and valve positioners serve much the same functionality as the booster relay in addition to interfacing with the electronic process controller. 

FIG. 8-87 Current-to-pressure transducer component parts. (a) Direct-cur-rent-pressure conversion. (b) Pneumatic booster amplifier (relay), (c) Block diagram of a modern I/P transducer. 

FIG. 8-73 Current to pressure transducer components parts (a) direct current to pressure conversion (b) pneumatic booster amplifier (relay) (c) block diagram of a modem I/P transducer. 

Transducers The current-to-pressure transducer (I/P transducer) is a conversion interface that accepts a standard 4- to 20-mA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range [normally 0.2 to... 

Since most valves are still actuated by air pressure, current signals are usually converted to an air pressure. An Z/P (current to pressure) transducer is used to convert 4 to 20 mA signals to 3 to 15 psig signals. 

Measurement, comparison, and adjustment constitute an entity called a control loop. Very schematically one may describe the control loop in terms of sensors, controllers, actuators, and the process to be controlled. Shown in Figure 12.18 is a level control system for a tank. Broken lines denote control signals. The controller (denoted by LC) receives the level signal from the level transducer (LT) that sends a signal to the current-to-pressure transducer (I/P) that then applies a pneumatic pressure to the valve diaphragm. 

Consider the schematic diagram of a controlled blending process shown in Fig. 8.1. The control objective is to control the mass fraction of the exit stream, x, by adjusting inlet flow rate, wi, using a feedback controller. The mass fractions of a key chemical component in the inlet streams, x and xi, are constant, and mass flow rate w is a disturbance variable. The liquid volume V is constant. The composition sensor/transmitter (AT) and the current-to-pressure transducer (I/P) are both direct-acting devices. 

The standard block diagram in Fig. 11.8 can be used to represent a wide variety of practical control problems. Other blocks can be added to the standard diagram to represent additional elements in the feedback control loop such as the current-to-pressure transducer in Fig. 11.7. In Fig. 11.8, the signal path from E to T through blocks Gc, Gv, and Gp is referred to as the forward path. The path from Y to the comparator through G is called the feedback path. 

The current-to-pressure transducer has negligible dynamics and a gain of 0.3 psi/mA. 

Current-to-pressure transducer The I/P transducer acts as a linear device with negligible dynamics. The output signal changes from 3 to 15 psi when the input signal changes fuU-scale from 4 to 20 mA. 

Suppose that the current-to-pressure transducer and the control valve operate as linear devices with negligible dynamics. Then its input-output relationship can be written as... 

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