Introduction to Analog converters


In this session we are gonna discuss about Analog converters.In instrumentation analog conveters plays a major role in the field lets take an example of a thermocouple ,the output of thermocouple is in mill volt range for signal transmission we want to convert it into mA range .so lets discuss about some analog converters

Pneumatic-to-Electronic Converters

The pneumatic-to-electronic transducer is used wherever pneumatic signals must be converted to electronic signals for
any of the following reasons:

1. Connecting existing pneumatic plants to computers
2. Transmission over large distances
3. Input to an electronic logger
4. Input to telemetering equipment
5. Instrument air not available at the receiver controller

In principle, any of the electronic pressure transmitters could be used, but in practice, special, improved-accuracy devices are used. The air signals are at low pressure levels (3 to 15 PSIG or 0.2 to 1.0 bar), and many of the standard
pressure detectors are not sensitive or not linear enough at these pressures.
A P/I transducer should be at least 1/2% of full-scale accurate and preferably 1/4% to preserve the integrity of the initial
signal. Since the total error is the square root of the mean squares of the individual component errors, the greater the
precision of the P/I transducer the better the signal. Because of this need for higher accuracy, most P/I transducers
use a bellows input and a motion balance sensor. A typical high-quality P/E converter is shown in Figure 

Current-to-Air Converters

Electropneumatic transmitters are also referred to as converters and transducers. They are important because they form the link between electrical measurements in a pneumatic control system. They also convert electronic controller outputs into air pressure signals for the operation of pneumatic control valves. Figure  illustrates one of these converter designs and also lists the various electric devices with which it can be combined. The input is usually a DC current in the range of 1 to 5, 4 to 20, or 10 to 50 milliamperes. An Alnico permanent magnet creates a field that passes through the steel body of the transmitter and across a small air gap to the pole piece.
A multi-turn, flexure-mounted voice coil is suspended in the air gap. The input current flows through the coil creating an electromagnetic force that tends to repel the coil and thus converts the current signal into a mechanical force.
Since the total force obtainable in a typical voice coil motor with such small current inputs is only on the order of some
ounces, a different approach, namely, the use of a reaction nozzle, is employed to convert the force into a pneumatic output pressure. In this circuit, the supply air flows through a restriction and out the detector nozzle. The reaction to the air jet as it impinges against the nozzle seat supplies the counterbalancing force to the voice coil motor. The nozzle back pressure is the transmitted output pressure. To make the transmitter insensitive to vibration, the voice coil is integrally mounted to a float submerged in silicone oil. The float is sized so that its buoyant force equals the weight of the assembly, reducing the net force to zero. The zero of the transmitter is adjusted by changing the force on a leaf-spring. Span is adjusted by turning the range adjusting screw, which changes the gap between the screw and the magnet, thus shunting some of the magnetic field away from the pole piece

Also read

I/P converter calibration

Working of I/P Transducer.

Millivolt-to-Current Converters

Millivolt-to-current converters are widely used in the measurement of temperature, using thermocouples or other millivolt generating sensing elements. They are also utilized in converting the output signals of analyzers into higher-level transmission signals.

Resistance-to-Current Converters

Resistance measurements are common in temperature measurements and also in resistance or strain gauge sensors. The circuits used are similar to those of millivolt-to-current converters except that the front end is a resistance bridge instead of a voltage bridge

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