Intro4u2u

1. A positioner ensures that there is a linear relationship between the signal input pressure from the control system and the position of the control valve. This means that for a given input signal, the valve will always attempt to maintain the same position regardless of changes in valve differential pressure, stem friction, diaphragm hysteresis and so on.
2. A positioner may be used as a signal amplifier or booster. It accepts a low pressure air control signal and, by using its own higher pressure input, multiplies this to provide a higher pressure output air signal to the actuator diaphragm, if required, to ensure that the valve reaches the desired position.
3. Some positioners incorporate an electropneumatic converter so that an electrical input (typically 4 - 20 mA) can be used to control a pneumatic valve.
4. Some positioners can also act as basic controllers, accepting input from sensors.

A frequently asked question is, ‘When should a positioner be fitted?’
A positioner should be considered in the following circumstances:

1. When accurate valve positioning is required.
2. To speed up the valve response. The positioner uses higher pressure and greater air flow to adjust the valve position.
3. To increase the pressure that a particular actuator and valve can close against. (To act as an amplifier).
4. Where friction in the valve (especially the packing) would cause unacceptable hysteresis.
5. To linearise a non-linear actuator.
6. Where varying differential pressures within the fluid would cause the plug position to vary.

To ensure that the full valve differential pressure can be accepted, it is important to adjust the positioner zero setting so that no air pressure opposes the spring force when the valve is seating.
Commonly, P to P positioner takes a pneumatic signal (P) from the control system and provides a resultant pneumatic output signal (P) to move the actuator.

One advantage of a pneumatic control is that it is intrinsically safe, i.e. there is no risk of explosion in a dangerous atmosphere, and it can provide a large amount of force to close a valve against high differential pressure. However, pneumatic control systems themselves have a number of limitations compared with their electronic counterparts.
To alleviate this, additional components are available to enable the advantages of a pneumatic valve and actuator to be used with an electronic control system.

The basic unit is the I to P converter. This unit takes in an electrical control signal, typically 4 - 20 mA, and converts it to a pneumatic control signal, typically 0.2 - 1 bar, which is then fed into the actuator, or to the P to P positioner, as shown in

Fig. 6.6.15 Pneumatic valve / actuator operated by a control signal using I to P converter and P to P positioner

With this arrangement, an I to P (electrical to pneumatic) conversion can be carried out outside any hazardous area, or away from any excessive ambient temperatures, which may occur near the valve and pipeline.

However, where the conditions do not present such problems, a much neater solution is to use a single component electropneumatic converter / positioner, which combines the functions of an I to P converter and a P to P positioner, that is a combined valve positioner and electropneumatic converter. Figure 6.6.16 shows a typical I to P converter / positioner.
Fig. 6.6.16 A typical I to P converter / positioner fitted to a pneumatic valve (gauges omitted for clarity) Fig. 6.6.16 A typical I to P converter / positioner fitted to a pneumatic valve (gauges omitted for clarity)

Most sensors still have analogue outputs (for example 4 - 20 mA or 0 - 10 V), which can be converted to digital form. Usually the controller will perform this analogue-to-digital (A / D) conversion, although technology is now enabling sensors to perform this A / D function themselves. A digital sensor can be directly connected into a communications system, such as Fieldbus, and the digitised data transmitted to the controller over a long distance. Compared to an analogue signal, digital systems are much less susceptible to electrical interference.

Analogue control systems are limited to local transmission over relatively short distances due to the resistive properties of the cabling.

Most electrical actuators still require an analogue control signal input (for example 4 - 20 mA or 0 - 10 V), which further inhibits the completion of a digital communications network between sensors, actuators, and controllers.