Intro4u2u

The output signals from most control systems are low power analogue signals but there is a growing use of digital systems such as ‘Fieldbus®’ or ‘Profibus®’.

An analogue system provides a continuous but modulating signal whereas a digital system provides a stream of binary numeric values represented by a change between two specific voltage levels or frequencies.

A comparison between digital and analogue systems can be made using Example 6.7.1 and Example 6.7.2:

Example 6.7.1
Imagine two people, person A and person B, each on opposite hilltops and each with a flag and a flag-pole. The aim is for person A to communicate to person B by raising his flag to a certain height. Person A raises his flag half way up his pole. Person B sees this and also raises his flag halfway. As person A moves his flag up or down so does person B to match. This would be similar to an analogue system.

Example 6.7.2
Now assume that person A does not have a pole but instead has two boards, one with the figure ‘0′ and the other with the figure ‘1′, and again wants person B to raise his flag half way, that is to a height of 50% of his flag-pole. The binary number for 50 is 110010, so he displays his boards, two at a time, in the corresponding order. Person B reads these boards, translates them to mean 50 and raises his flag exactly half way. This would be similar to a digital system.

It can be seen that the digital system is more precise as the information is either a ‘1′ or a ‘0′ and the position can be accurately defined. The analogue example is not so precise because person B cannot determine if person A’s flag is at exactly 50%. It could be at 49% or 51%. It is for this reason, together with higher integration of microprocessor circuitry that digital signals are becoming more widely used.

Digital addressing

Digital addressing allows a controller to send information over a set of wires onto which several receivers are connected and yet be able to communicate with only one of those receivers if required. This is done by allocating an address to each receiver, which the controller must broadcast first.

To explain this, consider the digital example above but now assume that there is another person, person C on a third hill. Person B and person C can both see person A, so person A must first indicate to whom he is communicating.

This is done with the first board. If the first board is a ‘0′ then all subsequent data is intended for person B who adjusts his flag accordingly. Conversely, if the first board is a ‘1′ then all subsequent data is intended for person C. Hence person B has a digital address of ‘0′ and person C has a digital address of ‘1′; each person knows that the first number to be seen by them refers to the address not the message.

What is HART®?
HART® stands for ‘Highway Addressable Remote Transducer’ and is a standard originally developed as a communications protocol for control field devices operating on a 4-20 mA control signal.

The HART® protocol uses 1200 baud Frequency Shift Keying (FSK) based on the Bell 202 standard to superimpose digital information on the conventional 4-20 mA analogue signal. Maintained by an independent organisation, the HART® Communication Foundation, the HART® protocol is an industry standard developed to define the communications protocol between intelligent field devices and a control system.

HART® is probably the most widely used digital communication protocol in the process industries, and:

• Is supported by all of the major suppliers of process field instruments.
• Preserves existing control strategies by allowing 4-20 mA signals to co-exist with digital communication on existing 2-wire loops.
• Is compatible with analogue devices.
• Provides important information for installation and maintenance, such as Tag-IDs, measured values, range and span data, product information and diagnostics.
• Can support cabling savings through use of multidrop networks.
• Reduces operating costs via improved management and utilisation of smart instrument networks.

What is PROFIBUS®?
PROFIBUS® is an open fieldbus standard for a wide range of applications in manufacturing and process automation independent of manufacturers. Manufacture independence and transparency are ensured by the international standards EN 50170, EN 50254 and IEC 61158.

It allows communication between devices of different manufacturers without any special interface adjustment. PROFIBUS® can be used for both high-speed time critical applications and complex communication tasks. PROFIBUS® offers functionally graduated communication protocols DP and FMS. Depending on the application, the transmission technologies RS-485, IEC 1158-2 or fibre optics can be used.

It defines the technical characteristics of a serial Fieldbus® system with which distributed digital programmable controllers can be networked, from field level to cell level. PROFIBUS® is a multi-master system and thus allows the joint operation of several automation, engineering or visualization systems with their distributed peripherals on one bus.

At sensor/actuator level, signals of the binary sensors and actuators are transmitted via a sensor/actuator bus. Data are transmitted purely cyclically.

At field level, the distributed peripherals, such as I/O tutorials, measuring transducers, drive units, valves and operator terminals communicate with the automation systems via an efficient, real-time communication system. As with data, alarms, parameters and diagnostic data can also be transmitted cyclically if necessary.

At cell level, programmable controllers such as PLC and IPC can communicate with each other. The information flow requires large data packets and a large number of powerful communication functions, such as smooth integration into company-wide communication systems, such as Intranet and Internet via TCP/IP and Ethernet.

What is Foundation™ Fieldbus?
Foundation™ Fieldbus is an all-digital, serial, two-way communications system that serves as a Local Area Network (LAN) for factory/plant instrumentation and control devices. The Fieldbus® environment is the base level group of the digital networks in the hierarchy of plant networks. Foundation™ Fieldbus is used in both process and manufacturing automation applications and has a built-in capability to distribute the control application across the network.

Unlike proprietary network protocols, Foundation™ Fieldbus is neither owned by any individual company, nor regulated by a single nation or standards body. The Foundation™ Fieldbus, a not-for-profit organization consisting of more than 100 of the world’s leading controls and instrumentation suppliers and end users, controls the technology.

While Foundation™ Fieldbus retains many of the desirable features of the 4-20 mA analogue system, such as a standardized physical interface to the wire, bus-powered devices on a single wire, and intrinsic safety options, it also offers many other benefits.

Device interoperability
Foundation™ Fieldbus offers interoperability; one Fieldbus® device can be replaced by a similar device with added functionality from a different supplier on the same Fieldbus® network while maintaining specified operations. This permits users to ‘mix and match’ field devices and host systems from various suppliers. Individual Fieldbus® devices can also transmit and receive multivariable information, and communicate directly with each other over a common Fieldbus®, allowing new devices to be added to the Fieldbus® without disrupting services.

Enhanced process data
With Foundation™ Fieldbus, multiple variables from each device can be brought into the plant control system to analyse trends, optimise processes, and generate reports. Access to accurate, high-resolution data enables processes to be fine-tuned for better productivity, less downtime, and higher plant performance.

Overall view of the process
Modern Fieldbus® devices, with powerful microprocessor-based communications capabilities, permit process errors to be recognized faster and with greater certainty. As a result, plant operators are notified of abnormal conditions or the need for preventive maintenance, allowing personnel to consider pro-active decisions. Lower operating efficiencies are corrected more quickly, enabling production to rise while raw material costs and regulatory problems fall.

Improved in plant safety
Fieldbus technology helps manufacturing plants keep up with stringent safety requirements. It can provide operators with earlier warning of potential hazardous conditions, thereby allowing corrective action to be taken to reduce unplanned shutdowns. Enhanced plant diagnostic capabilities also offer less frequent access to hazardous areas, thus minimizing the risks to personnel.

Easier predictive maintenance
Enhanced device diagnostics capabilities make it possible to monitor and track insidious conditions such as valve wear and transmitter fouling. Plant personnel are able to perform predictive maintenance without waiting for a scheduled shutdown, thus reducing or even avoiding downtime.

Reduced wiring and maintenance costs
The use of existing wiring and multi-drop connections provides significant savings in network installation costs. This includes reductions in intrinsic safety barriers and cabling costs, particularly in areas where wiring is already in situ.

Additional cost savings can be achieved through the decreased time required for construction and start-up, as well as simplified programming of control and logic functions using software control blocks built into Fieldbus® devices.

Pneumatic actuators are commonly used to actuate control valves and are available in two main forms; piston actuators (Figure 6.6.1) and diaphragm actuators (Figure 6.6.2).

Fig. 6.6.1 Typical piston actuators

Piston actuators
Piston actuators are generally used where the stroke of a diaphragm actuator would be too short or the thrust is too small. The compressed air is applied to a solid piston contained within a solid cylinder. Piston actuators can be single acting or double acting, can withstand higher input pressures and can offer smaller cylinder volumes, which can act at high speed.

Diaphragm actuators
Diaphragm actuators have compressed air applied to a flexible membrane called the diaphragm. Figure 6.6.2 shows a rolling diaphragm where the effective diaphragm area is virtually constant throughout the actuator stroke. These types of actuators are single acting, in that air is only supplied to one side of the diaphragm, and they can be either direct acting (spring-to-retract) or reverse acting (spring-to-extend).

Fig. 6.6.2 A pneumatic diaphragm actuator

Reverse acting (spring-to-extend)
The operating force is derived from compressed air pressure, which is applied to a flexible diaphragm. The actuator is designed so that the force resulting from the air pressure, multiplied by the area of the diaphragm, overcomes the force exerted (in the opposite direction) by the spring(s).

The diaphragm (Figure 6.6.2) is pushed upwards, pulling the spindle up, and if the spindle is connected to a direct acting valve, the plug is opened. The actuator is designed so that with a specific change of air pressure, the spindle will move sufficiently to move the valve through its complete stroke from fully-closed to fully-open.

As the air pressure decreases, the spring(s) moves the spindle in the opposite direction. The range of air pressure is equal to the stated actuator spring rating, for example 0.2 - 1 bar.

With a larger valve and / or a higher differential pressure to work against, more force is needed to obtain full valve movement.

To create more force, a larger diaphragm area or higher spring range is needed. This is why controls manufacturers offer a range of pneumatic actuators to match a range of valves - comprising increasing diaphragm areas, and a choice of spring ranges to create different forces.

The diagrams in Figure 6.6.3 show the components of a basic pneumatic actuator and the direction of spindle movement with increasing air pressure.

Fig. 6.6.3 Valve and actuator configurations

The direct acting actuator is designed with the spring below the diaphragm, having air supplied to the space above the diaphragm. The result, with increasing air pressure, is spindle movement in the opposite direction to the reverse acting actuator.

The effect of this movement on the valve opening depends on the design and type of valve used, and is illustrated in Figure 6.6.3. There is however, an alternative, which is shown in Figure 6.6.4. A direct acting pneumatic actuator is coupled to a control valve with a reverse acting plug (sometimes called a ‘hanging plug’).

Fig. 6.6.4 Direct acting actuator and reverse acting control valve

The choice between direct acting and reverse acting pneumatic controls depends on what position the valve should revert to in the event of failure of the compressed air supply. Should the valve close or be wide-open? This choice depends upon the nature of the application and safety requirements. It makes sense for steam valves to close on air failure, and cooling valves to open on air failure. The combination of actuator and valve type must be considered. Figure 6.6.5 and Figure 6.6.6 show the net effect of the various combinations.

Fig. 6.6.5 Net effect of various combinations for two port valves

Fig. 6.6.6 Net effect of the two combinations for three port valves

Effect of differential pressure on the valve lift
The air fed into the diaphragm chamber is the control signal from the pneumatic controller. The most widely used signal air pressure is 0.2 bar to 1 bar. Consider a reverse acting actuator (spring to extend) with standard 0.2 to 1.0 bar spring(s), fitted to a direct acting valve (Figure 6.6.7).

closed Fig. 6.6.7 Reverse acting actuator, air-to-open, direct acting valve - normally closed

When the valve and actuator assembly is calibrated (or ‘bench set’), it is adjusted so that an air pressure of 0.2 bar will just begin to overcome the resistance of the springs and move the valve plug away from its seat.

As the air pressure is increased, the valve plug moves progressively further away from its seat, until finally at 1 bar air pressure, the valve is 100% open. This is shown graphically in Figure 6.6.7.

Now consider this assembly installed in a pipeline in a pressure reducing application, with 10 bar g on the upstream side and controlling the downstream pressure to 4 bar g.

The differential pressure across the valve is 10 - 4 = 6 bar. This pressure is acting on the underside of the valve plug, providing a force tending to open the valve. This force is in addition to the force provided by the air pressure in the actuator.

Therefore, if the actuator is supplied with air at 0.6 bar (halfway between 0.2 and 1 bar), for example, instead of the valve taking up the expected 50% open position, the actual opening will be greater, because of the extra force provided by the differential pressure.

Also, this additional force means that the valve is not closed at 0.2 bar. In order to close the valve in this example, the control signal must be reduced to approximately 0.1 bar.

The situation is slightly different with a steam valve controlling temperature in a heat exchanger, as the differential pressure across the valve will vary between:

* A minimum, when the process is calling for maximum heat, and the control valve is 100% open.
* A maximum, when the process is up to temperature and the control valve is closed.

The steam pressure in the heat exchanger increases as the heat load increases. This can be seen in Tutorial 6.5, Example 6.5.3 and Table 6.5.7.

If the pressure upstream of the control valve remains constant, then, as the steam pressure rises in the heat exchanger, the differential pressure across the valve must decrease.

Figure 6.6.8 shows the situation with the air applied to a direct acting actuator. In this case, the force on the valve plug created by the differential pressure works against the air pressure. The effect is that if the actuator is supplied with air at 0.6 bar, for example, instead of the valve taking up the expected 50% open position, the percentage opening will be greater because of the extra force provided by the differential pressure. In this case, the control signal has to be increased to approximately 1.1. bar to fully close the valve.
Fig. 6.6.8 Direct acting actuator, air-to-close, direct acting valve - normally open Fig. 6.6.8 Direct acting actuator, air-to-close, direct acting valve - normally open

It may be possible to recalibrate the valve and actuator to take the forces created by differential pressure into account, or perhaps using different springs, air pressure and actuator combinations. This approach can provide an economic solution on small valves, with low differential pressures and where precise control is not required. However, the practicalities are that:

* Larger valves have greater areas for the differential pressure to act over, thus increasing the forces generated, and having an increasing effect on valve position.
* Higher differential pressures mean that higher forces are generated.
* Valves and actuators create friction, causing hysteresis. Smaller valves are likely to have greater friction relative to the total forces involved.

The solution is to fit a positioner to the valve / actuator assembly

The thrust available to close the valve has to provide three functions:

1. To overcome the fluid differential pressure at the closed position.
2. To overcome friction in the valve and actuator, primarily at the valve and actuator stem seals.
3. To provide a sealing load between the valve plug and valve seat to ensure the required degree of tightness.

Control valve manufacturers will normally provide full details of the maximum differential pressures against which their various valve and actuator/spring combinations will operate; the Table in Figure 6.6.10 is an example of this data.
Note: When using a positioner, it is necessary to refer to the manufacturer’s literature for the minimum and maximum air pressures.

Fig. 6.6.9 Two and three port formulae Fig. 6.6.9 Two and three port formulae

6.6.10 Typical manufacturer’s valve/actuator selection chart

For many applications, the 0.2 to 1 bar pressure in the diaphragm chamber may not be enough to cope with friction and high differential pressures. A higher control pressure and stronger springs could be used, but the practical solution is to use a positioner.

This is an additional item (see Figure 6.6.11), which is usually fitted to the yoke or pillars of the actuator, and it is linked to the spindle of the actuator by a feedback arm in order to monitor the valve position. It requires its own higher-pressure air supply, which it uses to position the valve.

Fig. 6.6.11 Basic pneumatic positioner fitted to actuator pillars (valve not shown) Fig. 6.6.11 Basic pneumatic positioner fitted to actuator pillars (valve not shown)

A valve positioner relates the input signal and the valve position, and will provide any output pressure to the actuator to satisfy this relationship, according to the requirements of the valve, and within the limitations of the maximum supply pressure.

When a positioner is fitted to an ‘air-to-open’ valve and actuator arrangement, the spring range may be increased to increase the closing force, and hence increase the maximum differential pressure a particular valve can tolerate. The air pressure will also be adjusted as required to overcome friction, therby reducing hysteresis effects.

Example: Taking a PN5400 series actuator fitted to a DN50 valve (see Table in Figure 6.6.10)

1. With a standard 0.2 to 1.0 bar spring range (PN5420), the maximum allowable differential pressure is 3.0 bar.
2. With a 1.0 to 2.0 bar spring set (PN5426), the maximum allowable differential pressure is increased to 13.3 bar.

With the second option, the 0.2 to 1.0 bar signal air pressure applied to the actuator diaphragm cannot provide sufficient force to move an actuator against the force provided by the 1.0 to 2.0 bar springs, and even less able to control it over its full operating range. In these circumstances the positioner acts as an amplifier to the control signal, and modulates the supply air pressure, to move the actuator to a position appropriate to the control signal pressure.

For example, if the control signal was 0.6 bar (50% valve lift), the positioner would need to allow approximately 1.5 bar into the actuator diaphragm chamber. Figure 6.6.12 illustrates this relationship.

Fig. 6.6.12 The positioner as a signal amplifier Fig. 6.6.12 The positioner as a signal amplifier

It should be noted that a positioner is a proportional device, and in the same way that a proportional controller will always give an offset, so does a positioner.

On a typical positioner, the proportional band may be between 3 and 6%. The positioner sensitivity can usually be adjusted. It is essential that the installation and maintenance instructions be read prior to the commissioning stage.

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.

Sometimes referred to as a SMART positioner, the digital positioner monitors valve position, and converts this information into a digital form. With this information, an integrated microprocessor offers advanced user features such as:

• High valve position accuracy.
• Adaptability to changes in control valve condition.
• Many digital positioners use much less air than analogue types.
• An auto stroking routine for easy setting-up and calibration.
• On-line digital diagnostics*
• Centralised monitoring*

*Using digital communications protocols such as HART®; Fieldbus, or Profibus.

The current industrial trend is to provide equipment with the capability to communicate digitally with networked systems in a Fieldbus environment. It is widely thought that digital communications of this type offer great advantages over traditional analogue systems.

Selecting a pneumatic valve and actuator
In summary, the following is a list of the major factors that must be considered when selecting a pneumatic valve and actuator:

1. Select a valve using the application data.
2. Determine the valve action required in the event of power failure, fail-open or fail-closed.
3. Select the valve actuator and spring combination required to ensure that the valve will open or close against the differential pressure.
4. Determine if a positioner is required.
5. Determine if a pneumatic or electric control signal is to be provided. This will determine whether an I to P converter or, alternatively a combined I to P converter/positioner, is required.

Rotary pneumatic actuators and positioners
Actuators are available to drive rotary action valves, such as ball and butterfly valves. The commonest is the piston type, which comprises a central shaft, two pistons and a central chamber all contained within a casing. The pistons and shaft have a rack and pinion drive system.

In the simplest types, air is fed into the central chamber (Figure 6.6.18a), which forces the pistons outwards.

The rack and pinion arrangement turns the shaft and, because the latter is coupled to the valve stem, the valve opens or closes.

When the air pressure is relieved, movement of the shaft in the opposite direction occurs due to the force of the return springs (Figure 6.6.18b).

It is also possible to obtain double acting versions, which have no return springs. Air can be fed into either side of the pistons to cause movement in either direction. As with diaphragm type actuators, they can also be fitted with positioners.
Fig. 6.6.18 Spring return rotary pneumatic actuator Fig. 6.6.18 Spring return rotary pneumatic actuator

Air supply
An adequate compressed air supply system is essential to provide clean and dry air at the right quantity and pressure. It is advantageous to install an individual coalescing filter / regulator unit ahead of the final supply connection to each piece of equipment. Air quality is particularly important for pneumatic instrumentation such as controllers, I to P convertors and positioners.

The decision to opt for a pneumatically operated system may be influenced by the availability and / or the costs to install such a system. An existing air supply would obviously encourage the use of pneumatically powered controls.

In order to position the control valve in response to the system requirements a modulating actuator can be used. These units may have higher rated motors (typically 1 200 starts/hour) and may have built-in electronics.

A positioning circuit may be included in the modulating actuator, which accepts an analogue control signal (typically 0-10 V or 4-20 mA). The actuator then interprets this control signal, as the valve position between the limit switches.

To achieve this, the actuator has a position sensor (usually a potentiometer), which feeds the actual valve position back to the positioning circuit. In this way the actuator can be positioned along its stroke in proportion to the control signal. A schematic of the modulating actuator is shown in Figure 6.6.21.

Fig. 6.6.21 Integral positioning circuit for modulating electric actuators

Pneumatic actuators have an inherent fail-safe feature; should the air supply or control signal fail the valve will close. To provide this function in electric actuators, ’spring reserve’ versions are available which will open or close the valve on power or control signal failure. Alternatively, fail-safe can be provided with battery power.

Electric actuators offer specified forces, which may be limited on spring reserve versions. The manufacturer’s charts should always be consulted during selection.

When sizing an actuator, it is wise to refer to the manufacturer’s technical data sheets for maximum differential pressure across the valve (see Figure 6.6.22). Another limitation of an electric actuator is the speed of valve movement, which can be as low as 4 seconds / mm, which in rapidly varying systems may be too slow.

Fig. 6.6.22 Typical manufacturer’s electric actuator selection chart

The market for intelligent process control valves is one which has grown rapidly in recent years, with these products offering huge advantages over manual valves. This is not to say there aren’t drawbacks, however. Many of these intelligent valves offer levels of sophistication that many users simply don’t need, and offer options such as fieldbus for monitoring and diagnostics that are unused in a large number of process applications. Some products even necessitate the use of fieldbus for their operation, yet only a small percentage of process valve users have actually adopted fieldbus communications. These added extras and unnecessary sophistication can add significant cost to the installation, and increase set up times.

Fluid handling equipment specialist Heap & Partners set out to design a product which would offer all the advantages of an intelligent valve, whilst addressing the need for easy installation, simple set up and maximum operating efficiency. And recognising that 4-20mA was still a much more widely used control option than fieldbus, the company determined that use of fieldbus should not be an implicit requirement for use of the valve - rather, it was decided that users should be able to specify options for the likes of fieldbus communications, diagnostics and flow monitoring.

The result of the development work is the Storm valve - a smart valve positioner, typically costing 30-40% less than alternative products. The design comprises an analogue pneumatic positioner, based around piezo pneumatic technology from Hoerbiger-Origa. Intelligence is provided by an in-built 16-bit processor, which is configured via an integrated three-button keypad and LCD display. Use of the RISC processor ensures accurate control, and enables a greater degree of functionality to be built into the unit as standard without adding cost.
Automatic calibration

Upon installation, the unit is automatically calibrated and requires no fine adjustment of either the air inlets or the electronic controls. With a typical setup time of just 2.5min, the Storm positioner can provide a time saving of around an hour per valve installation. The set-up routine includes auto-tuning of the PID control loop, and there is also an option to calibrate the 4-20mA loop.

Using a PID control algorithm is the key to providing enhanced operation. Three-term control eliminates overshoot whilst increasing the speed and accuracy of response. An attractive feature of the Storm positioner is that the standard unit can be readily programmed to suit a wide range of options, such as direct reverse, limited travel and limited range, all of which are often optional extras on alternative designs. The unit can also be supplied pre-programmed to match specific performance curves for individual applications.

At the heart of the positioner is a Hoerbiger-Origa piezo valve - a simple three port, two position valve in which the air flow is switched by a piezo element. Operating rather like a bimetallic strip, the piezo element deflects in proportion to an applied voltage, so that the valve may be used in a simple on/off switching function or in a proportional mode.
Ideal alternative

For the Storm positioner, the piezo technology offers an ideal alternative to solenoid operation. Because it operates on a minute voltage, the piezo valve can be driven directly from the 4-20mA data signal, removing the need for a separate power supply. And there is no requirement for the circuit protection normally associated with solenoid valves. The technology offers very fast operation, with reaction times typically less than 2ms. In addition, a power rating as low as 0.014mW means that piezo valves are intrinsically safe. Fully certified to EEx 1A IIC T5, the Smart positioner can be specified for use in hazardous areas without added explosion protection costs being incurred. Further protection is provided by a specially designed enclosure sealed to IP66.

Another advantage of the use of piezo technology is that it eliminates the problem of static air bleed. The pneumatic positioning system is extremely efficient in its air consumption, with the control system flowing measured volumes of air during the positioning operation only. Once in position, the air flow is totally closed off, which eliminates the air wastage associated with designs that require a constant air bleed.

Piezo technology explained

Pneumatics control valves are most commonly operated by solenoids, to facilitate the easy interfacing with electronic controllers. While convenient and relatively low cost, solenoids are not particularly efficient - they heat up, and even with low power solenoids, typically have an electrical consumption of several Watts.

In explosion-proof environments, remote control and battery operated systems, energy consumption is particularly important and demands careful consideration when specifying products.

By comparison, piezo-pneumatic operators offer a low energy electro-pneumatic interface. This technology has minute power demands, is extremely fast in operation and, with practically no moving parts, is extremely reliable.

At the heart of all Hoerbiger-Origa piezo operators is a tiny piezo valve - a simple three-port, two-position valve in which the air flow is switched by a piezo element. Operating rather like a bimetallic strip, the piezo element deflects in direct proportion to an applied voltage, so that the valve may either be used in a simple on/off switching function or in a proportional mode to produce an analogue pressure output.

By integrating the piezo valve into an ISO standard pilot operator, Hoerbiger-Origa offers a low energy piezo technology, as an alternative to solenoid operation for most standard pneumatic valves, providing significant application advantages:

* Fast operation - reaction times are typically less than 2ms
* No requirement for the circuit protection normally associated with solenoid valves
* A single pilot operator may often be used where two conventional solenoids were previously required, as piezo valves hold their switched position
* With a power rating as low as 0.014mW, piezo valves are inherently intrinsically safe
* Piezo valves can be actuated directly by data signals using simple two-wire technology and are compatible with most types of fieldbus systems
* Switching, proportional and pulse width modulation control modes are all possible.

Standard units are offered with protection to EEx ib II CT6/T5 and EEx ia II CT6/T5 and also with NAMUR connections.

Piezo technology is finding an increasing number of uses in a great variety of applications. Its low power consumption and non-sparking characteristic makes it ideal for usage in intrinsically safe environments where no spark can ever be tolerated due to the high risk of explosions. Its tiny size and virtually indestructible ruggedness means it is also being adopted for use in mobile equipment. The fact that it can emulate both analogue and digital systems is also proving a great user attraction.