Modern Positioners
Developments concerned directly with electro-pneumatic positioners

The ever more pressing demands for commercially viable and environmentally friendly production methods in all branches of industry have led over the past few years to rapid developments in process control technology. Automation has also made significant progress in the field of control valves.. the increasing integration of actuator, pneumatic drive and electro-pneumatic positioner into a functional whole and the inclusion of additional intelligence in the positioner, including bidirectional communications, is of tremendous benefit to the user from an ecological and economic point of view.

The developments in recent years in pneumatically driven valves have led to a completely new outlook on the part of users and manufacturers. Features such as compact design, reliable commissioning, improved control quality, bidirectional communication, diagnosis and maintenance, cost of ownership have been and remain at the forefront of the research and development activities, which, since the appearance of digital positioners more than five years ago, have experienced almost exponential growth. While technology such as digital signal processing in electro-pneumatic positioners or binary pneumatics with piezoelectric valves in two-wire circuits are already state-of-the-art, the following article examines current development trends.

Design Concept
In any process engineering application, the proper functioning of the final control element plays a key role not only in the level of quality that can be achieved but also in plant safety. In spite of the comprehensive standardisation of almost all aspects of control element quality, there have been numerous innovations in recent years that, with regard to the reliability of the valves, measurement of the amount of wear and adherence to accident prevention measures, have improved the situation still further. Among the improvements that affect the valve bodies directly can be counted the use of new materials, reduction in cavitation and the development of low-noise valves. In this article we are going to concentrate on the developments concerned directly with pneumatic drives and electro-pneumatic positioners.

Fitting the Positioner
A solid and vibration-free fitting of the positioner to the actuator is a significant factor in the control quality of the overall system. The compact installation with a short lever directly on the yoke support has proved to be of great benefit. In this arrangement, the travel setting elements of the positioner are protected by the yoke. An automatically adjusting roller ensures there is no play in the stroke on the drive spindle. This type of design means there are no freely accessible moving parts such as an external lever, so that in addition to compactness and functional reliability, the requirements in terms of accident prevention are also satisfied.

Air Distribution and Anti-Corrosion Measures
Air normally passes from the positioner to the pneumatic actuator through pipework or tubing. In addition to installation requirements and the space they require, a further disadvantage of these connections is their mechanical vulnerability: pipework and tubing are often misused as footholds or handles, with loose joints the result. A more elegant and reliable way of moving air between the positioner and the drive head is to have air connections integrated into the yoke of the pneumatic drive. This not only makes the airways more reliable but considerably simplifies the amount of work required when installing the positioner. A special add-on provided in modern instruments is the ability to mist the contact chamber as well as the drive head with the instrument air. This misting procedure provides reliable anti-corrosion protection of the positioner and spring housing, even in aggressive conditions.

Commissioning
The simple and reliable commissioning of the intelligent positioner is one of its main advantages compared to normal instruments. And its not just a question of "time is money": the adaptation performed by the positioner with respect to its valve and the resulting improved control quality is a far more significant factor in the long term on the commercial viability of the application. The commissioning of modern positioners can either be performed via a display and a dedicated keyboard on the instrument itself or by serial communications via a PC with the appropriate software. The following procedure is used in the instrument shown in Figure 1: once "initialisation" mode of the instrument has been selected, an automatic start-up is launched under the control of the positioner. First of all the zero point and the stroke range are determined by having the positioner drive the valve to its end positions. At the same time, the actuating times under the specified compressed air conditions are measured. The actuating times can be modified using adjusting screws on the positioner if this is expected to lead to an improvement in the control quality, e.g. in the case of very small drives and consequent very short actuating times. Suggestions for these modifications can be made by the positioner itself during commissioning. Once the end positions and actuating times have been determined, the most suitable dynamic parameters for position control using adaptive algorithms are determined. Although the total initialization procedure only takes a few minutes, it produces excellent control results that can always be checked on-line and modified if necessary to suit changing conditions.

Control Quality
Whereas commissioning procedures and operation of the new generation of instruments have become much easier and are also much more powerful, considerable improvements have also been made in the new technology with regard to control quality. The full benefit has not yet been seen in this area, however, because of the finite memory capacity of the microcontrollers being used. This applies in particular to problems with all types of hysteresis, such as:
- Drives with a high degree of friction
- Stuffing boxes with a high packing inversion effect
- Piston drives that tend to slap when they change direction
- Valves with a high breakaway moment
- Play or backlash in the drive mechanism
Positioners with the usual jet and baffle plate technology without any type of intelligent control have no chance of satisfactorily controlling disturbances in such cases. This is the big chance for intelligent instruments: Once a problematic drive/valve combination of this type has been identified, a software strategy specifically designed to counteract this effect and improve control quality can be implemented using appropriate control algorithms. However, the memory requirements required to counter such disturbances are considerable. firstly, the relevant disturbance from among all the known types must be identified. This is particularly difficult in cases where the disturbances is not of a pure sort or two or more such effects overlap each other. The choice of suitable countermeasures also requires a considerable amount of computing effort, as a large number of parameters have to be calculated. The amount of firmware that can be implemented is, however, dependent on the power of the microcontroller being used. Most intelligent positioners are available as 2-wire devices (4 - 20 mA) in which the electrical power input is limited. This rules out external memory modules, and the firmware required may not exceed the amount of integrated program memory available on the chip. This is probably the main reason that only limited success has been recorded in this area to date, although it has to be said that each success is in itself an encouraging milestone on the way to a comprehensive disturbance identification and removal concept. Nevertheless, even the most optimistic observer should be aware of one thing.. each complex software strategy can at best only alleviate the symptoms of these disturbances in problem valves, not remove the actual cause.

Bi-Directional Communications
The most widely used way of transmitting process engineering variables today is the 014 to 20 mA signal. Transmitters, controllers, recorders, positioners and many other types of instrument exchange data using this standardised current signal. The advantage of this signal lies in its ease of use and, of course, the fact that it is understood by practically every automation instrument in the world. The disadvantage is that a two-wire line has to be laid for every signal to be transmitted. In practice, this limits the extent to which more intensive communications between the process instruments and a visualisation level can be implemented by the plant operator. New standards have meanwhile established themselves that permit bidirectional communication between automation devices and which enable a practically unlimited amount of information to be sent down a pair of wires. The HART protocol, which is well known and extensively used in the world of transmitters, is now available for practically all intelligent positioners as well.

HART Protocol
The HART protocol (highway addressable remote transducer) uses the 4-20 mA signal as the carrier for additional information. A frequency modulated signal is superimposed on the current signal and uses the two fixed frequencies of 1200 Hz (digital "1") and 2200 Hz (digital "O"). This technology transmits data at the rate of 1200 bits per second. Other variables in addition to the analog information in the 4-20 mA signal can be transmitted. An advantage of HART communication is that it uses the established wiring technology and is easy to implement. All that is required to decode the HART information is a HART modem attached, for example, to a PC running the appropriate software. Handheld terminals can be used in the field as well as in hazardous areas. The disadvantage of HART is that the transmission of data is relatively slow and that every instrument in the field requires its own two-wire connection. Although multidrop operation, i.e. the connection of several instruments to the same two-wire connection, may be possible in principle, this is of very little practical significance as the most important process variables to be transmitted, such as a transmitter signal or a positioning signal, have to be transmitted cyclically using the HART protocol, which, owing to the low baud rate, is only of interest in processes with very high time constants.