Control Engineering Online Update for May 13, 2005

The control of automated production plants without digital data communication is nearly unimaginable today. In factory automation, signal transmission and control of all plant functions by means of bus-based data networks is state of the art.

However, in process automation, digital bus technology is gaining acceptance rather slowly1. The reasons go beyond long plant life spans and innovation cycles and into the unique requirements of these production facilities. The immense requirements on plant availability, operational reliability, and especially on safety in explosion/hazard environments call for dependable technologies and protection mechanisms. Additionally there is the desire for sensor and actuator power supply that employs the same cable used for transmission of measurement values and control signals, as with 4-20 mA technology.

Explosion/hazard areas
A widely used protection method for explosion protection in Zone 1 and Class I, Div. 1 is intrinsic safety Ex i. Using this method, energy is limited in the electrical loop to prevent the formation of sparks that could ignite surrounding gas mixtures. Devices designed in accordance with these intrinsic safety rules allow maintenance and repair work even under environmental conditions where an explosion hazard exists. With conventional 4-20 mA technology, the energy limitation in the loop of each individual field device is achieved by means of appropriate explosion protection barriers, or remote I/O interface modules. For each one of these loops, intrinsic safety must be individually confirmed and documented.

A significant benefit of fieldbus is the ability to connect several field devices with a single cable to the digital communication card of the control system, referred to as the host. By means of appropriate power supply modules, the segment can be supplied intrinsically safe. Unfortunately, the energy limitation required to achieve intrinsic safety dramatically reduces the number of devices that can be connected to one loop.

The approach used to confirm intrinsic safety by calculation is referred to as the Entity Model. This model allows six to eight devices on the segment instead of the maximum possible of 31. An empirical study, conducted by the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, resulted in the Fieldbus Intrinsically Safe Concept (FISCO)4. This concept allows up to 10 field devices to be connected to one intrinsically safe fieldbus segment, with significant restrictions on cable and spur lengths, and tough requirements on device parameters and cable quality.

Until recently this formed a significant obstacle for the general acceptance of fieldbus technology in process automation because the investment required to install intrinsically safe fieldbus technologies far outweighed the benefits. The visible advantages of fieldbus devices, such as enhanced diagnostic capabilities, preventive maintenance, and extended functionalities could not be realized due to considerable investment costs.

The latest developments in fieldbus installation technology allow users to realize the full benefits, even in explosion-hazard environments. One can even reduce the investment costs compared to conventional technology3.

This goal was achieved by stepping away from pure intrinsic safety, and moving toward a combination of two explosion protection methods: 1) increased safety Ex e Div. 2 wiring method and 2) intrinsic safety Ex i (I.S.).

These concepts divide the explosion protection by function. Intrinsic safety, with all its benefits, is limited to the necessary portion of the segment, namely the individual field device and its connection spur. The passive wiring level (the trunk cable and fieldbus distribution) is designed according to the protection method Ex e, or Div. 2 wiring method. Therefore, it is not subject to energy limitation, which allows the coupling of a high supply current onto the fieldbus trunk. Now the maximum number of participants and maximum cable lengths are possible in hazardous zones and in safe areas.

The central component of this concept is the fieldbus barrier, which integrates the functions of fieldbus distribution and intrinsic safety separation. The field barrier allows daisy chaining of the non-I.S. high power trunk, exactly like a typical junction box, but provides four intrinsically safe outputs for connection of the field devices. This field barrier concept (Figure 1) forms the basis of the FuRIOS study 5 (conducted by major users in the European chemical industry). As a result, any limitations that previously hampered the adoption of fieldbus technology in explosion-hazard areas no longer exist.

Adapted protection mechanisms
In case of a functionality fault with 4-20 mA technology, only the affected sensor or actuator will fail. Due to the conventional one-to-one wiring, a lead breakage or a short-circuit affects only one signal loop, which does not typically result in significant plant downtime. With fieldbus technology, however, this becomes more critical, because a failure at one segment affects several participants, which in a worst-case scenario could be 31 field devices.

Comprehensive fieldbus installation systems offer a range of protection mechanisms. To prevent negative feedback from short circuits in one connection spur to the remaining segment, one should use junction boxes with short-circuit current limitation. Designed according to Ex nA[L], these junction boxes provide non-incendiary outputs and can be used in safe areas as well as in Zone 2 Class I, Div. 2. Thus a multitude of field devices in various explosion protection methods can be integrated into the fieldbus communication.

The same short circuit protection of the intrinsically safe spurs in Zone 1 Class I, Div. 1 is integrated into the field barrier, which in itself can be installed in Zone 1 Class I, Div. 2. To connect field devices designed in accordance with flameproof explosion protection method Ex d, appropriate segment protectors are available.

As for the fieldbus power supply, a high safety margin against failures is achieved by using passive filter modules. These are highly reliable and dissipate little heat, thus burdening the environment only minimally. Furthermore, power supply components can be installed redundantly, with a power conditioner or power hub. The latter features various insulation levels up to fully galvanic isolation, and includes specific diagnostic modules for control of the fieldbus segment, and indication of unsafe operational conditions.

To guarantee safe, disturbance-free digital communication, fieldbus termination resistors are required for each fieldbus segment. Transmission cables need to have superb quality in terms of balance and shielding in order to protect the data transfer against external noise irradiation. A state-of-the-art fieldbus installation system allows the utilization of different shielding and earthing concepts, especially the capacitive grounding concept as recommended in the Profibus Technical Guideline2and in Revision 2 of the Foundation Fieldbus Application Guide AG-163.

Fieldbus installation in reality
Figure 2 shows a complete topology following the fieldbus barrier concept with integrated fieldbus process interfaces. Based on the FuRIOS study, several major chemical production plants throughout Europe have been equipped according to this concept, with smaller installations in operation since 2003.

A series of currently drafted plant designs that incorporate fieldbus barrier topologies and fieldbus process interfaces indicate the increasing acceptance of fieldbus technology in process automation. New plant designs are increasingly based on all-fieldbus communication and get optimized accordingly, thus gaining the full benefit of the FuRIOS study’s recommendations. For existing plants with 4-20 mA technology or remote I/O systems, specific migration strategies smooth the path to a fieldbus future6. These allow, over several years, the use of regular maintenance downtimes to revamp an existing plant to modern fieldbus communication without major disturbances to the normal production process.

Thomas Kasten is product marketing manager at Pepperl+Fuchs, Twinsburg, OH; www.am.pepperl-fuchs.com

1. Kegel, G.: The future of Automation, Elektronik magazine Edition 13, 2002

2. PROFIBUS PA User and Installation Guideline, PROFIBUS Nutzerorganisation e.V., 2003

3. Tauchnitz, T., Schmieder, W., Seintsch, S.: FuRIOS: Fieldbus and Remote I/O – a system comparison, atp Automatisierungstechnische Praxis 44 (2002), Edition 12

4. Johannsmeyer, U.: Investigations into the Intrinsic Safety of fieldbus systems, PTB-Bericht W-53e, Pysikalisch-Technische Bundesanstalt, Braunschweig 1994

5. Kasten, T.: The applicability of the FuRIOS study, atp Automatisierungstechnische Praxis 45 (2003), Edition 3

6. Westers, T., Münkel, M.: From star cabling to fieldbus, Chemietechnik no. 5, 2003