New FOUNDATION Fieldbus Interface Techniques Increased Availability and Reduced Costs. Andreas Agostin Pepperl+Fuchs

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1 New FOUNDATION Fieldbus Interface Techniques Increased Availability and Reduced Costs Andreas Agostin Pepperl+Fuchs Abstract InfraServ Hoechst and Aventis Pharma of Germany performed a comparison study between an installation using conventional technology and a comparable planned installation using fieldbus. Conclusion: fieldbus reduces overall costs if used with the herein described FieldBarrier. Existing fieldbus topologies for intrinsic safety (I.S.) applications in explosive atmospheres consist of a host controller, a Fieldbus power supply and the field instruments. The new topology replaces the traditional fieldbus power supply with two redundant power supplies and a failure-free fieldbus power conditioner. The spur connections of the field instruments are replaced by the FieldBarrier or the Segment Protector. The FieldBarrier is a field-mounted junction box that includes four integrated IS barriers, while the Segment Protector is a junction box that includes short circuit protection for the spur outputs. This topology has the following advantages: - Redundancy for the power supply - Standard power supplies with higher output current can be used - Increased number of instruments can be connected - Independent I.S. barriers for each spur means increased availability of instruments - In case of a failure at one spur (e.g. short circuit) the other spurs or the trunk are not affected The new concept is also beneficial in non-ex applications. The rationale will be covered in this document. Page 1 of 11

2 1. Introduction In recent years, fieldbus began to replace the conventional 4-20 ma signal interface technology. Several requirements need to be fulfilled by a fieldbus to be accepted by end users, especially in process industries: - Low power consumption - No separate power supplies for the instruments - Allow intrinsic safety (I.S.) installations The instrument manufacturers can fulfill the first two requirements while interface manufacturers like Pepperl+Fuchs can solve the intrinsic safety requirements. However, end users ask for two additional requirements in a fieldbus installation: - Increased reliability of the fieldbus power supply (implementing redundancy) - Increased availability of the field instruments and protection against failures 2. Fieldbus topology Figure 1 illustrates a typical fieldbus topology for a safe area application that shows a control system with a fieldbus communication controller, the fieldbus cable, a FF power supply and several field instruments. The device at the end of the bus cable is a fieldbus terminator that is needed to ensure proper communication. Most of the instruments are bus -powered and the communication signal is superimposed on the DC power on the same pair of wires (shielded twisted pair fieldbus cable). The communication controller in the DCS usually does not supply power for these buspowered devices. Hence, it is necessary to use a separate fieldbus-dedicated power supply. Andreas Agostin Page 2 of 11

3 FF interface H1 segment Terminator FF Power Supply Bus-powered instruments 24 VDC Fig 1. Typical fieldbus topology in non -hazardous areas A fieldbus power supply consists of a power converter with voltage regulator and a filter (conditioner). The power converter transforms the variable input voltage (not necessarily 24 Vdc) into the desired fixed output voltage. The filter decouples the communication signal from the voltage regulation unit. Without filtering, the power regulation would destroy the communication signal since the regulator tries to maintain a fixed output voltage. The fieldbus power supply and the field instruments are connected in parallel to the fieldbus cable. This results in the following problem - If there is a failure of the power supply or if there is a short circuit at any of the connected field instruments, signals from every device will be lost. The new topology provides two solutions. One solution covers the loss of the power supply while the other covers the availability of the field instruments. Andreas Agostin Page 3 of 11

4 3. Power Supply Redundancy (Non-Hazardous Area) 3.1 Introduction To create a reliable fieldbus installation, the above-mentioned topology requires redundancy for active power supplies. Active is stressed since passive devices are considered and proven to be more reliable and failure-free when compared to an active device. A typical passive device is, for example, a backplane motherboard equipped with screw terminals, module sockets, connectors, diodes and resistors. Examples of active devices include galvanically isolated power supplies, barriers or complex electronic components. Until recently, fieldbus power supplies have always been active. 3.2 Fieldbus Power Conditioner (FPC) Most fieldbus power supplies consists of a power converter and a filter (conditioner). The new approach is to divide the traditional fieldbus power supply into two components. Using this concept, there is no need to have a dedicated fieldbus power supply but only a standard power supply and a separate filter. The Fieldbus Power Conditioner (FPC) contains this filter. The filter is a passive device and contains only a few discrete, fixed-value components (inductors, capacitors and resistors). As such, the filter can be considered failure-free and does not need to be redundant. The standard power supply unit still must be redundant. Standard power supplies can be used but it is not possible to simply connect two independent power supplies in parallel. Since these supplies would not be compatible, each power supply attempts to regulate the output voltage concurrently. The connection can only be done through decoupling diodes. For this purpose, redundant power feed modules can be used (see Figure 2). Two, passive power feed modules are energized by two independent power supplies Andreas Agostin Page 4 of 11

5 and provide the voltage to the Power Rail. The FPC picks the voltage from the Power Rail and supplies it through the filter to the fieldbus cable. To fieldbus cable + - DIN rail with P+F PowerRail Power Feed Module FPC + - To power supply #1 + - To power supply #1 Fig 2. Redundant Power feed modules and FPC Whenever one power supply or one power feed module fails, the FPC will still be supplied by the other power feed module and power supply. Even triple redundancy is possible in the same manner. By using the FPC, not only is reliability increased but also the amount of hardware is reduced. As many as four FPCs can be energized by the two Power Feed Modules (see Figure 3). Fig 3. Redundant power feed modules and four FPC s Andreas Agostin Page 5 of 11

6 4. Intrinsic Safety (I.S.) Applications 4.1 Introduction With the fieldbus topology described in Section 2, the availability of all instruments depends on a single failure. The same problem applies to applications in hazardous areas (see Figure 4). FF interface H1 segment Terminator FF Power Repeater 24VDC Bus - powered instruments Fig 4. Typical fieldbus topology in hazardous areas The I.S. barrier (Power Repeater) not only isolates the safe area from the hazardous area (Ex and non-ex), it functions as a power supply for the bus-powered instruments. As such, the entire segment is lost if the I.S. barrier fails. Unlike the fieldbus power supply, the power repeater cannot be split into a power supply barrier and filter. From a safety perspective, it is not possible to connect multiple I.S. barriers for fieldbus (Johannsmeyer, 1994). Hence, it would be reasonable to connect only one instrument to one I.S. barrier. However this would make a fieldbus installation very expensive since additional wiring would be required and barriers would be under-utilized. One possible solution for this problem is the FieldBarrier. Andreas Agostin Page 6 of 11

7 4.2 FieldBarrier The FieldBarrier combines four I.S. barriers (channels) in a common IP66/Type 4 field housing. In the previous section, we described why it is necessary to connect only one instrument per barrier to ensure instrument availability. Figure 5 shows an alternate approach. Fig 5. FPC and FieldBarrier FF interface (host) 24 VDC Safe area FF Power Supply H1 segment FieldBarrier 2 Zone 1 or 2 Division 2 Zone 0 or 1 Division 1 Fig 5. FPC and FieldBarrier Each of the four channels provides enough current for up to four instruments, typically one or two devices would normally be connected in either FISCO (Johannsmeyer, 1994) or entity configurations. If one instrument is connected per channel, the maximum availability is achieved. If there is a failure along any of the four channels, the other FieldBarrier channels remain unaffected. Andreas Agostin Page 7 of 11

8 In addition, the outputs of the FieldBarrier are galvanically isolated from the host segment; therefore, potential equalization is unnecessary. Those who are familiar with this topic appreciate the tremendous cost savings of this feature. FieldBarrier itself consumes a limited amount of current from the bus to operate; hence, the connected field instruments can use almost all of the available current from the fieldbus power source. To increase the total number of instruments on the host, several FieldBarriers can be connected to the fieldbus trunk. Most of the available hosts support 16 devices on one communication controller segment. This number can only be achieved with the new FPC since this device produces as much as 3 times the amount of current than existing supplies. To allow this high current to be delivered into the hazardous area, the protection method Exe (increased safety) is used for the incoming bus connection. Alternatively, this same fieldbus segment can be installed within an acceptable wiring method for Division 2 applications. Mounting of the FieldBarrier is generally very close to the instruments. Since the FieldBarrier creates isolated spurs, the maximum distance between the instruments and the outputs is limited to 120 m. This distance is independent of whether the application involves a safe area or a hazardous area (Zone 1 or Division 2 mounting). This feature also allows operation without additional fieldbus terminators at each output and saves costs. Since all of the field instruments are directly connected to the FieldBarrier, it functions as a T junction box and there is no need to have additional junction boxes at the instruments. This is another cost saving factor. According to Fig 5 above, if the power supply fails, every signal from the system is again lost. As such, redundancy for the power supply is mandatory. Andreas Agostin Page 8 of 11

9 5. Power redundancy and increased instrument availability The FieldBarrier in conjunction with the FPC combines power redundancy and instrument availability, as shown in Figure VDC 24 VDC Safe area EExe or Div. EEx 2 e wiring FF H1 segment Long distance between the host and the field n Division 1 Division 2 Zone 0/1 Zone 1/2 Fig 6. FPC and FieldBarrier Several FieldBarriers can be connected to the trunk in order to use all of the available current provided by the power source. The 24 Vdc power source must be highly reliable (redundant). The FPC and redundant power feed modules described in section 3.2 must also be implemented. The redundancy of the power supply ensures the availability of power in the fieldbus segment while the current limitation and short circuit protection of the FieldBarrier ensures availability of the instruments in case of failure. Andreas Agostin Page 9 of 11

10 6. Power Redundancy and Increased Instrument Availability in Non-Ex Applications The Segment Protector for non-hazardous applications provides a topology similar to the FieldBarrier. Figure 7 shows this set-up. 24 VDC 24 VDC FF H1 segment Long distance between the host and the field n Segment Protector Safe area Fig 7. FPC and Segment Protector Andreas Agostin Page 10 of 11

11 7. Conclusion Although many fieldbus capable products have been available for years, the potential of fieldbus is just now being realized, mainly because end users and manufacturers needed to obtain knowledge and experience on the new technology. New generation fieldbus products like the FieldBarrier overcome the limitations of previous topologies and make it possible to incorporate the lower cost of this new technology. The unique features of the FPC, the Segment Protector and the FieldBarrier result in a new installation topology that can be used in hazardous and non-haz ardous applications. In addition to the improved reliability of the entire structure, and especially the availability of field instruments, a significant cost saving can be achieved. Reference Berge, J.: Fieldbus for Process Control: Engineering, Operation and Maintenance, ISA Press, 2002 Pepperl+Fuchs datasheets, brochures and catalogs: Andreas Agostin Page 11 of 11

Benefits derived from the FF specification FF-831

Foundation Fieldbus End Users Council Australia Inc. 9 Corcoran St Duncraig, WA 6023 P.O. Box Z5546 Perth, WA 6831 Benefits derived from the FF specification FF-831 Why we need it, and why it improves