The design, documentation and installation of I/O and marshalling can be a lengthy and labour intensive exercise, meaning that it often falls on the critical path of a project. This process has hardly changed since distributed control systems were first introduced. The introduction of electronic marshalling has enabled a paradigm shift in the industry, drastically reducing the time taken to execute projects as well as enabling a 50% reduction in controller cabinets, a 40% reduction in cabinet footprint, and a 90% reduction in intra-cabinet wiring.
Anybody who has ever been closely involved with an automation project knows that intensive engineering involvement is required. This has always been the case, and it didn’t change when distributed control systems (DCSs) were introduced in the 1970s.
A typical, traditional DCS project with conventional wiring requires engineers to list the I/O requirements and size the controllers, then design the power, grounding and fusing; design the I/O and the cabinets; draw up the P&IDs, conduit and cable layout; and define their spares needs. They must then write up the process narratives and scope out an installation package.
The next step is to sort out the marshalling cabinets and the junction boxes. This entails cabinet and junction box design, specifying the jumper and termination requirements and finally drawing up the wiring and cable layout diagrams. A change at any time to the I/O type or quantities has a ripple effect throughout the whole process, requiring a review of the work carried out to date and modifications as needed.
Installation also requires extensive engineering involvement. The multi-core cables from the field are brought into a cabinet, and then the technicians have to wire them over to the controllers. This sounds simple, but the process involved is anything but simple.
During the design phase, the designer has to draw the interfaces and produce a wiring diagram. The field technician has to follow this diagram, physically wire each and every device and then ’marshall‘ the wiring in a junction box or marshalling panel. Wires must then be installed from the marshalling panel to the correct I/O card and from there to the controller. This can be 2-3 more wiring ’hops,’ meaning that every single device and its associated wires are aligned to a very specific controller, with multiple wiring hops in between. In order for it to work correctly this process must be followed, to the letter, for every single device in the plant.
Cutting time and cost
The introduction of Emerson’s electronic marshalling technology has significantly changed the execution of automation projects. The field wires still come in to a marshalling panel, just like they did before, but from there on in everything is different. Instead of the cross wiring being used to transmit the signal to the right I/O card, the signal is carried up a backplane to a set of redundant I/O cards, where they are connected to controllers via a high bandwidth, high-speed copper Ethernet or fibre optic connection. The technician no longer needs to match up the wires 3-4 times and be exacting about each connection.
The secret to electronic marshalling is a little device called a CHARM—or CHARacterisation Module. It’s an A to D converter that allows the engineering team to dynamically characterise the needed I/O in a cabinet, regardless of whether it is an analogue or digital connection. The CHARM doesn’t care. Then, the connection to the controllers is accomplished through a digital connection. The technician no longer needs to worry about connecting an I/O point to a specific controller as any I/O can be associated with any controller.
The use of electronic marshalling will significantly cut the time and cost of cabinet wiring, and will also reduce the number of wiring mistakes. In addition, electronic marshalling will save time during the design phase of the project. Since a CHARM can characterise any signal, engineers don’t need to design custom cabinets. There is now only one design needed—a standard cabinet. There is also a remote junction box available where the same principles of electronic marshalling apply, only out in the plant as opposed to in the cabinet room.
A large amount of engineering work is eliminated in the design phases of a project by using electronic marshalling. Cabinet I/O and fusing designs are no longer required, as the cabinets are based on a standard design with no optional extras (none are required). The marshalling cabinet designs are eliminated altogether along with their associated wiring diagrams and cable layouts. Anybody who has ever seen the amount of wiring diagrams that are required for a typical project will know how significant that is.
Using electronic marshalling can result in a 50% reduction in controller cabinets, a 40% reduction in cabinet footprint, and a 90% reduction in intra-cabinet wiring.
Streamlining the design phase
The benefits that electronic marshalling brings to the design phase are even greater than just the cabinet design. If we look in detail at how a typical project is managed you will see just how labour intensive it is. When the process design is finished, the control system design can be completed, then there is a Factory Acceptance Test (FAT), before the system is shipped and installed followed by a Site Acceptance Test (SAT). After the SAT, the plant can be started up. All of these steps are sequential and are often on the critical path of the project.
Using a conventional approach, the system designers normally like to wait until the process design is about 99% complete. This is because every wire to every card and controller needs to be designed and documented. The ripple effect of any changes to the design is huge. If the process design is late, the delay will cause disruptions through the whole project.
A common event in an automation project is the addition of a new process unit. This requires a complete redesign of the controllers, I/O and cabinets for that area. This means change orders and also means that all the drawings that have been done to date have to be redone. This delays FAT and shipment and incurs change order costs.
Another frequent occurrence in a project is that some of the I/O specifications are wrong. These errors are generally found during installation. Drawings have to be changed and delivered to the cabinet electrician, who will wire them up. Changing drawings takes time and involves re-scheduling of the work on site, causing disruptions.
Using electronic marshalling to execute a project reduces all that complexity. Because control system design only needs to be based on the total I/O count and whether the marshalling is placed in a rack room or in the field, full completion of the process design is not necessary. Only 90% completion of process design would normally be acceptable before starting system design. Also, because the field cabinets are a standard design they can be shipped at any time, ready for the site personnel to start placing them.
Should a new process unit be added, that’s easy to handle too. As soon as total I/O count is known the correct number of CHARMS Junction Boxes can be shipped. The control configuration can be incorporated with no delay and the only cost will be the extra equipment.
Should any of the I/O specifications be wrong, that presents no problem either. All the technician needs to do is to plug in the appropriate CHARMS.
An example in Sweden
One European manufacturer who will be taking advantage of electronic marshalling is Perstorp, a speciality chemicals company based in Sweden. Electronic marshalling was an important factor in their selection of Emerson’s DeltaV v11 control system for their migration project. This was because they wanted to minimise installation time and costly plant downtime during the upgrade.
Perstorp will deploy 792 single-channel CHARMS to relay 3000 Device Signal Tags to 60 DeltaV S-series I/O modules. This unique wiring technology is expected to reduce downtime during this upgrade by two days, helping to reduce lost production. The migration procedure will be performed by Perstorp’s engineering team and is expected to take two weeks to complete.
The use of electronic marshalling does not preclude or supersede the use of other instrument communications technologies. For example, it is often possible to eliminate wires altogether using wireless instrument communicating via WirelessHART.
Alternatively, Foundation fieldbus is a good choice if customers would like to offload controllers by running control loops in the field. This gives the added advantage of maintaining smooth control functionality should communications with the controller be lost for some reason. Electronic marshalling is expected, however, to become a pre-eminent choice of customers seeking to eliminate confusion, extra work, and scheduling headaches. End users should be able to choose the exact type of field device protocol that is demanded by project circumstances without needing to worry about difficult engineering and installation issues.