Reliable and comprehensive data communications are essential elements in the modern process plant. Plant managers today demand access to more information, more quickly than ever before. Process operators must have the ability for full control at all times, 24 hours a day, 365 days of the year. Maintenance managers need the information required for efficient asset management.
By Mike Howard – Rotork
To meet these requirements, design engineers incorporate field communication networks to enable critical plant equipment to be controlled and monitored by a Distributed Control System (DCS). The DCS often controls multiple systems that are assigned to management, operational and maintenance tasks, exchanging data about the equipment and process under their control.
To assist the DCS and relieve it of some its workload, sites often use a master station. A master station provides the essential link between field devices and the site DCS or Programmable Logic Controller (PLC). Upstream communications between the master station and DCS is usually via a serial or Ethernet highway and for downstream communications between the master station and the field devices a 2-wire highway or loop is employed. As well as relieving the DCS of some of its workload, a master station’s main purpose is to monitor the field loop/highway health, collect and store feedback from the field devices and transfer commands from the DCS to the field devices. It can also be used as a local control station for field device operation and monitoring, and can remotely modify actuator parameters. Advanced capabilities should also enable master stations to support and be integrated with asset management systems. Access to field devices is simplified by a standard fixed database meaning that once a DCS has been configured with a master station database it can be copied multiple times for all other units on the site, greatly reducing DCS configuration time. This also boosts the efficiency of communication from the host and provides a single slave address for controlling and monitoring multiple field devices, freeing up data collection and processing time within the system.
The major advantage of using a master station is it provides the end user with a direct reduction in engineering effort and associated costs due to simple network design, plus significant savings on installation and commissioning time compared to traditional hardwired methods. It allows access to extensive actuator data, including asset management information, when compared to hardwired methods and as a result reduces downtime due to the increase in field equipment information. Available data can include history, status, diagnostics and alarm logs which can greatly assist the operator in determining valve condition and planning maintenance. This benefit can also make future expansion significantly quicker and easier to implement.
Master stations are usually available in either single, dual or hot standby configurations. The single option provides redundant host connections and redundant field network loops while the dual configuration sees two fully isolated independent units installed side by side. The hot standby variant offers full redundancy and every interface is replicated including Central Processing Units (CPU), network interfaces, customer control interfaces, power supplies and display. In the event of an error in the primary unit, a standby master station will automatically assume control of the network and receive all data.
Field Network Connectivity
With master stations, third party device integration is possible via either an open protocol or dedicated MOV control and monitoring protocol.
If a dedicated protocol is used, third party integration is achieved via hardwiring to an actuator or General Purpose Field Unit (GPFU). A dedicated MOV control and monitoring protocol will usually be simple and require no repeaters, terminators or line biasing. It should also allow for high node counts on long loop lengths (up to 240 devices on a 20 km loop are typical values), have good noise immunity and have a rapid MOV scan rate using a ‘report by exception’ protocol, meaning only new data is reported. All of these benefits should be achievable using low cost, standard instrumentation cable.
Open protocols, such as Modbus and Profibus, are widely used in industry but are general field communications protocols and offer no specific advantages for MOV control and monitoring. In theory, it should be possible to incorporate multiple vendors’ products on the same highway, with the above advantages, but in reality open protocols are limited by their governing bodies’ standards and rules for highway length, node count, line termination and biasing requirement and data transfer telegram formats. Rotork’s own product, the Rotork Master Station, supports both a dedicated MOV control and monitoring protocol (Pakscan™ Classic) as well as a Modbus RTU protocol for field communications, which both offer third party device integration.
Classic highways are always connected in a redundant loop configuration, whereas Modbus field highway configurations can be redundant in the form of a loop or dual channel highway, a single channel highway option is also available.
The Rotork Master Station uses Modbus for host communication via Ethernet (Modbus TCP) and/or a serial highway (Modbus RTU). The master station’s database presented to the host is independent of the field network protocol, giving a common open industry standard interface to the host for process control information. Rotork’s unique feature of mapping any slave device automatically into its standard database structure means configuration is both quick and simple, and existing sites with legacy Master Stations will already have the database structure pre-mapped.
An increased amount of complex information, such as historical and diagnostics data, can be reported using a 2-wire system compared to multi-core cabling. This means not only can the condition of an asset be shown but also the severity of the problem. Some systems also have in-built diagnostic features with automatic fault location indication and the availability of communication performance data to aid maintenance.
Valve actuators typically have up to 18 conductors to provide input and output signals for control, status, feedback and alarms. These are available as discrete signals at a terminal bung or connector block in the devices and are required for an automation control system to control the process.
Intuitive Touch Screen Display
Most master stations feature large touch screen displays to allow standalone operation in the event of unavailability of the host, as the master station can be used as a local control station for field device control and monitoring, and to modify actuator parameters remotely. The display provides a multilingual user interface to enable device setup, interrogation and issue resolution. The Rotork Master Station features multiple languages including Chinese. NAMUR standard diagnostic icons and coloring are incorporated for easy recognition of status and alarm severity, while screen to screen navigation is simplified by having identical screens for both web page access and the integral touch screen, each using the same menu structure.
Many master stations require the user to download specialist software to enable monitoring of the master station via a laptop and use a different menu structure on their local HMI. However, no such complications occur with the Rotork Master Station as no specialist system configuration software is needed, any web browser can access the Rotork Master Station. This simple interface is available on all the Rotork Master Station Ethernet ports, with two located on each side of the unit for redundant host connections and one specified for service use. This dedicated service Ethernet port maintains Local Area Network (LAN) separation between configuration or monitoring systems and the DCS systems controlling the process.
Essential capabilities for the modern field device control network include security, built-in redundancy, the ability to monitor and report status changes within a defined time, detect and isolate faults and communicate alarms to the supervisory system to enable identification and mitigation. This next generation of control systems provides operators with unprecedented access to data for asset management support.