26 August 2009
Ethernet hubris? There is no doubt industrial communications has overwhelmingly adopted and supported Ethernet as the medium of choice ever since it shed its 1980s legacy of coaxial cable and message collisions. But will it progress as far as some are now saying: becoming the medium for all plant communications, right down to the final instrument?
**************************
PHOTO: Ethernet can be made intrinsically safe, say several European companies. At Interkama 2008 MTL launched its intrinsically safe version of Power over Ethernet, called PoEx™. A key feature is the ‘Fast Switch’ technology which allows much greater amounts of power to be delivered to hazardous areas. Upon detecting the slightest increase of power on the line—the telltale indication of an emerging spark—the Fast Switch shuts down power immediately. Pepper+Fuchs introduced a similar product, called DART.
Speed and non-determinism used to be the major obstacles, but no more. SERCOS III, Profinet IRT, and EtherCAT are all Ethernet hybrids finding increasing use in high-speed motion control applications. They bring control to the microsecond level of achievement, and make it easy to integrate motion controllers with higher level supervisory systems.
And today, most PLCs connect with either Ethernet itself or one of the accredited Ethernet derivatives, such as Profinet. In so doing, an entire level of the celebrated plant pyramid has been annexed by Ethernet, filling all the area down from the top to the level of the programmable controllers.
Down below But now, what about that vast area below the PLCs, at the bottom of the pyramid where the PowerPoint artists draw in icons of motors, valves, and temperature transmitters? Not surprisingly, Ethernet’s twisted-pair tentacles are reaching down there, seeing what they can find.
Today a lot of this downward migration is via Ethernet based I/O systems. These are getting the stamp of approval from a growing number of vendors eager to supply the components. One of the latest is from Pepperl+Fuchs, which launched its Ethernet Remote I/O at Interkama this year (see photo below). It is basically the same I/O hardware used for RS-485 Modbus and Profibus networks, only now it has an Ethernet gateway that runs Modbus TCP.
The way P+F looks at it, Ethernet Remote I/O can be regarded as any other Ethernet device. It uses Ex e bus connections in Zone 1 or Ex nA in Zone 2 for explosion protection in the same way as the traditional RS-485 Profibus. The field loops are completely isolated and segregated from the Ethernet. Therefore any work carried out on the field loops will not have any adverse effect on neighbouring devices nor the bus.
The integration of the Ethernet Remote I/O into host DCS systems is accomplished with FDT technology and certified DTMs.
But is it safe down here?
The first question any instrument engineer working in the process automation area will ask is, is Ethernet safe enough for hazardous areas? Safety companies like MTL and HIMA say that it can be made so, and MTL released at Interkama 2008 its intrinsically safe version of Power over Ethernet (PoE) called the 9460-ET Series, along with a power supply that incorporates MTL’s ‘Fast Switch’ technology. MTL claims the technology has been proven in use in many mining applications, and with the ATEX and IEC-Ex certifications it can deliver the same benefits for the process industries.
The certifications permit the components to be mounted in a Zone 1 hazardous area with connectivity into Zone 0, or with suitable isolators and power supplies it can be mounted in a Zone 2 hazardous area and connected to intrinsically safe equipment in a Zone 0 or 1 hazardous area. And, following the accepted practice in the process industry of to providing both power and communications to the end device, The 9460-ET Series can deliver intrinsically safe power over Ethernet (MTL’s brand name is PoEx™) with a single Cat 5e or Cat 6 cable, allowing live connection and disconnection of the end device in Zone 0 and 1 hazardous areas.
So, the technology is there, and it can be done. Does anybody want to do it? There doesn’t seem to be a stampede in this direction, but you never know what the future will bring.
The great divide As noted above, one of the great divides of industrial automation is that of extending Ethernet connectivity below the PLC barrier.
While extending Ethernet to a PLC or DCS I/O block is very common, the idea of using it to connect to individual process or discrete sensing devices is relatively rare. But is that assessment changing?’ Carl Henning, deputy director, Profibus Trade Organization in the U.S., has typically suggested using multiple tools for networking applications. Is Ethernet driven instrumentation coming?
*************************
PHOTO: Pepperl+Fuchs extended its Remote I/O line to include Ethernet with Modbus TCP protocol with this product launch at Interkama 2009. P+F says it can be used as safely as any Modbus or Profibus network.
‘The cost does nothing but go down as usage goes up,’ he says. ‘It's going to become less expensive and therefore more practical to put industrial Ethernet into more devices. The limits you run up against are either environmental, such as the need for intrinsically safe instruments, or when the overhead associated with Ethernet is simply too much. It's nonsensical to use it to carry a one bit data load.’
And most instrument vendors, having survived the fieldbus wars, don’t want to hear about it. ‘We do not offer the Profinet bus system in our sensors,’ says Juan Garcia, a product manager at VEGA Grieshaber KG.
‘Profinet is normally used for bus systems not specially placed in the field area. It is a 4-wire system and our experience is that 2-wire sensors are favoured in the market. Profinet does not have an adequate Ex-concept, and this is of course mandatory for process automation applications.
‘Most of the Ethernet solutions do not have a standardised protocol which makes it difficult for us to decide what to use. This criticism is not specially relevant for Profinet, but it is for any Ethernet based bus system.’
Mr. Garcia says it is sufficient for an instrument maker to connect his device with Profibus-PA, Fieldbus Foundation H1, and possibly HART multidrop systems which are all designed for the difficult environments of process automation and the complex requirements of sophisticated devices like radar level sensors. If you want the data on Profinet, then connect your Profibus-PA system with the various couplers that are widely available. When connected the proper way, he says, ‘You have all the relevant information from the sensors available using adequate parameterisation software for example based on FDT/DTM (PACTware) technology.’
The question remains, will enough customers demand Ethernet connectivity to motivate instrumentation suppliers to produce it? While Ethernet networking and power over Ethernet (PoE) have spread to end devices in home, office, and commercial environments, industrial applications are still rare.
There are several main constraints:
Bandwidth overkill—Most sensors do not put out huge amounts of data, so they do not need the kind of bandwidth Ethernet offers. ‘Ethernet's primary advantage is that it is fast and can move large amounts of data quickly,’ says Charles Larson, director of technology for Moore Industries. ‘With typical process sensors, such as temperature and pressure transmitters, the data are not changing so rapidly that the speed advantage of Ethernet has any value. Compared to other options such as 4-20 mA with HART, Foundation Fieldbus, and Profibus, Ethernet requires faster and more costly microprocessors to support it. It is also limited to 100 m cable length without repeaters.’
Additional transmitter cost—‘Adding Ethernet (with internal switching) to field instruments would add an extra level of complexity and cost over and above 4-20mA HART,’ says Gareth Johnston, Wireless Product Manager of ABB. ‘An Ethernet solution at the instrument level would need to be as simple as 4-20mA HART to become successful. 4-20mA HART is still by far the dominant technology for field instrumentation despite the presence of more sophisticated digital only buses.’ Perhaps the cost differential isn't as great now as a few years ago, but there is still a gap
Hazardous environment limitations—One aspect that has limited process applications is that Ethernet is not intrinsically safe. More traditional fieldbus platforms such as Foundation Fieldbus and Profibus PA are better suited for those environments. However, HIMA, MTL, and Pepperl+Fuchs have demonstrated that even Power over Ethernet can be made intrinsically safe and have several products in this area. But this is emerging technology.
Appropriate DCS and PLC I/O hardware—Typical control systems in process applications may not have appropriate I/O hardware for high populations of Ethernet based devices. While most systems have flexibility to accept many different types of input signals via appropriate I/O cards, there is little capability to accept direct Ethernet inputs when there are hundreds and potentially thousands of field devices. This undoubtedly reflects a lack of demand rather than any particular technical constraint, but it is still a practical problem.
Wiring topology—Ethernet wiring to field devices is similar to traditional analogue wiring using a star topology, which is one of the drawbacks when compared to a fieldbus. However, some platforms such as Siemens’ newer Profinet devices have a two-port Ethernet switch in each device which facilitates a linear topology of a sort, if latency isn't a serious concern. Peculiarities of PoE—Historically PoE has not been compatible with industrial devices and available power levels are below typical fieldbus offerings. The conventional wisdom is that devices that generate enough data to need an Ethernet connection, such as process analysers, usually also need external power. Devices that can run on PoE usually don't need the bandwidth. Beckhoff makes an I/O junction that uses PoE for factory automation sensors communicating via EtherCAT. The junction converts normal PoE voltage to 24 V for the devices, but the maximum device power consumption is 350 mA with a total available of 15.4 W.
PHOTO: There are some instruments on the market that connect directly to Ethernet. This Aztec 600 is a complex instrument from ABB that analyses potable water for specific metallic elements by measuring the passage of light through a sample. It has an onboard high precision piston pump that provides the sample and reagent fluid for measurement, mixing, and disposal. Working at six samples per hour, it has a built-in Ethernet communications link with onboard web and ftp servers to enable remote monitoring, configuration, and data and log file access to the analyzer from a web browser.
PoE improvements are in the works including the coming PoE+ standard, which will support 25.5 W and even 40 W power levels. Dozens of companies, such as Microsemi, Linear Technology Corp. and ON Semiconductor have begun manufacturing support devices they expect will be compliant with the final version.
Ethernet-based instruments While not used to supply power, there are devices that compile information from a cluster of remote instruments and sensors. A common configuration is an I/O junction that collects data from a group of conventional devices, such as process sensors, and sends the data back on one Ethernet cable. For example, companies such as Acromag have introduced the EtherStax® series of rugged Ethernet I/O which features extremely high I/O density: 64 channels of analogue data, directly from the field, converted to digital via onboard 16-bit A/D converters, and shipped out over Ethernet (Modbus TCP/IP or UDP). One entire scan of all 64 channels takes only 10 msec.
And of course, there are instruments that are, by themselves, connected directly to Ethernet. They generally undergo complex applications and have large amounts of data output. Analysers in process applications are a common example. ABB’s Aztec 600 online analyser checks potable water for minerals such as aluminium, iron, and manganese; it does six samples per hour and posts results on its internal website. A built-in Ethernet communication link enables remote monitoring, configuration selection, data and log file access.
Emerson’s X-Stream multichannel gas analyser uses photometric technology to analyse up to four components and send the results off by Ethernet. Likewise Emerson’s Model 700 process gas chromatograph is another field-mounted analyser that can communicate its results directly over Ethernet.
Shhhhhh….. There is also quiet development going on in a number of areas.
For example, one protocol traditionally associated with hard-wired analogue devices has expanded to include wireless and soon Ethernet.
‘The HART Communication Foundation continues to move the technology forward,’ says Ed Ladd, director of technology programs for the foundation. ‘We're extending the gateway interface specification to include HART over IP based networks (TCP/UDP). This next step allows a HART enabled device to be built using POE that could be immediately integrated into HART systems that incorporate the HART server technology. This new addition to the standard is complete but still must be approved by the membership.’
For the moment Ethernet may still not be a serious contender below the PLC divide, but don't count it out completely.
—Peter Welander, Control Engineering, with contributions from the CE-Europe editorial staff
PHOTO: Emerson X-stream Emerson’s X-STREAM® series of gas analysers features multi-channel analysis using a variety of photometry, paramagnetic and electrochemical oxygen, and thermal conductivity sensor technologies. It can measure up to 4 components. The data may be output via RS-485 Modbus or Modbus TCP/IP through its Ethernet port.
Print this page | E-mail this page
This isn't a paywall. It's a Freewall. We don't want to get in the way of what you came here for, so this will only take a few seconds.
Register Now