摘 要：电力线数据传输网络技术是目前的热点之一，本文分析了三种用以太网协议同时传输电力和数据技术（POE、EOP和P&E），讨论适合这三种技术在工业自动控制领域的不同应用。

COMBINED TRANSMISSION of data and power on the same network or power line makes sense for many reasons. Chief among them are lower system installation costs, simplified wiring design and a smaller wiring footprint for device networks.  

There are currently three main options for combined data and power transmission: Power over Ethernet (PoE); Ethernet over power lines (EoP); and the hybrid approach of data and power on different conductors within the same cable (Power and Ethernet, P&E). There are most definitely pros and cons with each approach. There really has to be a constructive debate as to how industry standards will drive change going forward, and what the future holds for combined data and power transmission. 

Each option offers unique capabilities to the market, but these options are not a 'one size fits all.' Whether or not you adopt one of these technologies depends greatly on your data transmission needs.  

Power over Ethernet
Power over Ethernet is a method of transmitting electrical power and data to remote devices over standard twisted-pair cable in an Ethernet network. PoE is useful for powering a wide variety of devices. These typically include cameras, sensors, remote network switches, wireless LAN access points, and other devices that have lower power consumption. Existing Ethernet cabling does not typically require modification to support PoE.  

An IEEE standard, 802.3af, governs implementation of low-wattage PoE applications. The specification allows the powering of a 15- watt device to use 36 to 57 volts DC (48V is nominal). PoE can be used with both standard Ethernet applications and Gigabit Ethernet applications.  

While current PoE is ideal for low-wattage devices such as those described above, new PoE standards need to be developed for higher wattage devices. For example, the IEEE802.3at PoE+ standard will deliver up to 30W, supporting devices such as inverters, linear encoders, actuators, and simple motion devices. New technologies are being developed for these applications.  

For example, Molex has developed the 12 port RJ45 HyperJack 1000 PoE+ Integrated Connector Module, (ICM) which allows Ethernet switch and router manufacturers to provide 30W PoE+ output in a self contained plug and play module. The level of integration in the ICM includes high levels of protection circuitry against ESD and EMI, fully isolated communications to the module, Gigabit magnetics and thermal management, etc.  

While 30W is an improvement over the previous standard, significant additional market access would be gained by doubling that once again. The 802.3 standard does not rule out 60W but it is not a 60W standard. Many questions remain to be asked before implementing these power levels. Some of them include the cable specifications as well as needed connector coding, yet this level is clearly attainable in the near future with current chipsets and common industrial connector and pin configurations. The next doubling step of 120W is a much long distance away and not even being discussed currently within the working group.  

Ethernet over Power
Ethernet over power lines (EoP) is an emerging technology that, when connected to an existing network infrastructure, can provide transmission of Ethernet data packets over the same lines used to transmit power.  

EoP is especially useful for devices that consume large amounts of power but do not require large amounts of data interchange. Many industrial applications such as conveyance lines, welding cells, stamping and presses fit this category. Since the power cabling is already in place, adding special data taps into the lines to transmit simple signals can be a very cost effective solution for networking plant equipment. This approach is similar to what many have done in the residential markets with BPL (Broadband over Power Lines) with standards typically achieving between 500kbps to 190Mbps data rates via common electrical outlets and wiring.  

Power and Ethernet
A third approach that has already been tried in the industrial community is power and data transmitted through different conductors but the same cables and connectors. This approach makes wiring much easier, reducing installa- tion costs and allowing unskilled labour to successfully commission lines and equipment with fewer problems.  

A good example over the last 10 years has been the growth of DeviceNet, an industrial network sponsored by Rockwell Automation, Omron, Eaton and others (see www.ODVA.org) that transmits CAN-based signals and 24V power in a hybrid cable/connector system. It has been very successful and is currently the second most popular industrial fieldbus with 5 million nodes installed worldwide and growing.  

While it seems clear that many users found value in the power and data approach of DeviceNet, it is interesting to note that when the ODVA brought out their industrial Ethernet protocol, EtherNet/IP, no power standard was contemplated. Even more curious, the first versions of the leading rival standard, Profinet (see www.profibus.com), were based on hybrid connectors and cabling for combined data and power and adopted throughout European automotive markets.  

Both industrial standards groups are currently refraining from settling on an official approach to the issue while member vendors jockey with solutions exhibiting various degrees of elegance.  

While the installation benefits are clear, the issue with P&E in going forward is the cost of the hybrid cables and connectors. Since no such standard exists in the commercial world, any solution created for industrial will inherently have lower volumes and higher costs then leveraged solutions from the mass market. Despite the success of DeviceNet and early versions of P&E Profinet in the market, this cost dynamic makes it is difficult to see a scenario where P&E becomes the ultimate winner in this race to the tipping point of industrial power and data transmission.  

Industry standards
One of the challenges in combined data and power transmission relates to the relatively few industry standards that specifically govern these recently emerged applications. While the IEEE 802.3at standard governs commercial lowwattage PoE use, new standards are needed for industrial higher-wattage applications. These have been slow in developing both because of the technical challenges and the limited size of the industrial market.  

The IEEE P1675 standard being used by utilities as they test transmission of data over power lines seems destined to never see broad adoption or extension to useful applications due to EMI issues, concerns from regulators and niche lobbyists (like astro-physicists!). While EoP has been talked about longer than any of the other approaches, it seems to never get far beyond talkware.  

Agreement on a P&E approach that would create a standard for cabling and connectors still seems a long way off, yet this is crucial to attaining the economies of scale needed to see this horse emerge as the winner. An agreement on standardisation between the two main industry groups, the PNO and the ODVA, along with their member vendor support would go a long way to see this approach emerge as the industrial winner.  

Summary
It seems likely that the answer as to which technology will end up capturing the market lies with the major automation companies and the degree of resource that they invest in their chosen route. Rockwell, Siemens and Schneider have so far dabbled in custom applications but largely remain on the sidelines for now, working aggressively on end-user specification wins, safety extensions, completing product portfolios and leaving the power battle for a later date.  

Process leaders like Emerson, Honeywell, ABB and others have been largely focused on wireless technology and software structures. These areas of interest have of necessity been the priorities for establishing Ethernet as the industrial networking standard by the creators of de facto standards. The clear benefits of combining power and data for end-users will eventually draw the attention of the major players in the market to extend the standards and their offerings.  

All of us in the industry with a view to implementing combined data and power transmission should stay tuned for new developments in the coming year or so. New standards, new testing devices, and new switching, cabling and connection devices are being tested today at a factory near you and will soon emerge as the next significant trend in your connectivity infrastructure.