In the modern world sensors are used for recording everything from exhaust gas temperatures to the electrical activity in a human heart. Direct wiring methods are commonly used for simple on-off device control, where a circuit is either open or closed. Likewise, digital sensors are usually wired directly to monitoring panels in the world of industrial automation. As the distance increases, it becomes exponentially more difficult to wire these connections. The growing presence of Ethernet in many industrial sites has given system integrators other options for connecting digital sensors to remote display panels and other systems that need the information.
Sensing trouble out at the network edge article tells that those sensors are becoming intelligent: Embedded microcontrollers and built-in software are turning them into thinking, communicating, and active nodes of intelligent networks, systems that demand localized intelligence and real time, sensor-driven analytics. You don’t just connect them anymore; they communicate with you.
Earlier generations of sensors reported their data via basic analog or digital connections. These days they’re becoming Ethernet-aware, with all of the accompanying advantages. Their use of universal Ethernet communications protocols enables them to work with off-the-shelf technology, and their ability to communicate via Ethernet means that they can be placed just about anywhere.
And sensors that do not directly connect to Ethernet can be connected to Ethernet with help of PLC or peer-to-peer I/O servers as described in Connecting Sensors over Ethernet article. For simple, direct sensor connections over long distances, peer-to-peer I/O servers can save companies a tremendous amount of effort and expense, and can make new monitoring applications possible.
To communicate with those sensors connected to Ethernet we need Ethernet networks that work reliably on in harsh environment. And sensors tend to live way out on the network periphery, where conditions are the very worst. Designers will need to specify equipment built for the job (ruggedized equipment). For example, when installed in difficult environments, the popular RJ45 connector needs to be strengthened and protected by an over-connector.
Connecting sensors to Ethernet can be problematic in many real world scenarios. Copper wire-based Ethernet has a practical range limitation of 100 meters is a serious limitation on many applications. This limitation can be solved in many cases with a device called an Ethernet extender, which uses DSL technology to create a long-distance Ethernet bridge over virtually any available copper pair (typically capable for distances over thousands of feet while maintaining a quite serviceable connection rate of several Mbps). The flexibility provided by Ethernet extenders can represent an enormous savings when they allow re-using the existing wiring (let it be old telephone cabling, legacy coaxial cable or old automation system cabling), because the labor and materials involved in new cable installation are very high.
Ethernet connections help sensors communicate, but they can also leave sensors vulnerable to damage from power surges and spikes through the copper wiring. The greater the distance between two connected devices, the more likely it is that they will have different ground potentials and the associated risk of damaging ground loops.
Sensing trouble out at the network edge article recommended protecting using Ethernet isolators, but based on my experienced in many cases this kind of devices are not needed. Ethernet over UTP is very immune to small (or even some larger) ground potential differences because Ethernet connection has full transformer isolation at both ends (1500 volt isolation). Ethernet over UTP does not cause ground loop problems. But if you use shielded wiring for Ethernet, then the shield connections can easily cause ground loop problems that needs to be solved (shielded wiring is originally designed to be used in well grounded office environment where ground potential differences are less than 1V). If you use shielded cables, you need to be careful how you arrange shield grounding and you might need extra hardware like isolators.
Lightning strikes and other power surges can also travel on copper Ethernet cable to burn out integrated circuits and connections when the voltage on the cabling exceed the Ethernet isolation level (1500 volts). Sensors can be protected by Ethernet isolators and protectors, which allow data to pass unimpeded, but control electrical flows.
A second option for long distance communications is using fiber optic cable. To use fiber it typically needs that you pull the new fiber for your communication needs (having good already installed free to use fiber wiring is rare). Given that labor costs typically represent the largest expense in a cabling installation, A it often makes sense to go straight to single mode fiber, even if its capabilities exceed current requirements (it is somewhat more expensive but more future-proof). For example many fiber to home systems nowadays use single mode fiber even on quite short fiber runs. Fiber itself is also immune to ground loops, power surges and lightning strikes because the glass fiber does not conduct electricity. Please note that armoured construction fiber optic cable can have steel or other conductive material in their armour.
When cabling isn’t a practical solution, then you need to consider wireless options. Today’s WiFi can provide robust IP-based connectivity out to thousands of meters. Some wireless bridges can even connect at distances of many kilometers while maintaining connectivity rates well into the tens of Mbps. Some sensor manufacturers are embedding WiFi directly at the sensor level.
Interfacing Intelligent Sensors With Industrial Ethernet Networks article tells that while Industrial Ethernet has become the kingpin and standard for automation control networks, some devices such as smart sensors and actuators are relying on a simpler, point-to-point communications protocol to save on the cost, size, and complexity that an Ethernet solution requires. One increasing popular alternative is IO-Link technology. The technology is supported worldwide by a consortium of automation control suppliers. It is particularly popular in Europe, although it is also gaining acceptance worldwide.
Besides automation applications Ethernet sensors are increasingly used for physical security system applications. Cable planning for IP convergence article tells the basics of the art and science of building structured cabling systems for digital security applications. Typically you need a dedicated security network for security sensors and IP cameras. Physical Security IP convergence articles page gives links to several articles worth to take a look.