Digital device communication
In the December issue of Control Engineering, there are at least three articles discussing situations where digital communication is a critical element of making an application work. Maybe this seems like old news, but think of how different the world of automation would be if analog was your only choice.
Most of the installed base of process instrumentation is still analog using various current or voltage signal approaches. (A few devices, such as turbine flowmeters, use pulsation frequency, but this is a comparatively small selection.) The leader among these is the 4-20 mA current loop, and the bulk of field devices sold today still use it. Conventional transmitters are powered via the loop and send a continuous signal indicating the process variable (PV). Specific current levels translate to engineering units according to the scaling of the device. For example, for a given pressure sensor, an output of 17.45 mA means 112 psi. A technician with a multi-meter and applicable chart could tell you the PV. Each analog device has its own cable going back to the I/O rack of the host system.
That’s fine as far as it goes, and in many applications the primary PV is all that’s necessary. The problem is that a growing number of applications require additional information about what’s going on with that device. What is the sensor’s condition? What is the ambient temperature around the transmitter? What is the temperature of the process fluid?
Analog isn’t very good at sending larger amounts of data. While a clever engineer could figure out a way to send multiple variables via a single cable on a rotating basis, there are easier ways to do that sort of thing. HART (highway addressable remote transducer) is a hybrid combining analog and digital. The HART Communication Foundation describes it: “The HART Protocol provides two simultaneous communication channels: the 4-20 mA analog signal and a digital signal. The 4-20 mA signal communicates the primary measured value (in the case of a field instrument) using the 4-20 mA current loop—the fastest and most reliable industry standard. Additional device information is communicated using a digital signal that is superimposed on the analog signal. The digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc. Together, the two communication channels provide a low-cost and very robust complete field communication solution that is easy to use and configure.”
There are other methods that leave out the analog part entirely. Fieldbus protocols, such as Foundation fieldbus and Profibus PA, are purely digital. Early on these were promoted as a method to communicate with a group of devices via one cable in an effort to economize on wiring, but users soon found a fieldbus was an excellent means to send more information than was possible with analog or even HART. Users who are well trained on this technology and deploy it appropriately find it very practical and reliable.
Moving up from the device level, other digital protocols also emerged as effective ways to transport larger amounts of information. Modbus started out as a way for PLCs to talk to each other, but proved to be effective with field devices. Data gathering systems compile information from clusters of analog devices and send the data digitally on one cable similar to a fieldbus. This same approach has grown to Ethernet as those networks are deployed more broadly. Recent developments include a growing selection of individual field device transmitters equipped with native Ethernet connections.
Wireless field devices would be far less practical without digital communication. Even WirelessHART digitizes the analog signal component so it can be sent to the gateway. When captured at the host, the signal often has to be converted back to analog so it can communicate with legacy I/O systems.
Going forward, the importance of Ethernet will continue to grow. It is taking a more prominent role in all sorts of industrial applications, and is even being built into industrial communication standards. The fact that it is the basis for virtually all aspects of IT and enterprise-level networks is a mixed blessing. IT has moved onto the plant floor, like it or not.
Given the number of analog devices presently installed, it isn’t going away anytime soon, but don’t expect much growth in new applications. The world is fast becoming digital.
Peter Welander is a content manager for Control Engineering. Reach him at email@example.com