Talking to transmitters

I/O systems have to offer multiple communication options to accommodate different types of field sensors and transmitters.

01/11/2013


Field instruments have a variety of communication methods.Process control system vendors often have to provide a range of I/O (input/output) options to communicate with the variety of signal types that a user might find with process sensors and actuators deployed in a process unit. These include sensors of the “big four” process variables (flow, pressure, temperature, and level) along with control valves, safety sensors, analyzers, weight cells, and so on.

If all these devices had one way to communicate, life would be much simpler, but they don’t. Three of the most common communication formats include: analog with HART, digital (binary), and pulsations. (For the moment we’ll ignore various fieldbus protocols, wireless, and Ethernet.) Let’s look at all three.

Analog

The most common analog communication protocol is a 4-20 mA current loop. Adding HART communication is also typical for today’s field device offerings. Others may have a voltage signal, resistance signal, pulse signal, and possibly others that are more specialized. But what does all that mean?

A sensor measures its desired variable by modulating some electrical signal. It can cause a change in voltage, resistance, or capacitance. For example:

  • Thermocouples generate a voltage in relation to temperature
  • Strain gages, thermistors, and RTDs change resistance in response to movement or temperature, and
  • Some pressure sensor designs use changes in capacitance to quantify changes in pressure.

A raw signal directly from a sensor is usually not suitable to send to a control system as-is. It needs to go through some additional circuitry to control signal scaling, linearity, compensation, and amplification. A device with this capability is typically called a transducer. Thermocouples and RTDs are exceptions in that many control systems have I/O cards that can take such signals directly; however, these signals are very weak and difficult to transmit over any long distance. They are also subject to interference, so as a process unit and its control system become more complex, users will more often insert a transmitter to beef up the signal. Most temperature transmitters can also interface with a variety of thermocouple and RTD types. Keep in mind that the terms “transducer” and “transmitter” are often used interchangeably, but the latter suggests an ability to condition a signal and send a signal over a longer distance.

In most cases, this more robust signal will be a 4-20 mA current loop. The transmitter will draw a given current level at 24 Vdc from the supply in the I/O connection of the control system, corresponding to the variable. A zero reading of the variable will draw 4 mA, and the maximum reading draws 20 mA. Anything beyond those limits indicates a malfunction.

A two-wire device can power its internal circuits using current it draws from the supply, meaning everything can operate at 4 mA or less, so it needs no other power source. However, there are devices that cannot operate at 4 mA and they need an external power supply. These are four-wire devices where two wires transmit the process variable and two bring in power at whatever level it needs. Coriolis flowmeters are typical examples of devices that require external power. There are models that use a more creative approach and have changed the signal format to 10-20 mA. While this compresses the available signal range, it eliminates the need for an additional power connection.

Most manufacturers have standardized on this signal format, moving on from other current loop values and voltages. Older voltage signals have lost desirability due to their susceptibility to line loss through wire resistance.

Adding HART to the analog signal is a way to carry additional information by modulating the current signal in a way that allows digital information to be piggybacked on the analog without disrupting the basic process variable. (Read more about HART below.) A close look at the analog signal will show that it is not a straight line, but modulates at 1.2 or 2.2 kHz to carry binary sequences. The I/O connection cannot “see” the digital signal without a modem, so the normal process variable doesn’t change.

Digital

In some cases calling a signal “digital” suggests something like a fieldbus, Modbus, or Ethernet where data is sent in packets. However, in this context we’re talking about a binary signal, or one that is simply on or off. There are many process sensors that do not send out scalar data. They are simply set to change their signal state when the variable crosses a specific threshold. For example, a low-level alarm in a tank switches on when a liquid level gets too close to the bottom. Another might be a pressure switch that turns on a compressor when pressure in a tank reaches a given point.

Such devices may simply close or open a set of contacts like a relay. In other cases, an analog signal may change. On means 20 mA and off means 4 mA, but that’s really an analog device.

Pulse

Some sensors, particularly turbine flowmeters, send their signal as a pulsation, the frequency of which indicates the process variable. Such a flowmeter uses a proximity sensor that triggers each time the turbine propeller passes by. Higher flow makes the turbine spin faster and send pulses at a higher frequency. Counting the frequency provides an analog measurement.

This variety of signal formats grew out of a desire to have the best tool for a given job, but the practical result can be somewhat confusing for end users. At least the number of signal formats has thinned out in recent years, and system providers are doing a better job of accommodating those that remain. 

Peter Welander is a content manager for Control Engineering. Reach him at pwelander(at)cfemedia.com.

Online:

Subscribe to Process Instrumentation & Sensors eNewsletter at www.controleng.com/newsletters



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.