Measuring Signals in High Electrical Noise

A Reader Asks Our Experts for Best Practices About Signal Conditioning Device Selection and Tips About the Installation and Wiring. See What The Experts Had to Say

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We build small systems for chemical processing applications and pull a lot of analog signals out of sensors in high electrical noise areas. Measurement inconsistencies are a too frequent complaint, and we need to minimize them. We can't easily mount signal conditioners, let alone sensors, in protective, grounded enclosures all the time. We'd like some best practices about signal conditioning device selection and even tips about the installation and wiring where that's a critical issue.

—from November '10 CONTROL DESIGN

ANSWERS

Good Quality Counts

In 25 years of doing installations at chemical facilities, I have never had analog noise problems nor have I needed special signal conditioning provided I used shielded wiring and grounded those shields at one point. If the system had a 24 V power supply, I left the negative floating off ground to avoid ground spikes from the noise floor. I used metallic, well-grounded conduit and a metallic, grounded box for any control equipment, and bought good-quality instrumentation.

There is strong incentive as a skid manufacturer to purchase lower-cost sensors and transmitters. However, if that equipment is not adequately shielded, the output of the device will spike with RFI noise and no amount of conditioning will resolve that issue. As a test, take a plant radio and key it next to the device while monitoring the output. If you see a spike, consider a different manufacturer.

P. Hunter Vegas, Senior Project Leader,
Avid Solutions, www.avidsolutionsinc.com

Free Signals

To begin with, choose devices that can be DIN rail-mounted and be provided power from adjacent modules through a daisy chain. This helps keep signals free from ground loops, electrical noise and other interference.
A simple two-wire setup for input and output is easier and more reliable in industrial environments. Our modules provide these features while accepting voltage, current, thermocouple and RTD as input and pass voltage or current as output, detecting all analog signals from the sensors for further analysis.

Peishan Juan, I/O Product Manager,
Advantech, www.advantech.com

Best Practices

You can take a variety of actions to help maintain the integrity of analog signals in these environments. First, use a twisted-pair cable with shielding to decrease the amount of electrical noise interference. Second, the type of analog signal that you transmit can have a direct effect on how much noise you see. Low-level voltage signals are more susceptible to noise. If you use a thermocouple (mV), wheatstone bridge (mV), standard 0-5 Vdc or 0-10 Vdc signal, use a signal conditioner to convert them to a more robust 4-20 mA signal.

If you already use 4-20 mA, you could be experiencing issues with ground loops that occur with a difference in potential between your monitoring instrument ground and field device ground. A good solution is to insert a signal conditioner into the signal loop to provide galvanic isolation. One or all of the above best practices should help reduce the inconsistent measurements you are seeing on your analog signals.

Christopher Welker, Product Marketing Engineer,
Rockwell Automation, www.rockwellautomation.com

Remote I/O Modules

Remote I/O, or distributed I/O, offers an advantage over the traditional local I/O found on a PLC. It allows you to locate the I/O modules close to the process that is being monitored or controlled. This greatly improves noise immunity, as the weak sensor signals are converted to digital signals before being transmitted long distances through a noisy plant environment. Our universal remote I/O modules use a simple two-wire RS-485 link using Modbus RTU/ASCII protocol, which is supported by most PLCs.

Barry Faust, Engineer,
Omega Engineering, www.omega.com

Grounding Guidelines

Grounded loops (in signal and power lines) are the most common source of problems (ghosts) in instrumentation/signal management, especially low-level signal. Check them out and route them separately from other cables, preferably using shielded cables. And don't forget to ground the shield only at the receiving end.

Otto Fest, President and CEO,
Otek, www.otekcorp.com

Field Exposure

A critical consideration when selecting signal conditioning devices for use in high-electrical-noise areas is the degree of isolation protection they offer. Transformer isolation, field-side input protection, and transient protection—all are essential for measurement accuracy and consistency.

Equipment located in the field is exposed to a much greater range of electrical noise, power surges, temperature, humidity and corrosive environments than equipment in the control room, yet the field is where process variables are measured and where measuring and some signal conditioning equipment must be located. Because the wiring that connects the field and control room might be near heavy electrical equipment and run hundreds or thousands of feet, the likelihood of outside interference is greatly increased.

To help protect I/O signals, instrumentation wiring that connects field devices to the control room typically consists of heavy-duty (16-18 AWG) pairs, which are often twisted together to reduce magnetically coupled interference and run with other signal wires in a separate wiring tray. Another effective way to reduce the risk of data contamination, as well as to cut the high cost of system wiring, is the use of data concentrators. These devices collect large numbers of signals close to their origins in the field, perform signal conditioning and digital data conversion locally and then send the digitized information by communication links to a local area network or directly to control room equipment.

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