Eliminate Signal Gibberish

Several steps can help maintain the integrity of measurement and control signals.

By Greg Livelli, ABB Measurement Products

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• Signal reset. The user defines upper and lower signal thresholds. If five signal measurements exceed a threshold, the signal holds its value for a defined time (1.5 sec. or 0.5 sec.). This technique eliminates voltage spikes for which continuous feedback to the controller can't compensate.
• Prefilter. When considerable noise affects an input signal, the user can turn on a narrow frequency "notch" filter ahead of the main filter and select the width of the notch (in 0.15-Hz to 68-Hz increments). The narrower bandwidths provide higher noise reduction but require greater processing time, and vice versa. Signal processing in the prefilter takes half the time required in the main filter. So, prefiltering suits processes requiring fast response times such as many batches.

It generally is more cost-effective in the long run to install sensors correctly and minimize the sources of interference than to rely strictly on mathematics to separate data from noise. When constructing a control loop, apply data filters in the final stages of the project just before the loop tuning.

What about Wireless Network Interference?
Chemical plants are showing increasing interest in wireless measurement and control. After all, wireless devices dramatically reduce costs in wiring engineering, installation and maintenance while offering increased data gathering flexibility. Once plants install a wireless system, they can easily modify it, adding or deleting measuring points.

However, processing sites often have dense infrastructures, vehicle movement, large electrical equipment, and numerous sources of electromagnetic interference and radio frequency interference (RFI), including from other radio communication systems. Modern wireless networks for measurement and control must incorporate multiple capabilities to overcome possible communication interference.

The U.S. Federal Communications Commission permits use of the industrial, scientific and medical bands (902–928 MHz, 2.4–2.4835 GHz, 5.725–5.85 GHz) at power levels up to 1 W without end-user licenses. Spread spectrum radio transmissions operating at these frequencies have distinct advantages regarding immunity to noise and interference.

Two common methods used in these bands are frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS).

FHSS radio systems quickly hop through multiple frequency channels. The transmitters and receivers are synchronized. FHSS specifies a particular time slot and frequency for each transmission. This scheme anticipates competition with other radio systems and RFI from other sources. For example, if one frequency is affected or blacklisted in an FHSS system, the data switch to a clear channel.

DSSS radio systems spread a narrow frequency source by integrating it with a pseudo-random noise signal. The digital bits of the noise signal occur at a higher frequency than the original signal, spreading it into a wider band. The synchronized receiver processes the signal with the same pseudo-random sequence, reconstructing the original data. The technique adds redundancy to the original signal, permitting the receiver to recover data damaged during the transmission.

Additional techniques such as data checksums and redundant mesh routing further improve immunity to interference. The cyclic redundancy check (CRC), commonly used with data sent over wire, offers a unique digital signature to data. CRC ensures the data received are identical to the data sent. If the data don't match, the receiver automatically requests a repeat transmission. With redundant mesh routing, the network automatically reroutes transmission to unobstructed pathways whenever interference or other obstacles hinder communications.

Plants also can use antenna design to improve signal integrity. High-gain directional antennas can provide radio communications at long distances through a crowded chemical plant. Conversely, a low-gain antenna can keep radio signals from straying unwanted distances. Radio communications needn't be line of sight but objects in the path may attenuate the signal, so receiver sensitivity can become an important factor.

GREG LIVELLI is marketing manager for ABB Measurement Products, Warminster, Pa. E-mail him at greg.livelli@us.abb.com.

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