Advancements in Network-distributed Ultrasonic Sensors

Introduction

Industrial plant ultrasonic testing applications, such as those typical at refineries, chemical, nuclear and fossil power plants, sometimes require expensive, bulky or complicated instrumentation that does not easily lend itself to portability.  In some of those cases, including AUT Imaging, it is impractical or undesirable to bring the UT instrument very close to the area of test.  These applications therefore require the use of very long (> 150’ or 45.7 m) cables.  If dual-element probes or multi-element transducer arrays are needed for area coverage or other reasons, then the quantity and complexity of the cable bundles become both expensive and bulky to manage.  Cable breakage, wear-and-tear also become issues of availability and economics.

In addition, long cable lengths cause signal degradation especially at higher frequencies. The longer the cable the more chances for it to act as an antenna and pick up noise from the environment limiting the ability to measure small signals. The cable will attenuate the pulser signal as well as attenuate or limit the ability of the UT Transducer to propagate a received signal back to the UT instrument receiver. Attenuation rates can be as high as 3.3 dB at 10 MHz per 100’ (30.5 m) of RG-174 cable on both the pulse and receive signals. At longer lengths, the cable can look like a transmission line to the pulser (which contains higher frequency content) potentially resulting in reflections in the cables. The results of which could be multiple excitations of the UT transducer causing reduced resolution.  

An alternative approach to multiple, long coaxial analog cables is to place the entire UT system very close to the transducers, digitize the signal output, and then use a simpler, lower-cost twisted-pair cable such as RS485 or wireless technologies to send the data to a common location for analysis, review, and archiving.    

Development and Deployment of Digital Networks

Over the past few years, several digital network topologies have been developed which can be used individually or combined with each other and a common interface / web-based back-end.  

These include:

• Hard-wired Modbus over RS485
• LORA WLAN (Wireless Local Area networks)
• Cellular over standard commercial networks

Modbus with RS485: Modbus is an industrial communications protocol commonly used for field instrumentation such as temperature, pressure, flow measurement and bi-directional control of valves, etc.  In the case of a digital ultrasonic network, multiple UT devices can be daisy-chained together using standard RS485 cable. Typically, the UT device can be a single, small, low-power circuit board containing the pulser, receiver, amplifier, multiplexer, digitizer and communications hardware. The Modbus network can support up to 32 devices, 1,500’ (500m) of cable and can terminate into either a tablet, laptop, desktop PC or a plant’s Digital Control System (DCS).  This represents a closed “in the fence” network system with no connectivity to the internet or cloud unless the PC-captured data is intentionally uploaded.  Some sites or company’s IT departments require closed systems for cyber-security reasons and some applications such as buried pipe or process control require hard-wired Modbus solutions.