The vehicle carries out tests in a way that won’t damage the tank – so called non-destructive testing (NDT). One of the principal NDT methods is ultrasonic testing (UT), which can characterize the thickness and internal structure of a surface using high-frequency sound waves. Applied to a tank floor, UT can reveal potential weak spots created by corrosion, for example.
The robot is capable of hovering, unlike the “crawler” robots sometimes deployed to move across tank floors to do basic inspections. This means the Square Robot can avoid any obstacles on the floor, and also carry out high-definition visual inspections of a tank’s shell using on-board cameras.
Highly accurate velocity measurement for precise positioning of robot
But in order to precisely examine and map areas of concern, the robot needs to know exactly where it is in the tank. That requires highly accurate velocity measurement to aid navigation in a critical way – neither of which is straightforward in an enclosed tank full of liquid product.
To solve the problem, the company looked to its roots in the subsea industry. Square Robot’s founders and several other team members previously worked at a leading AUV developer and have had long careers in the subsea industry. So, they already knew what worked well for velocity measurement in the ocean and wondered whether it could be made to work in the harsh environment of a storage tank.
“We needed high navigational accuracy, and knew DVLs could provide this from our background in subsea,” says Underwood.
DVLs use the Doppler effect to calculate velocity by measuring the shift in wavelength between acoustic pulses transmitted to and reflected back from particles in a liquid or a surface such as a seabed.
There is more on how the DVLs are used in navigation in this in-depth guide.
The team were familiar with Nortek’s Doppler velocity logs, which had a reputation for being highly accurate, reliable and straightforward to use for underwater navigation, as well as offering the compact form needed to fit into the restricted space of the robot.
Crucially, Nortek’s 1 MHz DVL1000, the instrument Square Robot decided on, could operate accurately at a distance of only around 20 cm from a given surface – often the seabed, but in this case the tank floor. That is closer to the floor than many other DVLs on the market are capable of operating accurately, and a prerequisite for the inspection vehicle to do its job properly.
The fact that this DVL performs so well in such a difficult environment can be attributed to Nortek’s proprietary bottom-tracking algorithm.
Collaborative approach lays the foundation for technological innovation
But, while DVLs had proven their worth in the ocean, Square Robot was uncertain whether they would work accurately within the confines of a steel tank, the sides of which have very different acoustic properties from a typical subsea environment.
“Using a DVL in our operating environment is unique, so we’ve definitely provided some challenges for Nortek. There are very high noise levels because we are operating in what is really a constrained metal bottle – and not only that, but we’re operating in fluids that aren’t water,” says Underwood.
“We’ve been working with Nortek extensively to optimize it. It’s been a partnership, where we say, what kind of performance can we get out of you – let’s test things and move this technology forward.”
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The Square Robot team say they have been very happy with the fruits of the relationship, which has been enhanced by the close proximity of the Square Robot and Nortek offices in Boston’s port area.
“It’s really nice to have a vendor where you can just knock on their door when you have a problem and get an instant response. Often the vendor is a country away, and you have to ship materials back and forth,” says Underwood.
Fine-tuning the DVL for a harsh and challenging environment
The DVL1000 needs to be mounted in such a way that its beams are not blocked by other parts of the robot when working close to the tank floor, so its compact form was a major asset. The team also determined that there was no need to house the Nortek DVL in any extra protective casing, as it was made from materials that were compatible with the various fluids in which the robot operates.
The DVL1000 was tested at various heights, with adjustments made to reduce the amount of noise produced by the steel surfaces of a tank in measurements.
“We were able to tune the DVL to get the functionality we need. By adjusting the “Transmit Power” setting of the DVL, we don’t use as much power out as you would use for subsea application, which helps reduce noise,” Underwood says.
The team was also able to make the necessary adjustments to compensate for the differing density and acoustic characteristics of the fluids in which they were operating.
The flexibility and ease of use of Nortek’s software was also a plus. “Nortek has a lot of different protocols to help software integration. We started with one we were comfortable with, but eventually we migrated to Nortek’s native messaging system, as they make that pretty easy to use,” says Underwood.