Professor Massimo Ruzzene’s research on integrated vehicle health monitoring (IVHM) has made him bullish about the future.
He readily envisions a time when commercial airlines will be able to abandon conservative and costly maintenance schedules because their planes will be able to sense impending malfunction in one of their systems the same way a human might avoid a root canal by heeding the warnings of a simple tooth ache.
“The goal is condition-based maintenance and component-damage tracking. It’s all possible, but you need to think outside the box if you want to advance the state of the art,” he said recently.
“We can’t keep pushing the same legacy sensors – sensors we developed 30 or 40 years ago – and expect to realize new benefits. We need sensors that are built for the information they transmit.”
Ruzzene’s IVHM research is doing just that.
Over the last six years, his research team has developed three next-generation ultra-sonic sensors, each engineered to monitor the health and/or usage of modern aerospace, civil and mechanical systems.
One of his designs, the Frequency-Steered Acoustic Transducer (FSAT) has a patent pending. Two others - the Acoustic Wave Rosette (AWR) and the Impact Directionality Revealer (IDR) - were the subject of an award-winning paper, written by Ruzzene’s doctoral student Matteo Carrera (see box).
Dr. Ruzzene's doctoral student, Matteo Carrara has worked alongside his mentor to explore the next generation transducers. Carrara's recently authored paper on the subject, "Frequency-wavenumber Design of Spiral Macro Fiber Composite Directional Actuators" was selected for the 2015 Best Student Presentation Award at the SPIE 22nd International Symposium on Smart Structures and Materials and Nondestructive Evaluation and Health. Find out moreabout Carrara's award-winning work.
The Fourier Framework
What sets these transducers apart from their predecessors is their design, which is dictated by the Fourier Transform, a mathematical framework that gives an excellent representation of things that are periodically in time and space.
“The Fourier Transform allows us to specify the design in the ‘Fourier-Space’ and directly obtain the resulting shape of the transducer through a simple mathematical process,” he noted.
Fabricated from piezoelectric materials (PZT or PVDF), these transducers adopt a complex surface pattern that is, itself, determined by the Fourier Framework. The unique shape allows it to more efficiently and accurately convert sound waves into signals that will pinpoint areas of concern or risk. From a practical standpoint, it requires just two electrical wires (to receive and elicit signals) - not the myriad wires used in legacy sensors.
“The importance of reducing wires cannot be underestimated,” Ruzzene points out.
“If an aircraft manufacturer distributes legacy sensors throughout the airframe to monitor health and damage, they are bound to find that the added weight of the wires and hardware will negate many if not all of the advantages of the lighter weight composites they are using to build the structure.”
Once it is adhered to the surface, the new generation of transducers is ready to receive an acoustic wave that will begin the diagnostic. The unique deformation that results from this acoustic charge sends out elastic waves in different directions - each determined by the frequency of the original signal. All of the waves traveling through the material elicit an echo or return wave which is picked up by the sensor and can be analyzed. When an anomalous wave signal returns to the sensor, it is evidence of a defect of some type.
If this sounds like ultrasound technology of old, it is. With a twist: it works much better and requires much simpler hardware.
“We can create an image of the defect and know exactly where it is by directing waves in different directions,” notes Ruzzene. “But we are not bound to all of the wires and hardware that adds weight to legacy sensors.”
Eventually, Ruzzene thinks these sensors will be able to replace phased array technology - a system in which multiple sources send acoustic waves through the material at different points in time. The location of any anomalous results is determined by analyzing the constructive and destructive interference.
“This still works, but it has its weaknesses,” Ruzzene notes. “The wires themselves are not so robust, so they have to be maintained or replaced on a regular basis. And the diagnostic equipment is heavy, expensive, and not-transportable. You cannot take it onto the plane to conduct the assessment while you are in flight, for instance.”
The next generation begins: FSAT, AWR, IDR
The first of these, the Frequency Steered Acoustic Transducer (FSAT) takes acoustic information and transforms it into elastic waves that can be steered in different directions to detect defects.The three Fourier-designed sensors that Ruzzene’s team has already developed make a strong case for changing the way diagnostics are done. More models are bound to be developed, but the first three have shown promise in effectively monitoring hot spots or damage – an improvement which could significantly reduce maintenance, inspection, and, over the long-term, structure replacement costs.
The Acoustic Wave Rosette (AWR) takes the Fourier design a step further to house multi-component strain sensing capabilities in one device. When the underlying structure is in some way deformed, the AWR’s multi-band spatial filter detects it by monitoring peaks shift in the wavenumber domain. The resulting data can tell
The military has shown a lot of interest in the third Fourier-designed sensor, the Impact Directionality Revealer (IDR), which can deliver useful information on the location and source of damage caused by external objects (everything from space debris to bullets).engineers about its normal and shear strain components.
Where prior designs might require three sensors to determine the location of the impact and resulting damage, it takes just a single IDR device to produce data about the location, size, and directional origin of impact damage. It does this by using a combination of wave frequency and amplitude data.
“If you are trying to determine where the location where a bullet impacted the structure, and you have several bullet impacts, you can determine the direction of each one by measuring the frequencies of the waves that each impact creates,” Ruzzene said.
Ruzzene predicts that time, money, and accuracy will be the big winners when the next generation of sensing technology is fully integrated into industry. Safety will remain constant.
Is zero down-time maintenance attainable?
“People talk about zero down-time maintenance, and that may be possible, but wherever we are headed, it’s widely understood that the current schedule of structural maintenance stops is overly conservative,” he said.
Current best practices call for multi-day maintenance stops scheduled around the number of take-off-and landing-cycles a plane has completed. This approximates a certain number of flight hours, and assumes a certain amount of structural stress due to the repeated pressurizing and depressurizing of the cabin. It’s all averaged out into a profile.
And it’s all very expensive.
Maintenance stops mean everything comes to a halt. Flights are put on hold, and parts of the aircraft are taken apart for direct inspection. Expensive machines and trained engineers are brought out to run tests.
“It catches 9 out of 10 issues, sure, but it’s a little like going to the doctor every month instead of once a year when you are healthy. It’s worth asking if the cost of that visit – in terms of lost work time, inconvenience, dollars spent – could be better incurred by something that would improve your health instead of monitoring it,” he said.
“What we’re doing now is working, and that’s good. People are generally safe and planes are flying. But we can do better. By retiring the legacy sensors for this new design, we can establish a fundamentally better system that does not imply any additional complications right from the start. As engineers, that’s the kind of challenge we want to take on.”