Researchers at
the Indian Institute of Technology (IIT) Bombay have successfully made low-cost
piezo-resistive vibration sensors using polyurethane foam coated with carbon
nanomaterial-based ink. These sensors can be used for monitoring the health of
industrial machines and equipment and help identify incipient failures thereby
enabling efficient maintenance schedule planning.
The ink is made
of functionalised multi-walled nanotubes that are dispersed in a reduced
graphene oxide matrix. It is conductive due to the presence of large number of
multiwalled nanotubes. The ink, which uniformly coats the pores of the foam
when dipped-coated, imparts piezo-resistive properties. Conductive sheets were
pasted on the top and bottom sides of the foam and electrical wires connected
to the sheets for measurements. The ink and sensor were developed at the
Plastic Electronics and Energy Laboratory (PEEL), Department of Metallurgical
Engineering and Material Science, IIT Bombay.
“When the PU
foam coated with the ink is perturbed, in this case compressed, the air gaps
are removed and the foam gets thinner. This provides a conduction path for
electrical charges. The resistance drops as the foam is compressed and it
becomes more conductive,” says Amit Tewari at IITB-Monash Research Academy, IIT
Bombay, and one of the authors of the paper published in the journal IEEE
Sensors Letters. “The sensor is so sensitive that it can measure blood
pulse.”
“The total cost
of materials required for making the sensors works out to less than Rs.200 per
sensor, and can be reduced further if mass produced. The ink costs only about
Rs.7 per sensor. No sophisticated equipment is required for fabricating the
sensor,” says Prof. Dipti Gupta from PEEL, IIT Bombay and one of the authors of
the paper. In comparison, commercially available vibration sensors for such
applications cost more than $50 per sensor, which makes it prohibitively
expensive.
Characterisation
The team
validated the suitability of the foam-based material as vibration sensors. The
team was able to classify different machine operating conditions (good versus
bad bearing, and good versus bad gearbox) based on vibration signals.
In the absence
of a shaker table to characterise the vibration sensor, the researchers used a
portable Phillips Bluetooth speaker. The foam sensor was rigidly mounted to the
speaker and audio recordings were used to generate vibrations that were to be
studied. Acoustic recordings for different machine operating conditions were
played on the speaker and the sensor was validated.
Unlike a shaker
table, speakers will not produce high amplitudes, so the team focussed on the
frequency of the tones to validate the sensors. “Our interest was to locate the
frequency of the excitation vibration signal,” says Siddharth Tallur,
Department of Electrical Engineering, IIT Bombay, who is corresponding author
of the paper. “We were looking for these tones in the sensor output.”
Danger
signs
To be able to
identify the danger signs of a machine, the output of the vibration sensor has
to be captured and the Fast Fourier Transform (FFT) has to be computed. One
should then look where the peaks are located in the FFT.
Since the
vibration signal would vary from one gearbox to another, the vibration signal
prior to failure can be identified only when data for each gearbox in its good
state is available. It will then be possible to look for shifts in the
frequency as the machine ages. One way to make the measurements independent of
the variation in machines is by increasing the bandwidth studied. Another will
be to look for variation in particular frequency bands than specific
frequencies.
“In the
real-world scenario, we will be using the sensors directly on the machines,”
Prof. Tallur says. “It is not clear how many sensors are needed per machine and
the location of placement on the machines. If sensors are cheap we can deploy
more sensors per machine. And this is where the low-cost of our sensors becomes
particularly relevant.”
“We are jointly
working on developing more such novel improved sensors and exploring more
application spaces as well as deployment and field testing of such sensors,” he
says.
Source: THE HINDU-3rd December,2017
