Is there anything common to a concrete bridge, a human foot and the flow of traffic? When it comes to a low-cost sensor developed at the Indian Institute of Technology, Delhi, (IIT-D), to measure vibration patterns in structures, there may be a strong link.
The piezo-based sensor designed by the institute?s civil engineering department was primarily aimed at structural health monitoring, but its potential applications could extend to mapping foot-sole pressure for clinical applications or as a traffic monitoring sensor buried in the carriageway.
A sensor made of a piezoelectric material such as quartz, responds to mechanical stress by developing surface charges, a phenomenon called ?direct effect?. Conversely, an electric field causes it to undergo a mechanical strain. The prefix piezo is derived from Greek, which means ?squeeze? or ?press?.
In a recent research paper, a team led by associate professor of the civil engineering department Suresh Bhalla demonstrated that a piezoelectric ceramic (PZT) patch can complement existing techniques of measuring the vibration response of a concrete structure, such as a bridge, by playing the double role of determining strain for the structure as a whole and also at a local level. This can serve as a viable alternative to expensive accelerometers, which are widely used to carry out the health monitoring of such structures, the team says.
Bhalla, who heads IIT-D’s Smart Structures and Dynamics Laboratory, has come out with a ready-to-use composite piezoelectric sensor especially for reinforced concrete structures. ?The cost of such a sensor would be less than one-tenth of the cost of an accelerometer,? he says, adding that the sensor practically becomes part of the structure and can serve it for very long periods without any fear of accidental damage.
Their research paper, published in the Journal of Intelligent Material Systems and Structures, notes that the performance of surface-bonded patches like the piezo electric sensor have been shown to be as good as accelerometers and can capture frequencies as low as 1 hertz (Hz). ?The sensor has been tested in the laboratory environment successfully,? says Bhalla, adding that further laboratory trials and pilot studies are in progress.
Piezo composites have been in use as sensors and actuators in aerospace applications for over two decades though their entry into civil engineering is more recent. Currently, the types of sensors widely used for global structural health monitoring (SHM), or monitoring a structure as a whole, include electrical resistance strain gauges, fibre-optic sensors, piezo-electric accelerometers and ultrasonic pulse transducers.
?PZT is currently used for active sensing at an extremely local level to detect flaws and damages. The jury is still out on structural global level SHM using PZT, as the perforce are used for local-level response,? says IIT Rourkee director Pradipta Banerji. He is also on the executive council of the International Society for Health Monitoring of Intelligent Infrastructure.
In a subsequent paper published in the journal Experimental Techniques, Bhalla’s team demonstrated the use of the piezo sensors for monitoring traffic flow by embedding it below the carriageway. ?Compared with other traffic sensors available in the market, the proposed sensor is very cost-effective and warrants minimal data processing efforts and hardware costs,? the paper noted. The sensor had successfully passed tests under different traffic types and it could be calibrated for action under real situations, it said.
The team also collaborated with the Centre for Bio?Medical Engineering at IIT-D to adapt the same piezo sensor for biomedical applications such as mapping the foot-sole pressure and its distribution. ?This is specially relevant for diabetic patients whose foot pressure distribution is sometimes of concern. Such patients tend to lose their sensation on the sole and are prone to exerting very high foot pressures leading to foot ulcers,? he says.
The team has submitted proposals for transfer of the technology to a few companies under IIT-D?s Foundation for Innovation and Technology Transfer. Bhalla says the sensor has an edge because it can be used with minimum intervention and is less expensive.
Over the past few years, the quest for better sensors for SHM has driven research into areas such as wireless sensor networks and system identification techniques to interpret data and assess the condition of structures and for life extension of old structures. According to Banerji, some of the key challenges currently are to develop substantive and automated interpretation of online SHM data that can provide alerts when a structural or functional deterioration occurs in a structure, and the integration of data from various sources.