IBIS-S Interferometric Radar

INTRODUCTION

The modern conceptual design of civil engineering structures often includes the specification of the tests required for the Structural Health Monitoring (SHM) of the structure and sometimes the design of permanently installed monitoring system. Among the different techniques of experimental testing and monitoring, ambient vibration tests have recently become the main experimental method available for assessing the dynamic behaviour of full-scale structures.

Ambient vibration testing of full-scale structures is generally performed using piezoelectric or accelerometers since these sensors are relatively inexpensive, accurate and have adequate technical characteristics (i.e. bandwidth etc).

However, these devices exhibit well-known problems when used in vibration analysis applications, especially related to mounting and hardwiring from the transducer to the data acquisition system. When dealing with large structures, the effort associated with mounting and wiring the accelerometers is typically the most time-consuming task associated with the test. This can make Structural Health Monitoring a costly process to deploy in the field.

Also, accelerometers do not provide a direct measurement of displacement, which is often of interest for the Structural Monitoring of a structure in operational conditions. Within this context, the use of innovative non-contact systems for vibration measurement appears to be very attractive and especially applications of laser-based systems. Introducing the IBIS-S which is such a system.

TECHNOLOGY

The IBIS-S uses microwave interferometry, which is a new technology for remote static and dynamic Structural Health Monitoring of structures and/ or soil displacements. The IBIS-S is the fruit of a long term research activity performed in collaboration by IDS and University of Florence. The instrument can operate remotely, with no contact required whatsoever with the target to be monitored and that it can supply practically continuous deformation maps (not just punctual information as provided by current contact sensors), all with an unprecedented measurement speed and accuracy compared to current technology.

The radar beam from the IBIS-S illuminates the entire scenario to be investigated; the radar receiver then measures the signal reflected. The high resolution capacity over distance provided by the radar produces a displacement map showing the deflection of many targets over the entire range; in fact, the IBIS-S can sample the target as close as 0.5m. In principle this is as if a vast number of sensors were applied to the target, one every 0.5m.

In reality, the radar samples the target displacements, "detecting" even very small changes between scans. If the objective is to sample the displacement of predefined points, passive radar reflectors can be applied to the points of interest if required.

Thanks to the microwave technology used, the IBIS-S can detect displacements (ie structural defelctions for bridges towers etc) of each point on the target up to an oscillation frequency of 50 Hz and with a detection limit of less than 0.01 mm.

This great sensitivity in detecting deflections is due to the use of interferometry, measuring the phase differences of the radar waves reflected from each point. Each sub-millimetric displacement gives rise to a phase difference of the reflected wave, detectable by the IBIS-S.

APPLICATIONS

Static

• Structural load testing
• Structural displacement and collapse hazards
• Cultural heritage preservation

Dynamic

• Structural Monitoring
• Structural resonance frequency measurement (figures to the right show typical data output)
• Structural modal shape analysis and vibration analysis
• Real time monitoring of deformation including bridge deflections

ADVANTAGES OVER OTHER TECHNIQUES

• Remote sensing at a distance of up to 1 km.
• Displacement accuracy up to 0.01 mm.
• Real-time simultaneous mapping of deformations
• Fast installation and operation
• Static and dynamic monitoring
• Structural vibration sampling and analysis up to 200 Hz for modal testing
• Operates day-night, in all weather conditions
• Provides direct displacements, not derived quantities

EQUIPMENT

The IBIS-S radar system consists of a sensor module, a control PC and a power supply unit. The sensor module (see figure 1) is a coherent radar generator, which transmits  and receives the electromagnetic signals. The IBIS-S radiates at a central frequency of 16.75 GHz with a maximum bandwidth of 300 MHz. The sensor module is installed on a tripod equipped with a rotating head, allowing the sensor to be orientated in the desired direction. The module has an USB interface for connection with the control PC and an interface for the power supply module. The control PC is provided with the software for the system management and is used to configure the acquisition parameters, store the acquired signals, process the data and view the initial results in real time. The power supply unit is a 12 V battery unit.