This technology could transform the ability to monitor and strengthen existing infrastructure
By Abbe Mowshowitz and Jie Wei
The laser doppler vibrometer (LDV) is a promising tool for monitoring infrastructure as it can detect small vibrations (~5 nanometers) in objects from as far away as 300 meters. Signals representing vibrations can be analyzed in real time by specially designed software connected to the sensor. The results of the analysis provide a picture of the object’s condition that can be interpreted by trained personnel. For example, the LDV could be pointed at a portion of a trestle bridge while a train is passing, and record vibrations said bridge structure, providing critical information for assessing the stability of the bridge.
The main advantages of using an LDV are 1) the mobility of the device, which allows for successive inspections and measurements in many different locations for one item of interest and for many different items; 2) the ability to take measurements remotely at a safe distance, thus putting the engineers out of harm’s way; and 3) by simply aiming the laser focal point in the right direction, the ability to measure different key points on an object based on different scenarios and conditions that were not easily predicted or installed using other fixed measuring units (e.g., accelerometers) for strategic locations such as beams high above the ground or below a bridge.
The LDV sensor needed to perform laser measurements is easily portable, fitting in a midsized suitcase and able to be transported in a car and deployed in the field for monitoring infrastructure such as bridges, roads, rails, tunnels, buildings, etc. Assisted by a user-friendly software system, professionals in the transport sector and other fields could learn to master it with a minimal amount of training.
Being able to take measurements by aiming the laser beam of the LDV at an object of interest from a safe distance makes the technology especially useful.
The most sensitive vibratory laser comes at a steep price. An sLDV, for example, costs $200,000. However, considering the mobile and multiple uses of this instrument, the cost can be considered lower than that of a great army of fixed sensors that are unsafe to install, unreliable due to weather or wear and tear, and, more importantly, impossible to install in some locations of strategic importance.
The main challenge facing the use of LDVs for infrastructure monitoring is the development of a database of vibratory signatures for the various construction materials used in infrastructure. This information is essential for analyzing the effects of vibration. For example, if we were to record a certain vibration pattern produced by a vehicle traversing a bridge, we would need the vibratory signature of the concrete pillar supporting the bridge to assess the likely effect of bridge traffic on that pillar.
The work we are doing at City College is a start on the development of a database of vibratory signatures. Thus far we have performed experiments on ordinary materials such as aluminum, concrete, plastic and cardboard. In each case we have recorded the vibration patterns produced on these materials by human speech, hammer blows and footsteps.
This research marks a modest beginning of the effort needed to produce a reference catalog of vibratory patterns, and it could help determine vibratory signatures of materials used in infrastructure, and develop procedures and measurement techniques of practical importance in infrastructure monitoring.
Abbe Mowshowitz and Jie Wei are both professors at the Grove School of Engineering at the City College of New York. They can be reached at firstname.lastname@example.org and email@example.com, respectively. For more information, please visit www.ccny.cuny.edu.