A Wireless Ultrasonic Ndt Senor System

Ultrasonic condition monitoring technologies have been traditionally utilized in industrial and construction environments where structural integrity is of concern. Such techniques include active systems with either single or multiple transmit-receiver combinations used to obtain defect positioning and magnitude. Active sensors are implemented in two ways; in a thickness operation mode, or as an area-mapping tool operating over longer distances. In addition, passive ultrasonic receivers can be employed to detect and record acoustic emission activity. Existing equipment requires cabling for such systems leading to expensive, complicated installations. This work describes the development and operation of a system that combines these existing ultrasonic technologies with modern wireless techniques within a miniaturized, battery-operated design. A completely wireless sensor has been designed that can independently record and analyze ultrasonic signals. Integrated into the sensor are custom ultrasonic transducers, associated analogue drive and receive electronics, and a Texas Instruments Digital Signal Processor (DSP) used to both control the system and implement the signal processing routines. BlueTooth wireless communication is used for connection to a central observation station, from where network operation can be controlled. Extending battery life is of prime importance and the device employs several strategies to do this. Low voltage transducer excitation suffers from poor signal-to-noise ratios, which can be enhanced by signal processing routines implemented on the DSP. Routines investigated include averaging, digital filtering and pulse compression. Introduction: Ultrasonic Non destructive Testing (NDT) and in particular structural health monitoring cover a broad application spectrum, ranging from passive acoustic emission detection to active time-of-flight structure interrogation. Generally, the results of such testing are integral to planned maintenance schedules, however circumstances can lead to immediate operator intervention. Traditional structural health monitoring systems have two major shortcomings. Firstly they are cabled systems, using either copper wires or fiber optics. Not only does this lead to high installation and maintenance costs but can also lead to problematic network extension or reconfiguration. Furthermore, traditional cabled systems overcome the inherently large ultrasonic insertion losses with high excitation voltage and power levels. Time-of-flight pulse echo systems using pulsed excitation operate with voltage levels in the hundreds. Not only are these voltages impractical to generate and use in a battery-powered devices, but also when operating in hazardous environments they introduce health and safety implications. This work describes the design and implementation of a wireless, miniaturized batterypowered ultrasonic test unit. The sensor is fully autonomous while complying to typical matchbox dimensions of 35x50mm. The low voltage excitation operation must be addressed as the high insertion loss can lead to an unfeasibly low signal-to-noise ratio (SNR), rendering the measurement unreliable. This paper investigates a number of techniques to improve such situations. The simplest technique would implement an averaging algorithm to resolve the low SNR. Assuming the noise is random in nature, a characteristic of electrical noise [1], averaging will reduce its effects, however this could take hundreds of cycles unnecessarily consuming battery power. A digital Finite Impulse Response (FIR) filter can remove out-of-band noise and can provide a substantial SNR enhancement. A third technique that has found widespread favor is the pulse compression or matched filtering method [2]. Correlation between the received signal and the transmission waveform can provide a significant SNR enhancement.