Non-contact Ultrasonic Sensor for Density Measurement and Defect Detection on Wood

In this work the problem of measuring density and verifying structural integrity of wooden samples is approached by using non-contact ultrasonic transducers. The time of flight of the ultrasonic waves is measured in transmission mode using cross-correlation algorithms between emitted and received signals. Once known the times of flight, the propagation velocity through the wood, which is proportional to density, can be estimated. The conversion factor between velocity and density is experimentally established through a laboratory calibration. Moreover the non contact ultrasound technique is applied to detect the presence of knots by scanning the surface of the sample and measuring the integrated response proportional to signal energy. Introduction: In last years great efforts have been made on the development of measurement and testing techniques for wood and wooden structures with the aim of predicting their characteristics and ensuring their performances. Many experimental methods have been investigated and, among the different approaches, particular attention has been devoted to ultrasonic measurement. However, wood is a naturally grown, anisotropic and heterogeneous material that makes the use of ultrasonic measurement systems very challenging. In fact several parameters, e.g. the wood fibres disposition and the humidity content, can make the ultrasonic signals complex and difficult to be analysed. Nevertheless Non Destructive Techniques (NDT), mostly based on traditional contact ultrasounds application, have been widely applied in the evaluation of the wood quality in several characterization procedures, for diagnostic purposes and for the classification in working conditions. For instance, in order to evaluate the elastic modulus, the presence of defects and the bulk mass, a contact ultrasound system with waves propagating in the fibres direction can be used. A similar system, but with the waves propagating perpendicularly to the fibre direction, is utilized to detect the presence of wood rot directly on the tree. To determine the dynamic hardness and also the presence of wood rot an established measurement technique is the scleroscopy. One parameter of great interest in the evaluation of the mechanical properties of wood is the density, that is related to the wood behaviour against environmental forces like wind, rain, gravity, snow, and other. The evaluation of this parameter permits also to detect wood defects like decay, insect damage, splits, checks, knots which change the wood density substantially. Main limitations of traditional ultrasonic sensors are often connected with the contact and the coupling medium required to have a satisfactory SNR. As a consequence, one of the NDT measurement method used to locally evaluate the density is the X-Ray technique, which on the other side may present problems for use in industrial environment due to human exposure to radiations. In this paper the development of a measurement system based on innovative non-contact ultrasonic sensors for density measurement and knots location is proposed. The methodology is discussed and preliminary results are presented. The use of non contact ultrasonic probes for this kind of analysis is a new approach that could allow to monitor the density during, for example, the working process. Measurement technique: The developed experimental set-up is shown in Figure 1. The system is composed of two sensors, which can operate both as transmitter and as receiver. Therefore the system has two channels for data acquisition (with two amplifiers and two analog I/O boards), in such a way as to present four operation modes (two in reflection mode – one for each of the two sensors and two in transmission mode – one used as transmitter and the other as receiver and vice-versa). The boards for driving the ultrasonic probes are installed in a commercial industrial PC that performs generation, reception, acquisition and treatment of the ultrasonic signals acquired from the sensors. These signals will represent physical data such as the Time of Flight (time spent by the ultrasonic beam to cover the path) and Integrated Response (area underneath a selected transmission or reflection peak) of the ultrasonic waves. In Figure 2 an example of the cross-correlation signal measured, in transmission mode, on a wooden sample, is shown. Tc represents the time of flight of the first peak and tm is the difference between the ToF of the first and second peak. The arbitrary units are used because the information of interest is represented by the position of the peaks with respect to the time axis. The probes are non-contact piezo-electric transducers (spherical focused probes with 12.5 mm active area diameter), operating at about 1.0 MHz. The signals used to drive the transducers are chirps (very fast sinusoidal sweeps in a limited frequency band), with amplitude, time duration and frequency content tuneable, in such a way as to optimise the ultrasonic wave propagation in the different materials. To find the combination of settings that allowed to obtain the better signals for wood density measurement and knots location, several sets of parameters were tried leading to the following configuration: Central frequency: 750 kHz Bandwidth around the central frequency: 500 kHz Chirp Duration: 100 μs In the performed tests the system operated in direct transmission mode, i.e. ultrasound travels from the transmitting to the receiving transducer through ambient air and the sample. In order to measure the time of flight, a cross-correlation algorithm is employed, which allows to determine the delays between emitted and received waves. The system was used for two different purposes: to assess the density value of different kind of wooden samples and to detect the presence of knots. The measurement chain was optimized in order to have a satisfactory SNR (from 16 to 25 dB, depending on the wood) and a measurement time compatible with production requirements. In particular the distance and alignment of the sensors were carefully considered, as the losses at the interfaces (air-sample) significantly affect the achieved accuracy (see next section). Figure 1: Scheme of the measurement system (T: transducer, R: receiver) Results: In order to obtain the density values of the test material, it is necessary to measure the ultrasound velocity through the material. In order to find a relation between velocity and density, a dedicated calibration must be performed using reference samples whose density was previously determined by measuring the volume by a calibre and the mass by a balance with a resolution of 0.01 g. The samples have dimensions typical of those in the production line during working progress. The thickness of the different samples used is in the order of 5-7 mm. Several different kinds of wood were investigated. For each kind, about 4 or 5 samples at different density levels were used for calibration. In Table 1 the average reference density values of the samples calculated for each kind of wood are showed. Figure 2: Example of transmission mode cross-correlation signal measured on a calibration sample. The first multiple reflection is used to determine time of flight through the sample (tm) Wood Reference density (average) Sycamore figured 567.7 kg/m