High contrast ultrasound images of defects in food package seals

A real-time inspection system using a terahertz technique to detect microleak defects in the seal of flexible plastic packages

A method to detect production faults in flexible plastic packages with the use of terahertz radiation is presented. Relying on the large difference between the absorption coefficients of plastic and water (for water-filled channel defects) and on the refraction index difference between plastic and air (for air-filled channel defects), our technique consists of focusing and scanning a terahertz beam on the sealed area of the package, followed by detection of the transmitted signal. Compared with previous methods, such as visual and ultrasound inspection, our technique can be applied to optically opaque packages and does not require immersion in a matching liquid. We tested our system on defects that we fabricated as water-filled and air-filled channels imbedded in polyethylene films, with diameters in the range of 10 to 100 microm. The detection limit (the minimum size of a detectable defect) depends on the conveying speed; this relationship was determined and analyzed. The results show that our system has the potential for application in an actual production line for real-time inspection.

Experimental spatial sampling study of the real-time ultrasonic pulse-echo BAI-mode imaging technique

The ultrasonic pulse-echo backscattered amplitude integral (BAI)-mode imaging technique has been developed to inspect the seal integrity of hermetically sealed, flexible food packages. With a focused 17.3-MHz transducer acquiring radio frequency (RF) echo data in a static rectilinear stop-and-go pattern, this technique was able to reliably detect channel defects as small as 38 microm in diameter and occasionally detect 6-microm-diameter channels. This contribution presents our experimental spatial sampling study of the BAI-mode imaging technique with a continuous zigzag scanning protocol that simulates a real-time production line inspection method in continuous motion. Two transducers (f/2 17.3 MHz and f/3 20.3 MHz) were used to acquire RF echo data in a zigzag raster pattern from plastic film samples bearing rectilinear point reflector arrays of varying grid spacings. The average BAI-value difference (deltaBAI) between defective and intact regions and the contrast-to-noise ratio (CNR) were used to assess image quality as a function of three spatial sampling variables: transducer spatial scanning step size, array sample grid spacing, and transducer -6-dB pulse-echo focal beam spot size. For a given grid size, the deltaBAI and CNR degraded as scanning step size in each spatial dimension increased. There is an engineering trade-off between the BAI-mode image quality and the transducer spatial sampling. The optimal spatial sampling step size has been identified to be between one and two times the -6-dB pulse-echo focal beam lateral diameter.

A real-time approach to detect seal defects in food packages using ultrasonic imaging

The microbial integrity of many types of flexible food packages depends on a zero defect level in the fused seam seal. Human inspection for defects in these seals is marginal at best, and secondary incubation protocols are often used to spot packages with compromised integrity before releasing product for sale. A new type of inspection method has been developed and is being evaluated for robustness. The purpose of the study was to evaluate a new raster scanning geometry to simulate continuous motion, online ultrasonic inspection of the seal region in flexible food package seals. A principal engineering tradeoff of scanning inspection systems is between increased line speed that results from decreased spatial sampling (less acquired data to process) and decreased image quality. The previously developed pulse-echo Backscattered Amplitude Integral (BAI) mode imaging technique is used to form ultrasound images using the new scanning geometry. At an ultrasonic frequency of 22.9 MHz, 38- and 50-microm-diameter air-filled channel defects in all-plastic transparent trilaminate are evaluated. The contrast-to-noise ratio (CNR) of the processed BAI-mode image is used to quantify image quality as a function of spatial sampling. Results show seal defects (38- and 50-microm diameter) are still detectable for undersampled conditions, although image quality degrades as spatial sampling decreases. Further, it is concluded that the raster scanning geometry is feasible for online inspection.

Parametric modeling in food package defect imaging

A novel approach in food package defect detection is proposed based on system identification theory, in which the channel defect detection problem can be regarded as the conventional system identification problem, i.e., estimation of the system impulse response based on the input-output sequence using parametric and nonparametric models. The well-known parametric model ARX has been investigated in this paper. The data are collected with a focused ultrasound transducer (17.3 MHz, 6.35-mm diameter, f/2, 173 microm -6 dB pulse-echo lateral beam width at the focus) scanned over a rectangular grid, keeping the packages in the focus. Performance is measured in terms of detection rate, image contrast, and contrast-to-noise ratio. The results using the ARX model are compared with previous image formation techniques and also compared with the non-parametric method, i.e., spectral analysis. The results show that the ARX model has the comparable detection rate as RFCS and higher detection rate than BAI and RFS (except 6-microm air-filled channel in plastic trilaminate film) for channel in plastic trilaminate film. The ARX model has achieved the moderate contrast enhancement and ranks second in contrast-to-noise ratio enhancement among the compared techniques. The ARX model has a low detection rate for channel defects in aluminum trilaminate film, which shows that its performance is material-dependent. Finally, the parametric method, ARX model demonstrates better performance than the non-parametric method, spectral analysis for food package defect detection.