High contrast ultrasound images of defects in food package seals

Previous work to detect defects in food packaging seals using pulse-echo ultrasound inspired the backscattered amplitude integral (BAI) imaging technique, which could reliably identify channels with diameters 38 mum or larger at a center frequency of 17.3 MHz (lambda=86 mum). The current study presents two new processing techniques that more reliably reveal smaller channels ( approximately 6 mum in diameter). The RF sampling technique (RFS) displays a single, time-gated, pressure value from the received (not envelope-detected) RF waveform at each transducer position. The RF correlation technique (RFC) calculates the correlation coefficients of the RF signals with a reference signal that does not pass through a channel. The correlation coefficient can be calculated for the entire RF signal (RFCE) or over a short segment of the RF signal (RFCS). The performance of these imaging methods for detecting channel defects is investigated for plastic and aluminum foil trilaminate films with 6, 10, 15, 38, and 50 mum channels filled with water or air. Data are collected with a focused ultrasound transducer (17.3 MHz, 6.35 mm in diameter, f/2, 173 mum -6 dB pulse-echo lateral beamwidth at the focus) scanned over a rectangular grid, keeping the package in the focus. Performance is measured using detection rates, image contrast, and contrast-to-noise ratio (CNR). Both RFS and RFCS have improved detection rates relative to BAI for channels 15 mum or smaller. The RFCS technique is the most effective at smoothing the background, leading to the greatest CNR improvements.     

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.     

A solution to the diagonalization problem by constantprecompensator and dynamic output feedback

A solution is presented for the problem of diagonalization (row-by-row decoupling). The problem is solved using a constant precompensator and a dynamic output feedback compensator of a p×m linear time-invariant system. The solvability condition is compact and concerns the dimension of a single subspace defined via the concepts of essential rows and static kernels associated with the transfer matrix. A characterization of the set of all solutions to the problem is also given. In solving this dynamic feedback problem, a complete solution to its state-feedback counterpart, namely, the restricted state-feedback problem of diagonalization, is also presented     

Local convex directions for Hurwitz stable polynomials

A condition for a polynomial p(s) to be a local convex direction for a Hurwitz stable polynomial q(s) is derived. The condition is in terms of polynomials associated with the even and odd parts of p(s) and q ( s), and constitutes a generalization of Rantzer's phase-growth condition for global convex directions. It is used to determine convex directions for certain subsets of Hurwitz stable polynomials     

Fixed zeros of decentralized control systems

Considers the notion of decentralized fixed zeros for linear, time-invariant, finite-dimensional systems. For an N-channel plant that is free of unstable decentralized fixed modes, an unstable decentralized fixed zero of channel i (1⩽i⩽N) is defined as an element of the closed right half-plane, which remains as a blocking zero of that channel under the application of every set of N-1 controllers around the other channels, which make the resulting single-channel system stabilizable and detectable. The paper gives a complete characterization of unstable decentralized fixed zeros in terms of system-invariant zeros     

Global stabilization via local stabilizing actions

Stabilization of a linear, time-invariant system via stabilization of its main diagonal subsystems is the underlying problem in all diagonal dominance techniques for decentralized control. In these techniques as well as all Nyquist-based techniques, sufficient conditions are obtained under the assumption that the collection of the unstable poles of all diagonal subsystems is the same as the unstable poles of the overall system. We show that this assumption is by itself enough to construct a solution to the problem at least in cases where the diagonal subsystems have disjoint poles.     

Almost disturbance decoupling with internal stability: frequencydomain conditions

The problem is that of determining a dynamic measurement feedback which makes the H^∞-norm of the disturbance-input-to-regulated-output transfer matrix arbitrarily small while achieving internal stability. it is shown that the solvability condition in the frequency domain for this problem is a purely algebraic one and can be formulated in terms of a two-sided matrix matching equation involving polynomial system matrices. This is known to be a zero cancellation condition. A synthesis procedure for the compensator in the frequency domain is also given     

Reliable decentralized stabilization of feedforward and feedbackinterconnected systems

The synthesis of decentralized controllers that are reliable in the presence of breakdowns in the interconnections is considered for plants obtained via feedforward and feedback type interconnection schemes. The main objective is to determine the precise conditions on the plant which assure such reliable synthesis. It is shown that the central property for reliable decentralized stabilization is always an appropriate parity interlacing property among the poles of the interconnection subplants and certain blocking zeros of the overall plant. The conditions under which the stabilization of a two-channel plant is affected by the stabilization of its diagonal subplants are also obtained. The results also find application in multiple controller reliable synthesis problems. The discussion is restricted to two-channel plants     

Evaluation of defects in the seal region of food packages using the ultrasonic contrast descriptor, ΔBAI

A rapid, dependable method of on‐line package inspection will allow wider implementation of energy‐ and material‐efficient retortable pouches and trays, by reducing inspection costs. To evaluate high‐frequency ultrasonic imaging as a sensing method, the 17·3 MHz ultrasonic pulse‐echo Backscattered Amplitude Integral (BAI) method was used to visualize and evaluate major defects (channels and product inclusions, which compromise the seal integrity and must be reliably inspected) in the seal area of flexible food packages. The focus of this study was to evaluate the image contrast, denoted ΔBAI, for various packaging materials, defect types and sizes. Channel (6–100 µm in diameter) and inclusion defects (strands of mouse tail tendons, 20–150 µm) were created in the seal area of all‐plastic and foil‐containing packaging films. It has been shown that there is a direct relationship between the defect size and ΔBAI value, and that different defect types and packaging materials have a significant impact on the ΔBAI value. The utility of ΔBAI for detecting defects makes it a useful and reliable sensing method for package inspection. Copyright © 1999 John Wiley & Sons, Ltd.