You are here: Home | Publications | SAIEE Africa Research Journal | 2008: Vol 99 No 4:

Abstract: In practical image storage and distribution systems, JPEG is a common image format. In this paper, we propose a blind watermarking method for JPEG-compressed color images in the semi-decompression domain by integrating the JPEG coding process. The watermark is embedded by modifying the quantized DC coefficients based on the spread spectrum technique. There are some modifications on the basic spread spectrum watermarking scheme to meet the sensitivity of the quantized DC coefficients. Furthermore, Reed-Solomon coding is employed to improve the robustness of watermarking. Experimental results show that this watermarking scheme is robust to a very wide range of distortion attacks such as median filtering, noise adding, JPEG compression, malicious manipulation, and so on.

Abstract: This paper compares two techniques that are commonly used to prevent the pole-zero cancellation phenomenon inherent to the Riccati-based H_∞; mixed sensitivity approach. The first technique uses a strategy based on the reformulation of the standard mixed-sensitivity design problem, where the closed loop transfer function from the plant input to the plant output is penalized. The second technique combines the H_∞ optimal control with the bilinear transformation. The effectiveness of the techniques is illustrated by designing power system stabilizers for a simple power system. Computer simulations show that these techniques are effective in improving the damping of the system than the standard mixed sensitivity approach. However, the technique that combines the H_∞ optimal control with the bilinear transformation provides a better damping to the system and is more robust than the technique based on the reformulation of the mixed sensitivity design problem.

Abstract: In this paper, the authors propose a real-time multiple frequency fault detection, correction and identification system for vibration forces on the rotor of a rotational active magnetic bearing (AMB) system. Condition monitoring was performed on the displacement signals of a fully suspended 250 kW water cooling AMB pump, to obtain historical fault data. A pattern recognition system compared the realtime displacement error patterns with the displacement error patterns from the historical fault database. A fuzzy logic system used the patterns from the pattern recognition system to perform error correction. The Wigner-Ville distribution extracted the vibratory amplitudes and frequencies, which was used as input features to the pattern construction and pattern recognition systems. Experiments were performed on a double radial AMB test rack to demonstrate the effectiveness of the proposed system in the detection, correction and identification of vibration forces on the rotor of an AMB system. The detection and correction system was able to correct and minimize multiple frequency vibration forces to a stable working condition. The identification system calculated the type, parameters, vibratory level and zone of the vibration forces. The main advantage of this system is its capability to detect, correct and identify multiple frequency vibration forces.