The purpose of this study is to suggest a new formulation for active vibration control of a rectangular plate based on the optimal positions/orientations of piezoelectric actuators/sensors attached to the plate. The free vibration and modal properties are derived by using Rayleigh–Ritz and the transient response by assumed modes methods based on the classical plate theory. Three criteria are proposed for optimal location of piezoelectric patches attached to the simply supported plate. In other words, the optimal positions/orientations of piezoelectric patches can be determined based on spatial controllability/observability gramians of the structure, as well as the consideration of residual modes to reduce the spillover effect. These criteria are used to achieve the optimal fitness function defined for a genetic algorithm optimizer to find the optimal locations/orientations of piezoelectric sensors/actuators. To control the vibrations of the plate, a negative velocity feedback control algorithm is designed. The results of simulations indicate that by locating piezoelectric patches in the optimal positions, the depreciation rate of the structure increases and the amplitudes of the plate vibrations reduce effectively. The effect of number of piezoelectric devices on the active damping property of the system is also analyzed.

Due to inappropriate agricultural treatments, soils often turn into wastelands. After years, the return to cultivation is very difficult or even impossible. It is a serious problem for agriculture, also in Poland. In order to improve soil quality and to restore its value in use, different fertilizers are commonly used. As an alternative for artificial fertilizers, sewage sludge and biomass ashes might be applied. High cost of sewage sludge conditioning with the use of polyelectrolytes has resulted in the development of research dedicated to finding new and inexpensive conditioners. The article discusses the possibility of sludge conditioning by means of wood biomass ash. In the paper, the findings of sewage sludge dewatering with the use of wood ash on a laboratory and on a technical scale are presented. The analysis of results has shown that the addition of wood biomass ash improved sludge dewatering by 10–23% in a laboratory scale and by 15–27% on a technical scale. The application of wood ash also reduced the total bacteria number by 83–89% in a laboratory scale and by 40–53% on a technical scale. Additionally, the obtained mixture of sewage sludge and wood biomass ash might be applied in agricultural practices, especially in fertilizing perennial plants plantations. Thanks to that it is possible to obtain inexpensive and valuable natural fertilizer while reducing costs of sludge dewatering.