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The design of control system can be divided into two steps. First the process or plant have to be convert into mathematical model form, so that its behavior can be analyzed. Then an appropriate controller have to be design in order to get the desired response of the controlled system. In the continuous time domain the system is represented by differential equations. Replacing a continuous system into discrete time form is always an approximation of the continuous system. The different discretization methods give different digital controller performance. The methods presented on the paper are Step Invariant or Zero Order Hold (ZOH) Method, Matched Pole-Zero Method, Backward difference Method and Bilinear transformation. The above mentioned discretization methods are used in developing PI position controller of a dc motor. The motor model was converted by the ZOH method. The performances of the different methods are compared and the results are presented.

Hypophosphatasia is a rare hereditary metabolic disorder accompanying deficit of tissue nonspecific serum alkaline phosphatase. The incidence of overt forms is estimated about 1:100000 live births. In the prenatal manifestation the disease may cause severe damage to the foetus with intrauterine death. In children there is a defect of mineralization with rickets signs and the subsequent hypercalcaemia a hypercalciuria may lead to death. In adults the main manifestation is osteomalacia, skeletal deformities and fractures, early arthritis. In severe forms the heredity is autosomal recessive type. In mild forms the heredity may be dominant or recessive. In two case reports we present clinical course of the disease in two adult sisters, where diagnosis of hypophosphatasia was first time confirmed in Slovak population using molecular genetic methods.