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A haptic system consists of an articulated mechanical structure with motors and position sensors, as well as embedded electronics allowing force feedback. It is driven by a haptic interface, which enables the user to interact with an image in a virtual reality application, through the sense of touch. Thus, it is a hybrid dynamic system, which contains subsystems with continuous dynamics and subsystems with discrete dynamics that interact with each other. This paper develops a hybrid dynamic system model of the system, which takes into account the different sampling periods of the desired regulation, and the interaction between continuous, switching and discrete subsystems. It is interesting when very short electrical transients coexist with very long mechanical ones.

Fifteen patients with hypertension due to phaeochromocytoma and 35 controls with essential hypertension were studied to assess the diagnostic value of urinary and plasma biochemical determinations in phaeochromocytoma. In every case of phaeochromocytoma the urinary concentration of vanillylmandelate, metanephrines, or adrenaline plus noradrenaline was diagnostic of the disease irrespective of whether the patient was normotensive or hypertensive at the time. Plasma determinations of adrenaline and noradrenaline, however, gave falsely negative results on three occasions. These findings suggest that urinary biochemical determinations--particularly of metanephrines--are more reliable than plasma catecholamine measurements as a test for phaeochromocytoma. The test is particularly useful in patients with intermittent hypertension.