Abstract: A functional control device (1), designed to be used in primarily textile machines (20), in which the speed must be varied in a simple fashion. Its function involves a conversion of a pressure of a gaseous or liquid medium to a code, which is adapted to the machine. When speed control means, having plates (2, 3), designed to be depressed, e.g. in a sewing machines (20), are used, one of the pedals corresponds to a forward depressing movement and the other a backward depressing movement. When a nozzle (4) is used, said functions can also be provided. The so-called forward depressing movement comprises a presser foot downwards and 12 speed steps, from zero to a maximum rotational speed, which can be as high as 400-9,000 rev./min. A backward depressing movement comprises a presser foot as well as a cutting/locking function. The function of the pressure error interface can be varied in a simple way, for various types of machine control means, and for varying pressure sensitivities.