Abstract: A method and apparatus for measuring the tension of a thin, plane membrane (1). In accordance with the method, a burst of a membrane wave (2) is generated by a transducer element (3) e.g. a loudspeaker (3). The velocity of the membrane wave (2) is measured. The velocity measurement value of the membrane wave (2) is squared and multiplied by the basis weight of the membrane (1) in order to determine the tension. According to the invention, a light beam (10) is generated by at least one light emitting element (5), each light beam (10) is projected with help of projecting members (6 or 14, 15, 17) onto the membrane (1) in order to generate a light spot (4 or 19), the light spot (4 or 19) vibrating on the membrane (1) is converted into an electrical signal dependent on the position of the light spot (19) by means of at least one detector element (8 or 18 ) in order to determine the membrane wave (2) and the signal is processed in a processor (9) in order to determine the velocity of the membrane wave (2).

Abstract: A single-phase or multi-phase inverter protected with respect to the rates of increase of current and voltage, the said inverter comprising a filter capacitor and a choke protecting with respect to the rate of increase of current, as well as, members protecting with respect to the voltage increase rate. In order for it to be possible to discharge the energy stored in the magnetic field of the choke and in the capacitor, between the change-over steps of the inverter substantially without losses, for each phase, there is a series connection of the capacitor and a diode connected in parallel with only one semiconductor switch and a diode is connected between the connection point between the capacitor and the diode, and one terminal of a storage capacitor. The energy in this way transferred to the separate storage capacitor can then be transferred to the filter capacitor by use of a breaker.

Abstract: The speed of feed movement of a piston is controlled to an essentially constant value by direct digital measurement of the actual speed, by comparing the actual value to a preset value and by compensating for differences therebetween. Variations in the piston feed pressure and in the grinding power are taken into account by means of additional signals related to these factors and added to or subtracted from the signal representing the actual value of the speed of the piston.