Method and generator for characterizing an oscillatory system

A method for determining an electric capacitance Csw, of an ultrasonic oscillatory system (100) used for ultrasonic machining, the method being carried out during a machining break, the ultrasonic oscillatory system (100) comprising at least a generator (20), a converter comprising at least one piezoelectric element (10), and a sonotrode, the generator (20) supplying an alternating voltage that is converted by the piezoelectric element (10) of the converter into mechanical oscillation, the piezoelectric element (10) having an electrode and a counter electrode, wherein the sonotrode is brought into contact with a material to be machined in order to perform ultrasonic machining, and the sonotrode is not in contact with the material to be machined during the machining break, the method comprising the following steps:

• a) applying an electrical alternating voltage between the electrode and the counter electrode for the duration of an excitation interval in order to induce mechanical oscillation of the ultrasonic oscillatory system or of a sub-system of the ultrasonic oscillatory system so that after the excitation interval has expired, the ultrasonic oscillatory system or the sub-system performs a free oscillation without excitation,
• b) after the end of the excitation and during the free oscillation of the ultrasonic oscillatory system or of the sub-system without excitation:
• measuring a time curve of a voltage U (1) between the electrode and the counter electrode, and
• c) determining the electric capacitance Csw from the time curve of the voltage U (1), which time curve was measured in step b),
• wherein, in step b), a measuring device (30) having an internal capacitance Cprobe and an internal resistance Rprobe is used to measure the time curve of the voltage (1),
• and wherein step c) comprises the following sub-steps:
• aa) determining a time curve of a DC voltage portion UDC (2) from the time curve of the voltage U (1), which time curve was measured in step b),
• bb) determining a characteristic time interval τ, within which an initial value of the DC voltage portion UDC,O measured in step b) has fallen to the value UDC,O/e, wherein e is Euler's number,
• cc) calculating the electric capacitance Csw with the characteristic time interval I.