The present invention concerns a method of rapidly and accurately measuring the resonance frequency of a mechanical resonator, and it is more particularly interesting in the case of resonators consisting of solid metal bars of generally cylindrical form such as are used in electromechanical filters.
Usually, the resonance frequency of mechanical resonators, for example of steel bars, is determined in a bridge arrangement supplied by a variable frequency source. The resonator is disposed in a coil which is supplied by the variable frequency source constituting one of the arms of the bridge. In addition, the oscillating conditions under which the resonator is placed vary in accordance with the operational mode, which often involves the use of a delicate fixture. In the measurement, the bridge is balanced with the source frequency close to resonance frequency, and the frequency at which the unbalance of the bridge is maximum has to be found. This operation is relatively lengthy.
The method according to the present invention relies on the excitation of the resonator into vibration under the influence of a shock. The sensor consists of a solenoid which is operated with a weak magnetizing flux in which the resonator is positioned. The vibrations of the resonator modify the distribution of the flux in the coil and develop an induced voltage. Then counting the number of vibrations of the bar during a fixed period of time or measuring the duration corresponding to an integral number of vibrations, will provide the value of the resonance frequency. The duration of the measurement depends upon the damping of the vibrations. In the case of high performance resonators, it may reach about a hundred of milliseconds, and in this case a frequency measuring precision of the order to 10.sup.-6 can be obtained.
The measuring method needs a magnetizing flux in the solenoid. Depending upon the nature of the resonator, it may be created either by the resonator itself if it is magnetic or by an external magnet, or else by a direct current flowing through the coil constituting the solenoid. The excitation of the resonator by shock in accordance with the present invention must be so carried out as to develop the desired mode of vibration. It is well known to emphazize one mode of excitation with respect to other modes, even of near-by frequencies, by means of proper fixtures in or on the resonator. The shock by which the vibration of the resonator is brought about is of mechanical or magnetic origin, depending upon the embodiments.