Mechanical filter

A mechanical filter is an analog filter is used which is similar to that electrical filters for the selection and rejection of certain frequency components of the spectrum of a signal in the signal processing. In contrast to electrical filters, the filter effect is achieved by mechanical vibrations correspondingly shaped masses. Mechanical filters fall within the field of acoustics, even if the processed frequencies with them do not have to be audible. The interface between electrical and mechanical system represent converters, which convert the input, the supplied electrical oscillations into mechanical vibrations, and at the output of the mechanical vibrations back into electrical vibrations.

General

To calculate mechanical filter directly the methods and mathematical methods from the electrical circuitry can be used. In particular, the known from the electrical network analysis and established filter design methods as well as the methods thus can be transmitted with two gates directly into corresponding mechanical systems.

As the material is in mechanical filters in addition to steel and a variety of metal alloys such as iron-nickel alloys, ceramics, or quartz application. Mechanical filters are further divided according to structure and application such as surface acoustic wave (SAW ) filters in microsystems (MEMS ) miniaturized filter and the quartz crystals and ceramic resonators used in resonant circuits in the feedback loop.

In mechanical filters can be implemented very high filtration grades compared to passive analog filters. This follows from the fact that can not be prepared with high inductance coils and at the same time very low ohmic resistors, which represent losses in electrical systems. With mechanical filters and their relatively high quality factor Q, values ​​above 10,000 are common, resonant circuits can be realized with high frequency selectivity. This fact owes the quartz oscillator in electrical resonant circuits for frequency generation, such as in quartz watches, his opposite LC resonant circuits high frequency stability and accuracy.

Historical Development

Mechanical filters are not necessarily dependent on an electrical interface and making the necessary conversion. Thus, for example, through calculation of speakers without the electrical system. The first mechanical filters were in the 1920s when the then-emerging gramophone, developed as a purely mechanical systems without electrical interface.

In the 1950s began, mechanical filter as a separate filter module, as shown due to the high Q factors compared with LC filters in the above figure as an intermediate frequency filter for 455 kHz, uses in the former radio devices.

Electrical converter

The electrical transducer in mechanical filters are usually based on two effects, although transducers are not limited to: magnetostriction and piezoelectricity. In the quantum-mechanical effect of magnetostriction, one uses the fact that the magnetic flux passes through a similar material to a change in length. Usually one " magnetic bias " in the form of a permanent magnet is also required for this, which is why in newer and smaller transducers almost exclusively piezoelectric transducers made ​​of the material of lead -zirconate- titanate (PZT) or quartz are used. Also in these types of transducers, the piezoelectric material may be incorporated as an integral part of the mechanical filter system, as is the case for example with the SAW filters, resonators or the like quartz crystal.

Construction

Mechanical filters are among the passive filters. Electrical passive filters are formed with an electrical resistance ohmic resistors, capacitors, an electrical capacitance and with an inductance coil. In the mechanical system corresponding to the electric resistance R of the damping (D), the capacitance C, the mechanical compliance ( δ ), this size is the reciprocal value of the mechanical stiffness of S, and the inductance is expressed by the mass m. The mechanical equivalent of the electrical voltage (U) is the mechanical force (F), the electric current (I) is represented by the speed (v).

From this relationship can be described as analogous to the electrical system, a mechanical impedance (Z) as a function of angular frequency ω particular, the angular frequency is multiplied by the imaginary unit j. The identifiers x, v, and a stand for the distance x, velocity v and acceleration a in all, thus yield the following relations:

The above table represents the so-called impedance analogy with the help of the mechanical system can be described as an electrical circuit and calculated. It can thus directly apply the methods of complex AC circuit analysis on the mechanical system. Each mechanical element is next to a steaming According proportion friction from both a mass and a certain stiffness, which can be like the electrical circuits concentrated components constitute one of the desired properties only in approximation. Predominates in less attenuation of the Influence of the indulgence of the mass are " mechanical capacitors " formed. Typically, these consist of thin, long sticks as possible. " Mechanical coils ", as a counterpart, consisting of a small, flat discs of solid material, which try to maximize their mass and minimize sag.

• Schematic diagrams of mechanical filters

Mechanical filter with torsional resonators

Mechanical filter with Drumhead - Resonatren

Mechanical filter with Flexual resonators