A mechanical amplifier or a mechanical amplifying element is a linkage mechanism that amplifies the magnitude of mechanical quantities such as force, displacement, velocity, acceleration and torque in linear and rotational systems. In some applications, mechanical amplification induced by nature or unintentional oversights in man-made designs can be disastrous, causing situations such as the 1940 Tacoma Narrows Bridge collapse. When employed appropriately, it can help to magnify small mechanical signals for practical applications. No additional energy can be created from any given mechanical amplifier due to conservation of energy. Claims of using mechanical amplifiers for perpetual motion machines are false, due to either a lack of understanding of the working mechanism or a simple hoax.
Generic mechanical amplifiers Amplifiers, in the most general sense, are intermediate elements that increase the magnitude of a signal. These include mechanical amplifiers, electrical/electronic amplifiers, hydraulic/fluidic amplifiers, pneumatic amplifiers, optical amplifiers and quantum amplifiers. The purpose of employing a mechanical amplifier is generally to magnify the mechanical signal fed into a given transducer such as gear trains in generators or to enhance the mechanical signal output from a given transducer such as diaphragm in speakers and gramophones. Electrical amplifiers increase the power of the signal with energy supplied from an external source. This is generally not the case with most devices described as mechanical amplifiers; all the energy is provided by the original signal and there is no power amplification. For instance a lever can amplify the displacement of a signal, but the force is proportionately reduced. Such devices are more correctly described as transformers, at least in the context of mechanical–electrical analogies. Transducers are devices that convert energy from one form to another, such as mechanical-to-electrical or vice versa; and mechanical amplifiers are employed to improve the efficiency of this energy conversion from mechanical sources. Mechanical amplifiers can be broadly classified as resonating/oscillating amplifiers (such as diaphragms) or non-resonating/oscillating amplifiers (such as gear trains).
Resonating amplifiers
Any mechanical body that is not infinitely rigid (infinite damping) can exhibit vibration upon experiencing an external forcing. Most vibrating elements can be represented by a second order mass-spring-damper system governed by the following second order differential equation.
m x ¨ + c x ˙ + k x = F ( t ) {\displaystyle m{\ddot {x}}+c{\dot {x}}+kx=F(t)}
where, x is the displacement, m is the effective mass, c is the damping coefficient, k is the spring constant of the restoring force, and F(t) is external forcing as a function of time.
"A mechanical amplifier is basically a mechanical resonator that resonates at the operating frequency and magnifies the amplitude of the vibration of the transducer at anti-node location." Resonance is the physical phenomenon where the amplitude of oscillation (output) exhibit a buildup over time when the frequency of the external forcing (input) is in the vicinity of a resonant frequency. The output thus achieved is generally larger than the input in terms of displacement, velocity or acceleration. Although resonant frequency is generally used synonymously with natural frequency, there is in fact a distinction. While resonance can be achieved at the natural frequency, it can also be achieved at several other modes such as flexural modes. Therefore, the term resonant frequency encompasses all frequency bandwidths where some forms of resonance can be achieved; and this includes the natural frequency.
Direct resonators
All mechanical vibrating systems possess a natural frequency fn, which is presented as the following in its most basic form.
f n = 1 2 π k m {\displaystyle f_{n}={1 \over 2\pi }{\sqrt {k \over m}}}
When an external forcing is applied directly (parallel to the plane of the oscillatory displacement) to the system around the frequency of its natural frequency, then the fundamental mode of resonance can be achieved. The oscillatory amplitude outside this frequency region is typically smaller than the resonant peak and the input amplitude. The amplitude of the resonant peak and the bandwidth of resonance is dependent on the damping conditions and is quantified by the dimensionless quantity Q factor. Higher resonant modes and resonant modes at different planes (transverse, lateral, rotational and flexural) are usually triggered at higher frequencies. The specific frequency vicinity of these modes depends on the nature and boundary conditions of each mechanical system. Additionally, subharmonics, superharmonics or subsuperharmonics of each mode can also be excited at the right boundary conditions.
“As a model for a detector we note that if you hang a weight on a spring and then move the upper end of the spring up and down, the amplitude of the weight will be much larger than the driving amplitude if you are at the resonant frequency of the mass and spring assembly. It is essentially a mechanical amplifier and serves as a good candidate for a sensitive detector."
Parametric resonators
Parametric resonance is the physical phenomenon where an external excitation, at a specific frequency and typically orthogonal to the plane of displacement, introduces a periodic modulation in one of the system parameters resulting in a buildup in oscillatory amplitude. It is governed by the Mathieu equation. The following is a damped Mathieu equation.
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