Mobility analogy

The mobility analogy, also called admittance analogy or Firestone analogy, is a method of representing a mechanical system by an analogous electrical system. The advantage of doing this is that there is a large body of theory and analysis techniques concerning complex electrical systems, especially in the field of filters.^[1] By converting to an electrical representation, these tools in the electrical domain can be directly applied to a mechanical system without modification. A further advantage occurs in electromechanical systems: Converting the mechanical part of such a system into the electrical domain allows the entire system to be analysed as a unified whole.

The mathematical behaviour of the simulated electrical system is identical to the mathematical behaviour of the represented mechanical system. Each element in the electrical domain has a corresponding element in the mechanical domain with an analogous constitutive equation. All laws of circuit analysis, such as Kirchhoff's laws, that apply in the electrical domain also apply to the mechanical mobility analogy.

The mobility analogy is one of the two main mechanical–electrical analogies used for representing mechanical systems in the electrical domain, the other being the impedance analogy. The roles of voltage and current are reversed in these two methods, and the electrical representations produced are the dual circuits of each other. The mobility analogy preserves the topology of the mechanical system when transferred to the electrical domain whereas the impedance analogy does not. On the other hand, the impedance analogy preserves the analogy between electrical impedance and mechanical impedance whereas the mobility analogy does not.