Fail-safe speed sensor for london transport automatic trains

An electromagnetic speed sensor has been developed for use in automatic-train control on London Transport railways. The primary sensing element is a composite ferromagnetic-conductive rotor which introduces amplitude modulation, with frequency proportional to train speed, into a fixed audiofrequency carrier. A number of discrete rotational speeds are then defined by demodulation and low-pass filtering. An override system, operating by reducing the amplitude of a further AC signal with increasing train speed, provides integrity at low speeds, and a magnetic voltage reference unit maintains the power supply within predetermined safe limits. Fail-safe design principles are applied extensively on both component and system levels.