1. Field of the Invention
This invention relates in one aspect to vehicle wheel speed sensors particularly, but not exclusively, to wheel speed sensors for aircraft, and in a second, more general aspect to a method of converting a sine wave signal into a sequence of marker pulses, the marker pulses being produced for each quadrant of the sine wave signal.
2. Description of the Prior Art
Aircraft wheels are often provided with wheel speed sensors, generally in the form of a tachometer mechanically driven by the wheel axle. The tachometer may be used to provide a speed signal to an anti-lock braking means associated with the wheel. The hostile axle environment imposes strong demands upon the tachometer. Heat, dust and vibration are all encountered.
Wheel speed sensors are known in cars which comprise a toothed rotor associated with a car wheel, and a stationary inductive sensing means responsive to movement of the teeth past the sensing means.
With an aircraft it is desirable to be able to monitor accurately the aircraft wheel speed even at very low wheel speeds, such as 10 rpm. If a toothed or segmental rotor were to be used in conjunction with a conventional pulse counting circuit to count cycles of the sine wave generated by an inductive sensing means, a large number of teeth or segments, hereinafter referred to as `rotor elements`, would need to be provided on the rotor. Since the room for such a rotor adjacent to the axle is strictly limited, a larger number of rotor elements results in a smaller circumferential spacing between the elements. For a given dimension of inductive sensing means a smaller spacing between the rotor elements produces a reduced amplitude of the sine wave signal output since even a small sensing means tends to be influenced by several adjacent rotor elements.
Also, when a large number of rotor elements, such as 150 on a rotor of diameter 6.35 cm (2.5 inches), is attempted there is an increased requirement for close tolerance machining, and the slightest variation in the air gap between the sensing means and the rotor elements introduces spurious signals into the system.