The vehicle 110 monitors vehicle speed and generates a vehicle speed signal which is used in connection with various vehicle systems. According to one embodiment, the vehicle 110 is equipped with a speed sensor 222 that is operatively coupled to the vehicle's transmission 220 for monitoring the speed of rotation of a rotating shaft of the transmission 220. Preferably, the speed sensor 222 measures rotational speed of the output shaft of the transmission 220. The speed sensor 222 generates a quasi-sinusoidal signal which has a signal response proportional to the vehicle's speed. Alternately, a wheel speed sensor 224 may be employed to generate the quasi-sinusoidal vehicle speed signal by monitoring the wheel speed. Speed sensors 222 or 224 may include a hall-effect or variable-reluctance sensor such as model no. 1AV2A manufactured and sold by Honeywell.
The unbuffered vehicle speed signal 226 is applied to a vehicle speed signal buffer 228 which produces a square-wave vehicle speed signal 76. The square-wave speed signal 76 contains a series of pulses, and the number of pulse-per-second provides an indication of the vehicle speed. The square-wave vehicle speed signal 76 is made available as an input to various control modules within the vehicle to provide an indication of the vehicle speed. For example, the vehicle speed signal 76 may be used by the cruise control module 230, a digital speedometer module 232, a head-up-display (HUD) 234, a navigation system 236, and various other vehicle systems. When used in connection with the navigation system 236, for example, the vehicle speed signal 76 can be used to determine the vehicle speed by dividing the vehicle speed signal pulses-per-second by the pulses-per-mile ratio for the given vehicle, and factoring in a conversion factor of 3,600 seconds-per-hour. The vehicle speed equation is provided in the following equation: ##EQU1##
In order to determine vehicle speed, the system of interest, such as navigation system 236, must consider the pulses-per-mile ratio associated with that particular vehicle. Amongst the various vehicles, the pulses-per-mile ratio may vary among vehicle manufacturers and also between various automotive vehicle models. For example, one vehicle may be equipped with a speed sensor that has a pulse ratio of 4000 pulses-per-mile, while another vehicle equipped with a speed sensor may have a pulse ratio of 2000 pulses-per-mile. Installation of a navigation system in the two vehicles would realize two different vehicle speeds, unless the speed signal is properly calibrated. Accordingly, the vehicle speed signal may require calibration before the vehicle speed signal can be accurately used in a particular system.
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