Speed control systems for governing the speed of a vehicle

A speed control system for governing the speed of a vehicle comprising tachometer signal generating means for generating a tachometer signal having a frequency characteristic of the actual speed of the vehicle, desired speed signal generating means for generating a signal indicative of a desired speed for the vehicle, and vehicle control means governed jointly by the tachometer signal generating means and the desired speed signal generating means for governing the speed of the vehicle, wherein improved tachometer signal checking means comprises:

• (a) first and second parallel connected channel means responsive to the tachometer signal for checking that tachometer pulses are enough above a threshold detector value to assure accurate calculation of the actual speed of the vehicle when counting the tachometer pulses,
• (b) each of the channel means comprising in series operational amplifier means, threshold detector means, and tachometer pulse counting means for counting tachometer pulse outputs of each of the channels,
• (c) biasing means for each of the threshold detector means for requiring a minimum amplitude tachometer pulse input to each threshold detector means to deliver an output to the pulse counting means of the associated channel,
• (d) tachometer pulse attenuating means for acting on tachometer pulse inputs in each of the channels for periodically testing that attenuated tachometer pulses are at least above the threshold setting of the threshold detector for the associated channel,
• (e) attenuation enabling means for selectively rendering the attenuating means effective for the respective channels during predetermined intervals, and
• (f) comparator check means for checking that the substantially same number of tachometer pulses is counted by both channels before permitting the count of the tachometer pulses to normally generate an output enabling continued progress of the vehicle.