JPS6341082B

To decode signals picked up in a horizontal and vertical coil on the vehicle induced by a buried a-c energized cable, in which the vertical position of the coil above the cable is determined, a null sensing circuit is coupled to the horizontal coil to sense when the signal induced therein passes through zero. The signal induced in the vertical coil is likewise sensed, but rather than sensing the null, the peak value is determined. This peak value induced in the vertical coil will occur some time after the signal in the horizontal coil has passed through null or zero, and the peak value in the vertical coil is compared with the rise in signal in the horizontal coil. When the two signals (peak and rising horizontal coil signals) are equal, the time between the equality and the null point of the horizontal signal is determined, and used as a control parameter to steer the vehicle so that this time will be effectively null or zero, at which time the vehicle will be centered over the guide cable. The time can be determined by the count of a counter, the count state of which is transferred to a memory for read-out at a clock rate determined by a steering computer.