Acoustic ranging apparatus and method

Claim 1

An acoustic ranging apparatus, comprising: a means for producing a burst of acoustic energy along a predetermined path toward an object located a distance away from the ranging apparatus, the magnitude of the burst of acoustic energy decreasing as a function of distance from the ranging apparatus; a means responsive to acoustic energy for producing an electrical signal related to the magnitude of acoustic energy falling on the acoustic energy responsive means; a first order linear circuit means comprising a complex impedance means for producing a threshold signal which has a predetermined starting magnitude at a predetermined starting time with respect to the produced burst, the threshold signal decaying as a function of time in accordance with a predetermined time constant from the starting magnitude asymptotically toward a predetermined final magnitude, in which the predetermined starting magnitude, the predetermined starting time, the predetermined time constant, and the predetermined final magnitude are such that the threshold signal decreases in a manner which simulates the decrease in the magnitude of the burst of acoustic energy as a function of distance from the ranging apparatus; a means for comparing the electrical signal from the acoustic energy responsive means with the threshold signal and producing an electrical output signal based on the comparison, the threshold signal being independent of the output signal; a means responsive to the output of the comparing means for producing a signal related to the distance of the object from the ranging apparatus; and an amplifier means for increasing the magnitude of the electrical output of the acoustic energy response means; the final value of the threshold voltage being between (a) the magnitude of the smallest expected output of the amplifier means and (b) the sum of the amplitudes of a noise signal in the ranging apparatus and the d.c. offsets in the amplifier means and the comparing means; in which the threshold voltage starts at the time the burst of acoustic energy is produced and at a starting magnitude which is substantially equal to a saturation voltage of the amplifier means.