Method for determining a distance using a laser range finder
View Patent ↗A digital counting and display system and methods for use with a laser rangefinder that counts backscattered laser beams and displays a distance between a laser and a target. The laser rangefinder includes a laser configured to emit a pulsed laser beam, an afocal Gallilean telescope configured to receive backscattered laser pulses and generate a series of focused backscattered laser pulses, a silicon avalanche photodetector connected to the afocal Gallilean telescope, configured to generate a series of currents signal proportional to the series of focused backscattered laser pulses, a low noise, multistage amplifier connected to the silicon avalanche photodetector, configured to generate a series of linearly changing amplified voltage signals from the series of current signals, an analog-to-digital converter configured to convert the series of linearly changing amplified voltage signals to a series of digital voltage signals, and a digital counting and display circuit connected to the analog-digital converter.
1. A method for determining a distance with a laser rangefinder, comprising:
emitting, with a pulsed laser, a pulsed laser beam;
directing, with a beam splitter, a first portion of the pulsed laser beam towards a target;
receiving, with an afocal Gallilean telescope, a series of backscattered laser pulses reflected from the target, wherein the Gallilean telescope comprises
an achromatic convergent lens;
a divergent lens;
a biconvex lens;
an interference filter; and
an adjustable iris located in a focal plane of the afocal Gallilean telescope;
generating, with the afocal Gallilean telescope, a series of focused backscattered laser pulses;
receiving, with a silicon avalanche photodetector connected to the afocal Gallilean telescope, the series of focused backscattered laser pulses;
generating, by the silicon avalanche photodetector, a series of current signals proportional to the series of focused backscattered laser pulse;
receiving, by a low noise, multistage amplifier connected to the silicon avalanche photodetector, the series of current signals;
generating, by the low noise, multistage amplifier a series of linearly changing amplified voltage signals;
receiving, with an analog-to-digital, A/D, converter connected to the low noise, multistage amplifier, the series of linearly changing amplified voltage signals;
generating, with the A/D converter, a series of digital voltage signals;
receiving, by a digital counting circuit connected to the analog-digital converter, the series of digital voltage signals;
directing, with the beam splitter, a second portion of the pulsed laser beam towards a high speed silicon photodiode configured to convert the second portion to a second portion pulsed current signal;
converting, with an analog to transistor-transistor logic, TTL, converter circuit, the second portion pulsed current signal to a pulsed TTL signal;
receiving, by a digital gate of the digital counting circuit, the pulsed TTL signal;
incrementing a counter of the digital counting circuit;
determining, by the digital counting circuit, a time difference between receiving the series of digital voltage signals and receiving the pulsed TTL signal;
calculating a distance between the laser and the target based on the time difference and a counter total; and
displaying the distance between the laser and the target on a digital display connected to the digital gate.
2. The method of claim 1 , wherein the laser and the Gallilean telescope are mounted in a same plane in a monostatic biaxial configuration.
3. The method of claim 1 , wherein the low noise multistage amplifier includes:
a low noise preamplifier configured to have a fixed gain; and
a variable gain amplifier.
4. The method of claim 1 , wherein the analog-to-digital converter includes a high frequency Schmitt trigger circuit.
5. The method of claim 1 , wherein the high frequency Schmitt trigger circuit includes an operational amplifier having an inverting input connected to the low noise multistage amplifier and a non-inverting input connected to both a voltage reference signal and to a feedback path from an output of the operational amplifier, wherein the feedback path includes a variable resistor.
6. The method of claim 1 , wherein the digital display includes a plurality of seven segment light-emitting-diode, LED, displays.
7. The method of claim 1 , wherein the digital gate comprises:
a D flip flop integrated circuit, connected to the first monostable multivibrator, (MV 1 ).
8. The method of claim 7 , wherein the D flip flop integrated circuit includes:
a trigger input, C, configured to receive the TTL signal;
a data input, D, set to one volt;
a set input, 0, set to zero volts;
a reset input;
a first output, Q1; and
a second output, Q 1 , connected to the first monostable multivibrator, (MV 1 ).
9. The method of claim 8 , wherein the counter comprises:
a TTL integrated circuit which includes the display, a clock input, CLK, a latch strobe, LS , input, and a clear input, CLR .
10. The method of claim 1 , comprising:
unlocking a D flip flop of the digital gate when the pulsed TTL signal is received; and
locking the D flip flop when the digital voltage signal is received.
11. The method of claim 1 , comprising:
clearing the counter when a reset button is pressed.