Design of a laser rangefinder for automotive use

Abstract: Laser distance measurement includes three methods: pulse ranging, phase laser ranging, and pulse-phase laser ranging. The structural principles of a simple laser rangefinder encompass the ranging equation, laser reflector, and laser interface. Equipping vehicles with laser rangefinders not only enhances safety but also provides possibilities for intelligent vehicle instrumentation.

Keywords: laser distance measurement; laser rangefinder; laser ranging system

1. Introduction

With the rapid development of the automotive industry and public transportation, advanced scientific instruments are being equipped in vehicles, a practice already in the experimental stage in developed countries. Whether developing collision warning systems, automatic speed control systems, or ultimate vehicle autonomous guidance systems, real-time information, including road distances and relative speeds, must be obtained. This paper introduces the use of laser for distance measurement and the structural principles of a simple laser rangefinder.

2. Principles and Mechanisms

Laser distance measurement includes three methods: pulse ranging, phase laser ranging, and pulse-phase laser ranging. Pulse ranging is a technique based on measuring the propagation time of a pulsed laser beam to determine distance. If the pulsed laser beam encounters an object along its propagation path, it reflects back. By measuring the time t from the emission moment to the reflection returning to the emission point, the distance L to the object can be calculated using the formula: L = ct/2, where c is the speed of light (c = 2.9991x10⁸ meters/second). Methods to directly convert time t into distance readings include: (a) a digital method that counts the beat frequency of 150 MHZ (each reading corresponds to 1 meter); (b) an analog method that converts time t into an electrical quantity display. The final ranging accuracy using these two measurement methods is approximately cτ/2 (where τ is the pulse width of the laser). When t is 10 ns, the tolerance is about 15 meters. Additionally, the counter has a certain trigger level, and the amplitude of the return pulse is not consistent, causing the trigger point of the counter to fall at different positions on the pulse front, which also introduces measurement time errors and affects ranging accuracy. It can be seen that pulse ranging has significant errors and is not suitable for short-distance measurements.

Phase laser ranging, also known as precision ranging, requires placing a corner reflector (retroreflector) at the measurement point, which is also unsuitable for distance measurements between moving vehicles.

Pulse-phase ranging refers to a technique that uses both pulsed laser and high-frequency oscillating wave signals, strictly synchronized, to achieve precision ranging. The resolution of the measured distance is determined by the wavelength of the high-frequency oscillating wave and the precision of phase measurement.

A high-frequency master oscillator generates a high-frequency oscillating signal with a period T and wavelength λ, which is then frequency-divided to synchronously control the emission pulse.

The time interval between the emission pulse and the return pulse signal is t, which is also the time for the laser pulse to travel to and from the distance L to be measured. During this time, the phase of the high-frequency oscillation also changes by an amount ψ. There is a correspondence between distance, time, and phase. The value includes two parts: an integer multiple of 2π, corresponding to a distance of kλ/2 (where λ is the oscillator wavelength); and a fraction less than π, corresponding to a distance ΔL (which is less than λ), calculated as ΔL = λΔψ/(4π). The distance corresponding to kπ is measured using the pulse ranging method, while the distance corresponding to Δψ is measured using the phase method.

3. Instrumentation and Equipment

The basic operation of a laser rangefinder is illustrated in Figure 1.

Figure 1. Basic principles of laser rangefinders

3.1 Ranging Equations
The most important performance of a laser rangefinder is its ability to measure distance. The practical ranging equation is:

where R is the rangefinding capability of the rangefinder and K is a coefficient that takes into account the far-field shape of the emitted laser and all errors in tracking and aiming.