As autonomous driving matures, the eyes of the machine have moved from high-end applications into the mainstream. The contest between lidar and millimetre wave radar, the two sensors most used for sensing the surrounding environment, has become a widely discussed topic. This article offers a brief introduction to their principles, system architecture, detection methods, applications, advantages and drawbacks.

Lidar

Lidar is a system that combines three technologies - laser, global positioning system (GPS) and inertial measurement - and is also known as LiDAR, for Light Detection and Ranging.

Lidar uses a laser as its radiation source. It emits a laser beam towards a target to measure characteristics such as position and speed, then compares the signal reflected back from the target with the transmitted signal. After appropriate processing, information about the target - range, bearing, altitude, speed, attitude and even shape - can be obtained, enabling detection, tracking and identification of aircraft, missiles and other targets.

Working principle

Lidar emits a detection signal (laser beam) towards the target and compares the reflected signal (target echo) with the transmitted signal. After appropriate processing, information about the target is obtained, including range, bearing, altitude, speed, attitude and even shape.

System composition

A lidar system mainly consists of a laser transmitter, an optical receiver, a rotating platform and an information processing system. The laser transmitter converts electrical pulses into optical pulses and emits them; the optical receiver converts the reflected optical pulses back into electrical pulses and sends them to the display.

Detection methods

1. Direct detection lidar. This type receives the laser beam reflected or scattered by the target directly and derives target information from it. It is simple and reliable and suits long range detection and fast moving targets, but the receiver also picks up background noise and other interference, so interference suppression measures are needed.

2. Interferometric lidar. This type exploits laser interference. When two laser beams interfere, alternating bright and dark fringes appear, and measuring their number and variation determines target range and speed. It offers high accuracy and resolution but needs stable interference conditions and precise instruments, so cost is higher.

3. Scanning lidar. This type scans a laser beam across the scene, steering the beam mechanically or non-mechanically and receiving the reflected or scattered signal point by point. It delivers detailed one-dimensional shape and position information, but scanning and data acquisition take longer.

4. Phased array lidar. This type uses an array of small emitters and phase control to steer each emitter precisely. It offers high accuracy, high resolution and fast data acquisition, together with a wide field of view and strong interference immunity, but cost is higher and the control system is complex.

Applications

Lidar is widely used in aerial terrain surveying, mapping, autonomous driving, meteorological observation and remote sensing. Compared with ordinary microwave radar, lidar uses a laser beam at a much higher operating frequency, giving higher accuracy and resolution. Its low signature and strong interference immunity also give it broad prospects in military applications.

Advantages and drawbacks

Advantages: high resolution, capturing detailed target information such as speed, range and angle; strong interference immunity, because the signal is optical and less susceptible to electromagnetic and radio interference; and strong information processing capability, capturing shape, size and direction for target classification and recognition.

Drawbacks: relatively high cost, demanding environmental requirements, and difficulty coping with complex environments and adverse weather.

Millimetre wave radar

Millimetre wave radar operates in the millimetre wave band (30 to 300 GHz, wavelength 1 to 10 mm). It combines some advantages of microwave radar and electro-optical radar, offering high resolution, high sensitivity, strong interference immunity and anti-stealth capability. It also penetrates fog, smoke and dust well and works around the clock in all weather except heavy rain. As a result it is widely used for target recognition, imaging, tracking, measurement and positioning.

Working principle

A transmitter emits a millimetre wave signal, which reflects off the target and returns to the radar receiver. By measuring and comparing the frequency, phase and amplitude of the reflected signal, the target range, speed and bearing are obtained.

System composition

A millimetre wave radar system usually consists of an antenna, a transmitter, a receiver and a processor. The transmit system is the core, made up mainly of an RF generator, a power amplifier and a phase locked loop, and is responsible for generating and amplifying the millimetre wave signal for transmission. The receive system consists mainly of a low noise amplifier, a mixer and a filter, and is responsible for amplifying and filtering the received signal and processing it.

Detection methods

1. Pulse radar. Emits millimetre wave pulses and receives the reflected signal, calculating target range and speed from the time and phase difference of the return.

2. Continuous wave radar. Emits a continuous millimetre wave signal while receiving the reflected signal, calculating target range and speed from the frequency shift of the return.

3. Synthetic aperture radar. Emits millimetre wave signals and receives the returns, calculating target range and speed from the phase difference between multiple pulses.

Applications

With high resolution, high accuracy, high reliability and strong interference immunity, millimetre wave radar is widely used in automotive traffic, medicine, construction, aerospace and security monitoring, and plays a particularly important role in autonomous vehicles and smart cities.

Advantages and drawbacks

Advantages: strong penetration through fog, rain and snow; strong recognition capability, distinguishing shapes and sizes and telling vehicles from pedestrians; and moderate cost, suited to mass production.

Drawbacks: lower resolution, weaker detection of moving targets, and susceptibility to complex environments such as terrain and buildings.

Automotive AEC-Q100 and AEC-Q102 qualification

AEC-Q102 qualification for lidar

AEC-Q102 is a dedicated standard for the lasers and photodetectors at the heart of lidar, giving the technology a defined rulebook. AEC-Q102 specifically addresses the following for laser components:

1. Notes for testing laser components (Section 1.4.5).

2. Process change guideline for laser components.

3. Minimum parametric test requirements and failure criteria.

4. Qualification test methods in Table 3.

5. Test requirements in Table 3 that apply only to laser components.

AEC-Q102 test items at the NTEK automotive component laboratory

1. Parameter tests: optoelectronic performance, appearance, parameter verification, physical dimensions and thermal resistance.

2. Environmental stress tests: stress testing carried out according to military electronic device environmental standards and general automotive electronic environmental standards, including high temperature operating, high temperature reverse bias, high temperature high humidity operating, high temperature high humidity reverse bias, temperature cycling, power temperature cycling, intermittent operational life, low temperature operating life, pulse operation, vibration, shock, hermeticity, condensation, hydrogen sulphide and mixed gas.

3. Process quality evaluation: evaluation of component process quality covering packaging, subsequent electronic assembly processes and service reliability, including ESD, DPA, terminal strength, resistance to soldering heat, solderability, wire pull and shear, die push and whisker growth.

The table below summarises the Group A and Group B test items and sample requirements.