In order to meet automotive requirements, LiDARs need to be highly performant and scalable. MEMS-based LiDARs are a suitable solution. But how is the size of the mirrors defined and howabout the reliability of big-size MEMS mirrors?

“MEMS-based LiDAR sensors are usually less expensive, but they are not high-performance/reliability enough for use in autonomous vehicles.” We hear sentiments like this quite often. Now we will explain how our MEMS mirror array effectively invalidate this assumption, how we developed a MEMS technology for LiDAR, how we achieved the large-enough mirror size for better LiDAR structure, and what considerations factored in to system decisions.

(1)Needed for automotive applications: performance and scalability

For use in autonomous vehicles, LiDAR sensors have to meet two basic requirements: on the one hand, they have to deliver high performance, including long range and a wide field of view. On the other hand, they must also be scalable (reliable) so that millions can be produced and installed in vehicles. LiDAR manufacturers meet these challenges through various approaches. Mechanical LiDAR systems, whose beam deflection units are moved mechanically by polygon on motors, are still the widely used systems today. Although these devices boast a wide field of view — some up to 360° — and long range, their mechanics require regular maintenance and are large, heavy, and expensive to produce. Thus, mechanical LiDAR systems solve only the performance side of the two major demands placed on the sensor industry.

Another approach to meet these challenges is MEMS (microelectromechanical systems) technology. Here, components are produced in silicon, which has the advantage of scalability: since this technology has been tried and tested over many years, identical components can be produced in a cost-efficient manner and in large quantities. This approach is also used, among other things, in the production of microsensors.

But how do MEMS-based LiDAR systems meet the challenges of performance?

(2)Long range with the help of the right laser source

For autonomous vehicles to be able to travel at high speeds, they must be able to “see” and perceive the world around them — not only in their immediate vicinities, but also at greater distances. This is particularly important when driving on highways, as vehicles are moving faster and therefore objects, bends, and other vehicles must be reliably detected at greater distances in order to be able to react in good time. Sensors therefore require a long range in order to enable autonomous driving at highway speeds. In order to achieve this range with a LiDAR sensor, either the emitter or the detector needs to be optimized specifically for this application.

One possible starting point for such adjustments is the laser source. Typically, lasers with two different wavelengths are used in LiDAR sensors. Some LiDAR manufacturers rely on fiber lasers with wavelengths of 1550 nm. This wavelength cannot be focused by the human eye and can thus be used in an eye-safe manner even at high energy levels. This results in a longer range — the more energy used, the further the device “sees”. However, this type of laser source also has a decisive disadvantage: 1550 nm lasers are large and complex to manufacture, which leads to higher prices and large LiDAR housing dimensions.

Many LiDAR applications therefore use laser diodes that emit laser pulses with wavelengths of 905 nm. These have the distinct advantage of being very small and having been used for a long time in a wide variety of applications. As a result, these diodes are inexpensive and available on the market in large quantities. However, eye safety regulations require that the beam strength of the diodes be lower than that of 1550 nm lasers. The optimization on the emitter side is therefore limited.

(3)Searching for the right mirror size

So how can the detector be optimized? Here the aperture plays an important role in achieving long ranges. It describes the size of the detector. In the case of our MEMS mirror array design, the aperture corresponds to mirror size. In order to capture as much light as possible, a large aperture — in other word as large a mirror as possible — is required, usually upto 1-2 hundreds mm², and also it is necessary to calculate the optimal mirror size while taking these into account. These factors are: photon number to be received, collimation, deflection angle, and resonance frequency.

(4)Photon number

On the one hand, the size of the mirror used in LiDAR unit depends on how many photons have to be emitted in order for a sufficient number of photons to come back, thereby detecting an object. This minimum number of photons can be calculated accurately based on the link budget. This measure includes how many photons are lost at distance and through low reflective surfaces, homogeneous scattering of light, and detector inefficiency. In this way, it is possible to calculate how many photons must be emitted, or how large the aperture must be so that a minimum number of photons can be detected again. In addition, a coaxial design is usually structure, which means that only the light that comes back from the same direction in which it was emitted is recaptured. This is advantages in that it prevents other random light signals from being picked up and disturbing or falsifying the images. 

(5)Collimation

In order to obtain high-resolution data that reliably identifies even small objects, the lasers must hit objects in a collimated form. This collimation is achieved by placing a lens in front of the laser. Now the mirror size comes into play again: a mirror must be exactly large enough to deflect all the light collimated by the lens (usually 10mm or more in diameter is better). This also depends on the focal length required for optimal collimation and thus high resolution.

(6)Resonant frequency

MEMS mirrors oscillate at a certain resonant frequency. They are triggered by integrated actuators and therefore do not require motors. This is a clear advantage because motors with load-weight quickly wear out and require regular maintenance. These problems do not arise if the oscillation is triggered by integrated actuators or very small load-weight.

The resonant frequency at which a mirror oscillates depends on the size and mounting of the mirror. For this purpose we have developed a proprietary technique of mirror arrays in order to be able to work with particularly large aperture sizes. Due to these unusually large diameters, a large number of photons can be directed into the surroundings and back onto the detector, which allows LiDAR sensors to achieve accurate a long range. In addition, thanks to their larger size, these mirrors are more robust than conventional products, which are only a few millimeters in diameter. Mirrors worked with high resonant frequencies due to their array construction, which ensures that the greatest possible number of photons are returned to the detector: if the mirror oscillates too quickly or too slowly, photons will pass the detector due to the coaxial structure.

(7)MEMS technology specifically designed for LiDAR applications

In conclusion, mirror sizes are determined by a wide array of factors. In order to build the most high-performance LiDARs based on MEMS, mirrors must have specially developed compositions, sizes, and must be reliable enough for automotive test standard. And only if the MEMS technology is specifically developed with LiDAR applications in mind can the requirements of a long range, a wide field of view, and high resolutions be achieved.

（1）The bandwidth of the oscilloscope is defined by the point of sine-wave amplitude attenuation -3dB.

（2）The description of waveform and rise time in digital oscilloscope is obtained by real-time sampling circuit and high-speed A/D converter waveform data, and then obtained by interpolation operation.

（3）In the Tektronix oscilloscope, there is a real-time processing circuit to complete the so-called sine interpolation function, which is completed in the signal acquisition circuit. Of course, many oscilloscopes are also completed by the main processor of the oscilloscope mathematical operations, this time will spend more time.

（4）For the signal you measure, I am afraid that the use of 100MHz oscilloscope is not possible. 50MHz square wave, theoretically should use more than 450MHz oscilloscope in order to signal the most important harmonic below 9 times accurately re-, so as to ensure that the waveform is not distorted. What's more, you may have to consider the problem of signal rise time, in theory, the rise time of the oscilloscope should be more than 5 times faster than the signal.

（5）The probe is the same, because the ordinary probe will produce high frequency distortion when measuring high pressure, you should use a special differential probe or high pressure probe such as Tektronext P5205, P5100 for measurement.

(1) LiDAR stands for Light Detection And Ranging and operates in a very similar way to RADAR. A light source, often a laser, fires a beam of light, and the time between the beam being fired, bouncing back, and being detected by a sensor is carefully measured. From here, the distance between the light source and a distant object can be calculated. However, not all LiDAR systems utilise simple timing mechanisms, for applications that require accuracy in short distances, phase shifting is used whereby the difference in phase between the outgoing light and the incoming light can be used to infer distance.

(2)LiDAR is a technology that has many practical applications, including self-driving cars, fully autonomous drones, and 3D space mapping. However, LiDAR has several issues that hinder its ability to be used widely, and this also makes it expensive. The first issue comes from the need for a mechanical mirror to deflect the laser beam, and this uses a motor that spins the mirror around. Such a motor not only consumes large amounts of power, but it also makes the system incredibly bulky. Such mirrors are also often expensive, and the net result is a large, heavy system that can only be realistically integrated into large applications. While LiDAR can be made cheap by omitting a rotating mirror, the result is the incredibly narrow field of view, but many LiDAR applications require a wide field of view.

（3）Making LiDAR smaller and cheaper will help to integrate it into many applications, including IoT sensors, autonomous vehicles, and control systems which can map the immediate environment around them. Such capabilities allow for object tracking, object recognition, and object avoidance which can be difficult tasks to achieve with a single camera. LiDAR has advantages over camera systems as it provides distance measurements meaning that a single sensor can tell the difference between an object in the foreground or background without the need for AI. While cameras can recognise objects based on their appearance (colour, shape, and size), LiDAR can, in theory, be used with object outlines, but this would only work when objects are stood out on their own. The resolution of the LiDAR system is great enough to produce smooth outlines of objects. Overall, LiDAR is a fast technology that will be highly beneficial in autonomous vehicles that need instant distance measurements without the need for AI algorithms to try and recognise objects in flat 2D images. And also, Radar, 3D or 4D, the resolution is far less than LiDAR, so for the multiple corner cases, the camera and RADAR, LiDAR are complemented using in smart driving.

Light Detection And Ranging, or LiDAR, is a vision technology that uses a beam of light (almost always a laser), to measure the distance between itself and a target object. However, instead of ranging a single point source, LiDAR scans the area in front of them to create 2D distance maps of their surroundings, and this can allow a computer system to identify obstacles and objects.

In order for LiDAR to function, a laser beam is required to scan an area, and the first LiDAR systems would achieve this using rotating mirrors. While rotating mirrors are a cheap and effective method, it is also extremely bulky. The use of rotating mirrors also introduces wear and tear on bearings, and rotating systems can only operate so fast meaning that a LiDAR system using rotating mirrors can struggle with either resolution or frames per second.

However, LiDAR designers are beginning to move away from rotating mirrors towards MEMS mirror. A MEMS mirror is a miniature mirror fabricated on a semiconductor that is often attached to other mechanical components that flex under a voltage. As such, the mirror can be made to deflect at specific angles very fast, and the lack of bearings eliminates mechanical wear and tear. Of course, the most important benefit of a MEMS mirror is its size; a rotating LiDAR mirror system can be reduced in size from a large 20cm3 container to a device the size of a webcam.

(1) MEMS stands for Micro-Electro-Mechanical Systems and can be thought of as tiny machines powered by electricity. These can include cogs, gears, wheels, and cantilevers which are used to produce most sensors, including gyroscopes, accelerometers, and vibration sensors. MEMS mirrors are miniature mirrors that are adhered to a small mechanism which deflect upon passing an electrical current through them and can be used to point the mirror at different angles.

(2) Recognising that MEMS mirrors can be used for laser deflection, several years ago, a team of researchers from the University of Florida have created a prototype LiDAR system that not only has a wide field of view, but consumes very low amounts of power, and can be easily mounted in small applications. The LiDAR system takes advantage of MEMS mirrors to act as the scanning element of the LiDAR system, and the result is that a battery can power the LiDAR device due to the small power required in MEMS mirror LiDAR system. And also ,some of the LiDAR prototype also integrates a passive IR sensor (generally used for security systems), to detect the presence of individuals, and if none are detected, then the system powers down. This makes the prototype ideal for residential and commercial applications that may use smart environmental control systems (including heating, and air-conditioning systems). Keeping the design simple, the prototype utilises off-the-shelf parts to process distance data from the laser, but this is one of the bulkier and power-consuming components of LiDAR systems.