LiDAR sensors are rapidly growing in popularity as better manufacturing techniques and mass production are reducing the cost of this technology.
But since no single LiDAR solution is suited to all applications, choosing the right LiDAR can be tricky. A >£30k LiDAR is great for autonomous vehicles but it may not be able to properly detect an obstacle less than a metre away, while a <£700 LiDAR can do that job effectively but may have limited range.
If you are considering using a LiDAR for any of your projects keep reading to find out how to choose the right LiDAR for your application.
What is a LiDAR used for?
In architecture, for example, you can get a handheld LiDAR device that you carry while walking around a building, such as the Kaarta Stencil Pro. The device creates a 3D point cloud, so you can match the scan of the building to the CAD file of the construction design to see if the real building measures up. Also, assisting in the as-built process and survey.
In mining, LiDAR is used for safety and surveillance. Instead of putting humans in danger, LiDAR-equipped robots are sent into mines to detect anomalies like cracks in the walls or other physical objects that may be hazardous. Although you could do this with cameras as well, cameras do not perceive depth. Even if you can see a crack in the wall, a camera can’t tell you how deep it is. Cameras are also heavily influenced by lighting conditions, which may not be ideal in some environments.
Long-range vs. short and close range
The first thing to consider is the range of the LiDAR sensor. As a rule of thumb, long-ranging LiDARs with a range of 100m or more come with a greater cost.
For example, long-range LiDARs work well for navigating vehicles that are moving quickly, but they usually only detect objects at medium to long-range; they are not effective at close range, such as for detecting objects within 1 metre.
If your application involves a fast-moving vehicle, long-range LiDARs can help, because you have to detect objects from far away and take necessary actions before the objects come closer.
Indoors, or with slower-moving vehicles, you typically don’t need long-range. If you’re operating a slow-moving robot you typically need to see up to 25m, you can work with a short-range LiDAR.
Close-range LiDARs are used to detect objects in close vicinity. For example, you may need a close-range LiDAR on a vehicle to detect obstacles while opening the doors. Usually, you would combine a long or short-range LiDAR with a close-range unit.
2D vs. 3D
2D LiDARs will detect an object, whereas 3D LiDARs will produce a more detailed point cloud which can be used to determine the shape and depth of the object. Determining what it is, rather than just telling the robot or vehicle “there’s something there.” 3D gives you a better understanding of the surroundings, so your robot or vehicle can distinguish between a tree, a sign on the highway, or an oncoming truck.
The difference is in the information that can be extracted from them. For instance, using them in a building, 2D LiDARs can only give you a floor plan whereas, with 3D LiDAR, you can get the 3D map and location and height of every object in the building. So which one to use depends on the application.
If you are navigating in a flat environment and your robot is not tall, perhaps a 2D LiDAR is enough. However, for 3D outdoor environments where there are obstacles like hanging tree branches or pipes, you need 3D perception. 3D LiDARs are also useful in other applications like terrain classification and segmentation.
More considerations for different applications
Field of View (FoV)
Field of View is a critical feature of any LiDAR. It is measured separately along the horizontal and vertical axis. An autonomous vehicle is usually equipped with one or more LiDARs with 360° horizontal FoV, or a larger number of directional LiDARs to ensure the 360° degree view is available. It may not need a large vertical FoV, however, as it may only need to look at the road and perhaps 30° to 40° above the road surface.
If your application is obstacle avoidance within a constrained space you may not need a 360° LiDAR. The 360° surround capability is not an asset in all cases. If you only need to look straight ahead a 360°LiDAR would give you extra and non-essential data and hence put unnecessary load on system processing.
The number of channels
LiDAR with a greater number of channels produces a denser, more detailed point cloud. The amount you choose to get may boil down to your budget, processing capabilities, size of the objects, distance from the objects.
Vertical separation between adjacent channels
If you are trying to detect small objects you need to ensure that the separation is as small as possible, otherwise, there is a chance that the object may fall within the two adjacent channels and go undetected.
This can, however, be mitigated by moving the LiDAR so that the adjacent lasers fall on different points and are thus able to detect the object. Some LIDARs have smaller separation near the centre of the vertical FoV and greater separation away from the centre. This helps detect objects near the focus of the LiDAR.
Detecting low reflectivity objects
The range and accuracy of LiDARs vary considerably depending on the object’s reflectivity or how much light is reflected from the object. Known as Remission, it is expressed as a percentage. White drywall would have close to 90% remission while something like coal would have around 5% remission.
A LiDAR’s range is typically expressed at 80% remission, it can go down to 1/4th or even less at 10% remission. If your project involves looking at or detecting darker objects or objects that do not reflect a lot of light, you may want to ask the LiDAR manufacturer to provide you with the range and accuracy at different remissions. Unless you are looking at white drywall all the time!
Points per second
PPS or Points Per Second is another good spec to look at. It shows how many laser measurements are being taken per second. More points will result in a more detailed, denser and accurate point cloud.
PPS is primarily the function of the frequency of the LiDAR in Hz, number of channels and number of returns. A LiDAR operating in dual return mode would provide double the points per second. Similarly, a LiDAR operating at a higher frequency would yield higher PPS.
If your robot or autonomous guided vehicle needs to operate amongst humans, you might want something which is safety-rated. Some companies offer safety-rated 2D LiDARs but this option is very limited at this point. The safety-rated LiDAR would be just one component of the overall system and other considerations would need to be made to make the entire system safety certified.
LiDAR or a complete solution?
You might need a complete mapping solution, not just LiDAR. Your technical resources will determine how easy it is to put an extensive ecosystem into place. The LiDAR can provide you with a point cloud but to make sense out of it you may need localisation, time stamping and other valuable data. You may have to consider complete solutions that consist of LiDARs, IMUs, GPS, cameras and software. You may also need to do some kind of post-processing to use the data in a meaningful way.
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