Multi-beam Laser for Forward Looking Obstacle Detection along the Flight Path.

Laser range finders can be very effective in detecting even small obstacles at long distances from a moving UAV. To cover large areas the laser beam is usually scanned by direct drive or using mirrors. As an alternative to this approach, we present a multi-beam solution that has no moving parts and is therefore smaller and lighter than a scanning system. This new configuration, called the SF33, is ideal for "forward looking" obstacle detection along the flight path.

Starting with the electronics from our SF30 laser module, we added two additional laser sensors into the existing housing to create the 3-beam pattern that you can see in the picture above. These beams have 5 degrees of angular separation resulting in a fan shape that spreads out with distance. The beams can be arranged to give either a horizontal or a vertical pattern depending on how the housing is oriented, or they can be aimed 45 degrees downwards to give a "look ahead" indication of both obstacles and rising ground.

The SF33 is scorchingly fast so we are able to measure by cycling through each beam in succession to get independent distance results. In our prototype we set the cycling rate to 3000 times per second so that each beam could detect power lines from a UAV moving at 60 kph.

The "elastic band test" has become the benchmark for high speed obstacle detection, so we subjected the multi-beam laser to this indignity. We were expecting the elastic band to be detected three times as it passed through each of the three beams.

In the 'scope image below, the orange trace gives the distance as an analog voltage and the blue trace is a digital alarm signal that goes low when an obstacle gets too close. You can clearly see three sets of alarm events, one from each beam, as the elastic band shoots past.

We also need to consider that the beams are cycling 3000 times per second, so as the elastic band passes through the first beam, the second and third beams are still checking for other potential obstacles. This effect manifests as "gaps" in the alarm signal as can be seen in the zoomed-in traces of the first set of alarm events.

In this image, the three beams cycled 12 times as the elastic band traveled through the first beam. The alarm signal shows a 1:3 mark (low) space (high) ratio as beams 2 and 3 interleave their measurements with beam 1. A similar picture was produced by the elastic band as it traveled through the other beams.

The elastic band test clearly demonstrates that the SF33 multi-beam laser has adequate speed to detect obstacles with a high relative velocity and we have confirmed a useable range of around 50m when measuring to larger obstacles.

At this stage of development we can still make hardware and software changes. I know that a number of forum members have experimented with obstacle detection using lasers, so any suggestions on how to integrate multiple distance readings and alarm events into a meaningful communication message to the flight controller would be most welcome.

Special thanks to Jordi Munoz for his support and technical input on this project.

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Comment by Gary McCray on June 30, 2015 at 4:01pm

Hi LD,

This is great, one small suggestion, if possible, it might make sense to have your production unit of this be able to have the separation angle manually adjustable. maybe as much as 90 degrees, but certainly 45 degrees.

With a greater separation angle you could at least detect moving objects (or stationary ones for that matter) further to the side.

And I suspect that individual applications could benefit considerably from being able to customize it.

Of course this can also still be scanned.

I am now starting to set up a 3 axis gimbal to see what can be done with an X/Y scan device.

Best Regards,

Gary

Comment by Laser Developer on June 30, 2015 at 11:35pm

Hi Gary,

We're exploring "minimal configuration" laser sensors specifically for small payload UAVs as one of the many possible paths towards safer operation. Taking your 3-axis gimbal approach as representing a "complex solution" with high adaptability, we think that there is a spectrum of less complex systems that can provide very reliable sensing data but at lower weight and cost.

At this stage in UAV evolution, a number of solutions are beginning to emerge:

1. Static laser altimeters for final approach, hovering and landing

2. Static multi-beam lasers for obstacle detection along a fixed flight path

3. Scanning circular pattern lasers for SLAM during confined maneuvering

4. Scanning conical pattern lasers for adjustable forward looking obstacle detection and position holding 

5. Scanning downwards looking lasers for ground mapping

6. Scanning X/Y/Z lasers for 3D SLAM

Of course it's possible to include all or some of these configurations into a single, expensive device but we think that certain applications, such a power line monitoring, will only require a subset of these features and therefore a dedicated and more economical solution is more appropriate.

With UAV safety and high performance as our primary goals, LightWare will continue to push the boundaries of laser technology as fast as possible. Cheers,

LD

Comment by jean-luc on July 1, 2015 at 12:20am

Hi LD,

We look for a laser module application on our USV.(3meters long boat).

Do you have experience with the laser in sea environment (possibility of small water projection on the beam direction), do you think than the laser consider the water like an obstacle or not?

Regards,

Comment by Laser Developer on July 1, 2015 at 12:29am

Hi jean-luc,

That's a very interesting question. It is possible that "splash" will be detected as an obstacle but this may not be a serious problem.

Unlike the situation with UAVs where small obstacles could be dangerous, in a USV most obstacles will be fairly large and remain in position for some time. This would allow you to post-process the distance data and reject short signals that are a result of splash.

We actually have a "median filter" available in some of our units that can perform this short signal rejection for you.


Developer
Comment by John Arne Birkeland on July 1, 2015 at 1:42am

I just wanted to say keep up the great work. This type of technology is exactly what we need for the future of UAV.

Comment by Laser Developer on July 1, 2015 at 1:53am

Thanks John,

Developing new technology is brutally hard work and rarely leads to commercially viable products. But I love my job!

Comment by Laser Developer on July 1, 2015 at 2:42am


Moderator
Comment by Roberto Navoni on July 1, 2015 at 1:47pm

This project is similar to Leddar technology it produce unt 16 beams per point . LD are you already know this technology ? 

best

Roberto 

Comment by Laser Developer on July 1, 2015 at 10:19pm
Hi Roberto, there are many companies that make multi-beam sensors. However, the units that are fast enough to do obstacle detection are large and expensive and those that are small are too slow. We believe that the combination of speed, range, weight and price makes the SF33 ideal for UAV applications.
Comment by Thomas Stanley-Jones on July 2, 2015 at 1:29pm

In the talk about these small systems, I haven't noticed anything about the potential for eye damage.  In Canada, to fly with any laser system on a UAV, it has to be ok'd by a health and safety board.  Is there any problem looking down the barrel of one of these?

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