Back on December 15th, we got a look at the internals of a SICK Laser Rangefinder (LIDAR), a $6k device that employs a single laser diode to produce ~6000 points per second (~600 points per scan at ~10Hz) over a 180° field-of-view.  Now, we can compare that to the Rolls Royce of Laser Rangefinders -- the Velodyne Lidar, a $75k device employing 64 laser diodes to produce 1.3 million data points per second with a 360° horizontal field-of-view and a 26.8° vertical field-of-view.  Below is a video of Bruce Hall, President of Velodyne LIDAR, demonstrating the HDL-64E in operation and taking a look at its internals.  It may not be a complete disassembly (it does cost $75,000 afterall!), but it does provide some interesting insights into the Velodyne's internals.

You may recall that the Velodyne (below left) is a popular fixture on DARPA Urban Grand Challenge vehicles, producing the characteristic concentric laser scans (below right) that proved useful in everything from obstacle avoidance to curb and lane detection.  

So let's dig a little deeper and show how this amazing sensor functions.  First (below left) is an image showing the characteristic front lens assembly. Notice that there are two "blocks" -- a top and a bottom, which each contain 32 laser diodes (for a total of 64).  The laser beams exit the device on the outer lenses and return to photo-detectors through the middle lenses, using time-of-flight (TOF) to determine distance.  Below right is a view of the rear of the device.

There are a couple of interesting structures to note in the rear of the Velodyne.  For example, there are four banks of laser diodes, each containing 16 lasers; in the image (below left), Bruce is pointing to the "top right" laser diode bank.  The lasers are precisely (and painstakingly?) aligned with avalance photodiodes (a semiconductor approximation to a photo-multiplier tube) contained on a PCB behind the central lens.  Bruce is pointing to the top avalance photodiode board in the image (below center).  All of the timing, control, and reception signals are routed to a "main PCB" just under the top of the device.  Finally, counter-balancing weights are employed to keep the entire (spinning) system stable -- they are being pointed to in the image (below right).

OK, enough chatter.  You can watch the video if you like.

So, I have a few questions about the resiliency of these sensors...  Among my questions:

• What is the mean-time-between-failure (MTBF)?
• How critical is the laser diode and photodiode alignment?  What happens if the alignment drifts, and how often does this happen?  (How about cost of repair or maintenance?)
• Obviously, mass-balancing is pretty important for something with so much rotational inertia.  What happens if the device's balance is disrupted (say, it's hit and deformed with a ball) -- where does all that inertia go, and how dangerous is it to bystanders?

Anyway, it is a very compelling sensor -- I wish I could afford one.

Credit to Robot Central for pointing out this video.