The sensor dome sweeps in a full circle roughly once per second. (60 RPM) This sounded pretty good at first, but once I started moving the sensor it doesn’t look nearly as good. Laser distance plot is distorted because it’s moving while it’s sweeping, visibly so even at normal human walking speeds. Clearly a robot using this unit will have to post-process distance data generated by this sensor to compensate for speed. Either that, or just move really slowly like the Neato XV-11 robot vacuum this LIDAR was salvaged from.
The distance data is generated from a single narrowly focused beam. This generates detailed sweep data at roughly one reading per vertical degree of separation. However, it also means we’re reading just a very narrow one degree horizontal slice of the environment. It’s no surprise this is limiting, but just how limited wasn’t apparent until we started trying to correlate various distance readings with things we can see with our eyes.
Autonomous vehicles use laser scanners that spin far faster than this one, and they use arrays of lasers to scan multiple angles instead of just a single horizontal beam. First hand experimentation with this inexpensive unit really hammered home why those expensive sensors are necessary.
After the few handheld tests, the portable test unit was placed on top of SGVHAK Rover and driven around a SGVHAK workshop. There’s no integration at all…. not power, not structure, and certainly not data. This was just a quick smoke test that was very productive because it lead to more observations:
Normal household wall paint, especially matte or eggshell, works best. This is not a surprise given that it was designed to work on a home vacuum robot.
Thin structural pieces of shelving units are difficult to pick up.
Shiny surfaces like glass become invisible – presumably the emitted beam is reflected elsewhere and not back into the detector. Surprisingly, a laptop screen with anti-reflective matte finish behaved identically to shiny glass.
There’s a minimum distance of roughly 15-20cm. Any closer and laser beam emitted is reflected too early for detector to pick up.
Maximum range is over 4-5 meters (with caveat below). More than far enough for a vacuum robot’s needs.
The final observation was unexpected but obvious in hindsight: The detection capability is affected by the strongest returns. When we put a shiny antistatic bag in view of the sensor, there was a huge distortion in data output. The bag reflected laser back to the scanner so brightly that the control electronics reduced receiver sensitivity, similar to how our pupils contract in bright daylight. When this happens, the sensor could no longer see less reflective surfaces even if they were relatively close.
That was fun and very interesting set of experiments! But now it’s time to stick my head back into my ROS education so I can make use of this laser distance sensor.
The laser distance sensor I bought off eBay successfully managed to send data to my desktop computer, and the data looks vaguely reasonable. However, I’m not interested in a static scanner – I’m interested in using this on a robot that moves. Since I don’t have the rest of the robot vacuum, what’s the quickest way I can hack up something to see how this LIDAR unit from a Neato XV-11 works in motion?
Obviously something on the move needs to run off battery, and there’s already a motor voltage regulator working to keep motor speed correct. So that part’s easy, and attention turns to the data connection. I needed something that can talk to a serial device and send that data wirelessly to my computer. There are many ways to do this in the ROS ecosystem, but in the interest of time I thought I’d just do it in the way I already know how. A Raspberry Pi is a ROS-capable battery-powered computer and everything I just did on my computer would work on a Pi. (The one in the picture here has the Adafruit servo control PWM HAT on board, though the HAT is unused in this test.)
The Raspberry Pi is powered by its own battery voltage regulator I created for Sawppy, supplying 5 volts and running in parallel with an identical unit tuned for 3 volts supplying power to spin the motor. As always, the tedious part is getting a Pi on the wireless network. But once I could SSH into the Pi wirelessly, I could run all the ROS commands I used on my desktop to turn this into a mobile distance data station. Reading in data via FTDI serial port adapter, sends data out as ROS topic /scan over WiFi.
Using a Raspberry Pi 3 in this capacity is complete overkill – the Pi 3 can easily shuttle 115200 bps serial data over the network. But it was quick to get up and running. Also – the FTDI is technically unnecessary because a Pi has 3.3V serial capability on board that we could use. It’s not worth the time to fuss with right now but something to keep in mind for later.
Now that the laser is mobile, it’s time to explore its behavior on the move…
While waiting on my 3D printer to print a simple base for my laser distance scanner salvaged from a Neato robot vacuum, I went online to read more about this contraption. The more I read about it, the more I’m puzzled by its price. Shouldn’t these simple geometry-based distance scanners be a lot cheaper by now?
The journey started with this Engadget review from 2010 when Neato’s XV-11 was first introduced to fanfare that I apparently missed at the time. The laser scanner was a critical product differentiation for Neato, separating them from market leader iRobot’s Roomba vacuums. It was an advantage that was easy to explain and easy for users to see in action on their product, both of which help to justify their price premium.
Of course the rest of its market responded and now high-end robot vacuums all have mapping capability of some sort or another, pushing Neato to introduce other features like internet connectivity and remote control via a phone app. In 2016 Ars Technica reviewed these new features and found them immature. But more interesting to my technical brain is that Ars linked to a paper on Neato’s laser scanner design. Presented at May 19-23 2008 IEEE International Conference on Robotics and Automation titled A Low-Cost Laser Distance Sensor and listing multiple people from Neato Robotics as authors, it gave an insight into these spinning domes. Including this picture of internals.
But even more interesting than the fascinating technology outlined in the paper, is the suggested economics advantage. The big claim is right in the abstract:
The build cost of this device, using COTS electronics and custom mechanical tooling, is under $30.
Considering that Neato robot vacuums have been in mass production for almost ten years, and that there’s been ample time for clones and imitators to come on market, it’s quite odd how these devices still cost significantly more than $30. If the claim in the paper is true, we should have these types of sensor for a few bucks by now, not $180 for an entry-level unit. If they were actually $20-$30, it would make ROS far more accessible. So what happened on the path to cheap laser scanner for everyone?
It’s also interesting that some other robot vacuum makers – like iRobot themselves – have implemented mapping via other means. Or at least, there’s no obvious dome of a laser scanner on top of some mapping-capable Neato competitors. What are they using, and are similar techniques available as ROS components? I hope to come across some answers in the near future.
Since it was bought off eBay, there was an obvious question mark associated with the laser scanner salvaged from a Neato robot vacuum. But, following instructions on ROS Wiki for a Neato XV-11 scanner, results of preliminary tests look very promising. Before proceeding to further tests, though, I need to do something about how awkward the whole thing is.
The most obvious problem are the two dangling wires – one to supply motor power and one to power and communicate with the laser assembly. I’ve done the usual diligence to reduce risk of electrical shorts, but leaving these wires waving in the open will inevitably catch on something and break wires. The less obvious problem is the fact this assembly does not have a flat bottom, the rotation motor juts out beyond the remainder of the assembly preventing the assembly from sitting nicely on a flat surface.
So before proceeding further, a simple base is designed and 3D-printed, using the same four mounting holes on the laser platform designed to bolt it into its robot vacuum chassis. The first draft is nothing fancy – a caliper was used to measure relative distance between holes. Each mounting hole will match up to a post, whose height is dictated by thickness of rotation motor. A 5mm tall base connects all four posts. This simple file is a public document on Onshape if anyone else needs it.
Each dangling wire has an associated circuit board – the motor power wire has a voltage regulator module, and the laser wire has a USB to serial bridge. Keeping this first draft simple, circuit boards were just held on by double-sided tape. And it’s a good thing there wasn’t much expectation for the rough draft as even the 3D printer had a few extrusion problems during the print. But it’s OK to be rough for now. Once we verify the laser scanner actually works for robot project purposes, we’ll put time into a nicer mount.
I bought a laser scanner salvaged from a Neato robot vacuum off eBay. The promised delivery date is mid next week but the device showed up far earlier than anticipated. Which motivated me to drop other projects and check out the new toy immediately.
The first test is to verify the rotation motor works. According to instructions, it demands 3.0 volts which I dialed up via my bench power supply. Happily, the scanner turns. After this basic verification, I took one of the adjustable voltage regulators I bought to power a Raspberry Pi and dialed it down to an output of 3.0 volts. Since the connectors have a 2mm pitch, my bag of 4-pin JST-XH connectors could be persuaded to fit. It even looks like the proper connector type, though the motor connector only uses two pins out of four.
The instructions also had data pinout, making it straightforward to solder up an adapter to go between it and a 3.3V capable USB serial adapter. This particular adapter claims to supply 3.3V between 100-200mA. Since the instruction said the peak power draw is around 120mA, it should be OK to power the laser directly off this particular USB serial adapter.
With physical connection complete, it’s time to move on to the software side. This particular XV-11 ROS node is available in both binary and source code form. I chose to clone the Github source code because I have ambition to go in and read the source code later. The source code compiled cleanly and RViz, the data visualizer for ROS data, was able to parse laser data successfully.
That was an amazingly smooth and trouble-free project. I’m encouraged by the progress so far. I hope we could incorporate this into a robot and, if it proves successful, I anticipate buying more of these laser sensors off eBay in the future.
The biggest argument against buying a Monoprice robot vacuum for ROS hacking is that I already know how to build a two-wheeled robot chassis. In fact two-wheeled differential drive is a great simple test configuration that I’ve done once or twice. Granted, I have yet to build either of them into having full odometry capability, but I do not expect that to be a fundamentally difficult thing.
No, the bigger challenge is integrating sensing into a robot. Everything I’ve built so far has no smarts – they’re basically just big remote-control cars. The ambition is to ramp up on intelligent robots and that means giving a robot some sense of the world. The TurtleBot 3 Burger reads its surroundings with a laser distance sensor that costs $180. It’s been a debate whether I should buy one or not.
But at this past Monday’s SGVHAK meetup, I was alerted to the fact that some home robot vacuums use a laser scanner to map their surroundings for use planning more efficient vacuum patterns. I knew home robot vacuums have evolved beyond the random walk vacuum pattern of the original Roomba, but I didn’t know their sophistication has evolved to incorporate laser scanners. Certainly neither of the robot vacuums on clearance at Monoprice have a laser scanner.
But there are robot vacuums with laser scanners and, more importantly, some of these scanner-equipped robot vacuums are getting old enough to break down and stop working, resulting in scavenged components being listed on eBay… including their laser scanner! Items come and go, but I found this scavenged scanner for $54 and clicked “Buy It Now”. The listing claims it works, but it’s eBay… we’ll find out for sure when it arrives. But even if it doesn’t, Neato vacuums are available nearby for roughly the same price, so I have the opportunity for multiple attempts.
The unit off eBay was purportedly from a Neato XV-11 vacuum and someone in the ROS community has already written a package to interface with the sensor. The tutorials section of this package describes how to wire it up electrically. It looks fairly straightforward and I hope it’ll all come together as simply as I hope it will when the eBay item arrives in about a week and a half.