Some years ago the story went around that the laser from the Neato XV-11 would become available as an open source LRF/LIDAR: http://www.hizook.com/blog/2009/12/20/ultra-low-cost-laser-rangefinders-actualized-neato-robotics. For some reason this didn’t happen and subsequently many hacks have been presented to show how this laser unit works. Unfortunately, this hasn’t resolved the main issue of how users can get their hands on a open source, low cost LRF. For almost three years I’ve been looking for a way to fill the LRF part of this technology gap. The idea was to design an open source LRF that could be turned into a scanning LIDAR by the open community. So here it is…
In this thread I’ll cover the workings of the open source LRF module, imaginatively called the OSLRF-01, and to start with I’d like to dispel the notion that you need to have clocks running at GHz in order to make a time-of-flight LRF.
Measuring the round trip time of a laser pulse is not as simple as having a counter that starts when the laser fires and stops when the return signal comes back. Although this brute-force timing method can be used, the resolution of the result is going to depend on how fast you can clock the timer. It would take a 15GHz clock to achieve one centimeter resolution. In the 1960’s, Tektronix made a name for themselves with a sampling oscilloscope that could measure signals in the GHz range without using high speed clocks. How they did this is summarized here: http://www.cbtricks.com/miscellaneous/tech_publications/scope/sampling.pdf. The OSLRF-01 uses a modified version of “sequential-equivalent-time-sampling” (SETS). With this system it can easily achieve an “equivalent clocking rate” of more than 90GHz whilst using much lower frequencies.
The second issue that I’d like to address, is amplifying the very weak return signals that come back from the targeted surface. Since the laser pulse is very fast, typically around 10ns long with rise and fall times of a few nanoseconds, the detection and amplification circuitry would need to have a bandwidth of about 500MHz in order to preserve the shape of the pulse during amplification. This means that the “gain-bandwidth product” of a suitable amplifier would be more than 5GHz. Designing an amplifier circuit with this kind of specification is very difficult. The OSLFR-01 addresses this problem by amplifying the signals after the sequential-equivalent-time-sampling has taken place. This works because the SETS process expands the timebase of the signal, thereby reducing its bandwidth. Amplifying this slowed down signal can be done with conventional op-amps having a gain-bandwidth product of a few MHz.
The OSLRF-01 uses conventional electronics to solve the very unconventional design problems associated with a TOF laser rangefinder. It doesn’t include any signal processing but with the signals being so slow, a regular ADC is fast enough to digitize the entire waveform. You can add your own signal processing and timing algorithm using an Arduino or similar microcontroller.
In subsequent posts I’ll go into more detail about every element of the design and I’ll post a link to the circuit diagrams. In addition, www.lightware.co.za will soon be manufacturing complete units for sale at US$100 if you want to experiment with the device.
The bottom trace on the oscilloscope shows the outgoing laser pulse after SETS on a timebase of 5ms per division.