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I think the hard part would be getting a sensitive enough receiver. Even with a laser the receiver has to be able to accurately detect the signal over a long distance. Otherwise all you need is a high speed clock/counter and trigger circuit. The results can be read from a micro controller.
Even the counter and trigger probably aren't easy. To get to a crummy one-foot resolution the counter has to run near a ghz, the trigger needs a 1ns repeatability. As a hobby circuit, it would be like an introduction to design rules for microwave layout.
The laser would need a sharp and clearly defined rise time. The sensor would probably want to ramp up its gain over a hundred nanoseconds from the rising edge of the send pulse.
100mhz which is a little more practical would be 10 feet resolution, an FPGA could do that, though I don't know much about how the receiver would work.
I thought Mike's answer was fine. If somebody will actually google search for a circuit and do some research these guys will bend over backwards and help. If you are lazy and expect somebody to spoon feed you.... it's open season.
I thought lidar was based on hetrodyning (interference) and not time of flight. Do they claim any absolute distance measurements? I only saw velocity, which is quite easy with interference.
Is that how doppler works? Interference? I was wondering how they managed to detect such a small frequency shift.
THere are both TOF and doppler LIDARs just like there are both types for radars...just when people say LIDAR they are usually talking about police speedguns.
hi,
A simple hybrid logic/analog circuit works OK.
The TX pulse sets a F/F, a fast CC ramp runs and charges a cap.
On receipt of the RX pulse the F/F is reset and the fast down ramp [*1] is stopped and a slow CC ramp [*50] runs and discharges the cap.
During the slow up ramp a 30MHz clock is gated into a counter chain.
When the up ramp reaches zero, the count is stopped, the contents of the counter is the range value, to a resolution of 0.1mtr, assuming light speed of 300m/microsec.
Lidar can be both mneary, interferometry for speed and time of flight for distance. Yeah dk that's how simple radar works, if you mix the received signal with the transmitted signal you'll get a frequency in the audio range out that can be fed through a simple counter. Modern aircraft radar has much more sophisticated detection circuitry but that stuff is way over my head.
Even the counter and trigger probably aren't easy. To get to a crummy one-foot resolution the counter has to run near a ghz, the trigger needs a 1ns repeatability. As a hobby circuit, it would be like an introduction to design rules for microwave layout.
The laser would need a sharp and clearly defined rise time. The sensor would probably want to ramp up its gain over a hundred nanoseconds from the rising edge of the send pulse.
What they do is modulate the laser in MHz, then clean up the incoming received signal and measure the phase shift between the 2 signals, to give time of flight. Once the signal is cleaned it's not much harder that just using an AND gate to get the phase difference as a DC voltage.
The receiver circuit provides a pulse based on the energy in the middle of the peak, so it doesnt need to have super defined / or \ edges. This also helps because you need an input amplifier with variable gain etc etc.
It's still a really difficult task to get right. Even the commercial laser distance measuring devices I worked with in the surveying field needed a prism reflector.
That makes sense, I guess a fraction of a percent of phase shift is easier to detect than a nanosecond anything.
I hope these TOF cameras get real cheap real soon. This kind of thing would be a boon for robotics and such. Besides ranging, it has a terrific ability to do background subtraction. Anything out of range is transparent, so you don't need a green screen.
Prism reflector = corner cube reflector? A retroflector?
Just a cheap plastic reflector on a stick, like the automotive ones with hundreds of tiny corner prisms to return light in the same direction. I can't remember the proper scientific name for that type of reflector so I said "prism" reflector hoping I wouldn't look too stupid.
Those cameras sound interesting. One thing I want to get into soon is stereoscopic cameras for robotics, cameras are cheap now and there's plenty of cheap processing power now (dsPICs etc) to determine the distance to all the objects the cameras "see".
It's one of those cool areas in robotics that seems to be moving from the high $$ research arena to the "every robot hobbyist can do it at home" arena.
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