by lordtct » Wed Oct 13, 2010 2:03 am
Judging from your very short measurement distance, TOF laser rangefinding is not a wise idea for two reasons:
1. Your circuit must be able to shoot pulsed laser beams at a rate of at least 1/10th of your minimum time of flight. It takes only 66 ns for round trip at 10 m. At 5 m, you're looking at 33 ns. So yes, your lecturer is right: Your timing circuits needs to be clocked at a whooping 300 MHz. But the actual problem is not the FPGA, but rather HOW you are going to design a power circuit to fire such extremely accurate and fast pulses with near zero margin for timing errors.
2. The million dollar question: If you are able to shoot 300 MHz pulses, what are you going to use to sense the returning pulse? I don't think you can find any affordable photodetectors for the job! That's your final nail in the laser coffin.
On infrared distance measurement, I'm afraid it will not be very straightforward. If you calibrate distance according to IR receiver intensity, you're going to get in trouble because not all objects have the same specularity or reflectivity. If you calibrate the sensors w.r.t. a black object, don't expect the sensor to work for say, a piece of paper.
In the manufacturing industry, we do have something called a "displacement meter" for extremely accurate liquid level measurement. It works by shooting a powerful N-IR laser beam at the moving surface and the beam gets reflected to a 1 dimensional array of photodetectors. Changing heights of the measurant will cause the beam to fall at different positions on the 1D detector. There is of course, the need to calibrate the meter for different height ranges by varying the focusing lenses' powers and the position of detector itself.
If you want to try this yourself, you could aim an infrared laser at an object and measure the response from a 1D photodetector, say, Hamamatsu S3932 (with visible light filter). Knowing all your reference points, you could apply simple trigonometry to obtain the distance. You may need a concave lens to focus the beam in case it doesnt fall on the detector's edges.
Hope that helped!