22nd April 2003 04:34 PM
Handheld Range Finder
Wonder if anyone can help me. I've decided to build a hand held range finder. Using a laser I guess. For my HND project. I just need to know what the basic building blocks will need to be. Any comments would be of help. :?
23rd April 2003 01:38 AM
Laser range finding can be extremely difficult, or simplistic, depending on required accuracy and measurement frequency.
I believe there are three basic methods.. One relies on a device that can measure the incoming angle of the reflected laser beam. I am not too sure about this one, but I believe it is quite simple.
The two other methods are build on the principal that light takes time to travel. A laser beam is modulated with a sinewave and sent out of the unit. Before it is sent, a small portion of the beam is sent to a reference photodiode through a 90deg beam splitter. Then the beam is returned through a large lens and focused on another photodiode. The two signals after transimpedance amplification are compared for phase differences, then the resultant is calculated. I forget the equations, but it isn't a hard conversion. The problem with this is the modulations and the components required.
Another method, which I would consider if I had to do it, involves sending out a laser pulse. Once this is sent out, it triggers a counter to start counting at a high frequency. Then once it is recieved, the counter stops. This method is fairly simple, but involves a lot of highspeed components and fine tuning to perfect. You would need to use a FPGA for optimal results!
The optics involved in some of these systems are expensive... If you don't need good pinpoint accuracy, large distance measurement, good accuracy, then consider using one based on ultrasonics. You can use the same principals as the last method described, but you do not need a very fast counter.
To be aware of.. Both systems have weaknesses.. The laser light needs to be reflected back and detected.. so if you do not have very sensitive photodiodes or a medium that absorbs the laser or transmits it, then you are in a lot of trouble. Also, sound absorbing materials will not be detected with the ultrasonic method.
23rd April 2003 08:02 AM
Handheld Range Finder
Thank you for your quick reply to my query regarding the building of a range finder.
After your advice I've decided to go for the ultrasonic method. I believe students before me may have used this method for a distance meter for fitting to a car for help with car parking.
Any more advice on an ultrasonic range finder would be of help.
Thank you again for your assistance
23rd April 2003 12:44 PM
Yeah.. A laser range finder would be quite difficult, but with new technology, it remains fairly easily done. I have been checking it out lately, and I can make a measurement system that will be accurate to around 0.12m at ranges up to 2km. The accuracy isn't fantastic, but it is the best you can do easily.
Anyways, the ultrasonic rangefinder can be quite easy to design. It would be best to use a microcontroller for this, or even a pld would work okay. All you need to do is to send out a single pulse to an ultrasonic transducer. This generates a quick 'ping' or 'chirp', which is sent out and reflected (hopefully!) off of the target. Then you would use the same transducer to listen for the return of the ping. During this time, it would be best to run a counter of some sort either in software or in hardware. Then the returning ping will stop the counter. What you will have left is a value which needs to be processed.
This value needs to be divided by two some point in the calculations. Also, you need to know your actual counter frequency to know the real time. Then you simply multiply the real value of the count by the speed of sound, which is around 343M/S. That value will then need to be sent to a LCD display of some sort.
The speed of sound varies with atmospheric pressure and temperature. If you would like very nice accuracy, you can prepare a software lookup table and have an accurate temperature sensor to compensate for drifts in temperature. I recommend learning a higher level language and get a compiler for a microcontroller that will make things a lot easier for you. I recommend the BASCOM AVR/8051 BASIC compiler from www.mcelect.com , it isn't too expensive and works very nicely!
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