Doppler Sonar Sensor

[TrickyNekro]

Hello again,

I come here with a suggestion for a type of sensor yet to be seen...
Although typical sonars measures the distance of the body and the target they do not measure the velocity of the vehicle...

Although this may be very difficult, a sonar based on Doppler effect can give us such reading......
With sonar you don't need super fast electronics, that's for sure... And that makes everything possible...

If the body has a typical speed that isn't as fast as the sound (our case) and you can measure the receiving frequency then
by exploiting the Doppler effect you can find the velocity of the body!!!

Any suggestion, guys, on how to measure very small changes in frequency and if it is efficient to turn frequency
to voltage to do that...

Any suggestions are more than welcomed!!!
Lefteris, Greece

[dunk]

just an idea on where to start reaserching:
i *think* some burglar alarms use doppler sonar.


dunk.

[Webbot]

You wont be able to measure the true velocity of the object. You will only be able to measure its velocity towards/away from the sonar.

If your sonar is mounted on a moving robot then you will need to take into account the speed that your robot is moving within your calculations. In itself this could give a slight noise issue. ie if your robot is moving towards a wall then there will be some doppler shift - but this is caused by your robot moving - not the wall.

Since you are using a high frequency then you could use a low pass filter to filter out the main sonar carrying wave from your returned signal. The small frequency changes, ie the doppler shift, would pass through.

[paulstreats]

You would probably have to build your own sonar from scratch, I dont think any of the hobby modules do doppler shift detection. Maybe you could modify one though

[BuddingContraptionist]

isn't it just like, multiple pings in rapid succession at differing frequencies?

[TrickyNekro]

Well, modifying is an issue...
I have a SRF05 so that I can reverse engineer.... so it would be quite the same...

First I need to do some calculations... See the frequency sweep over speed...
Then I think of a method that might actually work...

Filters... the enable one frequency to pass, but this is only for perfect filters, since the freq sweep gonna be very low...
I can measure the output of two filters... one with a high and one with a low respective to 38kHz frequency...
So when we have forward speed the high filter will have more voltage that the lower... and so on...

What do you say??? Man, this is gonna be tough and may lead to nothing...
Still quite worthy trying...

Thanks for any replies, Best Regards
Lefteris

[Webbot]

Did some 'googling' and found this:


'Fast and Accurate Mobile Robot Sonar' at http://www.ecse.monash.edu.au/centres/irrc/LKPubs/Sensor_review99/sensrev.htm
It doesn't use doppler shift but describes a way of getting bearings etc - could be useful to others....


Also found an academic paper on a 'bat robot' that uses doppler shift (like bats do). It looked good but costs $31 to download !

[Admin]

Also found an academic paper on a 'bat robot' that uses doppler shift (like bats do). It looked good but costs $31 to download !

Use Google Labs to search for a free copy of the paper. If the author doesn't make the paper publicly available, it probably wasn't a good paper

[paulstreats]

isn't it just like, multiple pings in rapid succession at differing frequencies?

no its just a ping at a predefined frequency.

If the frequency of the rebounding ping is higher than that sent out then the object is travelling towards you and if its lower (or stretched) then its travelling away from you.

The prebuilt hobby modules send out a ping and then listen for the rebound. When they listen they listen for a whole range of frequencies and report them back. They dont make any calculations in frequency shift(so any frequencies detected are treat as just 1) and theres also no way to find out if there has been a frequency shift in software.

You need to really tune your equipment just right to accurately doppler detect.

A way to understand doppler shift is to listen to a car driving towards you. As it approaches the sound gets higher pitched, then when it passes and starts to move away the sound starts dropping in pitch. youve just experienced doppler shift. Or on the tv watching motor racing the cars go nnnnnneeeeeeeeoooooowwwwwnnnnn.   nnnnnnnneeeeeeeeeee is when the car approaches the camera and the ooooooowwwwwwwnnnnnnnnn is as its passed and travelling away.