calculating doppler speed using ultrasonic sensor

[Engr_student]

Hi i am an Engineering student and i am making a radar to detect the moving objects using arduino and ultrasonic sensor sr 04.
The problem i am facing is i want to calculate the speed and the Doppler frequency of the object.but i don't know how to interpret the Doppler frequency at the receiver side of the sensor in my code and how to use this frequency in writing a code, so that i could find the object velocity...

 

[BradtheRad]

What measurement calculation are you using?

Seems to me there are two ways to do this:
(a) Measure distance based on click's round trip time, compare to previous click's distance data,
or
(b) broadcast a frequency at target, measure percent difference of return frequency.

I think method 'a' will be easier to calculate.

 

[D.A.(Tony)Stewart]

Consider the speed of sound for local pressure, and temperature and the difference frequency detected is either pos or neg with direction of object travel. delta f / f * V(air) / 2 in desired units of measure. The Doppler effect works from sender to target and then the echo works the same from target back to send, so the result has a /2 factor.

 

[biff44]

unless the speed is very high, you will not get a very big shift in Doppler frequency using only ultrasonics.

Even at 10 GHz carrier, you only get tens of kHz of Doppler shift from typical movements.

 

[D.A.(Tony)Stewart]

THat's true biff but even low frequencies can be measured with time intervals as long (as you have enough time) but of course it only works with very narrow beam radiators or long tube helmholtz resonators.

Let's consider a 1 MHz Piezo power crystal.

1,234 kilometres per hour is air speed at sea level and room temp.

Thus for a resolution of 1 KPH (=0.6MPH) (from the mixer output signal which would output the filtered difference frequency)

using
v= delta f / f * V(air) / 2
delta f = 1KPH * 1MHz * 2/ 1234 KPH = 1620 Hz

Path loss will be an issue with air for range, power & safety. but can be sent as 5 ms pulses with low duty cycle.

If the path loss exceeds your needs, and you choose 50kHz there are biological implications but loss is a function of frequency. and you need high gain = narrow beam emitter/detector with impedance matching.

Now at 50kHz the resolution with 1 KPH is 20 Hz.

THe speed of light for GHz type Radar is much higher than sound so the sensitivty for frequency is similar but the SNR can be made much higher for long range with microwave.

 

[Gorgon]

If it is the HC-SR04 module you use, you have a range of 4m and an onboard electronics, that only gives you a proportional pulse of the distance, after you trigger it. There is no possibility to test the received 40kHz signal for doppler effects, as far as I can see. What you can do is to measure the distance change between measurements and calculate the speed of the object.

 

[Engr_student]

Consider the speed of sound for local pressure, and temperature and the difference frequency detected is either pos or neg with direction of object travel. delta f / f * V(air) / 2 in desired units of measure. The Doppler effect works from sender to target and then the echo works the same from target back to send, so the result has a /2 factor.

i am making an ultrasonic radar which detects the object's speed, angle and distance... the problem i am facing is i don't know how to use the receiving frequency to find the velocity of the object in my code...
do i have to store the freuency directly into a variable or what...???

 

[D.A.(Tony)Stewart]

YEs Gorgon that is a static range sensor not suitable for velocity using Doppler.

 

[kripacharya]

A possible method is to do a fourier analysis on the received signal (after mixing with Tx signal and LPfilter).

Then the power in your bins will tell you the doppler shift, and you can calculate your speed with some resolution.

I have made this system, so i can give you some good practical advice