Distance measurement using Ultrasonic sensors and phase shift method

[greatgrains]

Hi, 

I've recently started a project where the idea is that we have an ultrasonic transmitter and a receiver. The transmitter will send out a signal at 40 kHz that will hit some object, most likely a book, and reflect that signal to the receiver. The received signal should have a phase difference from the outgoing signal. Now, when we vary the outgoing signal, the phase shift should also change. Likewise, if we vary the distance to the object, the number of wavelengths that can fit in between the object and the transducers should also change. We would multiply the incoming and outgoing signals and measure the beat signal, but I'm not sure how we would do this in labVIEW. We can calculate the distance to the object by figuring out what d(phase)/d(frequency) is. So far,when I am having problems with actually getting an incoming signal and figuring what that phase shift is. I'm not sure how I could use simulate signal to accuately and precisely send out a 40 kHz sine wave, and then vary that signal by steps of 1 Hz. I have also tried using a signal generator, and having once BNC cable go to the input of the transmitter and the other going to input of the DAQ. I have another BNC cable going from the output of the receiver to the input of the DAQ. I had multiple channels on my DAQ, and used split signals to see the transmitted and received signal. But this doesn't seem to be working. I also tried to add gain to the received signal, but when adding an Op-Amp LF356 to the receiver, the signal completely disappears. Attached are some of the VI's I have, but none of them seem to be working. Thanks in advance for any help. 

[johnsold]

Have you done the basic physics and mathematics? For the speed of sound = 343 m/s and the frequency = 40 kHz the wavelength is 8.6 mm. At 40.001 kHz the wavelength is 214 nm shorter.

So, how do you propose to keep track of the number of wavelegnths if the target moves more than 8.6 mm?

To measure a phase shift you must have some kind of reference. Typically a fraction of the transmitted signal is used as a reference.  Since you probably do not know how much phase shift occurs in the power amplifier driving the transmitter or between the electrical signal at the transducer and the acoustic signal in air, it is usually best to measure both, ideally with the same receiver and amplifiers.

The LF356 should be good for modest amounts of gain. The transducers can look like a capacitance to the circuits. Do you have a DC offset which is saturating the amplifier?

Lynn