I'm working on a souped up ultrasonic range finder. I successfully amplified the signal driving the 40kHz piezo transmitter. The transmitter and receiver are located 0.75 inch apart. The problem is crosstalk directly between the transmitter and receiver, see scope trace 1 (no target). The crosstalk signal hides the target detection, see scope trace 2 ( target at 1 inch) and scope trace 3 ( target at 2 inches). I would like to use a PIC 16F877. Does anyone have a software or hardware suggestion of how to remove the unwanted crosstalk signal?

 

Are you talking about ultrasonic crosstalk, or electrical? If it is ultrasonic (which it probably is) you need to direct the ultrasonic waves more.

 

Thanks Caveman

The crosstalk is ultrasonic. Do you have a suggestion of how to "direct the ultrasonic waves more"?
I tried:
felt pads under the transducers
cotton balls between the transducers
paper dividers and cones between the transducers

 

Can you post you schematic so that we can see what the circuitry looks like?

hgmjr

 

How about some lead foil as a dampener? John

 

Given the distance, could it be that the receiver is simply continuing to vibrate after all that sound energy falls on it? The damping over time suggests something like that.

 

Would a resonant tube (cardboard tube cut to length, or multiple thereof) make the the signal more directional? I remember something similar using a bundle of straws glued together for a directional mike in an old Radio Shack article.

 

Try reducing the amount of amplification until you no longer receive the crosstalk.

Either that, or re-orient your receiver further away from the transmitter.

 

Can you post you schematic so that we can see what the circuitry looks like?

hgmjr

Thanks hgmjr
Here is the schematic you asked for.

 

How about some lead foil as a dampener? John

Sounds like a good idea. I will give it a try.

 

Given the distance, could it be that the receiver is simply continuing to vibrate after all that sound energy falls on it? The damping over time suggests something like that.

I think you are correct. The transmitter is driven at 40kHz for 8 cycles.

In trace1 you can see the crosstalk at the receiver with no target.

In trace3 you can see the crosstalk in the first 400us. You can also see the crosstalk + the signal reflected from the target in the next 700ns. The receiver continues to ring for another 500us.

I trying to develop code and/or a circuit to ignore the crosstalk and detect the signal reflected from the target.

 

Would a resonant tube (cardboard tube cut to length, or multiple thereof) make the the signal more directional? I remember something similar using a bundle of straws glued together for a directional mike in an old Radio Shack article.

Yes, I think a tube might make the signal more directional but unfortunately I need the full beam width.

 

Try reducing the amount of amplification until you no longer receive the crosstalk.

Either that, or re-orient your receiver further away from the transmitter.

Yes, reducing the strength of the driving signal and/or reducing the amplification does reduce the crosstalk. Unfortunately, it also reduces the ability of the sensors to detect both small targets and targets at a distance.

 

There may be another issue going on here. It looks like there is a "beat frequency" going on. You're not allowing the piezo to vibrate at it's own natural frequency; you're artificially forcing it to resonate at a frequency that's close. So, you're getting both the frequency you're stimulating it with, and the natural frequency of the piezo transducer.

But it also could be that the beat frequency is the difference between the transmitted signal frequency and the natural resonant frequency of the receiver transducer.

Rather than having your uC generate a frequency to blast your piezo transducer with, why not set up a driver using an inverter, with a couple of variable caps on either side of the transducer in the 10-50pF range to "tweak" it in?

 

There are a couple of things here.

As far as acoustic crosstalk, any sort of rigid structure will likely direct the beam. I used to work on ultrasonic motion sensors. That's what the grills on them do. And they will place plastic between the transducers to keep the direct path down.

The second issue is with your drive circuit. You should have a capacitor in series with the transmitting transducer. This is to remove any DC component which could damage the device. Otherwise it looks good.

 

There may be another issue going on here. It looks like there is a "beat frequency" going on. You're not allowing the piezo to vibrate at it's own natural frequency; you're artificially forcing it to resonate at a frequency that's close. So, you're getting both the frequency you're stimulating it with, and the natural frequency of the piezo transducer.

But it also could be that the beat frequency is the difference between the transmitted signal frequency and the natural resonant frequency of the receiver transducer.

Rather than having your uC generate a frequency to blast your piezo transducer with, why not set up a driver using an inverter, with a couple of variable caps on either side of the transducer in the 10-50pF range to "tweak" it in?

Wow, I hadn't noticed the "beat frequency" but I see it now that you point it out. The transducers are a matched pair rated at 40kHz and I think I'm driving them at that frequency.

As to your suggested solution, I'm afraid I have no idea what you are talking about. Can you be more explicit or better yet provide a schematic.

 

There are a couple of things here.

As far as acoustic crosstalk, any sort of rigid structure will likely direct the beam. I used to work on ultrasonic motion sensors. That's what the grills on them do. And they will place plastic between the transducers to keep the direct path down.

The second issue is with your drive circuit. You should have a capacitor in series with the transmitting transducer. This is to remove any DC component which could damage the device. Otherwise it looks good.

I had no acoustic crosstalk when driving the transmitter at ±5V but when I raised the voltage to ±10V the problem appeared. I have tried a wide variety of materials placed between the transducers and varied the space between the transducers from 0.75” to 2.25” all with little success.

Can you suggest size and type for the capacitor?

 

Just as a suggestion, what if you place a short across the receiver temporarily. If you continue to see a signal on the oscope, then you have to conclude that at least some of the output is electrical noise rather than acoustical in origin.

hgmjr

 

I had no acoustic crosstalk when driving the transmitter at ±5V but when I raised the voltage to ±10V the problem appeared. I have tried a wide variety of materials placed between the transducers and varied the space between the transducers from 0.75” to 2.25” all with little success.

Can you suggest size and type for the capacitor?

Something with a small impedance at 40kHz, like 0.1uF to 1uF range. Ceramic works well.

 

The lousy old LM358 is low power so it is slow. It can barely have a gain of 10 at 40kHz then its output is triangle-waves instead of square-waves.