Ultrasonic ears

[AllanBertelsen]

Look at this article. It's how to make an Ultrasonic Homing Device:

**broken link removed**

What I would like was a similar device. The only difference should be that it would find the angle to the transmitter.
My guess is that a MCU is not fast enough the measure this time difference.
The question is, if looking at figure 2, could the time difference from the signals from the two op-amps be used to charge a cap so that it could be measured afterwards? Or is there some other technique that would make this time measurement possible?

 

[dknguyen]

The uC is more than fast enough to measure the difference (after all, if it runs in the ~40MHz range it can measure differences of about 25 nanoseconds, while sound tends to work in the ms range). The hard part is making sure it's the same sound you are hearing. The ultrasonic rangefinders you normally see you some filter circuitry that just produces a generic spike (which translates directly, or through a transistor or something else as a HI) whenever an echo is received. In some cases the receiver has a narrow enough bandwidth that it doesnt need a filter (it might need an amplifier though). The uC can easily measure the time difference between these two spikes (it assumes that it's transmission is the only one out there, therefore the spikes represent the same sound). Whether or not the two incoming spikes represent the same sound is a much more tricky matter, but you usually don't need it.

 

[Sceadwian]

A pic at 40mhz 'runs' at 10, which is 100nano seconds actually =)

 

[ericgibbs]

hi allan,

The method of charging a capacitor over the time period of two pulses is in common use.

The important point is to charge the cap in a 'linear' way by using a constant current source.
So there is a direct correlation between the voltage and time.

Example: Pulse 'A' starts a linear ramp and pulse 'B' stops the ramp, a high input impedance opa isolates the voltage charge
on the cap from a measuring circuit.
As soon as the voltage measurement is complete the charge on the cap is dumped ie; Vcap=0v

By knowing the rate/slope of the voltage ramp on the cap, the time difference 'A' to 'B' can be calculated.

The same method is used in light/laser transmission range measurement.
The transmission pulse starts the Vcap ramp up at a high rate and the received 'echo' pulse stops the ramp
and starts a slow ramp down to Vcap=0.

The 'transit time' is measured during slow ramp down.

Does this help?

 

[gramo]

A pic at 40mhz 'runs' at 10, which is 100nano seconds actually =)

Its not that the PIC is ran at 40Mhz, that’s just a comparison to a PIC without PLL enabled.

The PIC uses a 10Mhz external crystal, and with PLL enabled, it will increment every oscillation rather than every forth, but yeah, every instruction will take 100nS

100nS = 34.029 um (Micro Meters) of sound travel

 

[AllanBertelsen]

Hi again
Thanks for the cap tips Eric. I was considering using two 74HC4060. That’s a 14-stage binary ripple counter with oscillator. It can count pulses at 80MHz. But with an 8 MHz oscillator I will be able use 16 of the 28 outputs to get a word variable representing a max distance difference at 257 cm with a precision at 0.0039 cm. If the distance between my "ears" is 1 m, I will be able to get cm accuracy in a distance at 20 to 30 m. (Theoretically!)
The problem is that the time difference for the signal to arrive at the two receivers, does not easily converts to an angle. At a given time difference, the transmitter will be positioned at a point on a parabola, not on a straight line.

 

[gramo]

It can count pulses at 80MHz.

Nice chip, never heard of it before learn something new everyday

Doesn’t the datasheet say 30Mhz-35Mhz? 4.5V - 5V

Found on Page 4

But with an 8 MHz oscillator I will be able use 16 of the 28 outputs to get a word variable

Perhaps we are looking at different datasheets?

representing a max distance difference at 257 cm with a precision at 0.0039 cm.

The example I did earlier provides 0.0034029 cm resolution

Accurate Captures could be used via the portb change interrupt, this way you can generate your 40Khz Signal at the same time..

Erm, before I go any further, are you familiar with PIC's?

 

[Nigel Goodwin]

Doesn’t the datasheet say 30Mhz-35Mhz? 4.5V - 5V

Don't forget, a PIC will count pulses to above 50MHz, even running with only a 4MHz clock - simply use the counter/timer hardware to count the pulses.

 

[AllanBertelsen]

Hi Gramo
I found my datasheet at:
http://www.ortodoxism.ro/datasheets/philips/74HC4060.pdf
I use PICAXE. Its preprogrammed PIC's. **broken link removed**
For this task they may be to slow. 10000 basic command per sec.

 

[AllanBertelsen]

Hi Nigel
You are right. Both interupt and pulse counting are supported (also in PICAXE's).

 

[gramo]

Don't forget, a PIC will count pulses to above 50MHz, even running with only a 4MHz clock - simply use the counter/timer hardware to count the pulses.

I thought Timer1 was limited to 200Khz? Or are you leading to a different direction?

 

[Nigel Goodwin]

I thought Timer1 was limited to 200Khz? Or are you leading to a different direction?

Check all the 50MHz PIC frequency counters - including the original MicroChip application note.

 

[AllanBertelsen]

Hi again
I have been looking for circuit drawings of an ultrasonic device that I have in mind.
An ultrasonic sender, that sends to a receiver at a distance of 10 to 30 meters.
The only ting I found was some commercial products. This one **broken link removed** can measure up to 18 meters. Round trip must be the double.
Does anybody know where to find such a circuit?
Another example: Sonin Combo Pro Distance Measuring Device **broken link removed** 250 feet!!