Netduino home hardware projects downloads community

Jump to content


The Netduino forums have been replaced by new forums at community.wildernesslabs.co. This site has been preserved for archival purposes only and the ability to make new accounts or posts has been turned off.
Photo

Double-check my reading of transducer spec sheet?

ultrasonic pwm

Best Answer CW2, 11 February 2013 - 10:21 PM

One thing I forgot to mention is that while driving the ultrasonic (piezo) transmitter is relatively easy, the detection and echo processing is a little bit more complicated. The test circuit in the datasheet does not demonstrate that, I would recommend looking at MaxBotix sensor schematic, which is reasonably functional and easy (two stage amplifier with band-pass filters, peak detector, etc.), so the output can be connected to a microcontroller. The voltage produced by the piezo element on reception is a few millivolts, which cannot be handled by the microcontroller directly.

Go to the full post


  • Please log in to reply
7 replies to this topic

#1 jdlogicman

jdlogicman

    Member

  • Members
  • PipPip
  • 12 posts

Posted 11 February 2013 - 03:14 AM

Thanks to you guys in this forum, I am past my last roadbock and onto my next - driving my ultrasonic transmitter.

 

 

Disclaimer: I'm a software guy, just learning DIY electronics

 

The transducer I want to use is model # T/R40-14.4A0-01. The spec sheet says two things that puzzle me:

 

1 - Vp-p 160

 

Am I really supposed to supply up to 160V to this thing to get maximum output? This is for salt water immersion!

 

2 - Pulse width 0.333mS, interval 20ms

 

Should I assume they meant microseconds (mu) not milliseconds (m)? If it's microseconds, I think I can do it with the Netduino 2 PWM and some amplifiers.

 

3 - Also, can anyone give me a hint on how to figure out what kind of current this thing will consume? I need to know that to spec my parts. I think I can design the circuit, but I don't know how to figure out the current for the device, so any pointers will be appreciated.

 

Thanks much!

 

 

 

 

 

 



#2 Mario Vernari

Mario Vernari

    Advanced Member

  • Members
  • PipPipPip
  • 1768 posts
  • LocationVenezia, Italia

Posted 11 February 2013 - 04:11 AM

Hello jdlogicman.

I found this sheet: http://www.futurlec..../USTR40-14A.pdf

The voltage could reach 140-160V, but bear in mind it's not a supply, rather a pulse amplitude. In the above document there's a circuit that well describe how to connect the sensor.

 

The timings are in milliseconds. Microseconds are expressed as the Greek "?" (mu) or even with the lower "u", to simplify.

 

The current has a relative meaning for a pulse generation. It's rather more interesting the energy used for a pulse.

Anyway, I'd follow the circuit and you should avoid any issue.

 

Cheers


Biggest fault of Netduino? It runs by electricity.

#3 jdlogicman

jdlogicman

    Member

  • Members
  • PipPip
  • 12 posts

Posted 11 February 2013 - 04:30 AM

Thanks for responding.

 

This spec sheet has the same issue that confused me originally. The schematic shows 40Khz on INPUT, but a pulse width of 0.5 milliseconds, interval of 20 milliseconds allows for (1000 ms/sec) / (20.5 ms/cycle) = 48.78 Hz, not the 40 KHz I expected. That's why I thought the units were off. That is, unless the transmitter "rings" for ~1000 cycles after an input pulse, so you only need to drive it at 40 Hz to get a 40 Khz signal out of it.

 

As for the diagram, I still can't see how you can get beyond 12V. C1 will never charge to more than that, if I understand how capacitors work. I could build some voltage doublers and use a very low duty cycle, if I knew that was the right thing to do....



#4 Mario Vernari

Mario Vernari

    Advanced Member

  • Members
  • PipPipPip
  • 1768 posts
  • LocationVenezia, Italia

Posted 11 February 2013 - 06:45 AM

Well, I'll try to explain, but I must go deeper in the physics of the components.

 

Have a look at the schematic: there's a transformer.

On the "primary" coil (the left side) there's a transistor, which acts as a switch upon the INPUT signal wants. When the transistor is closed, there's voltage on the transformer's coil, and this accumulates magnetic energy. However, when the transistor turns off, the energy accumulated can't flow through the transistor anymore, because it's like an open switch. Since that energy CANNOT be stored by the transformer, it must find another way to "escape" and find the way by discharging through the secondary coil, thus piezo and rest of the circuit.

 

Now, read the indication above the transformer: 1:7.

It means that the secondary coil has 7x turns than the primary one. This implies that the voltage seen at the secondary is 7x the one seen at the primary. Thus *around* 12V x 7 = 112V.

 

Unfortunately is not so simple, because there are other implications, such as the resonance frequency, the transformer tuning (it's adjustable), etc. For instance, if you cut off the 20k resistor in parallel to the sensor, the voltage across the piezo can get much higher than just 112V.

IMHO, you should use a scope for that circuit. If you don't have it, and/or don't want to deal with such a electronics, just turn to a ready-to-use ultrasonic sensor shield. I don't have experiences on what's a good one.

 

Also bear in mind that the piezo sensor is a thin ceramic-like surface between two metallic shields.

In open air (better: vacuum), you'll get a spark between two points just 1mm far when the voltage drop reaches 3kV approx. Thus, if you aren't aware to the piezo voltage, it'll break by excessive voltage.

 

I am sorry for the deep-tech description, but I had to answer you in some way!

Hope it helps.

Cheers


Biggest fault of Netduino? It runs by electricity.

#5 jdlogicman

jdlogicman

    Member

  • Members
  • PipPip
  • 12 posts

Posted 11 February 2013 - 08:00 PM

I am still learning my symbols - I thought that was a variable resistor, not a transformer. Now the voltage makes sense. Thanks for that.

 

So, am I correct that the units should be microseconds, and the transformer is getting enough time to recharge since the duty cycle is so short?



#6 CW2

CW2

    Advanced Member

  • Members
  • PipPipPip
  • 1592 posts
  • LocationCzech Republic

Posted 11 February 2013 - 09:55 PM

So, am I correct that the units should be microseconds, and the transformer is getting enough time to recharge since the duty cycle is so short?

 
I don't think so. The datasheet circuit uses the sensor driven with short pulses, it says 40 kHz 20 bursts on the INPUT - 20 bursts of 40 kHz pulses is 20*1/40000 = 0.5 ms, which is noted as maximum pulse width in the specifications table. The basic operation of the circuit is


    [*]The sensor is driven by 20 pulses at 40 kHz (its resonant frequency), the voltage of the transformer primary switched via transistor Q1 is multiplied 7 on the secondary winding, which excites the piezo element,
    [*]The transistor Q1 is then switched off, the piezo element rings for additional <= 1.2 ms,
    [*]When the sound wave is reflected and the echo signal comes back, the sensor produces small pulse, which is amplified by the operational amplifier, in the schematic there is just simplified symbol "Gain 330, TL084A" (not showing the actual op-amp inputs, feedback resistors etc.). One thing to note is that at this time the circuit on the transformer primary is switched off, so there is basically just the sensor connected to the amplifier. The clamping diodes D1 and D2 cuts the high voltage during transmission, protecting the op-amp input.
    [/list]

    The "20 ms" interval determines the maximum detection distance, there is 18.3 ms reception window, which translates to roughly 6 meters (air, speed of sound 346.13 m/s at 25C); but you'd need to know sensitivity/sound pressure level characteristic for more accurate calculation. The minimum distance is determined by the ringing time, 1.2 ms means about 40 cm (datasheet says ?35cm).

     

    The inductance of the secondary winding of the transformer is designed to tune out the reactance of the parallel capacitance of the ultrasonic transducer ("impedance matching transformer").

     
    Typical impedance of the piezo transducers is few hundred to thousand ohms, for example sensor with 100Vp-p which is usually noted as 20V RMS, and 500? nominal impedance will take about 40 mA - but this is average, there will be short spikes, so the power supply should be designed to withstand them.  

     
    If you are interested in more detailed information, have a look at Pro-Wave application notes or MaxBotix tutorials and product datasheets (with schematics).



#7 CW2

CW2

    Advanced Member

  • Members
  • PipPipPip
  • 1592 posts
  • LocationCzech Republic

Posted 11 February 2013 - 10:21 PM   Best Answer

One thing I forgot to mention is that while driving the ultrasonic (piezo) transmitter is relatively easy, the detection and echo processing is a little bit more complicated. The test circuit in the datasheet does not demonstrate that, I would recommend looking at MaxBotix sensor schematic, which is reasonably functional and easy (two stage amplifier with band-pass filters, peak detector, etc.), so the output can be connected to a microcontroller. The voltage produced by the piezo element on reception is a few millivolts, which cannot be handled by the microcontroller directly.



#8 jdlogicman

jdlogicman

    Member

  • Members
  • PipPip
  • 12 posts

Posted 11 February 2013 - 11:20 PM

Thanks - that gives me a lot to go on. My transmitter and by receiver will be separated, since this is for a following application, not echo detection, but it's good to know I can just hit it with a ton of voltage and let it ring a bit. The schematic for a transmitter board at engineeringshock is definitely following a different design philosophy, but I will need more volume than it would deliver.

 

Thanks again!







0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

home    hardware    projects    downloads    community    where to buy    contact Copyright © 2016 Wilderness Labs Inc.  |  Legal   |   CC BY-SA
This webpage is licensed under a Creative Commons Attribution-ShareAlike License.