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Close Range Ultrasonic Sensors Needed — Parallax Forums

Close Range Ultrasonic Sensors Needed

roboticsrobotics Posts: 90
edited 2010-07-11 20:37 in Accessories
I have an application that requires a close range single transducer head ( unlike the PING which has a separate emitter and receiver transducer head) ultrasonic sensor.

The distance from the senor to the target may vary from approx 1" ( 20mm ) to 6" ( 150mm ) and the sensor needs to enable distance determinations in approx 1mm steps or better.

The closest sensor that I found is the Daventech SRF01 ( http://www.robot-electronics.co.uk/htm/srf01tech.htm ) which can measure distances to targets as close as 0" (that's right "zero" inches); HOWEVER, the SRF01 enables distance measurements only in Centimeters and not Millimeters, which isn't accurate.

Suggestions are greatly appreciated!

Comments

  • roboticsrobotics Posts: 90
    edited 2010-07-10 05:19
    Per my original post, the end of the last sentence should read "which isn't accurate enough for this application"!

    Thanks
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-07-10 06:20
    Due to the wavelength of ultrasonic wave propagation in air, the uncertainty in detecting an echoed wavefront is likely to exceed 1mm by quite a lot. Are you sure that another technology (e.g. optical) might not be a better choice?

    -Phil
  • roboticsrobotics Posts: 90
    edited 2010-07-10 06:39
    Hi Phil,

    Thanks for your reply.

    As the target medium may be reflective as well as of varying light absorbency, optical would be problematic.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-07-10 07:45
    What is the target, exactly?

    -Phil
  • Beau SchwabeBeau Schwabe Posts: 6,559
    edited 2010-07-11 07:24
    Could the target itself have anything placed on it?

    In principle, a simple closed loop audio feedback circuit could be used to determine distance if you could place the receiver on the target or arrange the transmitter and receiver in such a way that the sound bounces off of the target to the receiver.

    As an example: think of when the speaker (<-person) at the microphone gets to close to the loud speaker and you hear the shrill audio feedback. If you were to measure that shrill frequency, you would find that it is directly proportional to the distance between the microphone and the speaker.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • roboticsrobotics Posts: 90
    edited 2010-07-11 16:51
    Hi,

    The target medium would be different type of products including food products such as meat cuts and fish fillets.
  • roboticsrobotics Posts: 90
    edited 2010-07-11 17:14
    Hi Beau,

    Thank you for your reply.

    The optimum configuration would be for the ultrasonic receiver to be placed away (ie., non-contact) from the target medium. However, if I understand your post correctly, is it the case that whether the ultrasonic receiver is physically placed on the target medium OR not, that the effective distance gauging ability of the sensor can be improved by use of a closed loop audio feedback circuit?

    In advance, thank you.
  • Beau SchwabeBeau Schwabe Posts: 6,559
    edited 2010-07-11 17:37
    robotics,

    "if I understand your post correctly, is it the case that whether the ultrasonic receiver is physically placed on the target medium OR not, that the effective distance gauging ability of the sensor can be improved by use of a closed loop audio feedback circuit?"

    This would no longer be in the realm of ultrasonic but instead audio that you could hear. ...But you could pulse it (enable it) so that the high frequency squeal would be perceived as a few 'clicks' to a nearby observer rather than a constant irritating squeal.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-07-11 19:27
    From my own experience (measuring the heights of apples in a fruit-packing plant), I would have to say that deploying ultrasonics above a moving conveyor is going to be a problem. The difficulty I ran into was with the mechanical noise interfering with the distance measurements, along with the fact that I was trying to achieve a measurement precision on par with the ultrasonic wavelength, which just doesn't work.

    In another thread (scroll down the page), I showed how to measure distance using a laser line generator and a TSL1401 linescan sensor. Perhaps something like this could work for your app.

    Beau's idea sounds promising as well, so long as the mechanical noise from the conveyor doesn't cause interference.

    -Phil
  • roboticsrobotics Posts: 90
    edited 2010-07-11 19:33
    Hi Phil,

    I really appreciate your thoughts on this. Have a great day!
  • localrogerlocalroger Posts: 3,451
    edited 2010-07-11 20:37
    You're not going to be able to do that with normal ultrasonic transducers. The wavelength of 40 KHz sound waves (the range these sensors normally use) is on the order of 1/4 inch and your shortest distance requires return detection in a matter of only about 4 cycles. If you want to use the same transducer for sending and receiving you're going to have a lot of trouble quieting it down after sending the ping in time to receive the reply.

    It might be theoretically possible with some kind of piezo transducer working above 200 KHz but I'm not aware of any standard product that works like that.
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