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Range-finding ultrasonic / infrared

Discussion in 'Electronic Basics' started by Danny T, Jan 3, 2005.

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  1. Danny T

    Danny T Guest

    What's the best (cheapest, quickest, easiest) way to detect obstacles
    for a small robot? The book I have starts with infrared, and later
    replaces them with ultrasonic. Infrared is way cheaper, but I don't full
    understand how it works..

    I've got some phototransistors here, that seem to work exaclty like
    LDRs, they just change the resistance with the light - except they also
    work with infrared (as well as normal light). I've seen infrared LEDs,
    and infrared phototransistors - am I right in thinking these are simple
    filtered to only detect IR, instead of normal light?

    Would these be accurate enough to be able to get a range, or would it
    simple be a yes/no if something's there? Ideally, I want to be able to
    detect a) if there's anything within, say, 6 inches, and also if there's
    anything within, say, 3 inches. That way, as things get near, I'll start
    turning my robot, but if it doesn't turn quick enough, it'll increase
    the turn when it gets closer.

  2. You can buy units that are designed as IR distance sensors. They give
    you either a voltage (which you can read with the comparator or a/d) or
    a binary output. The Sharp GP2D120 comes to mind. Here is a datasheet:

    You can get them at the various robotics webstores. However, they are
    cheaper at Arrow electronics. Make sure you get some JST connectors,
    because they are a bit difficult to run without them. Acroname sells
    them with the connectors and wires for $12.50 plus shipping:

    I've built robots with these before, and they work pretty well. One
    issue they have is that they are *really* directional. This is an issue
    when you are trying to detect obstacles, and mounting becomes critical.
    That is one thing to say for ultrasonics.

    You can also just use a bump sensor, like the roomba.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  3. Danny T

    Danny T Guest

    Sounds good, but Acroname don't have visible shipping costs for the UK!
    Cheapest I've found here is:

    But I'll have a look around. I'll probably need 2 or 3, but looks like
    they'll do the job! :)

    There's something more elegant about a robot that moves around without
    hitting things... Imagine seeing people walking around an office by
    bumping off walls ;-)
  4. Unfortunately, it would be like trying to navigate with tunnel vision.
    All you can tell is the distance to something, nothing else. That makes
    it difficult to build mental models of what's out there. Robots without
    vision processing are somewhat like flatlanders, living in a 2d world of
    walls. They tend to crash into walls, chair legs, and feet even with the
    IR sensors. Thus, bump sensors are fairly important for keeping the
    robot from getting stuck.

    One thing that is often done is to to measure the current through the
    motors. That way, you can use a sudden increase as a signal that you've
    hit something, and the motor has stalled. I was watching a roomba a few
    days ago, and it appears to use that strategy. It was trying to climb
    onto a persian rug that was too high for it, but which was lower than
    it's bump sensor. It got a bit onto it, but then got stuck, noticed
    this, and backed off. Not too bad. It also has wheels that can bob up
    and down, which helps with this kind of thing.

    You can notice increasing current by using a small (like a 1 ohm)
    resistor, in series with the motor, and by watching the voltage across
    it. By smoothing out the voltage spikes with a filter, you can keep
    track of the general trend of current. If it goes beyond a certain
    point, you are pretty sure that the motor is stalling.

    Unfortunately, with your driver circuit, you don't have the ability to
    back up. For that, you need a way to reverse the current through the
    motor. That is often done with an H-Bridge (or a 1/2 H). There are
    dedicated chips to handle this kind of thing (see the texas instruments
    SN754410, for example). You can also control the speed using PWM, which
    just means turning it on and off, and adjusting the amount of time it's
    on to control the speed of the motor. 'PWM' stands for Pulse Width
    Modulation. Some new PIC 16F chips have PWM built in, but only a single
    channel, which means you need two processors, working in tandem, to use
    this feature (which is pretty much useless). However, doing PWM in an
    interrupt routine is almost trivial, if the frequency is low.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
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