Shop OBEX P1 Docs P2 Docs Learn Events
Make: Drawbot — Parallax Forums

Make: Drawbot

ercoerco Posts: 20,257
edited 2014-05-23 12:54 in Robotics
Just saw this 2-servo drawing bot in Make magazine (volume 39, inside cover). No electronics, driven by a stereo audio signal. Very creative thinking, plus they used Parallax servos: http://makezine.com/projects/trs-drawbot

Mechanically, somewhat similar to my 3-servo BS2 printer: http://forums.parallax.com/showthread.php/139146-Robot-Arm-Printer-Project

Comments

  • Duane DegnDuane Degn Posts: 10,588
    edited 2014-05-21 15:26
    erco wrote: »
    No electronics, driven by a stereo audio signal

    I think that should be "using a stereo jack as a connector". The signal is still just normal servo pulses. One servo has the signal coming in over the right line and the other is using the left line.

    It uses a separate power line.

    IMO, the changed connectors just make the project harder than it needs to be. They're using two connections anyway why not just use the normal servo connections and not have to change the connector on the servos?

    IMO (again) your 3-servo BS2 printer is a huge step up from the Make arm. You didn't even have to change the servo connectors.

    Maybe they couldn't find any panel mount servo connectors and that's why they went through the trouble of adding unneeded connectors?

    Edit: I likely didn't understand the article (I just skimmed it.) I'll make sure and read it carefully before I offer another opinion about it. Thanks Rich.

    Edit again: Wow! I was way off. Sorry erco and other readers of my off base ramblings. I should read things more carefully before criticizing them.
  • jazzedjazzed Posts: 11,803
    edited 2014-05-21 15:47

    One man's faulty interpretation can still be a great idea!

    FCC approved R/C kind of works like that no?
  • W9GFOW9GFO Posts: 4,010
    edited 2014-05-21 18:41
    Duane Degn wrote: »
    I think that should be "using a stereo jack as a connector". The signal is still just normal servo pulses.

    Did you read this part?
    This trick works the other way, too: Plug a servo into an audio output (like a headphone jack), then play the right sound, and you can control the servo’s position directly, without a receiver. A “mono” signal can control a single servo, and a stereo feed can control two — one on each channel.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2014-05-21 18:44
    W9GFO wrote: »
    Did you read this part?

    No I didn't. I better reread the article.

    Thanks.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2014-05-21 18:55
    I'm embarrassed by how far I missed the point of the article.

    Thanks for the correction Rich. Thanks for posting the interesting article erco.

    There were a couple of things that happened today to make me wonder about my cognitive abilities. Spring fever? I like that excuse better than senility.
  • ercoerco Posts: 20,257
    edited 2014-05-21 19:45
    I was wondering why you hated it so much so quickly. Spring fever it must be! :)
  • mindrobotsmindrobots Posts: 6,506
    edited 2014-05-22 07:37
    So if you apply this to a rolling robot then with some onboard analog circuitry, you could have a robot that responded to sound. Either a remote using sonic or ultrasonic tones for forward, back, left, right or an autonomous robot that would try to get as close as it could to the mellow tones of your voice or go hide when it heard the shriek of a clarinet(just an example). Interesting way to control an analog-bot. It sure would be an interesting exercise in analog electronics.....how much can it do without a digital brain??
  • ercoerco Posts: 20,257
    edited 2014-05-22 08:55
    I don't think it's about tones, as the servo refresh frequency is always 50 hz. Driving two servos, you'd get two variable-width pulses each 20 ms, so it would sound something like 100 hz (AC hum is 60 hz for frequency reference). So it's square waves and the frequency is constant but the pulse width/duty cycle changes, prolly sounds like you're filtering the audio output. Worth listening to (or Youtubing, who'll do it first?) once but maybe no Grammy award this year.
  • Martin_HMartin_H Posts: 4,051
    edited 2014-05-22 19:04
    That's neat. I never thought about servo control pulses as an audio signal, but that's certainly one way to look at them.
  • WhitWhit Posts: 4,191
    edited 2014-05-23 06:32
    Cool project - Thanks for posting erco! (had forgotten about your printer too!)
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2014-05-23 09:13
    Analog servos can be driven at most any frame rate from 1 to around 300 Hz; the frame rate affects the power delivered to the servo, and not its control position. Above around 300 Hz (depends on servo), the next pulse comes in too soon and the internal RC network that creates the error signal can't compensate fast enough. The servo becomes jerky and unpredictable.

    For the DrawBot it's all about the "right kind" of audio signal that makes the difference. Electronics still control the servos, in this case a smart phone or other computer running software that generates the tones.

    In days long ago, a homebrew laser light show technique used R/C servos and a 556 timer. An audio signal is fed into the voltage pin of one half of the 556. The chip is wired to produce reasonably good 1-2 ms pulses at the desired 50 Hz frame rate, and the servo turns back and forth to the music. On a robot, music is too random to produce anything but a drunky bot, but it creates interesting effects when using mirrors mounted on X/Y servos and a laser. I included this project in an book I did back in the late 80s, and the idea was old even then. Not that old is bad, right Erco?

    The SVG-reading software used by the Drawbot is a super-duper cool thing. That ought to win a medal right there!
  • ercoerco Posts: 20,257
    edited 2014-05-23 12:10
    I see lots of simple "servo tester" circuits made from a pair of transistors or a 555 timer. This one, for instance.

    Most of these simple circuits can't possibly deliver a fixed pulse rate independent of the pulsewidth. They produce a 1-2 ms pulse but frankly my dear they don't give a damn about the 20 ms pause (ie 50 hz refresh rate) and somehow the servos survive. I assume they work. Here's one which says the off time is 40 ms. Hopefully old (!) analog servos don't mind the wait.

    I always heard that the rule was it was OK to send more pulses (faster than 50 hz) but not less.
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2014-05-23 12:39
    Less is okay for testing a servo. It just won't have much ooomph.

    The single-555 circuits can work okay as long as the pulse train is inverted, usually done with a diode, as is the case with the first circuit you pointed to. I've build a couple of these, and they work okay. The dual 555 or 556 offers a little more control.
  • ercoerco Posts: 20,257
    edited 2014-05-23 12:54
    That diode is a slick way to get to get a duty cycle of less than 50%. Halfway down the page at http://electronicsclub.info/555timer.htm . Love those hardware tricks.
Sign In or Register to comment.