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Need servo circuit help

Discussion in 'Electronic Design' started by Robert Wolcott, Nov 4, 2004.

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  1. I am part of a group at Oregon State University (Mechanical engineering) and
    we will be attending an international design competition next Sunday in
    Anaheim, CA. We will be competing with a robot we designed and constructed:

    We are currently having stability issues with the servo circuits we created
    that are based on a 556 timer IC. The circuits are very simple and they
    feed a signal to PWM motor controllers that drive small Speed 400 motors
    (robot drives exactly like a tank, with independent left and right speed
    control). This setup is extremely unstable and jumps all over the place.
    It also seems to wander (from neutral) as it warms up to operating
    temperature. When the drive electronics are working properly it is very
    easy to drive and control. Is there a better servo signal circuit out there
    that is more stable? What could be the cause of these problems we are
    having? The PWM controllers have a feature that calibrates them to max,
    min, and zero upon plugging into power. This wanders upon warming up

    Any help would be appreciated.

  2. A 556 is pretty stable and should work okay for this application. Did
    you bypass the control voltage to ground and put a good size (say 10uF
    in parallel with at least 0.1uF ceramic) bypass across the power
    supply for the timer chip?

    I hope you're using a film capacitor for the timing capacitor and not
    some horrible ceramic semiconductor type.

    Best regards,
    Spehro Pefhany
  3. Ban

    Ban Guest

    If the shoot-through of the bipolar timers is a problem you can maybe
    substitute them for the CMOS version (7555 or 7556). These behave much
    better and do not suck those current spikes when changing state. Otherwise
    Speff is right on point. Try to find out where the crosstalk comes from by
    watching critical points with the scope. If you use the dual timer, the
    second side can easily be triggered from the first one, here a good decouple
    cap is really needed. Better to use separate 7555 for each modulator.
  4. Terry Given

    Terry Given Guest

    all good stuff. Post your schematics, there are plenty of smart people
    at SED who will point out any obvious (and not-so-obvious) problems. The
    issues are probably a combination of circuit design (does it actually
    work under ideal conditions) and implementation (does it work properly
    in practice). How you build such a circuit, and where you place it, are
    quite important - a bad circuit layout, placed next to poorly wired
    motors (the motor leads should be tightly twisted together etc.) is a
    recipe for disaster.

    Nice robot though. Do you have an mpeg of it moving?

  5. Daniel Haude

    Daniel Haude Guest

    On Wed, 3 Nov 2004 23:00:07 -0800,
    That looks cute. What does it do? Grab something, I guess, and then?

  6. Tom Seim

    Tom Seim Guest

    Your servo system is unstable because you have too much loop gain.
    This gain drops as the motor warms up (copper has a positive tempco),
    causing it to fall within the stable range. You need to reduce your
    loop gain for it to be stable under all conditions.
  7. The circuit does work under ideal conditions but it tends to wander with
    time. I can't even find the original design but it is very basic. There is
    a voltage regulator with a 22uF electrolytic capacitor across it. This
    powers three servo circuits and I did use ceramic capacitors in the RC time
    constant portion. Can anyone supply me with a link to a decent circuit
    design? I'd like to redo these this weekend.

  8. Joerg

    Joerg Guest

    Hi Robert,
    I don't know how you want to do that without schematics. If you can see
    how the timers are done just jot it onto a piece of paper, take a
    digital camera shot of that schematic and post it.

    Anyway, as suggested before ceramics will drift. Replace them with good
    quality film caps. If you can't order any for the weekend just take the
    best you've got. Other erratic behavior can be caused by motor spikes.
    Place lots of bypass caps on the power supply lines of the timers.
    Ceramics are fine here, and actually preferred. Maybe a few 0.1uF as
    close to the chips as possible and 47uF or 100uF electrolytics next to them.

    Check the ground. If it's wires, oh boy. Then you need to beef that up
    with plenty of copper tape or at least some wide braid. If you have the
    time and enough parts rebuild the timers on experimenters board that has
    a ground plane, or better yet a ground and a power plane.

    The voltage regulator could also benefit from more caps, plus ceramic
    ones right at the pins. But be careful, some of the low drop-out variety
    can become unstable when there is too much capacitance on the output and
    not enough at the input. A little choke between battery and circuit
    board can't hurt either.

    Regards, Joerg
  9. Joerg,
    There is a picture of the servo circuit at this link:

    The batteries are on the robot and this is required by the rules. The
    control unit with the servo circuits is a gutted radio control box and this
    houses the servo circuits and other switches. So the power and return
    lines, as well as the signal lines all travel through an umbilical cord.
    The circuits are currently built on a prototype board (breadboard) that is
    epoxied in the RC box.

    My current plans:

    Redo the circuits on a circuit board (with solder)
    replace the ceramic caps with metal film caps (hard to find?)
    Add capacitors across the IC power inputs and VR inputs.

    Sound okay?

  10. Sorry, it's not that kind of servo, Tom.
  11. Thank you for the reply. Will the CMOS version of the 556 be enough of an
    improvement to warrant a digikey order?

    The servo gets its power directly from the battery pack so the servo circuit
    is only supplying the signal. I will go ahead and implement the other

  12. Working from the schematic in the book:

    1) It's best to use a separate supply for the timer
    and the servo. If that's not possible you may have
    to add a bunch of complexity. The ground current
    from the servo should not flow through the ground
    connection on the timer board (it should run
    directly to the servo supply).

    2) The CMOS version of the dual timer is preferable.
    Eg. TLC556CN Digikey: 296-1859-5-ND 1.04 1-off.

    3) Put another 0.01uF ceramic from pin 11 to ground.

    4) Decrease the 10uF to 0.1uF film You can use two
    BC 2222 370 11104 0.1uF/63V
    Digikey: BC1621-ND 0.98 for 10 pieces.

    5) Increase the 2.2K to 220K and the 150 ohms to 15K

    6) Put a 0.1uF ceramic across Vcc to GND near the chip.
  13. Terry Given

    Terry Given Guest

    very likely. The non-cmos version draws a huge current spike every time
    the output switches. This can (and often does) cause all sorts of
    problems, and is the main reason for placing a decent cap across the
    power supply pins.
    see below
    What Spehro specifically means is that the servo Vcc and 0V wires should
    go directly to the battery pack, and nowhere else. The Servo Vcc & 0V
    wires should also be twisted together - one twist per inch is fine.

    Likewise a separate pair of twisted wires for Vcc and 0V from the timer
    board to the battery.

  14. Add a 100uF electrolytic across the power supply (parallel with the
    0.1uF) and the bipolar type should be fine.

    But if you're ordering the film capacitors anyway, get the CMOS 556
    for the extra buck.
    Sounds good. Let us know how it turns out.

    Best regards,

    Best regards,
    Spehro Pefhany
  15. Joe

    Joe Guest

    In additiion to the missing connection listed by Sphero, I also noticed
    that pin 8 (timer 2 - trigger) wasn't on the schematic in the book. For
    astable operation, pin 8 is typically connected to pin 12 (timer 2 -
  16. Tom Seim

    Tom Seim Guest

    So you're saying it's not a servo system at all-that it's open loop.
    If that is the case then they ought to consider closing the loop.
  17. No, the loop is already closed inside the "servo" black box. This is
    an RC hobby type device. His circuit is suppling a pulse-width
    modulated signal as the setpoint input for the "servo". So, his
    problem is exactly that the pulse width is not stable.

    The stability of the repetition rate is not important for this kind of
    device- it's designed for multi-channel radio use where each channel
    gets refreshed with a pulse periodically.
  18. Pooh Bear

    Pooh Bear Guest

    And nice big wide ground traces. Avoid a common path for motor current and
    control circuitry.
    Who said 'metal film caps' ?

    They said 'film caps' meaning metallised plastic film. Polyester ( mylar ) film
    types will be fine. These are so easy to find it's untrue.

    They have stable capacitance unlike certain ceramic types.
    This is called 'supply decoupling' and helps reduce the effect of noise that may
    couple to the power supply. Absence of this can cause erratic operation.

  19. Joerg

    Joerg Guest

    Hi Spehro, Hi Robert,
    Really nice would be a very low impedance ground through the chassis.
    But if anodized aluminum was used that may not be possible this late in
    the game.

    Separate supplies as you suggested and decoupling through twisted pair
    as Terry had suggested can be nicely done with CAT-5 cable. It looks
    neat that way and you can write on the gray jacket with a small black
    Sharpie pen so it is clear which cable goes where. There usually is a
    spool of CAT-5 somewhere. Just don't run too much current through it or
    double up pairs for that.
    And maybe slide a few ferrite beads on the VCC line coming in. Secure
    them with soft glue so they don't rattle. Small chokes of a few uH may
    also do.

    Regards, Joerg
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