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Centering a futuba servo — Parallax Forums

Centering a futuba servo

MarkCrCoMarkCrCo Posts: 89
in BASIC Stamp
I needed more torque than the parallax continuous rotation servo so I modified a futuba high torque servo for continuous rotation ( a surprisingly easy task, about 2 minutes). Any way using the pulsout on a parallax a value of 750 is stopped with a little fine tuning from the centering screw. Futuba has no centering adjustment and the stop point seems to vary from servo to servo. Any suggestions other than modifying to code depending on which servo I have plugged in at the time?

Comments

  • PublisonPublison Posts: 12,366
    You will need to provide the Futaba servo part number to provide any answers.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2016-08-30 00:45
    What did you do with the original pot in the servo?

    On many of my CR servos I use the original pots to center the servo.

    Here's a photo of a couple CR servos with the pots hanging outside the servo body.

    95914.jpg

    Alternatively a multi-turn trimpot can be used to set the center position.

    attachment.php?attachmentid=89301&d=1328412621

    I used 5K trimpots but erco uses 10K and says they work just fine. The 10K versions are a bit easier to find than the 5K trimpots.

    I wasn't ambitious enough to install the trimpot inside the servo. There's usually enough room inside the servo body to mount the trimpot inside.

    If you used two resistors to center the servo then you'll need to make the center adjustments in software.
  • MarkCrCoMarkCrCo Posts: 89
    S3010 is the servo. The pot is still in it I just took out the plastic coupler from inside the gear to disengage it.
    cdn.instructables.com/F54/2J2O/FQWVUYNJ/F542J2OFQWVUYNJ.LARGE.jpg
  • Duane DegnDuane Degn Posts: 10,588
    MarkCrCo wrote: »
    S3010 is the servo. The pot is still in it I just took out the plastic coupler from inside the gear to disengage it.

    It's usually not hard to access the pot. Many servos have a single screw holding the pot in place but some servos have plasitic clips which hold the pot. A lot of servos have wires between the pot and the PCB. In these servos you can move the servo to the outside of the servo body.

    There's a (rambling) video embedded in this post. The video eventual tells you how to remove a servo from its original position.

  • MarkCrCoMarkCrCo Posts: 89
    Since I'm a programmer whose formal electronics training stopped at physics 101, I sometimes miss the obvious when it comes to electronics. I was thinking that since I had disengaged the pot from the gears the pot was irrelevant. Now I see that the pot in continuous rotation mode is setting the resistance to determine what pulse is full stop etc. I should have just set the pot to the correct position so that stop was at the point I wanted before I closed the servo back up. No big deal I opened it back up attached it to a stamp sending it my stop pulse, set the pot with some needle nose and viola. Just what I wanted!!!!
  • MarkCrCoMarkCrCo Posts: 89
    Ok next problem. Got it working except most of the time it draws to much power to run two servos from the controller. I need to supply them with a separate power source which should just require that I set up a common ground for controlling them (Right?). My next concern is do I need to put in a resistor so that I don't over power the servos or is that something I don't have to worry about? I'm going to power them with 4 rechargeable AA batteries (4.8V).
  • Duane DegnDuane Degn Posts: 10,588
    MarkCrCo wrote: »
    I need to supply them with a separate power source which should just require that I set up a common ground for controlling them (Right?)
    Correct. You need to make sure the Propeller and the servo share a common ground connection.

    MarkCrCo wrote: »
    My next concern is do I need to put in a resistor so that I don't over power the servos or is that something I don't have to worry about?
    You don't need (or want) a resistor on the power line. The servos will only draw the current they need.

    Some people suggest you use a resistor on the signal line. I personally don't do this and I know from experience a resistor on the signal line can cause problems where the Propeller's 3.3V logic doesn't communicate with the servo if there's a resistor in series. This is generally only a problem is the resistor value is too high (10K is too high). I've read from a reliable source (Beau Schwabe) a resistor can reduce noise in some situations (like when long wires are used).

    MarkCrCo wrote: »
    I'm going to power them with 4 rechargeable AA batteries (4.8V).
    This should work fine. You can also safely use 5 rechargeable batteries (NiMH or NiCd).

    There are some servos which tolerate two lithium ion cells (8.4V when freshly charged). I also know from experience some servos will go up in smoke with two Li-Ion (or LiPo) cells.

    Here are some suggestions I wrote about connecting servos to a Propeller.

    If you want your robot to move in a repeatable manner, you may need to use a regulated supply. I tried to get my cheap bot to perform a figure 8 without any luck until I switched to a regulated power supply. Here are some comments I wrote about getting the robot to drive in a consistent manner. There's a video of the robot in action in the top post of the thread.
  • MarkCrCoMarkCrCo Posts: 89
    edited 2016-09-02 02:01
    Things are going well. The separate power supply has helped tremendously. It could use the additional power of going from 4.8 to 6 volts though. I hope to eventually have a marketable product out of this. So here is the problem. What if Goof E. Consumer decides to ignore my warning to use only rechargeable 1.2 volt batteries and instead uses his 1.5 volt batteries he got at the dollar store. This will mean the servos will be operating at 7.5 volts instead of the max recommended voltage of 6.
    Will he:
    A.) have a product with a shorter life expectancy
    B.) have an immediately non-functioning product
    or
    C.) set his house on fire
  • Duane DegnDuane Degn Posts: 10,588
    MarkCrCo wrote: »
    So here is the problem. What if Goof E. Consumer decides to ignore my warning to use only rechargeable 1.2 volt batteries and instead uses his 1.5 volt batteries he got at the dollar store.

    I don't know what would happen.

    I've had some servo release smoke when connected to too high of voltage while other just didn't work well at the wrong voltage.

  • ercoerco Posts: 20,256
    I have found that the null position can vary with servo supply voltage, so having a stable power source is important. It's maddening when a servo drifts from the null point. The sensitivity/deadband varies between servos and brands.

    I was impressed at the exceptionally wide deadband of the motor driver I just posted about at http://forums.parallax.com/discussion/164984/neat-little-dual-motor-controller
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