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Feedback Servos — Parallax Forums

Feedback Servos

JwolfJwolf Posts: 74
edited 2019-04-03 12:33 in Robotics
I have a few analog servos, but their position relative to my settings keeps changing... i.e. I have it positioned horizontal, but overtime, the same analog value no longer represents horizontal.

Is there a servo with accurate positioning?
Right now I'm only needing 180deg movement, fast response preferred... I'm working on hovering and moving with thrust vectoring from a ducted fan.
Is something like a digital coreless what I'm looking for?

Comments

  • AwesomeCronkAwesomeCronk Posts: 1,055
    What microcontroller are you using?

    If it is a prop, then feedback 360 high speed servos are the ticket.

  • ercoerco Posts: 20,257
    If an analog servo's position changes, it is likely due to deterioration of its internal feedback potentiometer. Flying on an airplane may introduce harsh conditions: vibration, possibly shock loads, fuel (vs. electric ducted fan?), etc which may accelerate the problem.

    What type of servo deteriorated on you? Brand, size...
  • PublisonPublison Posts: 12,366
    The 360 servo will work with many MCU's, not just the Prop.
    Specifications

    RPM: +/-120 w/feedback control, 140 max (+/- 10) @ 6 V, no load
    Gears: POM
    Case: Nylon & fiberglass
    Spline: 25-tooth, 5.96 mm OD
    Peak stall torque @ 6 V: 2.2 kg-cm (30.5 oz-in)
    Voltage requirements: 6 VDC typical, 5–8.4 VDC max range*
    Current requirements: 15 mA (+/- 10) idle, 150 mA (+/- 40) no-load, 1200 mA stalled
    Control signal: PWM, 3–5 V 50 Hz, 1280–1720 µs
    Control signal zero-speed deadband: 1480–1520 µs (+/- 10)
    Feedback sensor: Hall effect
    Feedback signal: PWM, 3.3V, 910 Hz, 2.7–97.1% duty cycle
    Product weight: 1.4 oz ( 40 g)
    Cable length: ~ 9.8 in (250 mm)
    Dimensions: approx. 2.15 x 1.46 x 0.79 in (50.4 x 37.2 x 20 mm)
    Mounting hole spacing: 10 x 49.5 mm on center
    Operating temperature range: 5 to 158 °F (-15 to +70 °C)
    *5 VDC is absolute minimum required for no-load angular position control. 5.8 to 8 VDC is recommended for continuous rotation speed control.
  • JwolfJwolf Posts: 74
    edited 2019-03-07 19:10
    yes its for the propeller.
    the 360 servo looks great, a bit expensive and probably too large, but it has good features

    So for an analog servo, a feedback pin is required for positional feedback?
    What about digital servos... I suppose they need an ADC inside the servo, but do they have position feedback as a standard by chance?
  • iseriesiseries Posts: 1,496
    The 360 servo is nice but maybe all you need is a standard digital server that you can position.

    Standard Servo

    This servo just requires the correct PWM output to position the servo. No feed back is required. At 1500 PWM it is at 90 degrees and will position nicely between 0 and 180 every time.

    It is a rather large servo so you may want to just use a small one from hobbyking like HXT900.

    This unit does not have metal gears but works just like the parallax model just much smaller.

    Mike


  • JwolfJwolf Posts: 74
    edited 2019-03-08 07:48
    so generally a pulse rate should always represent the same position on an analog servo?
    This is my first time learning how to get accurate positioning on a servo... I need them to start at the same spot every time so that I can apply a certain amount of 'flap' degree(s).
    I'm guessing this is the reason no special 'feedback' servo is needed for general RC aircraft... otherwise the wing flaps would have to be re-calibrated before every flight, ya?

    I bought a pair of servos on ebay: "Emax ES08MAII Analog Servo"
    Using OBEX : http://obex.parallax.com/object/497
    I chose this obex because it has ASM drivers for up to 4 servos... however, every attempt so far to exceed 180deg rotation, results in the servo going crazy with weird jitters then a short bust of smooth movement followed by jitters and the servo repeats in a never ending rotation of jitters and smooth bursts... it wont stop doing that until I set the pwm above 45k or below 190k... (I did adjust the pulse low time from 10ms to 20ms, but it didnt make any diff).
    With that obex, I have to set the pwm between 190k & 45k, which represents only about 160deg movement, or else the servos go wild.
    So this is not working out... need new obex and servos?
  • iseriesiseries Posts: 1,496
    Looks like the reviews of that servo show it does not have a 180 degree travel length. Most servos do.

    Mike
  • kwinnkwinn Posts: 8,697
    Jwolf wrote: »
    so generally a pulse rate should always represent the same position on an analog servo?
    ....

    Not quite. It is the pulse width that determines the position of the servo, or the RPM if it is a continuous rotation servo. The pulse rate is usually about 50Hz
  • ercoerco Posts: 20,257
    Sounds like something else is the problem here. Analog servos have been working reliably for position control since the early 1970s.

    Jwolf, have you tested your servos with a dollar servo tester yet? Or a premade RC system?
  • W9GFOW9GFO Posts: 4,010
    Jwolf wrote: »
    What about digital servos... I suppose they need an ADC inside the servo, but do they have position feedback as a standard by chance?
    All servos have internal position feedback.

    A "digital" servo only means that there is a motor driver inside the servo that constantly feeds power to the motor. It has nothing to do with positioning feedback.

    An analog servo sends power to the motor at the same rate as the pwm signal. If the analog servo is in position then the motor gets no power. An analog servo will consume less power than a digital but it will not hold its position as firmly as a digital servo.


  • JwolfJwolf Posts: 74
    no I have only used these servos with the prop... got them to remotely flip a switch, which was accomplished and these were leftover servos.

    tried another obex, still get about 160deg, that is im guessing the range for these...
    although i was able to make one go in continuous circles, like 1080deg atleast... but it just rotates continuously instead of holding a position as I tried to set.
  • Mark_TMark_T Posts: 1,981
    Digital servos use a microcontroller internally, so their accuracy is determined by the internal RC oscillator
    of the microcontroller, rather than discrete analog components. This means they could be worse or better
    for position drift, depending on the specs of relevant components.

    Accuracy of a servo depends both on the position feedback pot and on the accuracy of converting incoming
    pulse-widths to set-point.
  • ercoerco Posts: 20,257
    I just got Feetech's 2019 spreadsheet with several interesting new servos. Excel screenshot attached. (Don't get too excited over the wholesale prices listed, there's no quantity discount and they charge lots for shipping from China). Several servos rotate 270-320 degrees with a magnetic position feedback line, much like Parallax's own Feedback 360 servo.

    screenshot.jpg
    1300 x 940 - 505K
  • The_MasterThe_Master Posts: 200
    erco wrote: »
    I just got Feetech's 2019 spreadsheet with several interesting new servos. Excel screenshot attached. (Don't get too excited over the wholesale prices listed, there's no quantity discount and they charge lots for shipping from China). Several servos rotate 270-320 degrees with a magnetic position feedback line, much like Parallax's own Feedback 360 servo.

    They have a regular servo that will spin 300 deg! And I found some such servos for sale on Amazon, too
  • ercoerco Posts: 20,257
    If you like 300 degrees of travel, you'll love 1260 (3.5 revolutions). No likey the $42 price though.

    https://www.amazon.com/Hitec-RCD-33785S-HS-785HB-Winch/dp/B000BOGI7E

    Search for sail which servos, which typically rotate several turns.
  • JwolfJwolf Posts: 74
    edited 2019-04-03 12:42
    Most of those Feetech servos are 320deg, and for the price... wow! nice find!

    That 1260 servo is made, I think, specifically for the Ragazza 1 Meter Sailboat to raise the sail.
    cool


  • The_MasterThe_Master Posts: 200
    edited 2019-04-03 21:15
    Jwolf wrote: »
    Most of those Feetech servos are 320deg

    Do you specifically know this as a fact? Because the sheet only showed a couple that do 320, both of which are rare/unavailable.
  • JonnyMacJonnyMac Posts: 9,159
    edited 2019-04-08 22:54
    Is there a servo with accurate positioning?
    I've been playing with the LX-16A -- it's like a lower-cost clone of the Dynamixel. It doesn't have the same feature set, but it is easy to program and the version with metal gears is about $15 on Amazon. I am writing a full driver in Spin because I have friends that want to use these in special effects projects. In servo mode, the rotation is 240 degrees with 0.24 degree resolution. Yes, it does have a motor mode. I'll post my code when it's complete.

    https://www.amazon.com/gp/product/B0748BQ49M
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