Shop OBEX P1 Docs P2 Docs Learn Events
Servos & Sweep angle — Parallax Forums

Servos & Sweep angle

PJAllenPJAllen BannedPosts: 5,065
edited 2006-07-05 19:47 in Robotics
OK, I have two standard servos (Parallax units.)
I have noticed that they sweep different angles given the same program (one does 180deg and the other <180, say 170deg.)
Also, one is at full stop with 2400 and the other at 2500.

Should they be more uniform, unit to unit?· Is there an adjustment?· [noparse][[/noparse]I have what's maybe an older doc, showing an adj. pot., but nonesuch on these two.]

This is my test program:
DO
 
  value = 2200
  FOR x = 0 TO 30
    PULSOUT 14, (value/2)
    PAUSE 15
    NEXT
  PAUSE 1000
 
  value = 400
  FOR x = 0 TO 30
    PULSOUT 14, (value/2)
    PAUSE 15
    NEXT
  PAUSE 1000
 
LOOP

Comments

  • Ryan ClarkeRyan Clarke Posts: 738
    edited 2006-07-05 14:36
    The Parallax servos are Futabas. Servos are only as accurate as the parts used to build them-

    First and foremost you should center your servos by removing the horns, and sending a centering pulse (1.5 ms) to both, then, while that pulse is being sent, reattach the servo horns (this is physical tuning of the servos)- after that you can determine experimentally how to fine tune in code. This is common practice when dealing with hobby servos.

    Ryan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Ryan Clarke
    Parallax Tech Support

    RClarke@Parallax.com
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-07-05 14:55
    ··There're some white nylon gears in these, I see.· Is the "horn" the black spinner part?· The nylon part that attaches to is fit on a stud or pinion or whatever it is, but it has a·flatted shaft.
    ··Do you have a link to a tutorial on the parts like?·
  • Ryan ClarkeRyan Clarke Posts: 738
    edited 2006-07-05 15:02
    You do not have to open the servo to get to the horn. The horn is the part that attaches (via a little black screw) on the outside. It generally looks like a cross:

    attachment.php?attachmentid=42354

    Ryan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Ryan Clarke
    Parallax Tech Support

    RClarke@Parallax.com
    servo-horns.gif
    500 x 285 - 20K
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-07-05 19:34
    Note that these Servo's are made by Futaba to control airplane surfaces via a radio remote-control, with a joystick, from the ground. They expect a person to be on the ground, getting feedback about how the plane is flying by watching it.

    Thus there is no requirement for each servo to move exactly the same amount for exactly the same control input. As long as they're "approximately" alike, it serves the purpose of the guy on the ground. Now, when we put them in robots, it would be awfully nice if they DID work exactly alike.

    If they're not "identical" enough for you, you'll have to put in some 'constants' so you can more easily 'tweak' your positioning routines.

    Like: MaxPos CON 2000

    Then, you can run your program, and test it. Then you can modify the MaxPos value:
    MaxPos CON 1980
    or
    MaxPos CON 2020

    to match the particular servo you've got installed.

    An alternative in more recent servos is to tweak the feedback resistor, so that you get "full scale" at 2500 on both servos.· That may or may not be practical -- it probably depends on the manufacturer of the servo.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-07-05 19:47
    · Yes, thanks, guys.· Didn't know what my expectations of them should be.· Like I said, I have what I presume is an older parallax(futaba) doc; it shows a centering/adj pot, but my units (fairly recent purchases) are without.

    · No worries, lives are not at stake, so I will compensate through programming (imagine.)
Sign In or Register to comment.