Page 1 of 3 123 LastLast
Results 1 to 10 of 27

Thread: Servo for camera follow focus system

  1. #1
    Join Date
    Mar 2009
    Location
    Phnom Penh, Cambodia
    Posts
    8
    Rep Power
    32

    Servo for camera follow focus system

    Hi, it's my first post. I'm an independant filmmaker and build all my own camera rigs.

    I've constructed a remote follow focus using R/C equipment. It includes a Hitec 785 boat winch servo and Futaba two channel transmitter with a dial.

    A large gear goes around one of various camera lens and is driven by a smaller gear that is attached to the servo hub. The aim is to be able to set two focus points (lens positions) and switch back and forth between them by rotating the lens using the transmittor's dial. Sometimes the lens has to rotate more than halfway around, so a servo with maximum rotation such as the 785 is needed.

    The system works except that the servo is slightly overshooting the mark and then oscillating. With high definition video this is a no-no. The servo needs to stop exactly where I expect it to with no oscillation so I can acheive critical focus and maintain it until the shot is over.

    It's been suggested that Trossen's Robotis AX-12 servo would be a better choice. Are these digital servos controlled using standard R/C equipment or is additional hardware and programming skills needed? Does this servo have the accuracy I'm looking for and the torque needed to turn the lenses?

    Martin

  2. #2
    Join Date
    Dec 2007
    Location
    Portland, OR
    Posts
    3,198
    Images
    952
    Rep Power
    139

    Re: Servo for camera follow focus system

    Welcome to the forums!

    Do you know exactly how much rotation you need on the servo itself?

    Oscillation can be caused by a couple of things, noise on the signal, noise on the RF link, over torqueing the servo (I doubt this is the case as it doesn't sound like a high torque app), etc. It could also be the pot itself getting a bad read internally on the servo, have you tried replacing the servo itself?

    The AX-12s have a 300 degree positional rotation and are pretty high torque, however they would not be compatible with your receiver directly (with uses PWM, AX-12s use TTL serial). You would have to implement a microcontroller as an in-between. The micro would read the PWM output from the receiver and output the proper TTL serial string to communicate with the AX-12.

  3. #3
    Join Date
    Mar 2009
    Location
    Phnom Penh, Cambodia
    Posts
    8
    Rep Power
    32

    Re: Servo for camera follow focus system

    Thank you Andrew, and thanks for allowing me to benefit from the knowledge here. I've attached a photo so you can more easily visualize the app in its current form.

    Obviously the larger the gear that is attached directly to the servo, the less rotation I need. But there is a limit to how large I can go, and the new gear will probably have to be custom made. But that's okay.

    Let's assume I achieve a better than 1:1 gear ratio between these two gears. The amount of rotation I need for a given shot then depends on the lens I am using and the length of a given focus change. Let's assume that 300 degrees is enough when using the following set-up methodology. The servo is disattached (swung away from) the lens. The lens is rotated to the counterclockwise extreme of the focus change and the gears are re-meshed. At this point the servo should also be moved to its counterclockwise limit, and this position would be marked on the transmitter dial. Now the servo is ready to move in the other direction. To set the second focus point it is rotated until that part of the image is in focus, and another mark is made on the transmitter dial. (These two marks are presumably within the rotational limits of the servo.) It must now be possible to move back and forth between them with good speed and a very high degree of accuracy.

    In answer to your question, no I have not tried another servo as this is the only one I have. The tech person at the vendor (to whom I also put this question) suggested another model with a smaller range of movement might be more accurate with less oscillation. Unfortunately here in Phhnom Penh there are no R/C hobby shops, so that complicates my R&D efforts.

    What is the advantage in using this actuator/servo? It is highly accurate? What is involved in creating the microcontroller solution you said is necessary? Does it mean programming and burning a chip and integrating it into a circuit with connectors etc? That's beyond my capabilities. However if I found someone to do it, would this then be compatible with a standard R/C transmittor and capable of the accuracy needed?

    Many of my colleagues in the world of DIY camera rig building will be following the results of this effort. The professional quality remote follow focus systems now on the market are beyond the means of many. I might be able to interest people in a group effort.

    Martin

  4. #4
    Join Date
    Apr 2008
    Location
    Sacramento, CA, USA Area
    Posts
    5,341
    Rep Power
    174

    Re: Servo for camera follow focus system

    Welcome to the forum!

    The AX12 is not meant to be R/C controlled. Could you design a circuit and programming to build a PWM to ttl serial control for it? Sure. But I wouldn't go there myself. R/C, especially that car R/C controller, just isn't meant for this.

    Myself, I'd use something like the AX12 interfaced to something like the Arduino, then use a serial trackball or variable resistor to control it. Something very high resolution. Even a variable resistor (as a "dial") would give you 1024 steps from the 10bit ADC on the Arduino.
    Last edited by Adrenalynn; 03-12-2009 at 10:24 PM.
    I Void Warranties�

  5. #5
    Join Date
    Mar 2009
    Location
    Phnom Penh, Cambodia
    Posts
    8
    Rep Power
    32

    Re: Servo for camera follow focus system

    Thanks. Ideally what we need however is a wireless solution. The follow focus will be used on devices such as a steadicam, camera crane, dolly, camera rigs that are moving around and that the focus puller needs to be able to control without being tethered to them. Can an Arduino project work wirelessly?

    Martin

  6. #6
    Join Date
    Dec 2007
    Location
    Portland, OR
    Posts
    3,198
    Images
    952
    Rep Power
    139

    Re: Servo for camera follow focus system

    What sort of radio are you using? Its entirely possible you're running into RF interference.

  7. #7
    Join Date
    Apr 2008
    Location
    Sacramento, CA, USA Area
    Posts
    5,341
    Rep Power
    174

    Re: Servo for camera follow focus system

    I fly a Steadicam, and am certified (and used to have my SAG card). I can relate to your requirements for pulling focus.

    I have an Arduino working with XBee wireless. You can literally get miles away with the right choice.

    Tyberius and Fergs have an Arduino-like board with built-in XBee wireless.

    Understand though that the project would still require programming. You need to bit-bang serial out to the servos from some DIO pins. And you'd probably be needing two arduinos. One for input/transmitter, one for output/receiver.

    That said, my Arduino -> SSC-32 wireless arm controller (soon to be published in SERVO with a build article) is basically the same idea.
    I Void Warranties�

  8. #8
    Join Date
    Apr 2008
    Location
    Sacramento, CA, USA Area
    Posts
    5,341
    Rep Power
    174

    Re: Servo for camera follow focus system

    The car R/C TX are notoriously inaccurate anyway, unless you have some like super-honking-butt-kicking Airtronics or something.
    I Void Warranties�

  9. #9
    Join Date
    May 2008
    Location
    TN
    Posts
    853
    Images
    99
    Rep Power
    71

    Re: Servo for camera follow focus system

    The Arduino has some libraries already put together for servos. Check out www.arduino.cc for more info.
    "If A is a success in life, then A equals x plus y plus z. Work is x; y is play; and z is keeping your mouth shut."
    -
    Einstein

    Don't be a HelpVampire

  10. #10
    Join Date
    Mar 2009
    Location
    Phnom Penh, Cambodia
    Posts
    8
    Rep Power
    32

    Re: Servo for camera follow focus system

    Will your build article include a parts list and tutorial? I can use a soldering iron and have done a bit of programming, but I would need to get on a new learning curve for this, and don't have any hardware for burning chips etc. I don't think I want, nor do I have the time to go to this level.

    I don't know what XBee wireless is.

    Am I likely to be able to find someone who can build this (or some of the elements) without paying a fortune?

    When you mentioned 1024 steps from the pot, is that the answer to my question about accuracy?

    When you say R/C TX are notoriously inaccurate are you talking about the transmitter? Does that imply that a wired in controller with perhaps a better R/C servo would produce a better result?

    Martin

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Question(s) Writing an educatonal book on robotics
    By darkback2 in forum Robotics General Discussion
    Replies: 107
    Last Post: 05-24-2010, 11:51 PM
  2. RC & Serial Servo Controller Kit
    By Al1970 in forum Garage Sale
    Replies: 0
    Last Post: 01-10-2009, 12:39 AM
  3. Question(s) Phidgets and servo voltage
    By Eric in forum Arbotix, Microcontrollers, Arduino
    Replies: 5
    Last Post: 05-12-2008, 08:32 AM
  4. Kondo web page translated.
    By Droid Works in forum Humanoids, Walkers & Crawlers
    Replies: 4
    Last Post: 10-06-2007, 05:49 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •