Deciding the position of rotary platform?

Discussion in 'The Projects Forum' started by jackua, Nov 1, 2010.

  1. jackua

    Thread Starter New Member

    Oct 27, 2010
    7
    0
    I want to measure the position of my rotary platform accurately. I am not interested in its speed but only its position with my reference (0 degree) point while I move my platform onwards and backwards by a step motor. I checked the internet and find that a rotary encoder would be very helpful for this job. However; I dont have any background about encoders or how to take feedback from them. It is very important to me to know my reference points position accurately.Having an error of 0.5 or so would be enough.

    The encoders I looked up till now are all seemed like industrial type. Thus I dont know if I could buy one for myself inexpensively. If you have any background about this issue could you help me?

    Thanks for your care..
     
  2. jackua

    Thread Starter New Member

    Oct 27, 2010
    7
    0
    Unfortunately, my platform will move in both directions. I wanted to post some picture or simulation but couldnt find anything suitable.

    I will move my platform automatically, till I find the correct position. Thus I need to move it in both directions. Actually, I use this system in order to check if my step motor working well. As I said accuracy in this project is very important to me. Thus I should be sure that when I tell it to move 20 degrees right it is moving exactly 20 degrees.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you need accuracy to 1/2 degree, then you'll most likely need an encoder capable of 1/4 degree resolution, or 360° * 4 = 1,440 steps, which would require 11 data bits.

    As an alternative, you might consider using a Hall-effect sensor monitoring the position of a fine-toothed steel gear (reluctor) that would act as a positive indicator that the motor's shaft had rotated a certain number of degrees. I have some of these types of reluctors that I picked up at a local salvage sales store for $1 US each.
     
    jackua likes this.
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
    928
    And to go along with the Sgt's advice, I would add that a fixed reference points,(a hardwired home position) should also be used to give the circuit concrete position information in cases of those inevitable digital hiccups, when computer controlled devices do unexplainable things. :) Limit switches as well, would be a wize ting. !
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,671
    899
    Does the platform have to go more than 360°? If not, isn't this just a simple servo-like problem that can be solved with a rotary potentiometer?

    John
     
  6. jackua

    Thread Starter New Member

    Oct 27, 2010
    7
    0
    I do have some hall effect sensors.. Do you have any related text which I can refer or get help? Because I neither use an encoder nor hall effect sensor for this kind of project before. Btw yes jpanhalt its just moving in 360° range. I will use this feedback mechanism in order to check if the motor is following my orders. I mean I want to check if it does move the exact same amount of degrees that I desire. I want the error to be less than 0.5 degrees
     
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    If you are using a step (stepper ) motor and know the home position you should know the position at all times.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    why wouldn't your platform move to the desired position assuming the step angle and geartrain allow it to land there? If your concerned about slippage, increase your torque.
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,671
    899
    I agree, it sounds like a servo to me with a stepper motor (why?). A simple multi-turn pot and 2:1 or greater gear/belt reduction will give more than the required accuracy.

    John
     
  10. jackua

    Thread Starter New Member

    Oct 27, 2010
    7
    0
    OK first of all I want to thank all of you for your helpfull feedbacks I read all of it and appreciate it. I ll try to add the pics when I could. I am controlling the platform in order to move a certain load to my desired location. When I decide the target where to move load, I enter the coordinates via MATLAB. Then platform turns to the target and I push my load with another mechanism.

    Now that it all works well with light loads. But problem is when I have higher loads, will my platform again rotate accurately? That is why I want a seperate feedback system to tell me by how many degrees my platform has turned? And what is its position w.r.t. my reference (zero) point?

    I thought to use

    * a hollow shaft rotary encoder which I connect to my shaft between the motor and platform.

    * designing my own optical encoder with cny70s

    * using electro compass (I have no idea how I will use it but someone just suggested that)

    However I have never used any of these before that is why I wanted to ask experienced seniors like you. I hope I can find the best solution

    Cheers.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    How heavy is the load, and how strong is the stepper?

    Is the load moving parallel or perpendicular to the stepper's shaft?

    This could be a bearing or torque issue, but when trying to move something to an exact spot, repeatedly, a closed loop solution is a bit better, either limit switches, or a rotary encoder.
     
  12. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    A hollow shaft encoder will cost you a bundle of money. You might consider driving an encoder off the shaft via a timing belt, however.

    Does this mechanism have to stop at any arbitrary position, or is there some limited number of positions needed? There are low-tech but very accurate ways to determine if you've got alignment to a particular place. To detect location anywhere around a circle isn't easy. Generally with a stepper system, you'd have a single switch to tell you when you passed a certain place (call it the 12 o'clock position) but from then on, you'd count motor steps. If the system miscounts steps, it's a sign that it was overloaded or (hideous thought) it wasn't properly designed.
     
Loading...