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IndustryArena Forum > Mechanical Engineering > Mechanical Calculations/Engineering Design > Adding a rotary encoder to a large Rotary Table
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  1. #1
    Join Date
    Sep 2008
    Posts
    13

    Adding a rotary encoder to a large Rotary Table

    Hi all,
    I need to add a degrees readout to a large rotary table.
    The table is around 1 meter in diameter, and I can only add the sensor to the outer rim, kind of like a trundle wheel.
    My initial idea was to just setup a rotary DRO with a rubber wheel running on the periphery, but then I though what about the gearing?
    The table is so large that I cant have a 1:1 wheel, so how to do it?

    I dont mind doing a bit of electronic jiggery pokery, but Ive not done anything like this before...

    all ideas and help gratefully recieved

    Dave

  2. #2
    Join Date
    Dec 2003
    Posts
    24216
    There must be some kind of mechanism that turns the table? An encoder could be fitted to this, the only problem is that backlash may exist.
    I recently fitted a servo to a 1.5m dia. air bearing table, in this case there was an adjustment in the gearing to take up any backlash.
    A Maple Systems 2 line display was used for the readout.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
    Join Date
    Sep 2008
    Posts
    13
    Forgot to mention that I cant modify any of the drive train, it is manually operated, and interlocked with other items.
    Currently the positioning is done by bloke counting to 10 then letting go of the button, and this adds a certain amount of variability into the process (some what understated...)
    The positioning doesnt have to be accurate, as long as we know where it has got to IYSWIM.



    Dave

  4. #4
    Join Date
    Jun 2007
    Posts
    3757
    Can you wrap a cog belt around the outside, and use it like a rack to drive an encoder?
    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.

  5. #5
    Join Date
    Dec 2003
    Posts
    24216
    What does the drive train consist of exactly, rough description?
    If there was any milling or something that resembled a gear tooth or rack gear on the underside etc, a proximity sensor could be used?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  6. #6
    Join Date
    Dec 2004
    Posts
    524
    What are the target resolution and accuracy?

    Is continuous rotation required?

    Ken
    Kenneth Lerman
    55 Main Street
    Newtown, CT 06470

  7. #7
    Join Date
    Dec 2011
    Posts
    8
    You can wrap the table in magnetic scales (the kind that is like tape) and use a standard magnetic dro sensor to pick up the ticks. You'll get very good resolution with this method.

  8. #8
    Join Date
    Jun 2007
    Posts
    3757
    And if you are machine ferrous material you will get a million whiskers.
    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.

  9. #9
    Join Date
    Dec 2003
    Posts
    24216
    The Hall effect sensor would be the better choice, the sensor material just has to be ferrous metal, you can even get non-ferrous type also.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  10. #10
    Join Date
    Dec 2003
    Posts
    24216
    Another couple of options is if an encoder is used, rubber friction wheels specially for encoder drive can be obtained, if the ratio was worked out correctly, you could use the marker pulse as a 1 pulse/degree counter, the other method may be a bit tedious and would entail drill and tap for a Allen head machine screw every degree and use a proximity switch as a counter.
    Also it has to be designed as to whether the table rotates 1 turn or continuous, the electronics would has to take account of this.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  11. #11
    Join Date
    Jan 2006
    Posts
    2985
    Encoder Products Company offers encoders with a rubber wheel and a spring loaded mount which would work well for this application. I use one to keep track of linear feet on a winding application. If you use a counter/timer readout you can use its scaling functions to divide the encoder down so that it reads in degrees.

    I am using this encoder: Model TR1 Tru-Trac

    And this counter: Rate/Total/Batch Meters

    I think that would get you pretty close to a working solution.

    Matt

  12. #12
    Join Date
    Sep 2008
    Posts
    13
    Lots of good info so far, thanks
    I started typing this last night, a bit more detail, not sure why it didnt submit...

    The Table top is 25mm thick, 1.2 meters in diameter, and plain disk on both faces. Its mild steel, with a thin stainless steel cover over it.
    The drive system is sealed inside the housing, and I have no access to it, but its a DC motor and I presume a worm/wheel affair.
    Wrapping a toothed belt round might work, but still has the gearing problem of 1 rotation of the table is many of the encoder spindle. I dont think it would be much better than a rubber contact wheel?

    The target resolution is better than 1 degree, 0.1 degrees would be ideal.
    The table can only rotate in a clockwise direction. I need to be able to cope with that without getting to a travel limit on the encoder, as it can never 'unwind' if you see what I mean.

    Dave

  13. #13
    Join Date
    Jan 2006
    Posts
    2985
    Dave

    If you use a counter for your readout, like I linked to, you can scale the encoder pulses so that 478 encoder pulses (for example) will display 1 (degree) on the readout. You can have a decimal point in there too so it would be accurate to a tenth of a degree. This counter will generally keep counting higher and higher. You could use a single hall effect sensor as your reference mark and the signal from this could reset the counter every rotation. When the hall effect sensor was almost activated, the readout would show 360 degrees and then it would be reset to 0 degrees once you hit the sensor. This would provide an absolute angle measurement and would limit cumulative error to that found in a single revolution.

    Matt

  14. #14
    Join Date
    Sep 2010
    Posts
    1765
    with that diameter, the epc or equiv rubber wheel encoder sounds perfect. what u dont seem to grasp is that u can buy a display that scales the encoder pulses to display any units you want. so about any encoder rubber wheel thing (say 1000 or 1024ppr) will work as long as ur display is scalable. newall in UK sells a SA100 simple display that is scalable, so do folks like EPC, danaher controls, and red lion. $ 300 for encoder with rubber wheel plus $ 200 for display and u r good to go. if u need specific part nos & prices as examples pm me and I will send or just google it all.

  15. #15
    Join Date
    Sep 2008
    Posts
    13
    Mike,
    cheers, Ive googled the SA100, and will give newall a call after the weekeknd

    I think the rubber encoder is the way to go, and Ive found a uk source.

    I had a feeling this should be simple, I was just out of my depth. :drowning:

    Dave

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