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Discussion in 'General Electronics Discussion' started by davidebt, Nov 6, 2013.

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  1. davidebt

    davidebt

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    Nov 6, 2013
    Hi Guys I'm new to the forum (as many often are when wanting something and I'm no different).

    I am a recently retired toolmaker who has set up a workshop in my garage. I know little or nothing about instrumentation electronics / robotics so please be gentle.

    I need to make a 12 station 5mm thick aluminium disc of 250 mm diameter index automatically and quickly to a point then stop and dwell for a variable interval of between 1 and 30 seconds to locate components under a small gas flame.

    What can I buy or make to achieve this? I am using a low voltage, 30 RPM motor geared through a Tamiya gearbox to drive the disc.

    Looking forward to your help and advice
    Kind regards David
     
  2. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    Take a look at using a stepper motor and controller.
     
  3. davidebt

    davidebt

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    Nov 6, 2013
    Thanks

    Thanks for response but you might as well speak Dutch to me as I don't even know what a stepper motor is let alone choose one and then a suitable controller.

    Needs to be an idiot proof response thanks!
     
  4. BobK

    BobK

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    1,688
    Jan 5, 2010
    A stepper motor moves in discrete, repeatable steps. So if you once know how the disc is poisitioned, you can calculate how many steps to advance, in which direction to get to the desired position, then perform that number of steps. You would need some method of finding one specific position on the disc to calibrate each time it is powered up. That could be an optical or magentic sensor.

    The other way to do it, with a normal motor, is to have the disc somehow report it's position. One way to do that would be to have have black and white stripes at each desired station that would identify it, read by a light source and detector. The electronics would then rotate the disc unitl it sees the desired sequence of black / white and then stop. You would need 4 stripes to uniqely identify the 12 positions. And probably one more to indicate the start of a marker.

    A third way is to use a rotary encoder, which sends a signal each time the disc has moved a specific angle. Then it become much like the stepper solution, you find a start position by some other means, then count the steps to get to the desired position.

    Bob
     
    Last edited: Nov 6, 2013
  5. davidebt

    davidebt

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    Nov 6, 2013
    Thank you Bob for your very interesting replies I think I can understand each option..

    Do you have any circuit diagrams for the options or is there a possibility to buy such things say from from somewhere like Maplin? Regards
    David
     
  6. BobK

    BobK

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    1,688
    Jan 5, 2010
    I don't think you are going to find a ready-made solution. Either concept is best implemented using a microcontroller which involves not only circuit design but programming as well. Not a project for a beginner.

    How do you envision opearting the device? Is there a rotary switch to select which position to go to? Or manybe a pushputton or two to just move it to the next station? The latter might be a lot simpler and doable without complex electronics.

    Bob
     
  7. davidebt

    davidebt

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    Nov 6, 2013
    Description




    Hi, if you have a look at the video you will see what I am trying to achieve.

    Kind regards
    David
     
  8. BobK

    BobK

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    1,688
    Jan 5, 2010
    Okay, so in that video, what I see is that the disc simply waits several seconds then automatically advances to the next station. Is that what you are looking for? Does the wait time need to be adjustable?

    Bob
     
  9. davidebt

    davidebt

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    Nov 6, 2013
    Hi Bob

    Yes the wait time needs to be adjustable between about 3 and 12 seconds according to the size of brass being annealed.

    Regards
    David
     
  10. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    So you need 3 things. A timer that pulses every 3 - 12 seconds, adjustable. A method of detecting when the disc is at one of the 12 positions. And a control circuit that runs the motor.

    The first is easily done with an NE555 timer chip. You might even be able to find one that would be suitable already made up.

    The second part could be done by various means. An optical sensor mounted under the disc that would see a reflector only at the index positions would give you a signal when the reflector was over the sensor. Or you could even use a mechanical level switch for this with a portrusion from the disc pushing it when it comes into position.

    Finally, the circuit to control the motor would run the motor whenever it sees the signal from the timer or when the sensor is not seeing light. This would activate a relay or electronic switch (MOSFET) to turn on the motor.

    Bob
     
  11. davidebt

    davidebt

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    Nov 6, 2013
    Thank

    Many thanks Bob that has been most helpful and given me one or two ideas to progress.

    Kind regards
    David
     
  12. Brianj_92505

    Brianj_92505

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    Nov 2, 2013
    It’s actually a simple problem. The turn table in your video shows a 12 sided polygon so you just mount a micro switch so that the 12 high points contact the switch pin. Use a NO/NC type switch. Here is a simple timer circuit using a NE555 timer. Capacitor C1 charges through R1 and Discharges through R2 and Variable resistor 2. With values of C1 at 100uf, R2 at 20K and VR2 at 100k you get a delay time from 2.2 seconds to 12.2 seconds. The output at pin 3 can drive the turn table motor if it’s under 200ma or drive a relay if you need more. Here’s how it goes. The circuit is set, the discharge pin is off so the capacitor is charging and output is high so the motor is spinning. C1 charges rapidly through R1 and is completely charged by the time the table lobe closes the switch. When the switch closes the common pole connects to Vcc and sends that to the threshold of the timer resetting it. The output goes low and the motor stops spinning. The discharge pin goes on and the capacitor begins discharging through R2/VR2. After 2.2 to 12.2 seconds the trigger voltage falls below trigger level and the the timer sets and begins the whole process again.

    [​IMG]
     
  13. davidebt

    davidebt

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    Nov 6, 2013
    Thanks

    Hi Brianj

    Solution sounds great - simplicity is my keystone! just what I am looking for.

    I think I am capable of building of what you show on a "breadboard" and give it a try.

    Many thanks.

    David:)
     
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