Automatically Reversible Motor

Discussion in 'The Projects Forum' started by robbo89, Jul 18, 2013.

  1. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    Hi all,

    I've been given a project to look into for my company and basically I need to come up with a design to automatically raise and lower a magnet using a small motor.

    The idea at the minute is to attach the magnet to a piece of string/wire and use a motor to wind up the string to raise the magnet. After the magnet has been raised to a set height, the motor then needs to reverse to lower the magnet. Ideally, this needs to happen continously for an 8 hour period without any user interaction other than initially turning the system on. The magnet is small so there are no significant loads involved. The magnet does not need to be raised particularly quickly but does need to have a vertical displacement around 600mm.

    I thought a stepper motor was ideal for this purpose as it can be set to turn a set number of revolutions and then perform the same number of revolutions in reverse. However, I am a Mechanical Engineer and must confess I know absolutely nothing about electronics! So bearing that in mind I would like to know exactly what is required to create the set up I mentioned above.

    I apologise for the extremely sketchy information but this is still in the very early design stages, and I will try and answer any questions as I go along. I would be extremely grateful for any help. Thanks very much.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    So basically a self-reversing garage door?

    Since you're moving a magnet to begin with, perhaps you could just place a reed switch - to be triggered by the magnet - at each end of travel? From that signal it would be relatively easy to reverse the motor.

    Have you chosen a motor? Can we assume you have mains power nearby, so that you could power it with a wall wart for instance?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    As a mechanical engineer I wonder why a simple cam setup with a continuously rotating motor was not selected? Only one direction is needed.
     
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  4. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    The unit has to be quite compact and a cam setup would require a large cam or lever system to give the amount of displacement needed. It is still an option but very much a last resort at this stage.

    The reed switches are a great idea though it may not be possible to mount them near enough to the magnet. How would I go about setting these up to reverse the polarity of the motor?

    A motor hasn't been chosen yet though I would expect there to her mains power nearby which would be ideal considering the motor has to run all day.
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    Steppers are fine, but you have to use some sort of mechanism to detect an initial position, and then to count steps. Alternatively, use limit switches top and bottom, and have electronics that just runs the motor from one to the other, no need to count steps at all.

    Alternatively, use a d.c. motor and limit switches (could be reed switches detecting the magnet) with a relay that gets set at one end of travel and reset at the other, which reverses the motor.
     
  6. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    This sounds like a very simple solution that would work well. However, please forgive me for being slow but when I said I knew nothing about electronics, I really meant it!

    As I understand it I need:
    • Power Supply
    • DC motor
    • 2 x limit/reed switches
    • 1 x relay

    If it's not too much trouble can I ask what type of relay would be required and how would I go about wiring everything up?
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    In theory the relay needs to be at least a 3-pole type, where two of the poles form a crossover switch that reverses the motor. The third pole would be a "holding" circuit that keeps the relay energized while the motor is running in reverse, and this current path would be broken, or shunted to Gnd, when the motor hits the end of its travel. I did say "in theory" though; 2-pole relays are very common, 3 or more poles not so much, so it might be worth doing some figuring to add the latching action to a 2-pole crossover circuit somehow. I'm not sure how that might work!

    This web page shows how the relay gets wired to drive the motor, but not how to control it:
    http://electronics.stackexchange.com/questions/12551/standard-way-to-reverse-a-motor
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How many times does the up/down movement need to occur in an 8-hour period?

    What dimensions do you require regarding being compact?

    What is the weight of the magnet?

    Is the magnet reacting with anything?

    Lot's of details always help to get the best solutions. :)
     
    Last edited: Jul 18, 2013
  9. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    So without a 3 poles, the relay will become de-enegized when the magnet moves away from the limit switch? Whereas with a 2 pole relay and a latching relay, the 2 pole relay will remain energized even when the magnet moves away from the limit switch, until it hits the limit switch at the opposing end of travel?

    Came accross this post on another forum that seems to show examples of both 3PDT relay and 2PDT relay with 2 coil latching relay:

    http://www.electro-tech-online.com/...otor-polarity-using-reed-switches-relays.html

    Would any of those work for me?

    I would estimate that we would like a full cycle (600mm horizontal displacement each way) to occur in less than 4 minutes so we're looking at at least 120 cycles throughout the day.

    The original idea was to fit the system inside a cylinder that has a bore of around 250mm and a height of 600mm. However, as the display piece will be on a table it will be possible to place the mechanism under the table out of view which allows the unit to take up a lot more room. That's the reason we may be able to fall back on a more mechanical system (and stick to what I know!) if we can't find a more elegant solution using electronics.

    The weight of the magnet is no more than 300g. It will be reacting with a magnetic flapper indicator system used on piston accumulators.
     
  10. John P

    AAC Fanatic!

    Oct 14, 2008
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    All right, here is my suggestion using a 2 pole relay.
    [​IMG]

    When you apply power, the relay isn't energized and the motor will run in the forward direction (+v to the + terminal, Gnd to the - terminal, via relay contact 3-4 and 8-6). The motor runs until switch S1 is activated, at which point the relay coil is powered and the contacts change over. Current to the motor now flows from +V via relay contacts 3-5 and from the motor to Gnd via contacts 7-6.

    The motor now travels in the reverse direction, and very soon, switch S1 opens and current can no longer flow that way. But with the relay activated, a path for current exists via R1 to Gnd via relay contacts 7-6, and this is sufficient to keep the relay operated (typically, a relay will remain held in the activated position with a current only half as much as the current needed for initial operation). So the motor runs until switch S2 closes. When this happens, the relay coil has +V on both terminals, so no current can flow, and the relay will become deactivated. In the instant before the relay drops, current will flow direct from +V to Gnd via S2, R1 and contacts 7-6, but the resistor limits this current to a safe level, and it really is just an instananeous condition.

    A reasonable value for R1 would be half the resistance of the relay coil. You need to select the relay first, then R1.
     
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  11. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    Thanks very much, that's a massive help. Just to confirm I will need a latching rather than non-latching relay right? Is the resistance of the relay important or can I just choose anything?
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
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  13. John P

    AAC Fanatic!

    Oct 14, 2008
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    No, that's just a regular relay in my diagram. The external components are what make it "latch"!

    You could use a real latching relay, though it's not necessary. They aren't as common, but as BillB3857 says, they're in the catalogs.
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    One of the latching relays I referenced is used by a local water company to do some remote control through a two wire system. A pulse of one polarity turns it on and a pulse of the opposite polarity turns it off. Diode steering selects which coil gets activated. The coils are also polarity sensitive, if I remember correctly.
     
  15. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    OK despite incorrectly ordering a single coil latching relay and then ordering a non-latching relay in order to construct John P's circuit, I believe the dual coil latching relay will be by far the simplest method. However, I'm having difficulty understanding exactly how they work.

    If I were to use the following relay:

    http://docs-europe.electrocomponents.com/webdocs/0b64/0900766b80b642f5.pdf

    It mentions "Both coils can be used either as set or reset coils". Can the two coils still work independently? For example onse switch to operate one coil to set the latch, and the other switch to operate the other coil to reset the latch. Or do both coils need to be activated simultaneously in the set position to operate the latch and vice versa?
     
  16. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Both coils can be used independently. Note the polarity of the coils, used to set/reset. The limit switches at each end can be assigned to their own coil, simplifying wiring. I am on my phone, so cannot provide a schematic at this time. Hopefully, you get the idea.
     
  17. robbo89

    Thread Starter New Member

    Jul 18, 2013
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    Okay I think I can understand the dual coil circuit easily enough but I now have the rather embarrassing problem about not knowing how to mount and connect everything. Is it a case of just soldering wires together and leaving everything loose? Is there a less permanent option like using something similar to a breadboard? Once again, sorry for the incredibly stupid questions!
     
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