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Author Topic: Optical or Hall Effect sensing the home position of a unipolar stepper motor  (Read 971 times)
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Hi all - I'm very new to the Arduino forum so feel free to correct my etiquette where appropriate. I've done some searching on google and the forum for answers to my question and haven't quite found my answer yet.

Here is the situation:

I have a project that I am using a unipolar stepper motor to rotate a split-flap display. These are the "click-clacking" boards that you used to see in railroad terminals. Here's an example image for reference:



Anyways, in my application the split flap display will be used to display a score achieved by a game player. So at the beginning of each game, the flaps have to reset themselves to 0. This could be after the power was turned off and came back on to the game, so there may be no stored reference to the "home" position at the time of the game reset. When the game starts up, the motor will start rotating until the "home" position is sensed and at that point it stops and waits for the gameplay to start. I'm thinking of using a photoelectric sensor or a hall effect sensor for this job, but am a little lost on how to implement it.

Do you know of any applications that use this type of home sensing for stepper motors? Maybe there is a DC motor application that I could modify and just haven't found yet? Any help you can provide on the general set-up would be appreciated.

Thanks
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You could put a magnet on the back of the 0 card and use a Hall Effect switch to sense it.
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@johnwasser - yeah that's exactly the kind of thing I'm looking for. How to mount the magnet (if the hall effect sensor is the way to go) so that it only triggers the home state at the proper time.

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You could put a magnet on the back of the 0 card and use a Hall Effect switch to sense it.

The problem I found after thinking about this suggestion is that the back of every card is actually the front of the next card, so I can't mount something there or it will cover my display. As an example, on the back of the card that has the top half of the numeral "0" will be the bottom half of the numeral "1". So when that flap flips over, you see and display that reverse side. Hopefully that makes sense. So attaching a magnet somewhere could still work, but it can't be on any face of the cards.

Maybe the magnet needs to be mounted in a stationary position and a notched wheel "reveals" the magnet at the right time to the sensor?

Another thought I had while reading over my post was to use a micro switch. Maybe the wheel holding the flaps has a notch or bump on it at the home position that presses a microswitch as it comes around.
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I didn't think of that.

Do you have the units already?  There may be something on the cards (like a tab) or the hub (like a notch, tab, or pin) that is designed to be sensed.
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Do you have the units already?  There may be something on the cards (like a tab) or the hub (like a notch, tab, or pin) that is designed to be sensed

I should've been clearer earlier, I'm actually building these from scratch. Probably using a CNC to create the split-flap assembly (cards + spindle/gears). Your suggestion about a tab/notch/pin got me thinking that I could actually put a small tab on the spindle holding the cards and align it with the home position. Then I can use a photo interrupter to register when that tab passes by.

This is the interrupter I'm thinking of using:
https://www.sparkfun.com/products/9322

I'll let you know how it works out!
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If going the optical route, for mechanical reasons you may not want to use a tab. If something gets mis-aligned during operation the tab might hit the side of sensor instead of passing through the slot. If you use a continuous wheel, (either an opaque wheel with a hole or a clear wheel with a darkened spot), then the wheel constantly being in the gap of the sensor would keep it lined up.

Good luck with your design.
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@Sembazuru - Thanks for the advice! I'll try a notched disc or something like it to reduce the chance of misalignment and part failure.
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This may give you some ideas
http://www.instructables.com/id/Arduino-based-Split-Flap-game/
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