[sudo-discuss] vending machine homing switch mystery solved

[Mitch Altman]

That is a bizarre way of detecting "home".  Since there's a switch, wouldn't it be easier to just connect the switch directly to the Arduino to detect "home"?


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> Date: Sat, 9 Apr 2016 17:42:06 -0500
> From: jake at spaz.org
> To: sudo-discuss at lists.sudoroom.org; adam at aperture.systems; organic_unity at yahoo.com; emmaselephants33 at gmail.com; echoudhry at gmail.com
> Subject: [sudo-discuss] vending machine homing switch mystery solved
> 
> so i've been working on the black sudoroom vending machine (vending1):
> 
> https://sudoroom.org/wiki/hack-o-mat
> 
> which had previously been hacked by students at UC Berkeley (i think)
> 
> I believe it's a Snacktron like Emma and Ed's machine.
> (unlike our brown vending machine which is a snackshop II)
> pictures of their machine can be seen here:
> http://spaz.org/~jake/pix/vend/
> 
> they had ripped out the original control unit, and wired it up with a 
> bunch of reed relays and an arduino.  The motors of the vending machine 
> are wired as a grid of rows and columns, so the controller needs to be 
> able to enable a single row and column in order to power one product 
> motor.
> 
> But it also needs to be able to detect when the product motor has rotated 
> a full turn, and is back at the "home" position where it needs to stop. 
> There's a "home" sensor on the back of the motor, but the way it's wired 
> was a mystery for a while.  Here's some pictures of the homing circuit:
> 
> http://spaz.org/~jake/pix/vend/IMG_20160401_015211.jpg
> http://spaz.org/~jake/pix/vend/IMG_20160401_015700.jpg
> 
> as you can see, the only difference between the button being pressed or 
> not pressed, is that the 420nF capacitor is in parallel with the motor or 
> not.  Since the whole thing is in series with a diode (to protect against 
> trying to turn a motor backwards) we were stumped as to how to detect 
> whether a motor was homed or not.
> 
> Thanks to Zach who donated some test equipment including this awesome 
> signal generator (along with a nice power supply and bench multimeter):
> http://imgur.com/LTSFfOe
> 
> I was able to connect an AC waveform in series with a 12VDC supply to one 
> of the motors, and figure out how to detect whether the home switch is 
> closed or not.  Basically, you make a 31KHz (or similar frequency) 
> squarewave in series with the power supply, and watch the current through 
> the motor.  If the button is open and the capacitor is not in-circuit, the 
> motor's inductance absorbs the AC and the current draw is relatively 
> smooth.  But when the motor arrives at the home position (when you need to 
> stop delivering product!) the capacitor gets switched in, and now you'll 
> see a lot of the AC on the current path of the system.
> 
> Here is a video showing the voltage across a 22-ohm series resistor while 
> the motor is rotating with 12VDC:
> 
> https://www.youtube.com/watch?v=xHIGDw5HRCg
> 
> apparently it takes that motor about 11 seconds to dispense a product and 
> finish returning to home position, assuming the product weighs nothing.
> 
> by the way, you'll notice that i'm using the old analog oscilloscope. 
> Unfortunately the LeCroy 9510 has started crashing, basically 
> bluescreening.  It probably needs to be opened up and cleaned, as it got 
> wet after a fire before it was donated to sudoroom.
> 
> Or maybe someone wants to donate a nicer oscilloscope to sudoroom?
> 
> anyway, now I know how to detect the motor homing signal, and I just need 
> to wire up this machine to its arduino to detect the signal and dispense 
> product more accurately.  If anyone wants to help, please let me know.
> 
> by the way, if you are wondering how the UC students detected when the 
> motors were finished dispensing product, the answer is absurd:
> 
> they put a laser beam across the path where product would fall when 
> dispensed, and they watched for the laser beam to be interrupted.
> 
> did it work?  I don't know.  probably the semester ended and they got 
> their grade and moved on, and then they donated the vending machine to us.
> 
> -jake
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