[sudo-discuss] vending machine homing switch mystery solved

[Jake]

bump!  does anyone want to help get one or both vending machines working?  This
would be a great way to vend arduinos, ESP8266's, candy bars, and whatever else
we want to sell to sudoers and others.  Much of the work has already been done.

-jake

---------- Forwarded message ----------
> Date: Sat, 9 Apr 2016 17:42:06 -0500
> From: jake at spaz.org
> 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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